Lesson Seven- The Nervous System
Assignment:
• Read Chapter 9 in the textbook.
• Read and study the lesson discussion.
• Complete the Check Your Understanding activity.
Objectives: After you have completed this lesson, you will be able to:
• Describe the neuron, the nerve impulse, the synapse, and explain the components of a reflex arc.
• Identify the major structures of the brain and name associated functions.
• Discuss the anatomy and function of the spinal cord.
• Compare and contrast the function of the secondary somatic system to the autonomic
nervous system.
• Differentiate between the two branches of the autonomic system.
• Discuss the clinical significance of the academic material learned in this chapter.
• Identify common nervous system disorders.
The Nervous System
According to the Merck Veterinary Manual,The nervous system is composed of billions
of neurons with long, interconnecting processes that form complex integrated electrochemical
circuits. It is through these neuronal circuits that animals experience sensations and respond
appropriately.
Neuronal processes that transmit electrical alterations to the cell body are called
dendrites. Dendrites have receptor sites that receive stimulation or inhibition from
outside sources. If electrical stimulation of the cell body reaches a critical threshold,
an electrical discharge called an action potential develops. The action potential
spontaneously travels away from the cell body along an outgoing process called an
axon. When the action potential reaches the terminal branches of the axon,
chemicals called neurotransmitters are released. Neurotransmitters either
stimulate or inhibit receptor sites on other neurons, muscles, or glands. Although
neurons may have a variety of shapes, each one has dendrites, a cell body, an axon,
and releases neurotransmitters.
The peripheral nervous system (PNS) is formed by neurons of the cranial and
spinal nerves. The central nervous system (CNS) is formed by neurons of the
spinal cord, brain stem, cerebellum, and cerebrum.
Groups of neuronal cell bodies in the PNS are called ganglia, while those in the CNS
are called nuclei. Nuclei form the CNS gray matter. Groups of axons in the CNS form
the white matter and are arranged into tracts. The tracts are usually named after
their site of origin and termination (ex: the spinocerebellar tract begins in the spinal
cord and ends in the cerebellum).
PNS sensory neurons carry
information such as … touch,
temperature, taste, hearing,
equilibrium, and vision to the spinal
cord or brain stem. CNS sensory
neurons carry information to the
cerebellum, brain stem, and cerebrum
for further interpretation. Important
spinal cord and brain-stem sensory
tracts include several spinocerebellar,
spinothalamic, and spinorecticular
tract systems. The spinoreticular
tracts begin in the spinal cord and
terminate in the reticular formation of
the medulla. Any alteration in
sensation may be due to either CNS or
PNS disease.
Reactions to sensory inputs are initiated
by motor neurons in the cerebrum and
brain stem called upper motor neurons
(UMN). The UMN axons descend to brain
stem and spinal cord segments in tracts
named after their site of origination and
termination … .
Motor neurons with cell bodies in the
brain stem, and spinal cord gray matter
and axons that travel in the PNS cranial
and spinal nerves, respectively, are
referred to as lower motor neurons
(LMN). Injury to either the UMN or LMN
results in paralysis. Brain-stem and
spinal cord reflexes are the oldest
responses of the nervous system. When
the eyelid is touched, it closes; when the
toe is pinched, the limb withdraws before
conscious perception intervenes. Only a
sensory neuron in the PNS, a connector
neuron in the CNS, and a LMN are necessary for a reflex to be present … . If a reflex is
depressed or absent, a lesion involves the sensory nerve or LMN at that particular
site.
The brain stem is divided into
four segments: the medulla
oblongata, the pons, the
midbrain, and the thalamus.
Lesions of the medulla
oblongata cause conscious
deficits and weakness on the
same side or both sides with
normal or hyperactive limb
reflexes similar to cervical
spinal cord lesions. The
cerebellum is attached to the
dorsal surface of the pons. The
cerebellum coordinates all
muscle activity and establishes
muscle tone. The cerebellum
also has equilibrium functions.
Autonomic Nervous
System
Because the language of the nervous system can get quite technical rather quickly, I
would like to share with you some basic information from The Columbia
Encyclopedia, Sixth Edition. This information will describe in more basic terms the
autonomic nervous system.
The autonomic nerve fibers form a subsidiary system that regulates the iris of the
eye and the smooth-muscle action of the heart, blood vessels, glands, lungs,
stomach, colon, bladder, and other visceral organs not subject to willful control.
Although the autonomic nervous system's impulses originate in the central nervous
system, it performs the most basic functions more or less automatically, without
conscious intervention of higher brain centers. Because it is linked to those centers,
however, the autonomic system is influenced by the emotions; for example, anger
can increase the rate of the heart. All of these fibers in the automatic nervous system
are motor channels, and their impulses arise from the nerve tissue itself, so that the
organs perform more or less involuntarily and do not require simulation to function.
Autonomic nerve fibers exit the CNS as
part of other peripheral nerves but branch
from them to form two more subsystems:
the sympathetic and parasympathetic
nervous systems, the actions of which
usually oppose each other. For example,
sympathetic nerves cause arteries to
contract while parasympathetic nerves
cause them to dilate. Sympathetic impulses
are conducted to the organs by two or
more neurons. The cell body of the first lies
within the CNS and that of the second in an
external ganglion. Eighteen pairs of such
ganglia interconnect by nerve fibers to
form a double chain just outside the spine
and running parallel to it. Parasympathetic
impulses are also relayed by at least two
neurons, but the cell body of the second
generally lies near or within the internal
ganglion.
The Nervous System and
Reflexes
In general, nerve function is dependent on
both sensory and motor fibers, sensory
stimulation evoking motor response. Even
the autonomic system is activated by
sensory impulses from receptors in the
organ or muscle. Where especially
sensitive areas or powerful stimuli are
concerned, it is not always necessary for a
sensory impulse to reach the brain in order to trigger motor response. A sensory
neuron may link directly to a motor neuron at a synapse in the spinal cord, forming
a reflex arc that performs automatically. Thus, tapping the tendon below the
kneecap causes the leg to jerk involuntarily because the impulse provoked by the
tap, after traveling to the spinal cord, travels directly back to the leg muscle. Such a
response is called an involuntary reflex action.
Commonly, the reflex arc includes one or more connector neurons that exert a
modulating effect, allowing varying degrees of response, according to whether the
stimulation is strong, weak, or prolonged. Reflex arcs are often linked with other
arcs by nerve fibers in the spinal cord. Consequently, a number of reflex muscle
responses may be triggered simultaneously, as when an animal shudders and jerks
away from the touch of an insect. Links between the reflex arcs and higher centers
enable the brain to identify a sensory stimulus, such as pain; to note the reflex
response, such as a withdrawal; and to inhibit that response, as when the arm is
held steady against the prick of a hypodermic needle.
Disorders of the Nervous System
Regarding disorders of the nervous system, the Merck Veterinary Manual notes,
Disease processes affecting the nervous system may be congenital or familial,
infectious or inflammatory, toxic, metabolic, nutritional, traumatic, vascular, [or]
degenerative. Congenital disorders may be obvious at birth or shortly after. Some
familial disorders cause a progressive degeneration of neurons in the first year of
life, while others may not manifest for two or three years. Infections of the nervous
system are due to specific viruses, fungi, protozoa, bacteria, rickettsia, prions, and
algae … . Toxicity of the nervous system is most frequently caused by
organophosphates, carbamates, metaldehyde, ethylene glycol, theobromines (found
in chocolate), and sedatives.
Most of the above mentioned materials are chemicals used to kill pests but can have
detrimental impacts on the nervous system of animals.
Summary
As you studied in your textbook, the nervous system in its entirety controls a majority of
an animal's body functions. That is why understanding the nervous system and its many
components is vital to one's success as a veterinarian.
Sources Cited:
Kahn, Cynthia, ed. "Nervous System Introduction." The Merck Veterinary Manual. Ninth Edition. New Jersey: Merck & Co. Inc. 200
28 Dec. 2006. <http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/100100.htm>.
"Nervous System." The Columbia Encyclopedia, 6th ed. New York: Columbia University Press, 2001-04. 28 Dec. 2006.
<http://www.bartleby.com/65/ne/nervouss.html>.