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Neurology_Adams. spontaneous activity that is characterized by irregularly discharging high-frequency (50to 100-Hz) biphasic spike discharges, 100 to 300 μV in amplitude (i.e., large enough to cause an isolated muscle action potential). These potentials have been termed endplate spikes and represent discharges of single muscle fibers excited by spontaneous activity in nerve terminals. They must be distinguished from fibrillation potentials (see later). Finally, insertion of the needle electrode into the muscle injures and mechanically stimulates a number of fibers, causing a burst of potentials of short duration (300 ms). This is referred to as normal insertional activity, but the extent of this activity is greatly raised in certain pathologic states as noted below.


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Neurology_Adams. When a muscle is voluntarily contracted, the depolarization potentials of motor units begin to appear. One can observe a pattern of force build up by watching the progressive recruitment of MUPs; the initial ones, representing smaller motor units, firing at rates of 5 to 10 per second. With increased force of contraction, there is an increased rate of firing (40 to 50 per second as well as a recruitment of larger, previously inactive motor units; Fig. 2-17A). Because individual MUPs can no longer be distinguished during maximal voluntary contraction, this activity is referred to as a complete interference pattern (Fig. 2-17A, right). This is seen not only as a summated signal pattern but is also heard as a mixed high-frequency clicking when the electrical activity is made audible. As muscles relax, an increasing number of units drop out. If a muscle is weakened by denervation or if electrical conduction is blocked, there will be fewer MUPs, but the remaining ones will still show a


Neurology_Adams

Neurology_Adams. relax, an increasing number of units drop out. If a muscle is weakened by denervation or if electrical conduction is blocked, there will be fewer MUPs, but the remaining ones will still show a rapid firing rate (reduced recruitment; see Fig. 2-17B). In contrast, with poor voluntary effort and with upper motor neuron lesions, the MUPs fire in decreased numbers, at slower rates, and often in an irregular pattern (termed poor activation).


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Neurology_Adams. The Abnormal Electromyogram Clinically important deviations from the normal EMG include (1) increased or decreased activity upon insertion of the needle; (2) the occurrence of abnormal “spontaneous” activity during the relaxed state (fibrillation potentials, positive sharp waves, fasciculation potentials, cramp potentials, myotonic discharges, myokymic potentials); (3) abnormalities in the amplitude, duration, and shape of single MUPs; (4) a decrease in the number of MUPs and changes in their firing pattern such as recruitment discussed above; (5) variation in amplitude and number of phases of MUPs during voluntary contraction; and (6) the demonstration of special phenomena such as in states of continuous muscle fiber activity or electrical silence during shortening of the muscle (physiologic contracture). The underlying physiology of these changes is discussed in Chap. 45, in relation to diseases of the muscle.


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Neurology_Adams. Insertional activity At the moment the needle is inserted into muscle, there is a normally brief burst of action potentials that ceases once the needle is stable, provided that it is not in a position to irritate a nerve terminal. Increased insertional activity, however, is an abnormal finding seen in most instances of denervation as well as in many forms of primary muscle disease and in disorders that dispose to muscle cramps. In cases of advanced denervation or advanced myopathy, in which muscle fibers have been largely replaced by connective tissue and fat, insertional activity may be decreased and there is a palpable increase in the mechanical resistance to the insertion of the needle.


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Neurology_Adams. Abnormal spontaneous activity With the muscle at rest, spontaneous activity of single muscle fibers and of motor units, known as fibrillation potentials and fasciculation potentials, is abnormal. The two phenomena may be confused. Fibrillation is the spontaneous contraction of a single muscle fiber. It occurs when the muscle fiber has lost its nerve supply and is ordinarily not visible through the skin (but may be visible in the tongue). Fasciculation represents the spontaneous firing of an entire motor unit, causing contraction of a group of muscle fibers, and may be visible through the skin. The irregular firing of a number of motor units, seen as a rippling of the skin, is called myokymia.


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Neurology_Adams. Fibrillation potentials Destruction of a motor neuron or interruption of its axon causes the distal part of the axon to degenerate, a process that takes several days or more. The muscle fibers formerly innervated by the branches of the dead axon—that is, the motor unit—are thereby disconnected from the nervous system. By mechanisms that are still obscure, the chemosensitive region of the sarcolemma at the motor endplate “spreads” after denervation to involve the entire surface of the muscle fiber. Then, 10 to 25 days after interruption of the axon, the denervated fibers develop spontaneous activity; each fiber contracts at its own rate and without relation to the activity of neighboring fibers. This spontaneous activity, fibrillation, is associated with a random conglomeration of brief dior triphasic fibrillation potentials (Fig. 2-18A) having a duration of 1 to 5 ms and rarely exceeding 300 μV in amplitude. When brief spontaneous fibrillation potentials of this sort are observed


Neurology_Adams

Neurology_Adams. dior triphasic fibrillation potentials (Fig. 2-18A) having a duration of 1 to 5 ms and rarely exceeding 300 μV in amplitude. When brief spontaneous fibrillation potentials of this sort are observed firing at two or three different locations (outside the endplate zone) of a resting muscle, one may conclude that the fibers are denervated. In addition, fibrillation potentials may take the form of positive sharp waves, that is, spontaneous, initially positive diphasic potentials of longer duration and slightly greater amplitude than the spikes of fibrillation potentials. Usually, fibrillation potentials discharge at an almost regular rate but after 6 to 8 weeks, irregularly firing fibrillation potentials may be observed.


Neurology_Adams

Neurology_Adams. Fibrillation potentials continue until the muscle fiber is affected by one of several processes: reinnervation by regeneration of the interrupted nerve fiber, by outgrowth of nearby remaining nerve fibers (collateral sprouting), or replacement of the atrophied fibers by connective tissue, a process that may take years. Diseases such as poliomyelitis, which damage spinal motor neurons, or injuries of the peripheral nerves or anterior spinal roots, frequently produce only partial denervation of the involved muscles. In such muscles, one electrode placement may record fibrillation potentials at rest from denervated fibers and normal potentials during voluntary contraction from nearby healthy fibers. Fibrillation potentials, while characteristic of neurogenic denervation, are not altogether specific; for example, they are seen in muscle diseases such as polymyositis and inclusion body myopathy which presumably damage the muscle fiber and make its membrane electrically unstable.


Neurology_Adams

Neurology_Adams. Fasciculation potentials A fasciculation is the spontaneous or involuntary contraction of the muscle fibers of a single motor unit or part of a unit. Fasciculation potentials are evidence of motor nerve fiber irritability and are generally markers of denervation and reinnervation of muscle. Such contractions cause a visible dimpling or twitching under the skin, and infrequently they are of sufficient force to move a small distal joint. They occur irregularly and may be infrequent so that prolonged inspection of the skin overlying a muscle may be necessary to detect them. The accompanying electrical form of any individual fasciculation potential is relatively constant. Typically, a fasciculation potential will have three to five phases (“polyphasic” as described later, in contrast to normal biphasic muscle activity), a duration of 5 to 15 ms (far longer than fibrillation potentials and more dispersed than a typical CMAP), and an amplitude of several millivolts (see Fig. 2-18B). The


Neurology_Adams

Neurology_Adams. biphasic muscle activity), a duration of 5 to 15 ms (far longer than fibrillation potentials and more dispersed than a typical CMAP), and an amplitude of several millivolts (see Fig. 2-18B). The combination of fibrillations and fasciculations indicates active denervation combined with more chronic reinnervation of muscle.


Neurology_Adams

Neurology_Adams. The precise mechanism of fasciculation is still contested. Forster and colleagues challenged the original belief that the discharge originated in anterior horn cells by demonstrating that in disease of the anterior horn cell, fasciculations persisted after nerve block. These observations favored a distal axonal site of generation. It seems that several regions of the axon are capable of spontaneous impulse generation, depending on the underlying disease. Most of the diseases that produce fasciculations involve the anterior horn cell or the motor root, but more distal sites in the motor axon may become spontaneously active in cases of nerve compression.


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Neurology_Adams. Occasional fasciculation potentials, particularly in the calves, hands, and periocular or paranasal muscles, occur in many normal persons. They can be almost constant for days or weeks on end, or even for years in some individuals, without weakness or wasting; therefore, they need not be taken as evidence of disease (“benign fasciculations”). Certain quantitative features of fasciculations, such as brief duration and a consistent pattern and location of firing on the needle EMG examination, favor benign over pathologic discharges. Shivering induced by low temperature and twitchings associated with low serum calcium levels are other forms of fasciculatory activity. The main diseases that cause fasciculations are discussed in Chap. 45 and other related sections of the book.


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Neurology_Adams. Other types of spontaneous and elicited electrical activity (See also Chaps. 45 and 46) These various phenomena can be classified according to their generating source. The muscle fibers themselves are the source of fibrillations, positive sharp waves, and complex repetitive discharges (CRDs). The motor axons produce fasciculation potentials, myokymic discharges, neuromyotonia, and cramp syndromes; and the CNS is the source of complex ensembles of continuous motor activity such as occur in the stiff man syndrome, all described just below.


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Neurology_Adams. The common phenomenon of complex repetitive discharges, referred to in the past as bizarre high-frequency discharges, consists of repetitive spontaneous potentials created by numerous single muscle fibers that fire in near synchrony; there is often an erratic configuration and abrupt starting and stopping of the discharges. They are seen in some myopathies, in hypothyroidism, and in certain denervating disorders, and are a mark of chronicity (lesions >6 months old). High-frequency coupling of action potentials into doublets, triplets, or higher multiples of single units, indicating instability in repolarization of the nerve fiber to a muscle, occurs in tetany and in the early stages of myokymia.


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Neurology_Adams. Myokymia is a persistent quivering and rippling of muscles at rest. The EMG picture is distinctive. The spontaneously firing MUPs are called myokymic potentials, or discharges and consist of groups of repetitive discharging units, each firing at its own rate, quasirhythmically, usually several times per second, followed by a briefer period of silence. The small motor unit discharges may occur singly or as doublets, triplets, or multiplets.


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Neurology_Adams. The phenomenon of myotonia, or neuromyotonia denotes a failure of voluntary relaxation of muscle because of sustained firing of the muscle membrane (see Chaps. 45 and 46), is characterized by high-frequency repetitive discharges generally having a positive sharp waveform. These myotonic discharges wax and wane in amplitude and frequency, producing a “dive-bomber” sound on the audio monitor. The discharges can be elicited mechanically by percussion of the muscle or movement of the needle electrode and are also seen following voluntary contraction or electrical stimulation of the muscle via its motor nerve. The MUPs may appear normal during voluntary contraction, but they are not followed by the silence that normally occurs on relaxation; instead, there is a “prolonged afterdischarge” consisting of long trains of fibrillation-like potentials that may take as long as several minutes to subside (Fig. 2-19A). These EMG findings can be seen with any myotonic disorder. If the muscle is


Neurology_Adams

Neurology_Adams. consisting of long trains of fibrillation-like potentials that may take as long as several minutes to subside (Fig. 2-19A). These EMG findings can be seen with any myotonic disorder. If the muscle is activated repeatedly at short intervals, the late discharge becomes briefer and briefer and eventually disappears (see Fig. 2-19B), as the patient becomes able to relax the exercised muscle (“warmup” effect).


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Neurology_Adams. In paradoxical myotonia the myotonia worsens after each of a succession of voluntary contractions. This is the converse of what happens in myotonia congenita (Thomsen disease). As shown by single-fiber EMG studies, myotonia is generated by single muscle fibers and the mechanism of the membrane instability, at least in some forms, seems to involve changes in the chloride conductance. These disorders are discussed in subsequent chapters. The cramp-like contracture of McArdle disease and phosphofructokinase deficiency is associated with electrical silence of contracting muscle. This feature is an important part of the definition of true physiologic muscle contracture (as distinguished from chronic shortening of a muscle and its tendon which, strictly speaking, is a pseudocontracture).


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Neurology_Adams. In the syndrome of continuous muscle fiber activity or Isaacs syndrome (see Chap. 46), which is a generalized form of myokymia, the EMG discloses high-frequency (up to 300-Hz) repetitive discharges of varying waveforms. In the stiff man syndrome, painful muscle spasms and stiffness are generated by a spinal mechanism; the EMG potentials resemble normal motor units but are abnormal by virtue of continuous firing at rest. Abnormalities in Amplitude, Duration, and Shape of Motor Unit Potentials Figure 2-20 depicts the ways in which disease processes affect the motor unit and the appearance of the MUP in the EMG.


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Neurology_Adams. Abnormalities in Amplitude, Duration, and Shape of Motor Unit Potentials Figure 2-20 depicts the ways in which disease processes affect the motor unit and the appearance of the MUP in the EMG. Motor unit potentials in denervation Early in the course of denervation of muscle by disease of the nerve, many motor units with functional connections to the spinal cord are unaffected, and although the number of MUPs appearing during contraction is reduced, the configurations of the remaining ones are quite normal. In time, the remaining MUPs often increase in size and in electrical amplitude, perhaps two to three times normal, and become longer in duration and sometimes polyphasic (more than four phases).


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Neurology_Adams. Such large and sometimes giant polyphasic potentials (Fig. 2-20C) arise from motor units containing more than the usual number of muscle fibers that are spread out over a greatly enlarged territory within the muscle. Presumably, new nerve twigs have sprouted from nodal points and terminals of undamaged axons and have reinnervated previously denervated muscle fibers, thus adding them to their own motor units. Soon after reinnervation, the MUPs generated will be low in amplitude, extremely prolonged, and polyphasic, findings that constitute a transitional configuration of early reinnervation. These amplitudes disappear as the motor unit is reestablished. Increased amplitude is usually associated with very chronic, proximal axon loss, for example, with remote poliomyelitis and chronic radiculopathy. These MUPs are to be differentiated from (1) polyphasic potentials of normal duration, which, as has been mentioned, make up as much as 10 percent of the total number of MUPs in normal


Neurology_Adams

Neurology_Adams. radiculopathy. These MUPs are to be differentiated from (1) polyphasic potentials of normal duration, which, as has been mentioned, make up as much as 10 percent of the total number of MUPs in normal muscle, and (2) polyphasic MUPs of short duration and low amplitude, which are characteristic of most myopathies and of myasthenia gravis and other disorders of neuromuscular transmission.


Neurology_Adams

Neurology_Adams. The motor unit potential in myopathy As Fig. 2-20B shows, diseases such as polymyositis, the muscular dystrophies, and other myopathies that randomly destroy muscle fibers or render them nonfunctional, and obviously reduce the population of muscle fibers per motor unit. Therefore, when such a unit is activated, its potential is of lower voltage and shorter duration than normal, and it may also appear polyphasic as the compound MUP becomes fragmented into its constituent single-fiber potentials. Slowing of the propagated muscle fiber action potential in affected muscle fibers also contributes to the changes in the “myopathic” MUP. When most of the muscle fibers are affected, the MUPs are very small and of short duration and are recruited out of proportion to the tension generated, the so-called early recruitment. Both types of alterations produce a characteristic high-pitched crackling sound from the audio monitor that has been likened to rain falling on a tin roof. They occur in all


Neurology_Adams

Neurology_Adams. early recruitment. Both types of alterations produce a characteristic high-pitched crackling sound from the audio monitor that has been likened to rain falling on a tin roof. They occur in all forms of chronic myopathies. Identical MUP changes are seen occasionally with other processes that cause disintegration of the motor unit, for example, early Guillain-Barré syndrome (because of conduction block along some of the terminal nerve fibers), and rarely with disorders of neuromuscular transmission (myasthenia gravis, other myasthenic syndromes), but they are most characteristic of primary muscle disease.


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Neurology_Adams. Abnormalities of the interference pattern Diseases that reduce the population of functional motor neurons or axons within the peripheral nerve decrease the number of motor units that can be recruited in the affected muscles. The decreased number of motor units available for activation produces a low-amplitude interference pattern with only a few remaining units firing at a moderate to rapid rate. A severe reduction in the interference pattern may result in the recruitment of only a single unit (see Fig. 2-17B). Structural damage to nerve, as well as demyelinating block, can produce this pattern of reduced recruitment; indeed, a reduced recruitment pattern coupled with the absence of denervation usually indicates a conduction block.


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Neurology_Adams. If muscle power is reduced in diseases such as polymyositis or muscular dystrophy, in which individual muscle fibers are affected, there may be little or no reduction in the number of motor units available for recruitment until the process is far advanced and entire MUPs have been lost as a result of random loss of all their constituent muscle fibers. Nonetheless, each motor unit will consist of fewer muscle fibers than normal, so more motor units must be activated to reach a certain degree of force. A modest effort can thus produce a full interference pattern despite marked weakness (increased recruitment). Because fewer muscle fibers are firing, the amplitude of the pattern will be reduced from normal. This type of full, highly complex interference pattern of less-than-usual amplitude in the face of dramatic weakness is the hallmark of myopathy (see Fig. 2-17C).


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Neurology_Adams. This is a special technique for the recording of single-muscle-fiber action potentials that has found utility in measuring muscle fiber density and in detecting the so-called jitter in disease of the neuromuscular junction, particularly myasthenia gravis. Jitter is the variability of the interpotential interval of successive discharges of two single muscle fibers belonging to the same motor unit. This phenomenon is largely a result of the very slight variability of delay at the branch points in the distal axon and by synaptic delay at the neuromuscular junction, especially in myasthenia gravis where it has found its main clinical use. Fiber density and jitter may, however, also be increased in neuropathic disorders that cause denervation with reinnervation. Both are usually normal or only slightly increased in myopathic disorders.


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Neurology_Adams. Testing for jitter is carried out by having the patient voluntarily contract a muscle to the slightest degree possible so as to activate only one motor unit (requiring a great deal of cooperation by the patient) or by stimulating an intramuscular nerve twig (requiring great patience on the part of the examiner). The EMG needle is advanced until two muscle fibers from the same motor unit are recorded. If the oscilloscope sweep is triggered by the firing of the first fiber, a fluctuating latency of the second fiber potential can be seen on the screen as a movement (jitter) of the second peak. The degree of jitter can be quantitated by measuring the interval between the activation of the two muscle fibers (the result of slightly differing lengths of the terminal axons) from which a mean interpeak interval is determined. Approximately 20 fiber pairs are sampled, and an average of the mean intervals can be derived. In a muscle such as the extensor digitorum communis, the average variation


Neurology_Adams

Neurology_Adams. interval is determined. Approximately 20 fiber pairs are sampled, and an average of the mean intervals can be derived. In a muscle such as the extensor digitorum communis, the average variation should be no more than 34 ms. The acceptable average is lower for large proximal muscles. Also, in disease of the neuromuscular junction, one muscle fiber in a pair may fail to fire intermittently as a result of a blocking of conduction. Further details of this technique and its clinical applications are discussed by Stålberg and Trontelj.


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Imaging of Muscle and Nerve

Neurology_Adams. Imaging of Muscle and Nerve


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Neurology_Adams. Imaging techniques—CT, MRI, and ultrasonography—enable one to measure muscle volume and to recognize qualitative changes in muscle structure (see review of Filler and colleagues). Such methods are finding some clinical and research use in the diagnosis of disorders of muscle and in gauging the effects of treatment. CT scans of dystrophic muscle show foci of decreased attenuation, representing masses of fat cells. The fatty masses spread gradually from multiple foci and eventually replace muscle fibers. The original shape of the muscle is retained; indeed, an enlarged weak muscle containing mostly fat confirms the clinical impression of pseudohypertrophy. In denervative atrophy, the muscles are obviously small and contain multiple punctate areas of decreased attenuation, which represent interstitial fat. Eventually, large portions of chronically denervated muscle may be replaced by fat. Blood, blood products, and calcium deposits are expressed by increased attenuation in CT. This may


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Neurology_Adams. fat. Eventually, large portions of chronically denervated muscle may be replaced by fat. Blood, blood products, and calcium deposits are expressed by increased attenuation in CT. This may be helpful in the diagnosis of muscle trauma, myositis ossificans, and dermatoand polymyositis.


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Neurology_Adams. Fat and bone marrow have a high-signal intensity in MR images, whereas fascia, ligaments, and cortical bone lack signal intensity. In T1-weighted images, normal muscle has a low signal and dystrophic muscle, a slightly increased signal; in T2-weighted images, dystrophic muscle has a slightly increased (brighter) signal. Given its sensitivity to these dystrophic changes in muscle, MRI is particularly effective in determining the topographic distribution of muscle involvement in a dystrophy (see Chap. 45). Spectroscopic MRI in metabolically determined myopathies has the capacity to quantitate levels of selected biochemical constituents of muscle, including intracellular pH and levels of metabolic intermediates such as phosphocreatine. This technique is particularly effective in demonstrating subnormal generation of intracellular acidosis after a limb is exercised in disorders of glycogenolysis and of glycolysis. Some individuals with mitochondrial disease of muscle will demonstrate


Neurology_Adams

Neurology_Adams. subnormal generation of intracellular acidosis after a limb is exercised in disorders of glycogenolysis and of glycolysis. Some individuals with mitochondrial disease of muscle will demonstrate rapid depletion of energy supplies and profound delays in recovery that can be quantified and used as an end point for treatments.


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Neurology_Adams. As addressed in an earlier section, magnetic resonance techniques have been developed that allow the imaging of nerves. This may be an aid in assessing traumatic nerve injury and in demonstrating neuromas and other tumors, hypertrophy or atrophy of a nerve trunk or plexus. NEUROPSYCHOLOGICAL TESTS, PERIMETRY, AUDIOMETRY, AND These methods are used in defining and quantitating the nature of the psychologic or sensory deficits produced by disease of the nervous system. They are performed most often to obtain confirmation of a disorder of function in particular parts of the nervous system or to quantitate, by subsequent examinations, the progression of the underlying illness such as a dementia. A description of these methods and their clinical uses is found in the chapters dealing with cerebral function (Chap. 21), developmental disorders of the cerebrum (Chap. 27), dementia (Chap. 20), and disorders of vision (Chap. 12) and of hearing and equilibrium (Chap. 14).


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Neurology_Adams. Numerous genetic markers of heredofamilial disease have become available to the clinician and have greatly advanced both diagnosis and categorization of neurologic disease. The main examples are analyses of DNA extracted from blood or other cells for the identification of mutations (e.g., muscular dystrophy, spinocerebellar atrophies, and genetically determined polyneuropathies, and the quantification of abnormally long repetitions of certain trinucleotide sequences, most often used for the diagnosis of Huntington chorea). The use of these tests is elaborated in Chap. 38. The study of mitochondrial genetics has allowed the detection of an entire category of diseases that affect this subcellular structure, as detailed in Chap. 37. BIOPSY OF BRAIN, NERVE, MUSCLE, AND OTHER TISSUE


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Neurology_Adams. The application of light, phase, and electron microscopy to the study of these tissues may be highly informative. The findings are discussed in Chaps. 36 (skin and conjunctivum in the diagnosis of metabolic storage diseases), 45 (muscle), and 46 (nerve). Temporal artery biopsy is indicated when giant cell arteritis is suspected (Chap. 33). Brain biopsy, aside from its main use in the direct sampling of a suspected neoplasm, may be diagnostic in cases of granulomatous angiitis, some forms of encephalitis, and infectious abscesses. Biopsy of the pachymeninges or leptomeninges may disclose vasculitis, sarcoidosis, other granulomatous infiltrations, or an obscure infection, but its sensitivity is low. This is usually performed in concert with a biopsy of the underlying brain. Biopsy is now generally avoided in cases of suspected prion disease because of the risk of transmitting the causative agent. Abdominal fat pad biopsy is used in the diagnosis of amyloidosis.


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Neurology_Adams. In choosing to perform a biopsy in any of these clinical situations, the paramount issue is the likelihood of establishing a definitive diagnosis—one that would permit successful treatment or otherwise enhance the management of the disease. Ali SZ and Cibas ES. Serous Cavity Fluid and Cerebrospinal Fluid Cytopathology. New York, Springer, 2012. Altenmüller EO, Münte TF, Gerloff C: Neurocognitive function and the EEG. In: Niedermeyer E, Lopes DaSilva F (eds): Electroencephalography: Basic Principles, Clinical Applications, and Related Fields, 5th ed. Philadelphia, Lippincott Williams & Wilkins, 2005, pp 661–682. American Electroencephalographic Society: Guidelines in electroencephalography, evoked potentials, and polysomnography. J Clin Neurophysiol 11:1, 1994. Avery RA, Shah SS, Licht DJ, et al: Reference range for cerebrospinal fluid opening pressure in children. N Engl J Med 363:891, 2010.


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Neurology_Adams. Avery RA, Shah SS, Licht DJ, et al: Reference range for cerebrospinal fluid opening pressure in children. N Engl J Med 363:891, 2010. Barrows LJ, Hunter FT, Banker BQ: The nature and clinical significance of pigments in the cerebrospinal fluid. Brain 78:59, 1955. Bigner SH: Cerebrospinal fluid (CSF) cytology: Current status and diagnostic applications. J Neuropathol Exp Neurol 51:235, 1992. Blume WT, Kaibaro, M: Atlas of Pediatric Electroencephalography, 2nd ed. New York, Raven Press, 1999. Cros D, Chiappa KH: Clinical applications of motor evoked potentials. Adv Neurol 63:179,1993. Chiappa KH, Ropper AH: Evoked potentials in clinical medicine. N Engl J Med 306:1140, 1205, 1982. Dawson GD: A summation technique for the detection of small evoked potentials. Electroencephalogr Clin Neurophysiol 6:65, 1954. Den Hartog-Jager WA: Color Atlas of CSF Cytopathology. New York, Elsevier-North Holland, 1980. DeWeerd AW: Atlas of EEG in the First Months of Life. New York, Elsevier, 1995.


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Neurology_Adams. Den Hartog-Jager WA: Color Atlas of CSF Cytopathology. New York, Elsevier-North Holland, 1980. DeWeerd AW: Atlas of EEG in the First Months of Life. New York, Elsevier, 1995. Ebersole JA, Husain AM, Nordi DR (eds): Current Practice of Clinical EEG, 4th ed. Philadelphia, Lippincott Williams & Wilkins, 2014. Filler AG, Kliot M, Howe FA, et al: Application of magnetic resonance in the evaluation of patients with peripheral nerve pathology. J Neurosurg 85:299, 1996. Fishman RA: Cerebrospinal Fluid in Diseases of the Nervous System, 2nd ed. Philadelphia, Saunders, 1992. Fishman RA, Ransahoff J, Osserman E: Factors influencing the concentration gradient of protein in cerebrospinal fluid. J Clin Invest 37:1419, 1958. Goldenshohn ES, Wolf S, Koszer S, Legatt A (eds): EEG Interpretation, 2nd ed. New York, Futura, 1999.


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Neurology_Adams. Goldenshohn ES, Wolf S, Koszer S, Legatt A (eds): EEG Interpretation, 2nd ed. New York, Futura, 1999. Hahn JS, Tharp BR: Neonatal and pediatric electroencephalography. In: Aminoff MJ (ed): Electrodiagnosis in Clinical Neurology, 4th ed. New York, Churchill Livingstone, 1999, pp 81–128. Horowitz AL: MRI Physics for Radiologists, 2nd ed. New York, Springer, 1992. Hughes JR: EEG in Clinical Practice, 2nd ed. Woburn, MA, Butterworth, 1994. Kanal E, Gillen J, Evans JA, et al: Survey of reproductive health among female MR workers. Radiology 187:395, 1993. Marsden CD, Merton PA, Morton HB: Direct electrical stimulation of corticospinal pathways through the intact scalp and in human subjects. Adv Neurol 39:387, 1983. Modic MT, Masaryk TJ, Ross JS, et al: Magnetic Resonance Imaging of the Spine, 2nd ed. St. Louis, Mosby-Year Book, 1994. Morelli JN, Runge VM, Ai F, et al: An image-based approach to understanding the physics of MR artifacts. RadioGraphics 31:849, 2011.


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Neurology_Adams. Morelli JN, Runge VM, Ai F, et al: An image-based approach to understanding the physics of MR artifacts. RadioGraphics 31:849, 2011. Polich J: P300 in clinical applications. In: Niedermeyer E, Lopes DaSilva F (eds): Electroencephalography: Basic Principles, Clinical Applications, and Related Fields, 4th ed. Baltimore, Williams & Wilkins, 1999, pp 1073–1091. Scher MS, Painter MJ: Electroencephalographic diagnosis of neonatal seizures. In: Wasterlain CG, Vert P (eds): Neonatal Seizures. New York, Raven Press, 1990. Stålberg E, Trontelj JV: The study of normal and abnormal neuromuscular transmission with single fibre electromyography. J Neurosci Methods 74:145,1997. Stockard-Pope JE, Werner SS, Bickford RG: Atlas of Neonatal Electroencephalography, 2nd ed. New York, Raven Press, 1992. Strupp M, Schueler O, Straube A, et al: “Atraumatic” Sprotte needle reduces the incidence of post-lumbar puncture headache. Neurology 57:2310, 2001.


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Neurology_Adams. Strupp M, Schueler O, Straube A, et al: “Atraumatic” Sprotte needle reduces the incidence of post-lumbar puncture headache. Neurology 57:2310, 2001. Vernooij MW, Ikran MA, Tanghe HL, et al: Incidental findings on brain MRI in the general population. N Engl J Med 357:1821, 2007.


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Neurology_Adams. Figure 2-1. Normal CT in the axial plane of the brain, orbits, and skull base. A. Image through the cerebral hemispheres at the level of the corona radiata. The dense bone of the calvarium is white, and fat-containing subcutaneous tissue is dark. Gray matter appears denser than white matter due to its lower lipid content. B. Image at the level of the lenticular nuclei. The caudate and lenticular nuclei are denser than the adjacent internal capsule. CSF within the frontal horns of the lateral ventricles as well as surrounding the slightly calcified pineal body appears dark. C. Image through the mid-orbits. The sclera appears as a dense band surrounding the globe. The optic nerves are surrounded by dark orbital fat. The medial and lateral rectus muscles lie along the orbital walls and have a fusiform shape. Air within the nasopharynx and paranasal sinuses appears dark. D. Image at the base of the skull, digitally adjusted to visualize bone (“bone window”), showing the basal occipital


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Neurology_Adams. a fusiform shape. Air within the nasopharynx and paranasal sinuses appears dark. D. Image at the base of the skull, digitally adjusted to visualize bone (“bone window”), showing the basal occipital and temporal bones, clivus, the bony structures of the posterior nasopharynx, aerated mastoid air cells, internal auditory canals and inner ear structures, as well as the sutures in the occipital bone.


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Neurology_Adams. Figure 2-2. CT myelogram and MRI of the lumbosacral spine. Sagittal (A) and axial (B–C) CT images of the lumbosacral spine obtained after the intrathecal administration of radiopaque contrast material. The vertebral bodies are separated by intervertebral discs and the spinous processes are seen posteriorly. Contrast contained within the thecal sac appears white. The conus medullaris terminates at the L2 vertebral level (A–B) and the nerve roots of the cauda equina are clearly seen within the posterior thecal sac (A–C). Sagittal (D) and axial (E–F) T2-weighted MRI of the lumbosacral spine shows hyperintense CSF surrounding the conus medullaris, which terminates at the L1 vertebral level (A–B). The nerve roots of the cauda equina are seen within the posterior thecal sac (A–C). In C and F, traversing nerve roots within the lateral recess of the spinal canal are seen.


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Neurology_Adams. Figure 2-3. Normal brain MRI. A. Axial T2-weighted MRI at the level of the lenticular nuclei. Gray matter appears brighter than white matter. CSF within the ventricles and cortical sulci is very bright. The caudate nuclei, putamen, and thalamus appear brighter than the internal capsule.


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Neurology_Adams. B. Axial T2-weighted MRI at the level of the pons. Subcutaneous fat and calvarial marrow appear relatively bright. CSF within the fourth ventricle and prepontine cistern, endolymph within the cochlea and semicircular canals, and ocular vitreous fluid appears very bright. Signal is absent (i.e., a “flow void”) within the basilar artery. C. Midline sagittal T1-weighted MRI of the brain. Note that white matter appears brighter than gray matter and the corpus callosum is well defined. The pons, medulla, and cervicomedullary junction are well delineated, and the pituitary gland is demonstrated with a normal posterior pituitary bright spot. The cerebral aqueduct is seen between the ventral midbrain and the tectum. The clivus and upper cervical vertebrae are noted as well. D. Axial T2-weighted fluid-attenuated inversion recovery (FLAIR) MRI of the brain at the same level as in A. Note that the hyperintense fluid signal from CSF is now suppressed, and the differentiation between brighter gray


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Neurology_Adams. inversion recovery (FLAIR) MRI of the brain at the same level as in A. Note that the hyperintense fluid signal from CSF is now suppressed, and the differentiation between brighter gray matter and darker white matter is accentuated.


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Neurology_Adams. Figure 2-4. Intracranial and cervical angiography. A. Oblique CT angiogram of the neck showing the carotid bifurcation and the cervical segments of the internal and external carotid arteries. Note the slightly dilated carotid bulb at the initial segment of the internal carotid artery. A small focus of calcified atherosclerosis is noted near the origin of the external carotid artery. Note that the external carotid artery has multiple branches within the neck. B. Coronal MR angiogram of the neck showing the aortic arch, the origins and cervical courses of the carotid and vertebral arteries, and the vertebrobasilar junction. The sigmoid sinuses and internal jugular veins are faintly visible. C–D. Sagittal dynamic CT angiography of the head. Bony and soft tissue structures as well as brain parenchyma have been digitally subtracted. The image C was acquired during the arterial phase; the carotid and basilar termini and the anterior cerebral arteries are enhanced. Venous phase imaging (D)


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Neurology_Adams. have been digitally subtracted. The image C was acquired during the arterial phase; the carotid and basilar termini and the anterior cerebral arteries are enhanced. Venous phase imaging (D) shows enhancement of the superior and inferior sagittal sinuses, straight sinus, vein of Galen, internal cerebral veins, basal veins of Rosenthal, and the transverse and sigmoid sinuses.


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Neurology_Adams. Figure 2-5. Blood oxygen level-dependent (BOLD) functional MRI. The image shown is from a subject performing repetitive motor functions (tapping a button) with his right finger. Superimposed upon the grayscale structural MRI image are areas of altered BOLD signal, in color, associated with the task. The most prominent signal (yellow) is in the left lateral cerebral cortex, corresponding to the right hand area of the precentral and postcentral gyri. Other sites of lesser signal (red, orange) include the supplementary motor area, which is near the midline anteriorly. (Image courtesy of Dr. Michael D. Fox. From Fox MD, Snyder AZ, Zacks JM, Raichle ME: Coherent spontaneous activity accounts for trial-to-trial variability in human evoked brain responses. Nat Neurosci 9:23, 2006. Reproduced with permission.)


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Neurology_Adams. Figure 2-6. Axial 18FDG-PET of a normal brain. The PET data is colorized and overlaid on a CT image. Brain areas with higher metabolic activity such as cortex and deep gray nuclei appear bright, and areas with lower metabolic activity such as white matter appear purple.


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Neurology_Adams. Figure 2-7 A. “10-20” is a measurement system designed to reliably reproduce electrode positions on different patients, regardless of head size. Electrodes are placed at intervals of either 10 or 20 percent of the hemi-circumference of the head. (Courtesy of Dr. Jay S. Pathmanathan.) B. Each channel represents the amplified recording of voltage changes over time between two electrodes. Normal alpha (8 to 12 per second) activity is present posteriorly (bottom channel). The top channel contains a large blink artifact. Note the striking reduction of the alpha rhythm with eye opening (arrow). C. Photic driving. During stroboscopic stimulation of a normal subject, a visually evoked response is seen posteriorly after each flash of light (signaled on the bottom channel). (continued)


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Neurology_Adams. Figure 2-7 (Continued) D. Stroboscopic stimulation at 14 flashes per second (bottom channel) has produced a photoparoxysmal response in this epileptic patient, evidenced by the abnormal spike and slow-wave activity toward the end of the period of stimulation. E. Large, slow, irregular delta waves are seen in the right frontal region (channels 1 and 2). In this case a glioblastoma was found in the right cerebral hemisphere, but the EEG does not differ basically from that produced by a stroke, abscess, or contusion. F. An EEG showing focal spike-and-wave discharges over the right frontal region (channels 1 to 3). The box isolates a single spike–wave transient. (continued)


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Neurology_Adams. Figure 2-7 (Continued) G. Phase reversal is shown between electrode pairs, F7-T3 and T3-T5, implying that the site of the spike generator is under the T3 electrode. (Courtesy of Dr. Jay S. Pathmanathan.) H. Localization of a spike in a montage that utilizes the right ear (A2) as a reference electrode. The amplitude of the transient at T3 is greater than at other locations, implying that the source of the spike is closest to the T3 electrode. (Courtesy of Dr. Jay S. Pathmanathan.) I. Absence seizures, showing generalized 3-per-second spike-and-wave discharge. The abnormal activity ends abruptly and normal background activity appears. (continued)


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Neurology_Adams. Figure 2-7 (Continued) J. Deep coma following cardiac arrest, showing electrocerebral silence. With the highest amplification, electrocardiogram (ECG) and other artifacts may be seen, so that the record is not truly “flat” or isoelectric. However, no cerebral rhythms are visible. Note the ECG (lower channel). K. Grossly disorganized background activity interrupted by repetitive “pseudoperiodic” discharges consisting of large, sharp waves from all leads about once per second. This pattern is characteristic of Creutzfeldt-Jakob disease. L. Advanced hepatic coma. Slow (about 2 per second) waves have replaced the normal activity in all leads. This record demonstrates the triphasic waves often seen in this disorder.


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Neurology_Adams. Figure 2-8. Pattern-shift visual evoked responses (PSVERs). Latency measured to first major positive peak (termed P100 because of its latency from the stimulus of approximate 100 ms) and marked by “o.” Upper two tracings: These, from the right and left eyes, are normal. Middle tracings: PSVER from the right eye is normal but the latency of the response from the left eye is prolonged and its duration is increased. Lower tracings: PSVER from both eyes show abnormally prolonged latencies, somewhat greater on the left than on the right. Calibration: 50 ms, 2.5 mV. Stimulus12345LATENCY, msec678910AMPLITUDE, µVIAuditorynerve2IICochlearnuclei(pons)Hair cellsIIISuperioroliveIVLaterallemniscusV Inferior colliculus (midbrain)VIMedialgeniculate(?)VIIAuditoryradiations(?)IIIIIIIVVVIVIICochlearnucleiNucleus oflaterallemniscusCochlear nerveOrgan ofCorti10.40.30.20.10


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Neurology_Adams. Figure 2-9. Short-latency brainstem auditory evoked responses (BAERs). Diagram of the proposed electrophysiologic–anatomic correlations in human subjects. Waves I through V are the ones measured in clinical practice.


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Neurology_Adams. Figure 2-10. Short-latency SEPs produced by stimulation of the median nerve at the wrist. The set of responses shown at left is from a normal subject; the set at right is from a patient with multiple sclerosis who had no sensory symptoms or signs. In the patient tracing, note the preservation of the brachial–plexus component (EP), the absence of the cervical cord (N11) and lower-medullary components (N/P13), and the latency of the thalamocortical components (N19 and P22), prolonged above the normal mean +3 SD for the interval from the brachial plexus potential. Unilateral stimulation occurred at a frequency of 5 per second. Recording electrode locations are as follows: FZ, midfrontal; EP, the Erb point (the shoulder); C2, the middle back of the neck over the C2 cervical vertebra; and Cc, the scalp overlying the sensoriparietal cortex contralateral to the stimulated limb. Relative negativity at the second electrode caused an upward trace deflection. Amplitude calibration marks denote 2


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Neurology_Adams. scalp overlying the sensoriparietal cortex contralateral to the stimulated limb. Relative negativity at the second electrode caused an upward trace deflection. Amplitude calibration marks denote 2 mV. (Reproduced by permission from Chiappa and Ropper.)


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Neurology_Adams. AA˜B˜C˜Ref.11˜2˜2BC Figure 2-11. The median nerve is stimulated percutaneously (1) at the wrist and (2) in the antecubital fossa with the resultant compound muscle action potential recorded over the abductor pollicis brevis (arrow).The motor waveform is recorded as the voltage between the surface electrode and a reference electrode (Ref.) more distally. Sweep 1′ on the display depicts the stimulus artifact followed by the compound muscle action potential. The distal latency, A′, is the time from the stimulus artifact to the onset of the compound muscle action potential and corresponds to conduction over distance A. The same is true for sweep 2′, where stimulation is at site 2 and the time from the artifact to the response is B′. The maximum motor conduction velocity over segment C is calculated by dividing the distance between the two stimulating electrodes, C, by the time C′.


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Neurology_Adams. Figure 2-12. The principal pathologic alterations of CMAP. A. The normal CMAP, representing the summed discharges from a group of motor units activated by a supramaximal stimulus, measured over the muscle. B. With loss of motor axons, fewer motor units are activated and the CMAP has reduced amplitude. C. With demyelination of motor axons, the same number of motor units activate, but over a prolonged duration; thus the CMAP has reduced amplitude because there is temporal dispersion of the waveform. Figure 2-13. SNAP recording. A. Electrical stimulation of the median nerve at the wrist with recording of sensory action potentials at two sites in the second digit. The responses are generated by antidromic propagation of action potentials from the site of stimulation. B. A SNAP recorded from G1. Sensory nerve conduction velocity can be calculated by dividing the distance between G1 and G2 by the difference in onset latencies from these two sites.


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Neurology_Adams. Figure 2-14. Late responses. A. The H reflex is elicited by stimulating a sensory nerve. The action potentials travel in an orthodromic fashion through the dorsal root into the spinal cord, where synapses occur with motor neurons. The motor axons innervate a muscle (the gastrocnemius) from which the late CMAP response is recorded. B. The F response is elicited by stimulating a motor nerve. Some of the action potentials that have traveled in an antidromic fashion though the anterior horn are volleyed back in an orthodromic fashion along the same motor neurons. The late CMAP response is recorded from the muscle innervated by these axons.


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Neurology_Adams. Figure 2-15. Repetitive stimulation of the hypothenar muscles. A. Patient with myasthenia gravis—typical pattern of decrement in first four responses followed by slight increment. At this rate of stimulation (3 per second), the decrement in response does not continue to zero. B. Patient with Lambert-Eaton syndrome and oat cell carcinoma—marked increase from low toward normal amplitude with rapid repetitive stimulation (20 per second). Horizontal calibration: 250 ms. AB˜°C2 msecRef.ABC Figure 2-16. The shaded areas on the muscle (A, B, and C) represent zones of the propagating action potential depicted by the dashed arrow. The correspondingly lettered portions of the triphasic muscle action potential displayed on the screen reflect the potential difference between the active (vertical arrow) and reference (Ref.) electrodes. Polarity in this and subsequent figures is negative upward as depicted.


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Neurology_Adams. Figure 2-17. Patterns of motor unit recruitment. A. Normal. With each increment of voluntary effort, more and larger units are brought into play until, with full effort at the extreme right, a complete “interference pattern” is seen in which single units are no longer recognizable. B. After denervation, only a single motor unit is recorded despite maximal effort. It is seen to fire repetitively. C. With myopathic diseases, a normal number of units are recruited on minimal effort, though the amplitude of the pattern is reduced. Calibration: 50 ms (horizontal) and 1 mV (vertical).


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Neurology_Adams. Figure 2-18. Abnormal spontaneous activity. A. Positive sharp waves and fibrillations recorded from a paralyzed, denervated muscle. A typical positive sharp wave is seen above the star. The fibrillations (arrow) are 1 to 2 ms in duration, 100 to 300 mV in amplitude, and largely negative (upward) in polarity following an initial positive deflection. B. Fasciculation. This spontaneous motor unit potential was recorded from a patient with amyotrophic lateral sclerosis. It has a serrated configuration and it fired once every second or two. Calibrations: 5 ms (horizontal) and 200 μV in A; 1 mV in B (vertical).


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Neurology_Adams. Figure 2-19. A. Myotonia congenita (Thomsen disease). The five lines are a continuous record of activity in the biceps brachii following a tap on the tendon. The initial response is within normal limits, but it is followed by a prolonged burst of rapid activity, gradually subsiding over a period of many seconds or minutes. B. Same electrode placement as in A. Response to the fifth of a series of tendon taps. “Warmup” has occurred, and the characteristic prolonged myotonic activity is no longer evident. (See Chap. 48 for a description of the disease.)


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Neurology_Adams. Figure 2-20. Normal, myopathic, and reinnervated motor units. The colored muscle fibers are functional members of one motor unit, whose axon enters from the upper left and branches terminally to innervate the appropriate muscle fibers. The action potential produced by each motor unit is seen to the right: its duration is measured between the two vertical lines. The normal-appearing but uncolored fibers belong to other motor units. A. There are five muscle fibers illustrated in the active unit. B. In this myopathic unit, only two fibers remain active; the other three (shrunken) were affected by one of the primary muscle diseases. C. Four fibers that originally belonged to other motor units and had been denervated are now reinnervated by terminal sprouting from an undamaged axon. Both the motor unit and its action potential are now larger than normal. Note that only under these abnormal circumstances do fibers in the same unit lie next to one another.


Neurology_Adams

Neurology_Adams. In this major second part of the book, the cardinal manifestations of neurologic disease are described. In order to understand the signs and symptoms created by disordered function of the nervous system, it is necessary to first describe the normal anatomy and physiology, as they pertain to disease. Each chapter, therefore, begins with a consideration of these basic facts, followed by the manner in which they are affected by disease states and give rise to observable changes such as weakness, incoordination, abnormal movements, sensory loss, and pain. Disorders of Motility CHAPTER 4 Disorders of Movement and Posture CHAPTER 5 Ataxia and Disorders of Cerebellar Function CHAPTER 6 Disorders of Stance and Gait


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Neurology_Adams. CHAPTER 6 Disorders of Stance and Gait The control of motor function, to which much of the human nervous system is committed, is accomplished through the integrated action of a vast array of segmental and suprasegmental motor neurons. As originally conceived by Hughlings Jackson in 1858, purely on the basis of clinical observations, the motor system is organized hierarchically in three levels, each higher level controlling the one below. It was Jackson’s concept that the spinal and brainstem neurons represent the lowest, simplest, and most highly organized motor centers; that the motor neurons of the posterior frontal region represent a more complex and less closely organized second motor center; and that the prefrontal parts of the cerebrum are the third and highest motor center. This scheme is still regarded as being essentially correct, although since Jackson’s time the importance of the parietal lobe and basal ganglia in motor control has been recognized.


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Neurology_Adams. More recently, functional imaging has analyzed motor organization and found it to be remarkably complex. Motor and sensory systems, although separated for practical clinical purposes, are not independent entities but are closely integrated. Without sensory feedback, motor control is ineffective. Furthermore, at the higher cortical levels of motor control, motivation, planning, and other frontal lobe activities that subserve volitional movement are preceded and modulated by activity in the parietal sensory cortex.


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Neurology_Adams. Physiologic studies, cast in their simplest terms, indicate that the following parts of the nervous system are engaged primarily in the control of movement and, in the course of disease, yield a number of characteristic derangements. The large motor neurons in the anterior horns of the spinal cord and the motor nuclei of the brainstem, the axons of which comprise the anterior spinal roots, the spinal nerves, and the cranial nerves innervate the skeletal muscles. These nerve cells and their axons constitute the lower motor neurons, complete lesions of which result in a loss of all movement—voluntary, automatic, postural, and reflex. The lower motor neurons are the final common pathway by which all neural impulses are transmitted to muscle. The motor neurons in the frontal cortex adjacent to the rolandic fissure (motor strip) connect with the spinal motor neurons by a system of fibers known, because of the shape of its fasciculus in the medulla, as the pyramidal tract. Because the motor


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Neurology_Adams. the rolandic fissure (motor strip) connect with the spinal motor neurons by a system of fibers known, because of the shape of its fasciculus in the medulla, as the pyramidal tract. Because the motor fibers that extend from the cerebral cortex to the spinal cord are not confined to the pyramidal tract, they are more accurately designated as the corticospinal tract, or, alternatively, as the upper motor neurons, to distinguish them from the lower motor neurons. Several brainstem nuclei project to the spinal cord, notably the pontine and medullary reticular nuclei, vestibular nuclei, and red nuclei. These nuclei and their descending fibers subserve the neural mechanisms of posture and movement, particularly when movement is highly automatic and repetitive. Two subcortical systems modulate the activity of all movement; these are the basal ganglia (striatum, pallidum, and related structures, including the substantia nigra and subthalamic nucleus) and the cerebellum. Each of these systems


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Neurology_Adams. activity of all movement; these are the basal ganglia (striatum, pallidum, and related structures, including the substantia nigra and subthalamic nucleus) and the cerebellum. Each of these systems plays an important role in the control of muscle tone, posture, and coordination.


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Neurology_Adams. These structures and their disorders are the subjects of the following four chapters. Paralysis means loss of voluntary movement as a result of interruption of one of the motor pathways at any point from the cerebrum to the muscle fiber. A lesser degree of weakness is spoken of as paresis. The word plegia comes from a Greek word meaning “to strike,” and the word palsy is from an old French word that has the same meaning as paralysis. One generally uses paralysis or plegia for severe or complete loss of motor function and paresis for partial loss.


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Neurology_Adams. Each spinal and cranial motor nerve cell, through the extensive arborization of the terminal part of its efferent fiber, comes into contact with a variable number of muscle fibers, ranging from only a few to 1,000 or more; together, the nerve cell, its axons, and the muscle fibers they subserve constitute the motor unit. All variations in the force, range, rate, and type of movement are determined by the number and size of motor units called into action and the frequency and sequence of firing of each motor unit. Much of the sequence and coordination of firing is modulated by subcortical structures or the basal ganglia and cerebellum. Smaller movements involve relatively few motor units; powerful movements recruit many more units that accumulate to an increasing size.


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Neurology_Adams. The motor nerve fibers emanating from a group of anterior horn cells in one segment of the spinal cord constitute the ventral spinal root. These roots intermingle with neighboring ones to form plexuses and then form the peripheral nerves. Although the muscles are innervated in patterns largely corresponding to segments of the spinal cord (a myotome), each large muscle is usually supplied by two or more roots. In contrast, a single peripheral nerve usually provides the complete motor innervation of a muscle or group of muscles. For this reason, paralysis caused by disease of the anterior horn cells or anterior roots has a different topographic pattern than paralysis following interruption of a peripheral nerve. These patterns follow the distribution shown in Table 43-1. For example, section of the L5 motor root causes paralysis of the extensors of the foot with a foot drop and weakness of inversion of the foot, whereas a lesion of the peroneal nerve, which also causes foot drop, does


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Neurology_Adams. of the L5 motor root causes paralysis of the extensors of the foot with a foot drop and weakness of inversion of the foot, whereas a lesion of the peroneal nerve, which also causes foot drop, does not affect the invertors of the foot since they are supplied by L5 but via the tibial nerve.


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Neurology_Adams. All motor activity, even the most elementary reflex type, requires the synchronous activity of many muscles. Analysis of a relatively simple movement, such as clenching the fist, conveys some idea of the complexity of the underlying neuromuscular arrangements. In this act the primary movement is a contraction of the flexor muscles of the fingers, the flexor digitorum sublimis and profundus, the flexor pollicis longus and brevis, and the abductor pollicis brevis. In the terminology of Beevor, these muscles act as agonists, or prime movers. For flexion to be smooth and forceful, the extensor muscles (antagonists) must relax at the same rate as the flexors contract (reciprocal innervation, or Sherrington law). The muscles that flex the fingers also flex the wrist. If it is desired that only the fingers flex, the extensors of the wrist must be brought into play to prevent its flexion; the extensors are synergists. During this action of the hand, appropriate flexor and extensor muscles


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Neurology_Adams. the fingers flex, the extensors of the wrist must be brought into play to prevent its flexion; the extensors are synergists. During this action of the hand, appropriate flexor and extensor muscles stabilize the wrist, elbow, and shoulder; muscles that accomplish this serve as fixators. The coordination of agonists, antagonists, synergists, and fixators is effected mainly by segmental spinal reflexes under the guidance of proprioceptive sensory stimuli. In general, the more delicate the movement, the more precise must be the coordination between agonist and antagonist muscles.


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Neurology_Adams. Motor activities include not only those that alter the position of a limb or other part of the body (isotonic contraction) but also those that stabilize posture (isometric contraction). Movements that are performed slowly are called ramp movements. Very rapid movements, which are too fast for sensory control, are called ballistic (also termed phasic).


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Neurology_Adams. All voluntary ballistic movements toward a target are accomplished by the activation of ensembles of motor neurons. Large motor units participate mainly in triphasic movement, which are characterized by an initial burst of activity in the agonist muscles, then a burst in the antagonists, followed by a third smaller burst in the agonists. The strength of the initial agonist burst determines the speed and distance of the movement, but there is always the same triphasic pattern of agonist, antagonist, and agonist activity (Hallett et al). Smaller motor units are more efficiently activated by sensory afferents from muscle spindles, more tonically active, and more readily recruited in reflex activities, postural maintenance, walking, and running. The basal ganglia and cerebellum set the pattern and timing of the muscle action in any projected motor performance. These points are discussed further in Chaps. 4 and 5.


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Neurology_Adams. Unlike the phasic movements just described, certain basic motor activities do not involve reciprocal innervation. In support of the body in an upright posture, when the legs must act as rigid pillars, and in shivering, agonists and antagonists contract simultaneously. Locomotion requires that the extensor pattern of reflex standing be inhibited and that the coordinated pattern of alternating stepping movements be substituted; the latter is accomplished by multisegmental spinal and brainstem reflexes, the so-called locomotor centers. Suprasegmental control of the axial and proximal limb musculature (antigravity postural mechanisms) is mediated primarily by the reticulospinal and vestibulospinal tracts. These aspects of motor function are elaborated further on.


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Neurology_Adams. Muscle stretch (tendon) reflex activity and muscle tone depend on the status of the large motor neurons of the anterior horn (the alpha motor neurons), the muscle spindles and their afferent fibers, and the small anterior horn cells (gamma neurons), whose axons terminate on the specialized intrafusal muscle fibers (nuclear chain fibers) within the spindles. Some gamma motor neurons are tonically active at rest, keeping the intrafusal muscle fibers taut and more sensitive to active and passive changes in muscle length. Each anterior horn cell has on its surface membrane approximately 10,000 receptive synaptic terminals. Some of these terminals are excitatory, others inhibitory; in combination, they determine the activity of the neuron. Beta motor neurons effect cocontraction of both spindle and nonspindle fibers, but the physiologic significance of this innervation is not fully understood.


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Neurology_Adams. A tap on a tendon stretches causes a vibratory wave of the spindle and activates its nuclear bag fibers. Afferent projections from these fibers synapse directly with alpha motor neurons in the same and adjacent spinal segments; these neurons, in turn, send impulses to the skeletal muscle fibers, resulting in the familiar monosynaptic muscle contraction or monophasic (myotatic) stretch reflex, commonly referred to as the tendon reflex or “tendon jerk” (Fig. 3-1), more correctly called the muscle stretch or proprioceptive reflex. All this occurs within 25 ms of the sudden stretch. The alpha neurons of antagonist muscles are simultaneously inhibited but through disynaptic rather than monosynaptic connections. This is accomplished in part by inhibitory interneurons (reciprocal inhibition), which also receive input from descending pathways. Renshaw cells also participate by providing negative feedback through inhibitory synapses of alpha motor neurons (recurrent inhibition).


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Neurology_Adams. Thus the tension in the spindle fibers and the state of excitability of the alpha and gamma neurons (influenced greatly by descending fiber systems) determines the level of activity of the tendon reflexes and muscle tone (the responsiveness of muscle to stretch). Other mechanisms, of an inhibitory nature, involve the Golgi tendon organs, which detect the tension produced by active contraction of muscle. These encapsulated receptors, which lie in the tendinous portions of muscle, activate afferent fibers that end on internuncial cells, which project to alpha motor neurons, thus forming a disynaptic reflex arc. Golgi tendon receptors are silent in relaxed muscle and during passive stretch; they serve, together with muscle spindles, to monitor or calibrate the length and force of muscle contraction under different conditions. They also play a role in naturally occurring limb movements, particularly in locomotion.


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Neurology_Adams. The alpha motor neurons are located in the anterior gray matter (anterior horn) of the spinal cord. The medial parts of the anterior horn supply trunk or axial muscles, and neurons of the lateral parts supply the appendicular muscles. The largest neurons innervate large muscles with large motor units. Smaller anterior horn cells innervate small muscles and control more delicate movements, particularly those in the fingers and hand. Both groups of alpha neurons receive projections from the propriospinal neurons via the large fasciculi proprii from adjacent spinal segments. All the facilitatory and inhibitory influences supplied by cutaneous and proprioceptive afferent and descending suprasegmental neurons are coordinated at segmental levels. For further details the reader may consult Burke and Lance and also Davidoff (1992).


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Neurology_Adams. There is considerable information concerning the neurochemistry of motor neurons. The large neurons of the anterior horns of the spinal cord contain high concentrations of choline acetyltransferase and use acetylcholine as their transmitter at the neuromuscular junction. The main neurotransmitters of the descending corticospinal tract, in so far as can be determined in humans, are aspartate and glutamate. Glycine is the neurotransmitter released by Renshaw cells, which are responsible for recurrent inhibition, and by interneurons that mediate reciprocal inhibition during reflex action. Gamma-aminobutyric acid (GABA) serves as the inhibitory neurotransmitter of interneurons in the posterior horn. There are also descending cholinergic, adrenergic, and dopaminergic axons, which play a less well-defined role in reflex functions. Paralysis due to Lesions of the Lower Motor Neurons


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Neurology_Adams. If all, or practically all, peripheral motor fibers supplying a muscle are interrupted, the voluntary, postural, and reflex movements of that muscle are abolished. The muscle becomes lax and soft and does not resist passive stretching, a condition known as flaccidity. Muscle tone—the slight resistance that normal relaxed muscle offers to passive movement—is reduced (hypotonia or atonia). The denervated muscle undergoes extreme atrophy, being reduced to 20 or 30 percent of its original bulk within 3 to 4 months. The reaction of the muscle to sudden stretch, as by tapping its tendon, is lost (areflexia). Damage restricted to only a portion of the motor fibers supplying the muscle results in partial paralysis, or paresis, and a proportionate diminution in the force and speed of contraction. The atrophy will be less and the tendon reflex reduced instead of lost. The electrodiagnosis of denervation depends upon finding fibrillations, fasciculations, and other abnormalities on needle


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Neurology_Adams. The atrophy will be less and the tendon reflex reduced instead of lost. The electrodiagnosis of denervation depends upon finding fibrillations, fasciculations, and other abnormalities on needle electrode examination as mentioned in the previous chapter. However, some of these abnormalities do not appear until several days or a week or two after nerve injury.


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Neurology_Adams. Lower motor neuron (infranuclear) paralysis is the direct result of loss of function or destruction of anterior horn cells or their axons in anterior roots and nerves. The signs and symptoms vary according to the location of the lesion. In any individual case, the most important clinical question is whether sensory changes coexist. The combination of a flaccid, areflexic paralysis, and sensory changes usually indicates involvement of mixed motor and sensory nerves or of both anterior and posterior roots. If sensory changes are absent, the lesion must be situated in the anterior gray matter of the spinal cord, in the anterior roots, in a purely motor branch of a peripheral nerve, or in motor axons alone (or in the muscle itself). At times it may be impossible to distinguish between nuclear (spinal) and anterior root (radicular) lesions.


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Neurology_Adams. Preserved and often heightened tendon reflexes and spasticity in muscles weakened by lesions of the corticospinal systems attest to the integrity of the spinal segments below the level of the lesion. However, acute and profound spinal cord lesions and, to a lesser extent, corticospinal lesions in the brainstem and cerebrum, may temporarily abolish spinal reflexes (“spinal shock”; see Chap. 42). This is probably caused by the interruption of descending tonic excitatory impulses, which normally maintain a sufficient level of excitation in spinal motor neurons to permit the peripheral activation of segmental reflexes. The attenuation of spinal shock by opiate antagonists, such as naloxone, suggests that the phenomenon is at least in part mediated by the release of previously stored endogenous opiates from the distal terminals of neurons in the periaqueductal gray matter. Once the stored opiates are depleted, the presynaptic inhibition of motor neurons ceases, heralding the end of spinal


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Neurology_Adams. opiates from the distal terminals of neurons in the periaqueductal gray matter. Once the stored opiates are depleted, the presynaptic inhibition of motor neurons ceases, heralding the end of spinal shock and the beginning of the period of spasticity.


Neurology_Adams

Neurology_Adams. The terms pyramidal, corticospinal, and upper motor neuron are often used interchangeably, although they are not altogether synonymous. The pyramidal tract, strictly speaking, designates only those fibers that course longitudinally in the pyramid of the medulla oblongata. Of all the fiber bundles in the brain, the pyramidal tract has been known for the longest time, the first accurate description having been given by Türck in 1851. It descends from the cerebral cortex; traverses the subcortical white matter (corona radiata), internal capsule, cerebral peduncle, basis pontis (ventral pons), and pyramid of the upper medulla; decussates in the lower medulla; and continues its caudal course in the lateral funiculus (column) of the spinal cord—hence the alternative name corticospinal tract (Fig. 3-2). This is the only direct long-fiber connection between the cerebral cortex and the spinal cord. The indirect pathways through which the cortex influences spinal motor neurons are the


Neurology_Adams

Neurology_Adams. tract (Fig. 3-2). This is the only direct long-fiber connection between the cerebral cortex and the spinal cord. The indirect pathways through which the cortex influences spinal motor neurons are the rubrospinal, reticulospinal, vestibulospinal, and tectospinal; these tracts do not run in the pyramid. All these pathways, direct and indirect, are embraced by the term upper motor neuron or supranuclear, meaning above the anterior horn cells.


Neurology_Adams

Neurology_Adams. A major source of confusion about the pyramidal tract stems from the traditional view, formulated at the turn of the twentieth century, that it originates entirely from the large motor cells of Betz in the fifth layer of the precentral convolution (the primary motor cortex, or area 4 of Brodmann1) (Figs. 3-3 and 21-1). However, there are only some 25,000 to 35,000 Betz cells, whereas the medullary pyramid contains about 1 million axons (Lassek). Thus most of the fibers of the pyramidal tract arise from cortical neurons other than Betz cells, particularly in Brodmann areas 4 and 6 (the frontal cortex immediately rostral to area 4, including the medial portion of the superior frontal gyrus, that is, the supplementary motor area); in the primary somatosensory cortex (Brodmann areas 3, 1, and 2); and in the superior parietal lobule (areas 5 and 7).


Neurology_Adams

Neurology_Adams. Data concerning the origin of the pyramidal tract in humans are less robust than for animals. In the monkey, by counting the pyramidal axons that remained after cortical excisions and long survival periods, Russell and DeMyer found that 40 percent of the descending axons arose in the parietal lobe, 31 percent in motor area 4, and the remaining 29 percent in premotor area 6.


Neurology_Adams

Neurology_Adams. Fibers from the motor and premotor cortices (Brodmann areas 4 and 6, Fig. 21-2), supplementary motor cortex, and portions of parietal cortex (areas 1, 3, 5, and 7) converge in the corona radiata and descend through the posterior limb of the internal capsule, basis pedunculi, basis pontis, and medulla. As the corticospinal tracts descend in the cerebrum and brainstem, they send collaterals to the striatum, thalamus, red nucleus, cerebellum, and reticular formations. Accompanying the corticospinal tracts in the brainstem are the corticobulbar tracts, which are distributed to motor nuclei of the cranial nerves ipsilaterally and contralaterally (see Fig. 3-2). It has been possible to trace the direct projection of axons of cortical neurons to the trigeminal, facial, ambiguus, and hypoglossal nuclei (Iwatsubo et al). It appears that no axons terminate directly in the oculomotor, trochlear, abducens, or vagal nuclei. Insofar as the corticobulbar and corticospinal fibers have a similar


Neurology_Adams

Neurology_Adams. nuclei (Iwatsubo et al). It appears that no axons terminate directly in the oculomotor, trochlear, abducens, or vagal nuclei. Insofar as the corticobulbar and corticospinal fibers have a similar origin and the motor nuclei of the brainstem are the homologues of the motor neurons of the spinal cord, the term upper motor neuron may suitably be applied to both these systems of fibers.


Neurology_Adams

Neurology_Adams. The corticospinal tracts decussate at the lower end of the medulla, although some of their fibers may cross above this level. The fibers destined for the upper limb neurons cross first (more rostrally). The proportion of crossed and uncrossed fibers varies to some extent from one person to another (Nyberg). About 75 to 80 percent of the fibers cross and the remaining fibers descend ipsilaterally, mostly in the uncrossed ventral corticospinal tract. In exceptional cases, these tracts cross completely; equally rarely, they remain uncrossed. These variations are probably of functional significance in determining the amount of neurologic deficit that results from a unilateral cerebral lesion such as capsular infarction. A few well-studied cases are found, such as the one described by Terakawa and colleagues, of acute stroke of the cerebral hemisphere causing hemiplegia on the same side. Also, Yakovlev found instances of completely uncrossed pyramids among 130 autopsies of developmentally


Neurology_Adams

Neurology_Adams. and colleagues, of acute stroke of the cerebral hemisphere causing hemiplegia on the same side. Also, Yakovlev found instances of completely uncrossed pyramids among 130 autopsies of developmentally delayed neonates but considering the maldevelopment of these brains, the finding may not be generalizable.


Neurology_Adams

Neurology_Adams. The corticospinal tract is phylogenetically relatively new, being found only in mammals, which probably accounts for its variability between individuals as compared to the older vestibulospinal, rubrospinal, and reticulospinalparapyramidal systems, which are invariant among persons. Uncrossed fibers in the corticospinal tract account for mirror movements that are seen during efforts at fine motor tasks, particularly in children, and also in some disorders of the nervous system such as the Klippel-Feil syndrome and the Kallmann syndrome. For a more complete discussion of the crossing of the various tracts of the nervous system, the reader is referred to the review by Vulliemoz et al.


Neurology_Adams

Neurology_Adams. Beyond their decussation, the corticospinal pathways descend as well-defined bundles in the anterior and posterolateral columns of white matter (funiculi) of the spinal cord (see Fig. 3-2). The course of the noncorticospinal motor pathways (vestibulospinal, reticulospinal, and descending propriospinal) has been traced in humans by Nathan and his colleagues. The lateral vestibulospinal tract lies at the periphery of the cord, where it occupies the most anterolateral portion of the anterior funiculus. The medial vestibulospinal fibers mingle with those of the medial longitudinal fasciculus. Reticulospinal fibers are less compact; they descend bilaterally, and most of them come to lie just anterior to the lateral corticospinal tract. The propriospinal pathway (also called spinal-spinal) consists of a series of short fibers (one or two segments long) lying next to the gray matter.


Neurology_Adams

Neurology_Adams. The somatotopic organization of the corticospinal system is of importance in clinical work, especially in relation to certain stroke syndromes. As the descending axons subserving limb and facial movements emerge from the cortical motor strip, they maintain the anatomic organization of the overlying cortex; therefore, a discrete cortical–subcortical lesion will result in a restricted weakness of the hand and arm or the foot and leg. More caudally, the descending motor fibers converge and are collected in the posterior limb of the internal capsule, so that even a small lesion there will cause a “pure motor hemiplegia,” in which the face, arm, hand, leg, and foot are affected to more or less the same degree (see Lacunar syndromes in Chap. 33). The axons subserving facial movement are situated anterior in the posterior limb of the capsule, those for hand and arm in the central portion and those for the foot and leg, posteriorly (as detailed by Brodal).


Neurology_Adams

Neurology_Adams. This topographic distribution is maintained in the cerebral peduncle, where the corticospinal fibers occupy approximately the middle of the peduncle, the fibers destined to innervate the facial nuclei lying most medially. More caudally, in the basis pontis (base, or ventral part of the pons), the descending motor tracts separate into bundles that are interspersed with masses of pontocerebellar neurons and their cerebellipetal fibers. A lesser degree of somatotopic organization can be recognized here as well, exemplified by selective weakness of the face and hand with dysarthria, or of the leg, which may occur with pontine lacunar infarctions. Anatomic studies in nonhuman primates indicate that arm–leg distribution of fibers in the rostral pons is much the same as in the cerebral peduncle; in the caudal pons, this distinction is less-well defined. In humans, a lack of systematic anatomic study leaves the precise somatotopic organization of corticospinal fibers in the pons less certain.


Neurology_Adams

Neurology_Adams. in the caudal pons, this distinction is less-well defined. In humans, a lack of systematic anatomic study leaves the precise somatotopic organization of corticospinal fibers in the pons less certain. Restricted pontine lesions may cause a pure motor hemiplegia that is indistinguishable from the syndrome of the internal capsule. However, a study conducted by Marx and colleagues using MRI mapping techniques of patients with hemiplegia due to brainstem lesions suggests that the usual somatotopic organization breaks down in the base of the pons, and there is a concentration of fibers innervating proximal muscles lying more dorsally and those exciting distal parts of the limbs, more ventrally.


Neurology_Adams

Neurology_Adams. Another point of uncertainty has been the existence and course of fibers that descend through the lower pons and upper medulla and then ascend again to innervate the facial motor nucleus on the opposite side. Such a connection must exist to explain occasional instances of facial palsy from brainstem lesions caudal to the midpons. A discussion of the various hypothesized sites of this pathway, including a recurrent tract (Pick bundle), can be found in the report by Terao and colleagues. They conclude from imaging studies that corticobulbar fibers destined for the facial nucleus descend in the ventromedial pons to the level of the upper medulla, where they decussate and then ascend again; but there is considerable variation between individuals in this configuration.


Neurology_Adams

Neurology_Adams. The descending pontine bundles, now devoid of their corticopontine fibers, reunite to form the medullary pyramid. The brachial–crural pattern may persist in the pyramids and is certainly reconstituted in the lateral columns of the spinal cord (see Fig. 7-3), but it should be emphasized that the topographic separation of motor fibers at cervical, thoracic, lumbar, and sacral levels is not as discrete as usually shown in schematic diagrams of the spinal cord. The corticospinal tracts and other upper motor neurons terminate mainly in relation to nerve cells in the intermediate zone of spinal gray matter (internuncial neurons), from which motor impulses are then transmitted to the anterior horn cells. Only 10 to 20 percent of corticospinal fibers (presumably the thick, rapidly conducting axons derived from Betz cells) establish direct synaptic connections with the large motor neurons of the anterior horns.


Neurology_Adams

Neurology_Adams. Motor, Premotor, and Supplementary Motor Cortices and Cerebral Control of Movement The motor area of the cerebral cortex is defined physiologically as the region of electrically excitable cortex from which isolated movements can be evoked by stimuli of minimal intensity. The muscle groups of the contralateral face, arm, trunk, and leg are represented in the primary motor cortex (area 4 in Fig. 3-3), those of the face being in the most inferior part of the precentral gyrus on the lateral surface of the cerebral hemisphere and those of the leg in the paracentral lobule on the medial surface of the cerebral hemisphere. The parts of the body capable of the most delicate movements have, in general, the largest cortical representation, as displayed in the motor homunculus (“little man,” a term first suggested by Wilder Penfield) shown in Fig. 3-4.


Neurology_Adams

Neurology_Adams. Area 6a, the premotor area, is also electrically excitable but requires more intense stimuli than area 4 to evoke movements. Stimulation of its caudal aspect produces responses that are similar to those elicited from area 4. These responses are probably produced by transmission of impulses from all of area 6a to area 4 (as they cannot be obtained after ablation of area 4). Stimulation of the rostral premotor area elicits more general movement patterns, predominantly of proximal limb musculature. The latter movements are effected via pathways other than those derived from area 4 (hence, “parapyramidal”). Very strong stimuli elicit movements from a wide area of premotor frontal and parietal cortex, and the same movements may be obtained from several widely separated points. From this it may be assumed that the premotor cortex includes several anatomically distinct subregions with different afferent and efferent connections. In general, it may be said that the motor–premotor cortex is


Neurology_Adams

Neurology_Adams. be assumed that the premotor cortex includes several anatomically distinct subregions with different afferent and efferent connections. In general, it may be said that the motor–premotor cortex is capable of synthesizing agonist actions into an almost infinite variety of finely graded, highly differentiated patterns. These are directed by visual (area 7) and tactile (area 5) sensory information and supported by appropriate postural mechanisms.


Neurology_Adams

Neurology_Adams. The supplementary motor area is the most anterior portion of area 6 on the medial surface of the cerebral hemisphere (area 6b in Fig. 3-3B). Stimulation of this area may induce relatively gross ipsilateral or contralateral movements, bilateral tonic contractions of the limbs, contraversive movements of the head and eyes with tonic contraction of the contralateral arm, and sometimes inhibition of voluntary motor activity and vocal arrest.


Neurology_Adams

Neurology_Adams. Precisely how the motor cortex controls movements is still a controversial matter. The traditional view, based on the interpretations of Hughlings Jackson and of Sherrington, and elaborated by Denny-Brown, has been that the motor cortex is organized not in terms of individual muscles but of movements, that is, the coordinated contraction of groups of muscles. Jackson visualized a widely overlapping representation of muscle groups in the cerebral cortex on the basis of his observation that a patient could recover the use of a limb following destruction of the limb area as defined by cortical stimulation. This view was supported by Sherrington’s observations that stimulation of the cortical surface activated not solitary muscles but a combination of muscles, and always in a reciprocal fashion—that is, in a manner that maintained the expected relationship between agonists and antagonists. He also noted the inconstancy of stimulatory effects; the stimulation of a given cortical point that


Neurology_Adams

Neurology_Adams. is, in a manner that maintained the expected relationship between agonists and antagonists. He also noted the inconstancy of stimulatory effects; the stimulation of a given cortical point that initiated flexion of a part on one occasion might initiate extension on another.


Neurology_Adams

Neurology_Adams. These interpretations must be viewed with circumspection, as must all observations based on the electrical stimulation of the surface of the cortex. It has been shown that to stimulate motor cells from the surface, the electric current has to penetrate the cortex to layer V, where these neurons are located, inevitably activating a large number of other cortical neurons. The elegant experiments of Asanuma and of Evarts and his colleagues, who stimulated the depths of the cortex with microelectrodes, demonstrated the existence of discrete zones of efferent neurons that control the contraction of individual muscles; moreover, the continued stimulation of a given efferent zone often facilitated rather than inhibited the contraction of the antagonists. These investigators have also shown that cells in the efferent zone receive afferent impulses from the particular muscle to which the efferent neurons project. When the effects of many stimulations at various depths were correlated with the


Neurology_Adams

Neurology_Adams. cells in the efferent zone receive afferent impulses from the particular muscle to which the efferent neurons project. When the effects of many stimulations at various depths were correlated with the exact sites of each penetration, cells that projected to a particular pool of spinal motor neurons were found to be arranged in radially aligned columns approximately 1 mm in diameter.


Neurology_Adams

Neurology_Adams. The columnar arrangement of cells in the sensorimotor cortex had been appreciated for many years; the wealth of radial interconnections between the cells in these columns led Lorente de Nó to suggest that these “vertical chains” of cells were the elementary functional units of the cortex. This notion received strong support from Mountcastle’s observations that all the neurons in a column receive impulses of the same sensory modality, from the same part of the body. It is still not entirely clear whether the columns contribute to a movement as units or whether individual cells within many columns are selectively activated. Both Henneman and Asanuma summarized the evidence for these disparate views.


Neurology_Adams

Neurology_Adams. Evarts and his colleagues also elucidated the role of cortical motor neurons in sensory evoked or planned movement. Using single-cell recording techniques, they showed that pyramidal cells fire about 60 ms prior to the onset of a movement, in a sequence determined by the required pattern and force of the movement. But other, more complex properties of the pyramidal cells were also noted. Some of them received a somatosensory input transcortically from the parietal lobe (areas 3, 1, and 2), which could be turned on or off or gated according to whether the movement was to be controlled, that is, guided, by sensory input. Many neurons of the supplementary and premotor cortices were activated before a planned movement. Thus pyramidal (area 4) motor neurons were prepared for the oncoming activation by impulses from the parietal, prefrontal, premotor, and auditory and visual areas of the cortex. This preparatory “set signal” could occur in the absence of any activity in the spinal cord and


Neurology_Adams

Neurology_Adams. by impulses from the parietal, prefrontal, premotor, and auditory and visual areas of the cortex. This preparatory “set signal” could occur in the absence of any activity in the spinal cord and muscles. The source of the activation signal was found to be mainly in the supplementary motor cortex, which appears to be under the direct influence of the “readiness stimuli” (Bereitschaft potential) reaching it from the prefrontal areas for planned movements and from the posterior parietal cortex for motor activities initiated by sensory perceptions. There are also fibers that reach the motor area from the limbic system, presumably subserving motivation and attention. Roland has used functional cerebral blood flow measurements to follow these neural events.


Neurology_Adams

Neurology_Adams. Thus the prefrontal cortex, supplementary motor cortex, premotor cortex, and motor cortex are all responsive to afferent stimuli and are involved prior to, and in coordinated fashion with, a complex movement. As remarked later on, the striatopallidum and cerebellum, which project to these cortical areas, are also activated prior to or concurrently with the discharge of corticospinal neurons (see Thach and Montgomery for a critical review of the physiologic data). Termination of the Corticospinal and Other Descending Motor Tracts


Neurology_Adams

Neurology_Adams. Termination of the Corticospinal and Other Descending Motor Tracts This has been studied in the monkey by interrupting the descending motor pathways in the medulla and more rostral parts of the brainstem and tracing the distribution of the degenerating elements in the spinal gray matter. On the basis of such experiments and other physiologic data, Lawrence and Kuypers proposed that the functional organization of the descending cortical and subcortical pathways is determined more by their patterns of termination and the motor capacities of the internuncial neurons upon which they terminate than by the location of their cells of origin.


Neurology_Adams

Neurology_Adams. Three groups of motor fibers were distinguished according to their differential terminal distribution: (1) The corticospinal and corticobulbar tracts, which project to all levels of the spinal cord and brainstem, terminating diffusely throughout the nucleus proprius of the dorsal horn and the intermediate zone. A portion of these connect directly with the large motor neurons that innervate the muscles of the fingers, face, and tongue; this system provides the capacity for a high degree of fractionation of movements, as exemplified by independent finger movements. As alluded to above, a large fraction of the fibers in the corticospinal tracts originate from the sensory cortex and appear to function in the modulation of movement by afferent neurons. (2) A ventromedial pathway, which arises in the tectum (tectospinal tract), vestibular nuclei (vestibulospinal tract), and pontine and medullary reticular cells (reticulospinal tract) and terminates principally on the internuncial cells of


Neurology_Adams

Neurology_Adams. in the tectum (tectospinal tract), vestibular nuclei (vestibulospinal tract), and pontine and medullary reticular cells (reticulospinal tract) and terminates principally on the internuncial cells of the ventromedial part of the spinal gray matter. This system is mainly concerned with axial movements—the maintenance of posture, integrated movements of body and limbs, and total limb movements. (3) A lateral pathway, which is derived mainly from the magnocellular part of the red nucleus and terminates in the dorsal and lateral parts of the internuncial zone. This pathway adds to the capacity for independent use of the extremities, especially of the hands.


Neurology_Adams

Neurology_Adams. Reference has already been made to the corticomesencephalic, corticopontine, and corticomedullary fiber systems that project onto the reticulospinal, vestibulospinal, rubrospinal, and tectospinal nuclei. These control stability of the head (via labyrinthine reflexes) and of the neck and body in relation to the head (tonic neck reflexes) as well as postures of the body in relation to limb movements. Lesions in these systems are less well understood than those of the corticospinal system. They cause no paralysis of muscles but result in the liberation of unusual postures (e.g., hemiplegic dystonia), heightened tonic neck and labyrinthine reflexes, and decerebrate rigidity. In a strict sense these are all “extrapyramidal,” as discussed in the next two chapters. Paralysis Caused by Lesions of the Upper Motor Neurons


Neurology_Adams

Neurology_Adams. The corticospinal pathway may be interrupted by a lesion at any point along its course—at the level of the cerebral cortex, subcortical white matter, internal capsule, brainstem, or spinal cord. Usually, when hemiplegia is severe and permanent as a consequence of disease, much more than the long, direct corticospinal pathway is involved. In the cerebral white matter (corona radiata) and internal capsule, the corticospinal fibers are intermingled with corticostriate, corticothalamic, corticorubral, corticopontine, cortico-olivary, and corticoreticular fibers. It is noteworthy that thalamocortical fibers, which are a vital link in an ascending fiber system from the basal ganglia and cerebellum, also pass through the internal capsule and cerebral white matter. Thus lesions in these parts can simultaneously affect both corticospinal and extrapyramidal systems. Attribution of a capsular hemiplegia solely to a lesion of the corticospinal or pyramidal pathway is therefore not entirely


Neurology_Adams

Neurology_Adams. can simultaneously affect both corticospinal and extrapyramidal systems. Attribution of a capsular hemiplegia solely to a lesion of the corticospinal or pyramidal pathway is therefore not entirely correct. The term upper motor neuron (supranuclear) paralysis, which recognizes the involvement of several descending fiber systems that influence and modify the lower motor neuron, is more appropriate.


Neurology_Adams

Neurology_Adams. In primates, lesions limited to area 4 of Brodmann, the motor cortex, cause mainly hypotonia and weakness of the distal limb muscles. Lesions of the premotor cortex (area 6) result in weakness, spasticity, and increased stretch reflexes (Fulton). Lesions of the supplementary motor cortex lead to involuntary grasping. Resection of cortical areas 4 and 6 and subcortical white matter in monkeys causes complete and permanent paralysis and spasticity (Laplane et al). These clinical effects have not been as clearly defined in humans.


Neurology_Adams

Neurology_Adams. The one place where corticospinal fibers are entirely isolated is the pyramidal tract in the medulla. In humans, there are a few documented cases of a lesion more or less confined to this location. The result of such lesions has been an initial flaccid hemiplegia (with sparing of the face), from which there is considerable recovery. Similarly in monkeys—as was shown by Tower in 1940 and subsequently by Lawrence and Kuypers and by Gilman and Marco—interruption of both pyramidal tracts results in a hypotonic paralysis; ultimately, these animals regain a wide range of movements, although slowness of all movements and loss of individual finger movements remain as permanent deficits. Also, the cerebral peduncle had in the past been sectioned in patients in an effort to abolish involuntary movements (Bucy et al). In some of these patients, a slight degree of weakness or only a Babinski sign was produced but no spasticity developed. These observations indicate that a pure pyramidal tract


Neurology_Adams

Neurology_Adams. (Bucy et al). In some of these patients, a slight degree of weakness or only a Babinski sign was produced but no spasticity developed. These observations indicate that a pure pyramidal tract lesion does not result in spasticity. Animal experiments suggest that the corticoreticulospinal pathways are particularly important in this respect, because their fibers are arranged somatotopically and influence stretch reflexes. Further studies of human disease, possibly using diffusion tensor imaging techniques, are necessary to settle problems related to volitional movement and spasticity.