Tutorial: Neurophysiology
Cell body, Dendrite(s), and Axon(s) | Multipolar Neuron | Unipolar Neuron | Sensory Neuron | Motor Neuron | Action Potential | Sodium-Potassium Pump | Action Potential (unmyelinated) | Action Potential (saltatory) | Synapse | Neurotransmitter | Neurotransmitter Action | Reflex Arc | White vs Gray Matter | Divisions of the Nervous System | Overall Divisions | Brain Anatomy | Association | Eye | Ear | Drugs | Quiz

Copyright © Steve Kuensting, 2004, All Rights Reserved.
This web tutorial may not be distributed by any means
without the expressed permission of the author!

Cell body, Dendrite(s), and Axon(s)

The structural basis of the nervous system is the neuron. Each neuron has 3 basic parts: the cell body, dendrite(s), and axon(s). The cell body contains the nucleus and associated organelles. The dendrites and axons receive chemicals from the cell body and contain only small organelles. Draw the neuron below in your notes and label it.

Neuron structure

Unipolar Neuron/Multipolar Neuron

There are 3 structural types of neurons in the nervous system. They are categorized by the numbers of "extensions" emanating from the cell body. They are: unipolar, bipolar, and multipolar.

Neuron types

Sensory Neuron/Motor Neuron

Unipolar neurons are also called sensory neurons because they pass sensory information from the skin and other organs to the spinal cord. Multipolar neurons include motor neurons which pass impulses from the spinal cord to the muscles and other organs. Bipolar neurons are not as common and include the cone and rod cells of the eye.

Sensory neuron

Multipolar Neurons

For ease of discussion, we will focus on the multipolar neuron. Multipolar neurons have special cells wrapped around their axons called Schwann cells. There is a space between adjacent Schwann cells called nodes of Ranvier. A large amount of the Schwann cell consists of a fatty substance called myelin. The purpose of the Schwann cell and myelin is to insulate the axon. Sensory neurons have Schwann cells on both dendrites and axons.

Schwann celsl

Draw the following motor neuron in your notes:

Motor neuron

Dendrite vs Axon

Dendrites and axons are often structurally identical so they are difficult to distinguish. They are functionally very different, since dendrites always pass nerve impulses towards the cell body and axons always pass nerve impulses away from the cell body. The arrows below indicate the direction of nerve impulse flow.

Axons and dendrites

Action Potential

A neuron can pass information by transmitting nerve impulses which are better referred to as action potentials. A neuron can do this because its surface is electrically polarized. To be electrically polarized means to have separated charges. On the surface of the neuron are proteins involved in the active transport of charged particles called ions. The active transport proteins pump sodium ions out while they pump potassium ions in. Na = sodium, K = potassium.

Action potential

Sodium-Potassium Pump

For several reasons, the outside of the neuron becomes positively charged when compared to the inside. This is partially because each time the pumps move ions, they move three sodium ions OUT and only two potassium ions IN. Also, the neuron allows the potassium to leak back out and will not normally let the sodium in (except during an action potential). Lastly, the inside of the neuron has large negatively charged proteins that make the inside more negative. The link below animates the sodium-potassium pump.

NaK pump animation start
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =

Action Potentials

Because the neuron is polarized, it can pass action potentials (nerve impulses) down its dendrites, cell body, and axons to communicate with other neurons or organs. Action potentials occur when the neuron briefly allows sodium to pass inward which discharges or depolarizes the neuron membrane. An action potential is thus best defined as a "wave of depolarization".

Animation explanation

Action Potential Direction of Flow

Action potentials flow from one end of a neuron to another. After the action potential passes one area of the neuron, the membrane recharges itself by repumping the sodium out. And, during an action potential, only a specific area of the neuron is affected, not the whole neuron. Just as a wave has a specific place in the surf (not the whole ocean is affected by one wave) so also an action potential flows like an ocean wave, except that is only an electrical ripple and not a mechanical ripple.

Wave of depolarization

Action Potential (unmyelinated)

The hyperlinks below animate how an action potential might look if one could see the charges of a neuron. Actually, only a voltmeter can register an action potential since it only involves the movement of charged particles which are too small to see with even the most powerful microscopes. The action potential animated is non-saltatory since no Schwann cells are present to insulate the axon.

Action potential start
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =


Note that the action potential flowed from one end to the other in the animation. The dendrites and cell body were left out for the sake of simplicity. One of the most important developments in the evolution of vertebrates was the invention of Schwann cells. Schwann cells with their myelin are important because they speed up action potential speed, from less than 50 mph to over 200 mph.

Animation explanation

Action Potential (saltatory)

The Schwann cells do this by causing the action potential to "jump" across them - essentially allowing the action potential to skip sections of the axon/dendrite. This is called saltatory conduction. This effectively shortens the distance that the action potential must actually flow and thus allows it to get to the end of the neuron faster. The next links animate saltatory conduction.

Saltatory Action Potential Animation

Saltatory action potential

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =

Saltatory vs Nonsaltatory Speed

Did you notice how much faster the action potential traveled? Without actually having to flow the entire length of the neuron it was able to get from one end to another much faster.

Animation explanation


One neuron must be able to communicate with another neuron or organ. Neurons are not directly connected to each other for obvious reasons - if one had an action potential it would spread to all neurons and cause spastic muscle contractions and organ stimulations - not good! Neurons communicate with other neurons at small junctions called synapses.



At the synapse, the two neurons do not touch but lie very close to one another. The end of an axon, called an axon terminal, lies extremely close to either a cell body, dendrite, or cell of an organ. To communicate an action potential across the synapse the axon terminal "squirts" a chemical called a neurotransmitter on the cell body, dendrite, or cell of the organ. The neurotransmitter stimulates an action potential on the cell body, dendrite, or organ cell and is then immediately destroyed or reabsorbed by the axon terminal.


Neurotransmitter Storage

The neurotransmitter is stored in the axon terminal in small sacs called vesicles. When an action potential reaches the tip of an axon terminal, several vesicles will fuse with the tip of the axon membrane and release neurotransmitter into the synapse. The neurotransmitter will diffuse across and stimulate the dendrite to start an action potential and the communication is complete. To prevent constant stimulation, the neurotransmitter is removed from the synapse immediately after it is used.


Neurotransmitter Action

The next link animates the release of neurotransmitter in a synapse. Since action potentials on myelinated neurons can travel at over 200 mph, and neurotransmitter relay across synapses occurs in hundredths of a second, muscles and organs can quickly be coordinated to respond to a stimulus.

Synapse animation
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.


One of the simplest neuron pathways is called the reflex. A reflex is a muscle response to a stimulus that involved no brain input. Thus, reflexes occur without thought and sometimes after becoming aware of the stimulus that caused them. A hot object can initiate an arm-jerk reflex that involved pulling the arm away from the hot object before you are even aware of the pain from the burn! Reflexes usually involve a sensory neuron receiving the stimulus, an interneuron in the spinal cord, and a motor neuron carrying the action potentials to a muscle for the response.

stimulus ---> sensory neuron --> interneuron --> motor neuron --> action

Reflex Arc

The sensory neuron receives the stimulus, converts the stimulus to action potentials, and sends the action potentials from the point of stimulus to the spinal cord. Within the spinal cord, the axons of the sensory neuron squirt neurotransmitter on the interneuron, which relays the action potential to the motor neuron by squirting neurotransmitter on it. The motor neuron carries the action potentials out of the spinal cord to the appropriate muscle(s) so the response can take place. All of this occurs in less than a second for many reflexes! The next link animates the patellar reflex, which is initiated by striking the patellar ligament (below knee) by a hammer. The leg kicks out in response.
Synapse animation start
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.

White vs Gray Matter

As a last consideration of study, we will look at the structure of the spinal cord and brain. The spinal cord is best studied through cross-section. Below is such a cross section. The cross section reveals both gray and white matter. Gray matter contains mostly unmyelinated neuron parts (cell bodies and some dendrites and axons) while the white matter contains mostly myelinated axons and dendrites.

Gray vs white matter

Spinal Cord Structure

Ganglia (plural of "ganglion") are the clusters of cell bodies of incoming sensory neurons. They are located in protected spaces between the vertebra of the human backbone. Remember that the cell body houses the nucleus of the neuron, the most important part of the neuron. Notice below that the sensory and motor neurons split and are separated as they exit and leave the spinal cord. The arrows indicate the direction that action potentials flow.

Spinal cord structure

Central vs. Peripheral Nervous System

The human nervous system is divided into the Central and Peripheral Nervous Systems. The Central Nervous System consists of the brain and spinal cord. The Peripheral Nervous System consists of the sensory and motor neuron structures that lie outside of the central nervous system.

Central Nervous System

By Shao and --Schekinov Alexey Victorovich (talk) 02:30, 4 February 2009 (UTC) (uk:CNS.jpg) [Public domain], via Wikimedia Commons

Sensory and Motor Divisions

The Peripheral Nervous System is further subdivided into the sensory and motor divisions. The sensory division relays impulses from the sense organs to the brain/spinal cord. The motor division relays impulses from the brain/spinal cord to the muscles or glands. The structures of the spinal cord were discussed above and the structures of the brain will be discussed below.


Somatic and Autonomic Divisions

The motor division is further subdivided into the somatic and autonomic divisions. The somatic system innervates the skeletal muscles of the body which are mostly under cerebral (conscious) control. It includes reflexes which are mostly under the control of the spinal cord. The autonomic system controls the smooth and cardiac muscle of the body which wraps the tubes and chambers of the interior organs of the body. The churning of a stomach, pulsing of an artery, rate of heart beat, blood pressure, etc. are under autonomic control.

Somatic vs autonomic
Mucles: anterior
Muscles: Anterior

By User:Mikael Haggstrom (Image:Gray190.png) [Public domain], via Wikimedia Commons
GI tract
GI Tract

By Mikael Haggstrom [Public domain], via Wikimedia Commons

Sympathetic vs Parasympathetic Nervous System

The Autonomic Nervous System is further subdivided into the Sympathetic (fight or flight) and Parasympathetic (calming) Nervous Systems. The sympathetic system raises blood pressure, slows the digestion of food, slows the kidney filtration of blood, increases sight and hearing sensitivity, dilates the pupils, increases blood flow to arms and legs, and more. This prepares the body for "fight or flight". The parasympathetic system does the opposite. It lowers the blood pressure, decreases sight and sound sensitivity, increases food digestion and kidney filtration of blood, constricts the pupils, decreases the blood flow to the arms and legs, and more. Both systems are "wired" into most organs, so that each organ can be controlled as needed.


Sympathetic Sleep

To summarize, the nervous system is divided as follows:
CNS divisions

Brain Anatomy

The brain is best shown via sagittal section, the view obtained by cutting the brain into two halves, left and right. Note the names of the brain parts and their functions in the field below.

Brain Anatomy and Physiology
spinal cord: connects the brain to the organs of the body; controls many reflexes.
meninges: outer covering of the central nervous system that protects the brain and spinal cord.
parietal lobe: lobe of cerebrum that senses and remembers pain, heat, cold, and touch - the skin sensations.
frontal lobe: lobe of cerebrum that controls muscles, is responsible for thinking, speech, word formation, logic, etc...
occipital lobe: lobe of cerebrum that senses and remembers visual information.
temporal lobe: lobe of cerebrum that senses and remembers sounds.
ventricle: circulates cerebrospinal fluid beneath meninges and throughout core of brain and spinal cord; for cushioning the brain and spinal cord.
corpus callosum: allows the 2 hemispheres of the cerebrum to communicate with one another (association).
hippocampus: important to processing short-term memory and spatial information.
thalamus: part of the reticular formation which controls the rate of cerebral activity and determines which action potentials are allowed to reach the cerebrum.
hypothalamus:controls the activity of the pituitary gland, which controls puberty, the thyroid gland and metabolism, the adrenal glands, the gonads, and more.
midbrain: controls eye and ear reflexes that involve moving the head in response to sights or sounds, and movement of eyes in response to body movement.
cerebellum: coordinates muscle actions that the frontal lobe controls, such as walking, sitting, running, jumping, writing, etc...
pons: serves as a connection for the cerebellum to the brain.
medulla oblongata: controls heart rate, breathing rate, and blood pressure.

Brain anatomy 1=frontal lobe of cerebrum; 2=corpus callosum; 3=ventricle; 4=parietal lobe of cerebrum; 5=thalamus; 6=occipital lobe of cerebrum; 7=cerebellum; 8=medulla oblongata; 9=pons; 10=midbrain; 11=temporal lobe of cerebrum; 12=pituitary gland; 13=hypothalamus.
(Image from Biodidac)

Cerebrum - Saggital and Inferior View

Study the color diagrams of the cerebrum below and see if you can name the various parts that were labeled in the smaller picture above.

Brain sections
By Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC-BY-2.5], via Wikimedia Commons
Brain inferior view
Brain human normal inferior view

By Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC-BY-2.5], via Wikimedia Commons

Reticular Formation

The thalamus, midbrain, pons, and medulla oblongata together perform a very important function for the cerebrum. They determine the level of cerebral activity. In other words, they determine whether you feel awake or asleep. The most important part of these structures is the thalamus, which also screens sensory input from the eyes, ears, and skin. Action potentials that are considered unimportant are screened out so that you don't hear them. Thus, highway noise, rattles, and wind noise are not heard while driving for long periods in a car because the thalamus prevents their impulses from reaching the temporal lobe.


By Images are generated by Life Science Databases(LSDB). [CC-BY-SA-2.1-jp], via Wikimedia Commons

Association and Memory

Many parts of the brain exhibit memory - the storage of information. The cerebellum "remembers" how to walk, run, sit, stand, throw, etc. The cerebrum also remembers past events, but since the cerebrum is composed of 8 lobes the memories must be integrated between them. A typical memory of a childhood event consists of sights, sounds, and other feelings that are stored in the different lobes of the cerebrum. Association is the term used to describe the interaction of different lobes to remember a past event.

Cerebral lobes
Human Cerebrum Lobes

By Mysid [Public domain], via Wikimedia Commons

The Hippocampus

The hippocampus, at the base of the temporal lobes, is involved in short term memory. It is also involved in association of spatial information. Without a hippocampus, we can't remember events that just happened, and we can't figure out where we are... Adjacent to the hippocampus is the amygdala, which is important to the memories of emotional events. Fear, joy, and sorrow are all processed by the amygdala. The amygdala is also important to the strength of memory consolidation - the more emotion associated with the memory - the stronger the long-term memory.

By User:Washington irving (Own work) [Public domain], via Wikimedia Commons

Association and the Corpus Callosum

When you remember a conversation with a friend, the occipital lobe remembers what they looked like, their voice is stored in the temporal lobe, and the words they spoke are stored in the frontal lobe. Failure of association causes the inability to remember the name of a person when you hear their voice only, or the name of a person when you see their face only. If you've ever forgotten someone's name but remember their face, your lobes failed to associate the information for you. When you're reminded of their name, you realize you knew their name, but couldn't associate it with their face. The corpus callosum serves as the relay between the lobes and allows the association of different information to produce a specific memory of an event.

Corpus callosum
Corpus callosum

By modified image created by user:Looie496. original images created by John A. Beal, Ph.D. [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

The Senses

Sensory organs allow environmental stimuli to be converted to action potentials. The eyes convert light to action potentials; the ears convert sound waves to action potentials; the nose converts chemical stimuli in the air to action potentials; the tongue converts chemical stimuli in food to action potentials; skin receptors convert heat, cold, touch, and pain to action potentials. We will consider the detailed anatomy of the eye and the ear as an example.


Below is a diagram showing a sagittal view of a human eye.
The functions of the numbered parts are found below.

Human eye
Human Eye, sagittal view

By Talos, colorized by Jakov (deutsche Version ohne Farben (siehe unten)) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

1. Sclera: The tough white outer covering which protects the eye.
6. Iris: The muscle ring that regulates the size of the pupil according to light levels.
5. Cornea: The clear protective covering over the pupil area of the eye.
7. Pupil: The opening within the iris that allows light to enter the rear of the eye.
8,12. Humor: The fluid that fills the spaces that light must pass through within the eye.
14. Optic Nerve: The nerve that carries action potentials from the retina to the thalamus of the brain.
13. Retina: Converts visible light to action potentials with photoreceptors.
11. Lens: Focuses light on the retina by adjusting its size according to the distance of the object being viewed.

Vision Problems

Imperfections in the eye can cause vision loss and the need for eyeglasses/contact lenses. Astigmatism is the unevenness of the cornea of the eye causing images to be blurred as they enter the front of the eye. Near-sightedness is caused by an eyeball that is too long so that the image is focused in front of the retina. Far-sightedness is caused by an eyeball that is too short so that the image is focused behind the retina. Both near- and far-sightedness can be corrected with glasses or contacts which prefocus the light to the correct distance. Radial keratotomy is a surgical technique of reshaping the front of the eye to correct near-sightedness. A small circular ring of sclera is removed and the entire eyeball is shortened to correct the focal distance.


By National Eye Institute [Public domain], via Wikimedia Commons
By National Institute of Health [Public domain], via Wikimedia Commons


Below is a diagram showing a frontal view of a human ear.
The functions of the labeled parts are found below.

Human earHuman ear, cochlea mapped
By Inductiveload [CC-BY-SA-2.5], via Wikimedia Commons

Auricle: The outer sound-collecting structure of the ear.
Auditory Canal: The tube through which sound passes to reach the tympanum.
Hammer (Malleus): The first of the small bones of the middle ear that transmits vibrations from the tympanum to the anvil.
Anvil (Incus): The second of the small bones that transmit sound vibrations from the hammer to the stirrup.
Semicircular Canal: The fluid-filled tubes that monitor movement of the head.
Oval window: The inner eardrum that is vibrated by the stirrup to create pressure waves in the cochlear fluid.
Acoustic nerve: The nerve that transmits action potentials from the cochlea to the thalamus of the brain.
Cochlea: The organ of hearing consisting of tiny hairs in a coil that produces action potentials in response to the pressure waves in the cochlear fluid.
Stirrup (Stapes): The third of the small ear bones that transmit sound vibrations from the anvil to the oval window.
Tympanic membrane (eardrum): The external eardrum that vibrates in response to sound and moves the first of the small ear bones, the hammer.

Hearing Loss

Hearing loss can be caused by any number of factors interfering with action potentials reaching the thalamus. Earwax can prevent the tympanum from vibrating and block the auditory canal. Scar tissue (from middle ear infections) on the tympanum or around the ear bones can prevent their normal movement. The small hairs in the cochlea can be broken off when exposed to extremely high volume sounds, causing inability to hear specific sound frequencies. Cochlear damage is the most severe and difficult to correct form of hearing loss.

Human ear
Human Ear

By User:Dan Pickard [Public domain], via Wikimedia Commons

Drugs and the Nervous System

A drug is substance that changes the structure or function of the body. Recreational drugs are chemicals that affect synapses; most of them act on the reticular formation, especially the thalamus. There are 4 basic categories of recreational drugs, and many fit into more than one classification category!!
Stimulantsincrease actions regulated by the nervous systemdirectly act on the reticular formation and increase action potential frequencyamphetamine, methamphetamine, cocaine, nicotine, caffeine, ephedra, ecstasy (MDMA -methylenedioxymethamphetamine)
Depressantsdecrease rate of brain-controlled functionsdirectly act on the reticular formation to reduce action potential frequencytranquilizer, barbiturate, GHB (gamma hydroxybutyrate, date rape), ketamine, alcohol
Opiatespain killers; mimic endorphins which overcome the sensation of pain; very addictive due to lack of endorphin production as a result of taking opiatesdirectly act on the connection between the thalamus and the cerebrum, as well as the reticular formationopium, morphine, codeine, heroin, oxycodone
Psychoactivesa heterogenous group of compounds that can alter the perception of reality and have profound impacts on mood and behavior patterns (includes the hallucinogens)directly act on the cerebrumLSD, LSA, ecstasy, mescaline, psilocybin, THC (tetrahydrocannabinol)

Special Cases:
Alcohola depressant; slows down reflexes, disrupts coordination, impairs judgment; 40% of the 50,000 highway victims are involved in accidents where at least one driver has been drinking; FAS is destructive to the unborn - heart defects, malformed faces, delayed growth, poor motor development
Marijuanafrom Cannibis sativa; hashish comes from the flowers; active ingredient = THC (tetrahydrocannabbinol); long-term use results in memory loss, inability to concentrate, and lowered testosterone (in males)
Cocaineacts on the pleasure center of the brain, causing dopamine release; is a strong stimulant; highly addictive due to lack of dopamine due to taking the drug (crack is even more addictive)

Drug Abuse

Most recreational drug use leads to addiction - an uncontrollable dependence on a drug. There are two general types of dependence - psychological and physical. Withdrawal with psychological dependence is manifested in craving. Withdrawal with physical dependence is manifested in pain, nausea, chills, and fever. Psychological dependence is much less severe than physical dependence.


  1. What type of neuron has many proccesses leaving the cell body?

  2. What type of structural neuron is a sensory neuron?

  3. What fatty chemical is located within Schwann cells?

  4. What are the gaps between Schwann cells called?

  5. What chemicals are released from axon terminals?

  6. What is the junction of 2 neurons called?

  7. What is the name of action potential conduction that jump from node to node?

  8. What specific part of the brain is responsible for reasoning?

  9. What part of the brain coordinates muscle movements?

  10. What part of the spinal cord contains mostly myelinated axons?

  11. What part of the brain senses and remembers sights?

  12. What part of the brain screens sensory input to the cerebrum?

  13. What part of the brain senses pain and touch?

  14. What part of the brain controls heart rate and breathing rate?

  15. What part of the eye controls the amount of light reaching the retina?

  16. What part of the eye focuses light on the retina?

  17. What function does the retina perform in the eye?

  18. What are the 3 bones (outer to inner) of the middle ear?

  19. What is the important function of the cochlea?

  20. What is the purpose of the semicircular canals?

Diagram 1: Name the structures:
Quiz 1

Diagram 2: Name the structures:
Quiz 2

Diagram 3: Name the structures:
Quiz 3

Diagram 4: Name the structures:
Quiz 4

Diagram 5: Name the structures:
Quiz 5