The neuron, or nerve cell, is the "functional unit" of the nervous
system. The nervous system is composed of millions of neurons. The neuron can be
divided into three main parts. Neurons are quite similar even in different areas of the nervous
Parts of the neuron
1) Soma: The cell body, soma, contains the nucleus and other organelles. When
the soma dies the nerve cell dies. It was once believed that nerve cells never divide or
regenerate after birth, that has now been proven wrong. The soma appears gray in color.
2) Dendrite: The dendrite is a small process extending from the soma, usually
there is more than one per cell. It carries messages (nerve impulses) toward the soma.
Dendrites have many types of specialized receptors at their terminal portion.
(An individual dendrite has only one type of receptor.) Different
dendrites are able to pick up different types of stimuli. For example, the dendrites located in the skin perceive the outside environment.
a) What are several types of stimuli that we can differentiate at nerve
3) Axon: There is only one axon per neuron. It carries the nerve impulse away
from the soma, to the effecter organs. The axon can be very long. At the axon end are
vesicles which release neurotransmitters (substances which stimulate the effecter organ to
work). Most axons are white due to a lipid substance, myelin, which covers the axon.
Myelin acts to insulate the axon and allows the nerve impulse to move at "lighting
speed" (measured at 268 miles per hour!)
4) Synapse: The area or opening between the effecter end of the nerve and
The diagram below represents the terminal end of an axon containing vesicles
and the receptor area of the effecter.
remember what is released from a? from c (similar to
the muscular system?)
There are other types of cells associated with the nervous
neuroglia cells act as support cells. Types of neuroglia cells include the
Schwann cell, which form the myelin sheath around axons and the astrocyte, which help form
the blood brain barrier.
a1) What is the blood-brain barrier?
Functions of neuron:
Neurons carry messages to and from the brain and spinal cord via nerve
impulses. The impulse is a form of electrical energy which is set in motion by a stimulus.
The actual nerve impulse is caused by sodium ions ( with their strong positive charge)
flowing into the nerve fibers and displacing the weaker potassium ions, this causes a
slight negative charge on the outside of the neuron. This action is termed depolarization.
Depolarization progresses down the nerve fiber as the strong positively charged sodium
ions flow into the fiber, one after another. This cascading effect of the sodium ions into
the nerve fiber causes the outside of the fiber to change to a negative charge for a
millisecond as the electrical energy races down the fiber. Again, the nerve
impulse has been timed at moving 286 miles per hour.
Shortly after a nerve impulse travels down the nerve the sodium and potassium
rebalance and return to their original positions on the cell, this is termed
A stimulus can be received by many specific types of receptors (nerve endings).
This occurs in both the autonomic nervous system and peripheral system. The stimulus (some
form of energy) then begins the nerve impulse, which is carried to the spinal cord (and
usually on to the brain.)
b) What could be a stimulus for the autonomic nervous system?
Graphics reprinted with
permission by the copyright owner, Hill's Pet Nutrition Inc.
Cross section of brain
a=cerebellum, b=cerebrum, c=thalamus, d=brain stem
The brain is the first organ formed in the embryo and the most complex in the
body, it is also the least understood. We will cover some basic established facts about
the brain that help us understand clinical situations involving the nervous system.
Parts of the brain have been classified on the basis of their "gross"
anatomy (those parts that can be viewed with the naked eye) , their function and their embryonic origin.
The diagrams below and above show a few functional areas and structural parts
of the brain, with a brief basic description of function.
of cerebrum, 2=thalamus, 3=pineal gland, 4=hypothalamus, 5=anterior pituitary, 6=posterior
pituitary, 7=cerebellum, 8=brain stem
Cerebrum (cerebral hemispheres): This is the largest portion of the brain in
higher animals, it is divided into two halves by a deep mid line fissure. The cerebral
surface appears gray and "furrowed" by the
sulci (valleys) and gyri (hills ). The
cerebrum is the seat of the highest mental activity including: memory, language, reason,
emotion (along with the limbic system), voluntary body control and interpretation of
sensations. Usually the largest cerebrums occur in the most intelligent animals.
c) What do the gyri and sulci provide?
Cerebellum: The cerebellum "little brain" is located at the back of
cerebrum and appears similar to the cerebrum but smaller. It coordinates the
execution of movement and is involved in the unconscious control of balance. (receives
information from receptors in joints and muscles)
Pineal body: It is located above the pituitary gland, deep in the brain. It is
actually an endocrine gland and secretes releasing factors to the pituitary
gland and the hormone melatonin.
Thalamus: The thalamus is the "switchboard" to the cerebrum, it
controls and integrates the millions of messages sent to the brain.
Hypothalamus: It acts in the regulation of some autonomic functions including
hunger, thirst and body temperature. It is very important in regulation of the pituitary
gland. We will cover this structure more with the endocrine system.
Pituitary gland: The major endocrine gland of the body, it
protrudes from the bottom of the brain, thus demonstrating the close connection between
the nervous and endocrine systems.
Medulla oblongata: The medulla or brain stem is a primary regulator of
autonomic functions including respiration, blood pressure, heart rate, coughing and
The limbic system:
The limbic system is called the seat of emotion. This area of the mid brain
includes several structures and is involved with instinctive behavior, motivation and
emotional feelings. Early brain researchers implanted electrodes in the limbic system of a
rat, when the rat pressed a bar in its cage an electrical jolt was sent to this area of
the brain. The rat pressed the bar continually to get limbic system stimulation and
ignored food and water until death. This area is believed to be the "pleasure
center" of the brain and the release of endorphins probably occurs from here
d) What are endorphins?
The brain is composed of millions of neurons and glial cell. The outside
of the brain in higher animals appears "folded". The inner portion of the brain
contains cavities (the site of cerebral spinal fluid production, among other things), much
of the inside of the brain is white.
e) Why is the outside of the brain gray and the inside white?
Basically the larger the brain to body ratio the smarter the animal. It is
known that intelligence and memory is stored in the "gray matter" of the brain,
but one of the mysteries of life is how the brain actually stores all this information,
how it reasons and solves problems. One theory states that intelligence and memory may be stored
chemically in certain soma of the brain. Nerve endings coming from various receptors might
connect to these soma and activate a specific emotion or memory. For example the stimulus
for the emotion fear can come from different receptors - your eyes, (as that large mean dog
approaches), or your ears (as it growls deeply). But the nerves carrying the message
to the brain might all connect to the "fear" area and activate the fear chemicals
inside the brain.
Much of the brain of some species (for example, man and cats) has been
"mapped". Those areas that control certain functions, such as motor functions
and language, have been identified. When trauma or disease effects these areas a loss of
certain mental functions can be predicted.
The brain needs a rich supply of oxygen and glucose to function,
because it uses a huge
amount of energy. The nervous system receives about 20% of the oxygen in the body,
yet is only about 1- 2% of the total body weight. Brain metabolism is 7.5 times faster
than other organs.
There are many more neurotransmitters in the brain than in other parts of the
nervous system. Most of the new drugs which are helping humans and animals with mental and
behavior problems act on specific brain neurotransmitters.
The brain needs a constant supply of glucose to survive. To assure this
constant supply is available the brain has a unique feature. It is the only organ that can
absorb glucose without insulin present.
f) Hypoglycemia effects the brain first. What are the signs?
The cerebral spinal fluid (CSF):
The CSF is produced inside the brain ventricles (the hollow space
in the center of the cerebrums) and originates from clusters of
blood vessels. Cerebral spinal fluid moves from the center of the brain to
the space below the middle layer of the meninges. It bathes the outside of the entire central nervous system (there is about
150 mls. in the average sized human) and acts as a "liquid cushion" to help
There are two areas that the fluid is concentrated, one at the base of the
brain (in cisterns) and the other in the subarachnoid space of the sacral area. In
veterinary medicine the CSF can be "tapped" at those areas and examined to aid
in the diagnosis of various conditions.
g) What evaluations can be done on the CSF ? What is "water on the
The reflex arc
A reflex arc demonstrates involuntary control over a group of muscles
or usually a limb, it is a protective mechanism and occurs instantaneously
and without thought.
1) A stimulus is perceived by receptors (nerve endings).
2) A nerve impulse is sent from the receptor to the spinal cord through the
dorsal root of the spinal cord.
3) The message is relayed inside the spinal cord (synapses with another nerve)
and sent to the effecter organ (usually muscles).
4) The nerve impulse (message) is sent out
the ventral root to a muscle (or other organ) for
When an impulse gets to the spinal cord it always enters by the dorsal root
(top branch). The stimulus that is received only goes as far as the spinal cord before
sending the message back to the effecter organ. This split second response, which does not
require processing by the brain, is called a reflex. It is a protective mechanism to
escape danger quickly.
h) Have you ever experienced a "reflex arc"?
The spinal cord
Cross section of
vertebrae and spinal cord
a=spineous process of vertebrae, b=spinal cord (gray matter in center, white
matter on the outside) c=dorsal root of peripheral nerve, d=ventral root of peripheral
nerve, e=intervertebral disk
The spinal cord is encased by the vertebral column.
It runs from the brain to
the end of the spine. It is white on the outside and is covered by three tissue layers,
the meninges. The cord is segmented and a pair of spinal nerves branch off (one to the
left and the other to the right) at each vertebra. The cord is gray in the center due to
the concentration of neuronal soma. When nerves enter the cord they form tracts,
or groups of
nerves with similar functions.
Only sensory nerves enter the dorsal roots and only motor,
muscle controlling nerves, exit the ventral roots.
The three meninges which cover the central nervous system are the:
1) Pia mater,
the thin inner most layer
2) arachnoid, the "spider web" which adheres to pia (arachnoid
means spider in Latin)
3) dura mater, the tough protective outer layer
The area under the arachnoid is the sub
arachnoid space, where "spinal taps" are taken. The epidural area is below the
vertebral wall and above the dura mater. This is the area where anesthetic agents are
i) What is the advantage in the dorsal (sensory) and ventral (motor) nerve
arrangement of the spinal cord?
The peripheral nerves
The entire body surface is "covered" by nerves from the spinal cord.
These nerves receive stimuli from the outside environment and respond to protect the
The Cranial nerves: Mammals have 12 pairs of nerves which originate from the
brain. These are some of the most important nerves in the body including the receptors for
vision, hearing, and smell. Some of these nerves are "mixed", which means that
the nerve bundle carries both sensory fibers towards the brain and motor (effecter) fibers
away from the brain.
5) Trigeminal 9) Glossopharengeal
2) Optic *
6) Abducent 10) Vagus**
8) Vestibulocochlear* 12) Hypoglossal
*know the functions of the nerves highlighted with an
**the vagus nerve will also be discussed with the autonomic nervous system
Cranial nerve one (which mediates the sense of smell) and nerve two (sight) are
The seventh nerve is mixed, providing sensory enervation to the side of face
and taste buds, plus parasympathetic enervation to the salivary glands.
The eighth nerve is also called the vestibulocochlear because it mediates both
the sense of hearing and balance.
The tenth nerve (vagus) is mixed, sensory to the throat and parasympathetic to
the organs in the thorax and abdomen.
Spinal nerves: A pair of spinal nerves exit each side of each vertebrae and
then enervate a portion of the body. At different areas of the body groups of nerves come
together to form a plexus.
The front leg is supplied by the brachial plexus (which contains nerves C -3/4
+ T -1/2, there is species variation)
The hind leg is supplied by the lumbo-sacral plexus (which includes nerves
leaving L - 4/7 + S - 1/2 vertebra, also species variation)
j) What does C, T, L, & S stand for?
At the termination of the spinal cord a group of nerves extends into the tail
and perianal region termed the cauda equina (the horse's tail)
old Olympus towering tops a Finn and German viewed some hops"
l) Why do veterinary students memorize this saying when studying
The autonomic nervous system (ANS):
The autonomic nervous system, in conjunction with the endocrine system,
controls internal organ functions and "homeostasis" of the body.
The ANS is
divided into the sympathetic and parasympathetic portions. There are both functional and
structural differences between these portions of the system.
a=parasympathetic (in black) include the vagus nerve leaving brain,
(in red) from thoracic and lumbar spinal cord
The sympathetic nerve fibers exit the spinal cord in the thoracic
and lumbar region. Sympathetic nerves from the thoracic spinal cord enervate
organs in the cranial part of the body, while sympathetic nerves from the
lumbar spinal cord enervate organs and structures in the caudal part of the
The messenger or "neurotransmitter" is epinephrine (or norepinephrine). This portion of the system is often thought of as the "gas pedal"
because it speeds up many internal functions of the body. There is a group of sympathetic
ganglia that form a line outside and ventral to the vertebral column along the thoracic
and lumbar spine. The sympathetic nerves "synapse" here at various locations and
the bulge formed by the interconnections between soma and dendrites is termed a ganglion.
Parasympathetic: The nerves forming this portion arise from cranial nerves
(3,7,9,&10) and the caudal part of the central nervous system. Only the vagus nerve
leaves the cranial area. It travels from the head down to the major organs of the
cranial part of the body.
The neurotransmitter for this system is acetylcholine. This portion of the autonomic system
slows most body functions down, it is the system "in control" during rest and
The autonomic system effects most organs in a predicable manner.
Below is a list
of how several organs are effected by the sympathetic and parasympathetic nerve
digestive tract decreases
pancreas (insulin & enzymes) increase both
depends on stage of cycle for both
m) Which portion of the autonomic system is in control during the
"fight, flight or freeze" reaction?
Lesson #6 assignment - e-mail answers to Dr. Bidwell
Please copy and paste the questions into "word" then save
file and answer.
Clinical Textbook for Veterinary Technicians by Bassert and
Please answer these questions and return answers by e-mail to Dr.
Bidwell @ firstname.lastname@example.org
Assignment will be "spot checked" and logged in and answered e-mailed
back to student after receiving assignment. The student is responsible
for noting the correct. The questions might be included on exams.
1. If a dog has no dendrites on its nerve cells in the foot what would
probably happen if it stepped on something hot?
2. What is another name for "motor" nerves?
What do they do?
3. How does the local anesthetic lidocaine work in the body?
4. What is the function of the post-synaptic neuron?
5. Give an example how the same neurotransmitter can have an
excitatory and inhibitory affect.
6. Why is epinephrine considered a hormone by some experts?
7. Why does the patient often die if there is serious injury to the
8. Inflammation of the pia mater is termed
9. What is an important advantage of epidural anesthesia over general
10. In anatomy terms what is the blood brain barrier?
11. If the autonomic nerves arising from the brain were dysfunctional
what would probably happen to the heart rate?
12. If a drug affects the Beta-2 adrenergic sympathetic receptors of
the sympathetic nervous system which body structure is affected?
13. In the diagnosis of intervertebral disk disease the determination
of hypo- or hyperreflexia is often made, why?
Please do the question in the clinical
aspects section as well!