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Senses are the physiological methods of perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields. Sense is a faculty by which outside stimuli are perceived.
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The senses of Gustation (taste) and Olfaction (smell) fall under the category of Chemoreception. Specialized cells act as receptors for certain chemical compounds. As these compounds react with the receptors, an impulse is sent to the brain and is registered as a certain taste or smell.

Gustatory System





Clich and see a short video on the tounge."Taste Buds Video"

Structure of Taste Buds







Types of Taste

Arguably the simplest receptor found in the mouth is the salt (NaCl) receptor. An ion channel in the taste cell wall allows Na+ ions to enter the cell. This on its own depolarizes the cell, and opens voltage-regulated Ca2+ gates, flooding the cell with ions and leading to neurotransmitter release. This sodium channel is known as EnAC and is composed of three subunits. EnAC can be blocked by the drug amiloride in many mammals, especially rats. The sensitivity of the salt taste to amiloride in humans, however, is much less pronounced, leading to conjecture that there may be additional receptor proteins besides EnAC that may not have been discovered yet.

Disorders of the Tongue

A range of tongue disorders


Loss of taste
Sore tongue
Disorders such as diabetes, aneamia, some types of vitamin deficiency and certain skin diseases can include a sore tongue among the range of symptoms.
Black hairy tongue
While the term ‘black hairy tongue’ suggests the tongue surface looks black, it may also be dark yellow, brown, green or white. The tongue papillae are constantly renewing themselves and, usually, the old cells are shed as the new cells emerge. Black hairy tongue, a comparatively rare condition, is caused by the failure of the old cells to shed. The overgrowth of papillae trap food and bacteria, which create the characteristic dark ‘coat’ on the tongue’s surface, while the tongue looks furred because of the layering of unshed papillae. The cause isn’t known.
Risk factors include:
Poor oral hygiene
Cigarette smoking
Particular antibiotics
Chemotherapy and radiation treatment for cancers of the head and neck
Poorly managed diabetes.
Black Hairy Tongue
A condition characterised by a burning sensation on the tongue.
Benign migratory glossitis
Risk factors are thought to include:
Mineral or vitamin deficiencies
Local irritants, such as strong mouthwashes, cigarettes or alcohol
Certain forms of anaemia
Certain medications
Ankyloglossia, commonly known as 'tongue tie'.
Is a congenital oral anomaly which may decrease mobility of the tongue tip and is caused by a short, thick frenulum. Ankyloglossia varies in degree of severity from mild cases characterized by mucous membrane bands to complete ankyloglossia whereby the tongue is tethered to the floor of the mouth.

Olfactory System






An olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction cell in the olfactory system. Humans have about 40 million olfactory receptor neurons. In verebrates, olfactory receptor neurons reside on the olfactory epithelium in the nasal cavity. These cells are bipolar neurons with a dendrite facing the interior space of the nasal cavity and an axon that travels along the olfactory nerve to the olfactory bulb.

Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite into the mucus covering the surface of the olfactory epithelium. These cillia contain olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell contains only one type of olfactory receptor, but many separate olfactory receptor cells contain the same type of olfactory receptor. The axons of olfactory receptor cells of the same type converge to form glomeruli in the olfactory bulb.



In the brain




Pheromonal olfaction



Olfaction and Gustation


Disorders of Olfaction


While termed as a disability, anosmia is often viewed in the medical field as a trivial problem. This is not always the case - esthesioneuroblastoma is a cancerous tumor originating in or near the olfactory nerve. This is a serious condition - if you suspect that you have lost your sense of smell, see an Otolaryngologist (Ear, Nose and Throat doctor) immediately.


Losing an established and sentimental smell memory (e.g. the smell of grass, of a grandparent's attic, of a particular book, of loved ones, or of oneself) has been known to cause feelings of depression.



Decreased ability to smell
Things do smell differently than they should.

The Sense of Vision

Vision needs to have the work of both the eyes and the brain to process any information. The majority of the stimuli is done in the eyes and then the information is sent to the brain by the way of nerve impulses. Atleast one-third of the information of what the eye sees is processed in the cerebral cortex of the brain.

Anatomy of the Eye


Cross-section of the human eye.
Cross-section of the human eye.



Extraocular muscles

Eye Movement


Try This Experiment

How we see an object

  • These neuron signals are transmitted through the optic nerve,

Depth Perception



Diseases, disorders, and age-related changes


Color Blindness
Night Blindnes
Day Blindness
A cataract is an opacity which develops in the crystalline lens of the eye or in its envelope. Cataracts typically progress slowly to cause vision loss and are potentially blinding if left untreated. Cataracts form for a variety of reasons, including long-term ultraviolet exposure, secondary effects of diseases such as diabetes, or simply due to advanced age; they are usually a result of denaturation of lens proteins. Genetic factors are often a cause of congenital cataracts and may also play a role in predisposing someone to cataracts. Half of all people between the ages of 65 and 74 and about 70% of those over 75 have some cataract formation. Cataracts may also be produced by eye injury or physical trauma.
Impression of floaters, as seen against a blue sky.

Impression of floaters, as seen against a blue sky.
Also known as "Muscae Volitantes" are deposits of various size, shape, consistency, refractive index, and motility within the eye's normally transparent vitreous humour. Floaters are suspended in the vitreous humour, the thick fluid or gel that fills the eye. Thus, they generally follow the rapid motions of the eye, while drifting slowly within the fluid. Floaters located slightly to the side of one's direction of gaze can be particularly annoying. When they are first noticed, the natural reaction is to attempt to look directly at them. However, attempting to shift one's gaze toward them can be difficult since floaters follow the motion of the eye, remaining to the side of the direction of gaze. Floaters are, in fact, visible only because they do not remain perfectly fixed within the eye. Floaters are particularly noticeable when lying on one's back and gazing at blue sky. Despite the name "floaters", many of these specks have a tendency to sink toward the bottom of the eyeball, in whichever way the eyeball is oriented; the supine position tends to concentrate them near the fovea, which is the center of gaze, while the textureless and evenly lit sky forms an ideal background against which to view them. Floaters are not uncommon, and do not cause problems for most sufferers. However, floaters are more than a nuisance and a distraction to those with severe cases, especially if the spots seem to constantly drift through the field of vision. The shapes are shadows projected onto the retina by tiny structures of protein or other cell debris discarded over the years and trapped in the vitreous humour. It should also be noted that they can even be seen when the eyes are closed on especially bright days, when sufficient light penetrates the eyelids to cast the shadows. It is not, however, only elderly people who suffer from floaters; they can certainly become a problem to younger people, especially if they are myopic. They are also common after cataract operations or after trauma. In some cases, floaters are congenital. There are various causes for the appearance of floaters, of which the most common are described here. The most common cause of floaters is shrinkage of the vitreous humour: this gel-like substance consists of 99% water and 1% solid elements. The solid portion consists of a network of collagen and hyaluronic acid, with the latter retaining water molecules. Patients with retinal tears may experience floaters if red blood cells are released from leaky blood vessels, and those with a posterior uveitis or vitritis, as in toxoplasmosis, may experience multiple floaters and decreased vision due to the accummulation of white blood cells in the vitreous humour.
Visual Agnosia


The specific symptoms can vary depending on the cause of the agnosia. Some sufferers are unable to copy drawings, but are able to manipulate objects with good dexterity.Commonly patients can describe objects in their visual field in great detail, including such aspects as color, texture, and shape, but are unable to recognize them. Similarly, patients can often describe familiar objects from memory despite their visual problems


The Senses Of Hearing

The ear is the sense organ that detects sound and plays a major role in the sense of balance and body position. The sensory receptors for both hearing and equilibrium are to be found in the inner ear consisting of hair cells that have stereocilia (long mircovilli). The stereocillia are extremely sensitive to mechanical stimulations, which are known as mechanoreceptors.

Anatomy of the Ear

The ear has three divisions: the outer ear, middle ear, and the inner ear.

Anatomy of the human ear.

Anatomy of the human ear.
The fluid inside this duct is moved, flowing against the receptor cells of the organ of Corti, which fire. These stimulate the Spiral Ganglion, which sends information through the auditory portion of the eighth cranial nerve to the brain.
Hair Cell

Process of Hearing

Detection of sound motion is associated with the right posterior superior temporal gyrus. The superior temporal gyrus contains several important structures of the brain, including: (1)marking the location of the primary auditory cortex, the cortical region responsible for the sensation of sound. Sections 41 and 42 are called the primary auditory area of the cerebrum, and processes the basic characteristics of sound such as pitch and rythym. The auditory association area is located within the temporal lobe of the brain, in an area called the Wernicke's area, or area 22. This area, near the lateral cerebral sulcus, is an important region for the processing of acoustic energy so that it can be distinguished as speech, music, or noise. It also interprets words that are heard into an associated thought pattern of understanding. The gnostic area of the cerebum, (areas 5, 7, 39 and 40) helps to integrate all incomming sense patterns so that a common thought can be formed (correlated) using all arriving sensory information.

Hearing Under Water


Localization of Sound by Humans


Two mechanisms are known to be used.


Process of Equilibrium





Disorders with the Ear

The word deaf can have at least two different meanings. The first term is used to indicate the presence of enough hearing loss such that an individual is not sensitive to sound. Someone with a partial loss of hearing is more likely to be referred to as hearing impaired or the qualified partially deaf. The second term is used to indicate someone who was born without the sense of hearing or total deafness.
Otitis Media
Vertigo (dizziness)
Motion sickness
Meniere's Disease

Microtia (meaning 'small ear') is a congenital deformity of the pinna (outer ear). It can be unilateral (one side only) or bilateral (affecting both sides). It occurs in 1 out of about 8,000-10,000 births. In unilateral microtia, the right ear is most typically affected. There are four grades of microtia [1]:

Grade I: A slightly small ear with identifiable structures and a small but present external ear canal Grade II: A partial or hemi-ear with a closed off or stenotic external ear canal producing a conductive hearing loss Grade III: Absence of the external ear with a small peanut vestige structure and an absence of the external ear canal and ear drum Grade IV: Absence of the total ear or anotia. Grade III is most common, and can be corrected by surgery. Typically, testing is first done to determine if the inner ear is intact and hearing is normal. If hearing is normal, the next step (if a canal is not visible externally) will be to determine if a canal exists, by MRI. For younger patients, this is done under sedation. Surgery may not necessarily be attempted until at a later age (generally about age 8, when the ear is full adult size). Cosmetic surgery (building a new outer ear) is done first to ensure position in relation to the other side; the patient's own cartilage is used (taken from ribs or elsewhere). Surgery to build a canal and/or canal opening can then proceed.


Haptic From the Greek Haphe, means pertaining to the sense of touch. It is the earliest sense to develop in the fetus, and the development of infants.
Thermoception is the sense of heat and the absence of heat (cold), also by the skin and including internal skin passages. The thermoceptors in the skin are quite different from the homeostatic thermoceptors which provide feedback on internal body temperature.




Meissner's Corpuscle

Meissner's corpuscles (discovered by the anatomist Georg Meissner (1829-1903)) are a type of mechanoreceptor and more specifically, a tactile corpuscle (corpusculum tactus). They are distributed throughout the skin, but concentrated in areas especially sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face, nipples and the external skin of the male and female genitals. They are primarily located just beneath the epidermis within the dermal papillae.

Meissner's corpuscles are encapsulated unmyelinated nerve endings, which consist of flattened supportive cells arranged as horizontal lamellae surrounded by a connective tissue capsule. A single nerve fiber meanders between the lamellae and throughout the corpuscle. Any physical deformation in the corpuscle will cause an action potential in the nerve. Since they are rapidly adapting or phasic, the action potentials generated quickly decrease and eventually cease. If the stimulus is removed, the corpuscle regains its shape and while doing so (ie: while physically reforming) causes another volley of action potentials to be generated. (This is the reason one stops "feeling" one's clothes.) Because of their superficial location in the dermis, these corpuscles are particularly sensitive to touch and vibrations, but for the same reasons, they are limited in their detection because they can only signal that something is touching the skin. Feelings of deep pressure (from a poke, for instance) are generated from Pacinian corpuscles (the only other type of phasic tactile mechanoreceptor), which are located deeper in the dermis, and some free nerve endings. Also, Meissner's corpuscles do not detect pain; this is signalled exclusively by free nerve endings.

Pacinian Corpuscles

Pacinian corpuscles detect gross pressure changes and vibrations. Any deformation in the corpuscle causes action potentials to be generated, by opening pressure-sensitive sodium ion channels in the axon membrane. This allows sodium ions to influx in, creating a receptor potential.

These corpuscles are especially susceptible to vibrations, which they can sense even centimeters away (Kandel et al., 2000). Pacinian corpuscles cause action potentials when the skin is rapidly indented but not when the pressure is steady, due to the layers of connective tissue that cover the nerve ending (Kandel et al., 2000). It is thought that they respond to high velocity changes in joint position.

Pacinian corpuscles have a large receptive field on the skin's surface with an especially sensitive center (Kandel et al., 2000). They only sense stimuli that occur within this field.


Merkels Discs

Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal. This is sometimes referred to as a Merkel cell-neurite complex, or a Merkel disk receptor. A single afferent nerve fibre branches to innervate up to 90 such endings. They are classified as slowly adapting type I mechanoreceptors.


Ruffini Corpuscles


The physiological response of a sense organ to a chemical stimulus.
Specialized epithelial cells which are small projections on the top of the tounge.
The lack of olfaction, or a loss of the sense of smell.
The phenomenon of smelling odors that aren't really present. (AKA Phantom odors)
The white outer coating of the eye made of tough fibrin connective tissue which gives the eye its shape and helps to protect the delicate inner parts.
Outer Ear
The part of the ear visable to externally.
The auricle of the ear.
Auditory Canal
The tube from the auditory meatus or opening of the ear to the tympanic membrane.
Middle Ear
The main cavity of the ear; between the eardrum and the inner ear.
Tympanic Membrane
Like a tympanum or drum; acting like a drumhead; as, a tympanic membrane. The membrane in the ear that vibrates to sound.
Oval Window
Fenestra that has the base of the stapes attached to it.
Round Window
Fenestra leading into the cochlea.
The there small bones of tympanum of the ear.
One of the small bones in the tympanum of the ear; the hammer bone.
One of the small bones in the tympanum of the ear; the stirrups bone.
Auditory Tube
Inner Ear
Complex systems of interconnecting cavities; concerned hearing and equilibrium.
Semicircular Canals
Certain canals of the inner ear.
Is concerned with hearing, resembling a shell of a snail.



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