Basic Psychology, Biology of Psychology


Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord.

All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 2) dedicated to processing that particular sense.

How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species is largely similar, owing to the inherited similarity of their nervous systems; however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ.

Illustration A shows side view of a human brain. The thalamus is in the inner, middle part. Illustration B shows the location of sensory processing regions in the brain. The visual processing region is at the back of the brain, the auditory processing region is in the middle of the brain, and the somatosensory processing region is located in a sliver-like region in the upper part of the brain and extending halfway down.

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Types of Sensations

1.Gustation (Taste)

Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae(singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance (Figure 2): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.

The left panel shows the image of a tongue with callouts that show magnified views of different parts of the tongue. The top right panel shows a micrograph of the circumvallate papilla, and the bottom right panel shows the structure of a taste bud.

Figure 2. The Tongue. The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)


Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.

2.Olfaction (Smell)

Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 3). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.

The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.

The top left panel of this image shows the side view of a person’s face with a cup containing a beverage underneath the nose. The image shows how the aroma of the beverage passes through the nasal cavity. The top right panel shows a detailed ultrastructure of the olfactory bulb. The bottom panel shows a micrograph of the nasal cavity.

Figure 3. The Olfactory System. (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

3.Audition (Hearing)




Equilibrium (Balance)


The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure 10). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.


Figure 10. Linear Acceleration Coding by Maculae. The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.




4.Somatosensation (Touch)






Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 12). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

This diagram shows the lateral view of the eye. The major parts are labeled.

Figure 12. The Eye in the Orbit. The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

Basic Psychology, Biology of Psychology

Sensory Receptors

Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.

Structural Receptor Types

The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 1). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.

This figure shows the different types of receptors. The top panel shows a neuron receptor with free receptor endings, the middle panel shows a neuron receptor with encapsulated nerve endings, and the bottom panel shows a specialized receptor cell.

Figure 1. Receptor Classification by Cell Type. Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons.


Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move.

Functional Receptor Types

A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.

Receptor cells can be further categorized on the basis of the type of stimuli they transduce.

  • Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell.
  • Osmoreceptors respond to solute concentrations of body fluids.
  • Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor.
  • Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor.
  • Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.

Sensory Modalities

Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.

Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.

Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.

Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.

Basic Psychology, Biology of Psychology



The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away (some baleen whale sounds can propagate for many kilometers). For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances.


The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 1) possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Recall that in the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.


Illustration A shows a closed gated ion channel embedded in the plasma membrane. A hair-like tether connects the channel to the extracellular matrix outside the cell, and another tether connects the channel to the inner cytoskeleton. When the extracellular matrix is deflected, the tether tugs on the gated ion channel, pulling it open. Ions may now enter or exit the cell. Illustration B shows stereocilia, hair-like projections on outer hair cells that attached to the tectorial membrane of the inner ear. The outer hair cells are connected to the cochlear nerve.

Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

Encoding and Transmission of Sensory Information

Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus, and this segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus—a sound.

The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals.

Basic Psychology, Biology of Psychology

Sensation and Perception

A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.

Describing sensory function with the term sensation or perception is a deliberate distinction.

Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern.


Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.



Basic Psychology, Biology of Psychology

Endocrine System

The glandular system regulates the activities of the body by secreting chemical substances (hormones) into the bloodstream. These secretions come from a variety of glands which influences almost every cell, organ, and function of our bodies. The endocrine system is instrumental in regulating mood, growth and development, tissue function, metabolism, sexual function and reproductive processes.The endocrine system uses chemicals to “communicate” to and from various parts of the body. These chemicals are known as hormones. Each hormone’s shape is specific and can be recognized by the corresponding target cells.Low concentrations of a hormone will often trigger the gland to secrete. Once the concentrations of the hormone in the blood rise this may cause the gland to stop secreting, until once again hormone concentrations fall. This feedback mechanism (which is characteristic of most glands) causes a cycle of hormone secretions.Once hormones have served their function on their target organs/tissues they are destroyed. They are either destroyed by the liver or the actual tissues of the target organs. They are then removed by the kidneys. 

The major glands that make up the human endocrine system 


  • The hypothalamus is located in the lower central part of the brain.
  • It is the main link between the endocrine and nervous systems.
  • Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary. It is the gland that monitors what is happening in the body and then instructs the pituitary gland on what needs to be done.

Pituitary gland

  • It’s often called the “master gland” because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons.
  • The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands.
  • The posterior lobe of the pituitary releases antidiuretic hormone, which helps control the balance of water in the body.
  • The posterior lobe also produces oxytocin, which triggers the contractions of the uterus in a woman having a baby.

Pineal gland

  • The pineal gland is located in the middle of the brain and is stimulated by nerves from the eyes.
  • The pineal gland secretes melatonin at night when it’s dark, thus secretes more in winter when the nights are longer.
  • Melatonin promotes sleep and also affects reproductive functions by depressing the activity of the gonads.
  • Additionally, it affects thyroid and adrenal cortex functions.


  • The thyroid, located in the front part of the lower neck, is shaped like a butterfly and produces the thyroid hormones thyroxine and triiodothyronine.
  • These hormones control the rate at which cells burn fuels from food to produce energy.
  • The more thyroid hormone there is in a person’s bloodstream, the faster chemical reactions occur in the body.
  • Thyroid hormones regulate metabolism, therefore body temperature and weight. The thyroid hormones contain iodine, which the thyroid needs in order to manufacture these hormones.
  • If a person lacks iodine in his/her diet, the thyroid cannot make the hormones, causing a deficiency. In response to the body’s feedback loops calling for more thyroid hormones, the thyroid gland then enlarges to attempt to compensate (The body’s plan here is if it’s bigger it can make more, but that doesn’t help if there isn’t enough iodine.). This disorder is called goitre.
  • Under-secretion of thyroid hormone in children produces cretinism; the children show stunted growth (dwarfism) and fail to develop mentally. Under secretion in adults results in a low metabolic rate.
  • Over secretion in adults gives rise to exophthalmic goitre and the metabolic rate is higher than usual. Such persons may eat well but burn up so much fuel that they remain thin. This is usually accompanied by a rapid pulse rate.
  • This gland, therefore, has a profound influence on both mental and physical activity.

The Parathyroid Glands

  • Attached to the thyroid are four tiny glands that function together called the parathyroids.
  • They release parathyroid hormone, the function of which is to raise the blood calcium as well as maintain the balance of calcium and phosphorus in both the blood and bone structures.
  • Under secretion gives rise to a condition known as tetany in which the muscles go into spasm, and over secretion causes calcium to be lost to the blood from the bones giving rise to softened bones, raised blood calcium and a marked depression of the nervous system.
  • Calcium is important, not only for bones and teeth, but also for nerve functioning, muscle contractions, blood clotting and glandular secretion. If we don’t have enough calcium for these functions, the body will take it from the bones, causing them to easily fracture.

The Thymus Gland

  • This gland lies in the lower part of the neck and attains a maximum length of about 6 cm.
  • It is made up of two lobes that join in front of the trachea.
  • Each lobe is made of lymphoid tissue, consisting of tightly packed white blood cells and fat.
  • The thymus enlarges from about the 12th week of gestation until puberty.
  • After puberty, the thymus begins to atrophy so that in the adult only fibrous remnants is found.
  • Its secretion is thought to act as a brake on the development of sex organs so that as the thymus atrophies, the sex organs develop.
  • Its function is to transform lymphocytes (white blood cells developed in the bone marrow) into T-cells (cells developed in the thymus).
  • These cells are then transported to various lymph glands, where they play an important part in fighting infections and disease.
  • Swelling of lymph glands and fever are a signal that immune cells are multiplying to fight off invaders of the body: bacteria, fungi, viruses or parasites.
Thymus Gland

Adrenal glands

  • The outer part, the adrenal cortex, produces hormones called corticosteroids. There are three main types: those which control the balance of sodium salt and potassium in the body (salt and water levels); those which raise the level of sugar in the blood; and sex hormones. This cortex, or outer, yellow layer, takes its instructions from the pituitary hormone ACTH.
  • The smaller, inner region is part of the sympathetic nervous system and is the body’s first line of defense and response to physical and emotional stresses. This inner, reddish brown layer makes two types of hormones and takes all its instructions from the nervous system, producing chemicals which react to fear and anger and are sometimes called “fight or flight” hormones called catecholamines.
  • One of these, called epinephrine or adrenaline, increases blood pressure and heart rate when the body experiences stress.

The Pancreas

  • The pancreas is a long, narrow, lobed gland located behind the stomach.
    This organ has two functions.
  • It secretes digestive enzymes into the small intestine, which break down fats, carbohydrates, proteins and acids.
  • It also secretes bicarbonate, which neutralizes stomach acid as it enters the duodenum.
  • It also has endocrine cells which are arranged in clusters throughout the Pancreas, these are known as Islets of Langerhans.
  • They secrete insulin and glucagon to regulate the blood sugar level.
  • Glucagon tells the liver to take carbohydrate out of storage to raise a low blood sugar level.
  • Insulin tells the liver to take excess glucose out of circulation to lower a blood sugar level that’s too high.
  • If a person’s body does not make enough insulin (and/or there is a reduced response of the target cells in the liver), the blood sugar rises, perhaps out of control, and we say that the person has diabetes mellitus.
The Pancreas

Male Hormone System

    FSH and LH are produced by the pituitary gland.
  • FSH is necessary for sperm production (spermatogenesis), and LH stimulates the production of testosterone.
  • Testosterone as well as sperm cells, are produced in the testes.
  • The prostate is a gland of the male reproductive system that produces fluid for semen, which helps to transport sperm during the male orgasm.
  • The prostate is made up of about 30% muscular tissue; the rest is glandular tissue.
  • The prostate is about the same size and shape as a walnut and is located in front of the rectum and just below the bladder.
  • The prostate wraps around the urethra, which is the tube that carries urine out from the bladder through the tip of the penis.
Male Hormone System

Female Hormonal System

  • The hypothalamus produces gonadotropin-releasing hormone, which stimulates the pituitary gland to produce luteinizing hormone and follicle-stimulating hormone.
  • These hormones stimulate the ovaries to produce the female sex hormones, estrogen and progesterone, and some male sex hormones (androgens).
  • (Male sex hormones stimulate the growth of pubic and underarm hair at puberty and maintain muscle mass in girls as well as boys.) After childbirth, the hypothalamus signals the pituitary gland to produce prolactin, a hormone that stimulates milk production.
  • The adrenal glands produce small amounts of female and male sex hormones.
  • A woman’s reproductive cycles last from menarche to menopause.
    Menopause occurs when a woman’s reproductive cycles stop.
  • The changing hormone levels are responsible for the symptoms associated with menopause.

Basic Psychology, Biology of Psychology

Important Neurotransmitters & Their Functions

bout 50 neurotransmitters have been discovered so far. Out of these, a few of the most important neurotransmitters and their functions are discussed below.


  • This neurotransmitter was discovered in the year 1921, by Otto Loewi.
  • It is mainly responsible for stimulating muscles.
  • It activates the motor neurons that control the skeletal muscles.
  • It is also concerned with regulating the activities in certain areas of the brain, which are associated with attention, arousal, learning, and memory.
  • People with Alzheimer’s disease are usually found to have a substantially low level of acetylcholine.


  • Dopamine is the neurotransmitter that controls voluntary movements of the body, and is associated with the reward mechanism of the brain. In other words, dopamine regulates the pleasurable emotions.
  • Drugs like cocaine, heroin, nicotine, opium, and even alcohol increase the level of this neurotransmitter.
  • A significantly low level of dopamine is associated with Parkinson’s disease, while the patients of schizophrenia are usually found to have excess dopamine in the frontal lobes of their brain.


  • Serotonin is an important inhibitory neurotransmitter, which can have a profound effect on emotion, mood, and anxiety.
  • It is involved in regulating sleep, wakefulness, and eating.
  • It plays a role in perception as well.
  • The hallucinogenic drugs like LSD actually bind to the serotonin receptor sites, and thereby block the transmission of nerve impulses, in order to alter sensory experiences.
  • A significantly low level of serotonin is believed to be associated with conditions like depression, suicidal thoughts, and obsessive compulsive disorder. Many antidepressants work by affecting the level of this neurotransmitter.


Gamma-aminobutyric Acid (GABA)

  • GABA is an inhibitory neurotransmitter that slows down the activities of the neurons, in order to prevent them from getting over excited.
  • When neurons get over excited, it can lead to anxiety. GABA can thus help prevent anxiety.
  • It is a non-essential amino acid, that is produced by the body from glutamate.
  • A low level of GABA can have an association with anxiety disorders.
  • Drugs like Valium work by increasing the level of this neurotransmitter.



  • Glutamate is an excitatory neurotransmitter that was discovered in 1907 by Kikunae Ikeda of Tokay Imperial University.
  • It is the most commonly found neurotransmitter in the central nervous system. Glutamate is mainly associated with functions like learning and memory.
  • An excess of glutamate is however, toxic for the neurons.
  • An excessive production of glutamate may be related to the disease, known as amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease.


Epinephrine and Norepinephrine

  • Epinephrine (also known as adrenaline) is an excitatory neurotransmitter, that controls attention, arousal, cognition, and mental focus.
  • Norepinephrine is also an excitatory neurotransmitter, and it regulates mood and physical and mental arousal.
  • An increased secretion of norepinephrine raises the heart rate and blood pressure.



  • Endorphins are the neurotransmitters that resemble opioid compounds, like opium, morphine, and heroin in structure.
  • The effects of endorphins on the body are also quite similar to the effects produced by the opioid compounds. In fact, the name ‘endorphin’ is actually the short form for ‘endogenous morphine’.
  • Like opioids, endorphins can reduce pain, stress, and promote calmness and serenity. The opioid drugs produce similar effects by attaching themselves to the endorphin receptor sites.
  • Endorphins enable some animals to hibernate by slowing down their rate of metabolism, respiration, and heart rate.



Basic Psychology, Biology of Psychology


A neurotransmitter is defined as a chemical messenger that carries, boosts, and balances signals between neurons, or nerve cells, and other cells in the body. These chemical messengers can affect a wide variety of both physical and psychological functions including heart rate, sleep, appetite, mood, and fear. Billions of neurotransmitter molecules work constantly to keep our brains functioning, managing everything from our breathing to our heartbeat to our learning and concentration levels.

How Neurotransmitters Work

In order for neurons to send messages throughout the body, they need to be able to communicate with one another to transmit signals. However, neurons are not simply connected to one another. At the end of each neuron is a tiny gap called a synapse and in order to communicate with the next cell, the signal needs to be able to cross this small space. This occurs through a process known as neurotransmission.In most cases, a neurotransmitter is released from what’s known as the axon terminal after an action potential has reached the synapse, a place where neurons can transmit signals to each other.

When an electrical signal reaches the end of a neuron, it triggers the release of small sacs called vesicles that contain the neurotransmitters. These sacs spill their contents into the synapse, where the neurotransmitters then move across the gap toward the neighboring cells. These cells contain receptors where the neurotransmitters can bind and trigger changes in the cells.After release, the neurotransmitter crosses the synaptic gap and attaches to the receptor site on the other neuron, either exciting or inhibiting the receiving neuron depending on what the neurotransmitter is.  The neurotransmitters act like a key and the receptor site act like a lock. It takes the right key to open specific locks. If the neurotransmitter is able to work on the receptor site, it triggers changes in the receiving cell.

Sometimes neurotransmitters can bind to receptors and cause an electrical signal to be transmitted down the cell (excitatory). In other cases, the neurotransmitter can actually block the signal from continuing, preventing the message from being carried on (inhibitory).So what happens to a neurotransmitter after its job is complete? Once the neurotransmitter has had the designed effect, its activity can be stopped by different mechanisms.

  1. It can be degraded or deactivated by enzymes
  2. It can drift away from the receptor
  3. It can be taken back up by the axon of the neuron that released it in a process known as reuptake

Neurotransmitters can be classified by their function:

Excitatory neurotransmitters: These types of neurotransmitters have excitatory effects on the neuron, meaning they increase the likelihood that the neuron will fire an action potential. Some of the major excitatory neurotransmitters include epinephrine and norepinephrine.

Inhibitory neurotransmitters: These types of neurotransmitters have inhibitory effects on the neuron; they decrease the likelihood that the neuron will fire an action potential. Some of the major inhibitory neurotransmitters include serotonin and gamma-aminobutyric acid (GABA).

Some neurotransmitters, such as acetylcholine and dopamine, can create both excitatory and inhibitory effects depending upon the type of receptors that are present.

Modulatory neurotransmitters: These neurotransmitters, often referred to as neuromodulators, are capable of affecting a larger number of neurons at the same time. These neuromodulators also influence the effects of other chemical messengers. Where synaptic neurotransmitters are released by axon terminals to have a fast-acting impact on other receptor neurons, neuromodulators diffuse across a larger area and are more slow-acting.

Basic Psychology, Biology of Psychology

How neurons communicate with each other

Neurons are unique-looking little cells. Like all animal cells, a neuron has a cell body, called a soma, where the DNA-carrying nucleus sits, providing directions for the cell to make various proteins. In a neuron, however, this is just the beginning of the cell structure. On one end, the soma sprouts branch-like dendrites for receiving signals, while a long –- up to a meter long –- axon stretches away in the other direction, branching out into multiple axon terminals for sending signals.


Key words

Axon – The long, thin structure in which action potentials are generated; the transmitting part of the neuron. After initiation, action potentials travel down axons to cause release of neurotransmitter.

Dendrite – The receiving part of the neuron. Dendrites receive synaptic inputs from axons, with the sum total of dendritic inputs determining whether the neuron will fire an action potential.

Resting membrane potential – The resting membrane potential of a neuron is about -70 mV (mV=millivolt) – this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

Action potential – Brief (~1 ms) electrical event typically generated in the axon that signals the neuron as ‘active’. An action potential travels the length of the axon and causes release of neurotransmitter into the synapse. The action potential and consequent transmitter release allow the neuron to communicate with other neurons.

Neurotransmitter – A chemical released from a neuron following an action potential. The neurotransmitter travels across the synapse to excite or inhibit the target neuron. Different types of neurons use different neurotransmitters and therefore have different effects on their targets. 

Synapse – The junction between the axon of one neuron and the dendrite of another, through which the two neurons communicate.

Key facts

  • Neurons communicate with each other via electrical events called ‘action potentials’ and chemical neurotransmitters.
  • At the junction between two neurons (synapse), an action potential causes neuron A to release a chemical neurotransmitter.
  • The neurotransmitter can either help (excite) or hinder (inhibit) neuron B from firing its own action potential.
  • In an intact brain, the balance of hundreds of excitatory and inhibitory inputs to a neuron determines whether an action potential will result.


These axon terminals are often located close to the dendrites of another neuron, forming a connection known as a synapse — despite the fact that the axon terminals do not physically touch the other neuron’s dendrites. Any given neuron will have about a thousand synapses with neighboring neurons, connecting the cells and allowing them to send messages from neuron to neuron. The synapses in a single human brain outnumber the stars in the Milky Way.

But if the synapses are empty space, with no direct connection between one neuron’s axon terminals and another’s dendrites, then how does the message travel? The cells must send chemical signals across the gap. Within each axon terminal are sacs, known as vesicles, filled with one of 50 different chemicals called neurotransmitters. Each neurotransmitter sends a different type of message to the next neuron, which recognizes the neurotransmitters with specialized receptors on the surface of the dendrites.

These receptor sites are like locks that can only be open by specific neurotransmitter keys. Once these keys have opened the lock, they drift back into the space between neurons, where they are either destroyed by enzymes or pumped back into their original neuron’s axon terminal by transporters. Back inside the cell, the neurotransmitters are again either destroyed or returned to a vesicle where they can be reused. Different neurotransmitters serve different functions, and they are also recycled differently.

When the neurotransmitters fill the receptors of the receiving neuron’s dendrites, they can either be excitatory (encouraging the receiver to pass the signal on), or inhibitory (preventing the receiver from continuing the message.) Any individual neuron’s dendrites might receive neurotransmitter signals from one of many other neurons — and if the excitatory signal is strong enough and then inhibitory signal is not, it triggers the receiving neuron to fire, passing the message on.

Although chemicals are required to send a message from neuron to neuron, it takes a different medium to transmit that message from the receiving neuron’s dendrites to its own axon terminals: electricity. When the neurotransmitters trigger the receiving neuron to fire, it sends an electrical “action potential” along its length the way that an electrical pulse flows down a metal wire. Like wires, some axons even have an insulating coating, the fatty myelin sheath, to make the signal travel faster.

So how does a clearly non-metallic human cell manage to conduct an electrical signal? The neuron has to alter its own charge relative to the outside of the cell. In order to change its charge, a neuron manipulates the charged ions on the inside and the outside of the cell membrane. When the neuron is at rest, with no signal in the pipeline, the ions are distributed so that the inside of the cell is more negatively charged than the outside, which creates an electrical potential, called the resting membrane potential, across the cell membrane. Sodium and potassium channels in the cell membrane control the flow of positively charged sodium and potassium ions into and out of the cell, maintaining that negative rest charge.

But when certain neurotransmitters have entered the receptor sites, it changes the makeup of the axon’s cell membrane: In the section of the axon closest to the soma, the cell membrane becomes more permeable. This allows positive sodium ions to enter the cell and give the inside of that section of axon a positive charge relative to the outside. Although the sodium pumps work to move these positive charges out of the neuron and restore the resting state, the influx has already triggered the same behavior in the neighboring section of the cell. Gradually, this positive charge on the inside of the cell moves down the length of the axon to the axon terminals.

When the signal reaches the axon terminals, the electrical charge changes once more: instead of sodium ions, it is the positively charged calcium ions that enter the extra-permeable cell membrane. In the axon terminal, the arrival of calcium triggers the vesicles filled with neurotransmitter to drift to the cell membrane, fuse with it, and then release the appropriate neurotransmitters outside the cell. Although this process seems lengthy, an action potential moves incredibly quickly. If you had an axon the length of a football field, a fast action potential could traverse it in one second flat.

And then the next neuron in the chain has to repeat the whole process.


Basic Psychology, Biology of Psychology



Parts of a Neuron



