PDC Test - The Nerves

You can earn 0.25 PDC by passing the exam following this article, which has been approved for publication by NCRA's Council of the Academy of Professional Reporters.

The questions are based on the material in the article but some may require additional research. Send your answer sheet to NCRA's Continuing Education Office, 8224 Old Courthouse Road, Vienna, VA 22182, and enclose a check for $40 (member) or $50 (non-member) to cover the processing fee. 

The Nerves

Scientists for years have been studying nerve cells and how they transmit messages, but they are just beginning to understand how the vast system works. The parts of the body we commonly call nerves are really bundles of nerve fibers. Usually, the impulse from a receptor travels along a sensory nerve fiber to the brain, and the order to the muscle goes out from the brain along a motor nerve fiber. In the case of a very simple reflex action, the impulses may travel only to the spinal cord and back.

Under a microscope, a nerve cell, or neuron, may appear as a tiny blob with one or more threads extending from it. The threads are the nerve fibers, and the blob is the cell body that serves as local headquarters for the nerve unit. One fiber is longer than the others. It is called the axon, and it always carries nerve impulses away from the cell body. The shorter fibers are called dendrites. They carry nerve impulses toward the cell body.

A fiber may be coated with a layer of fatty material called myelin. The myelin, in turn, may be covered by a thin membrane called a neurilemmal sheath. The purpose of the myelin and the protective sheath apparently is similar to that of insulation on an electric wire. The larger fibers with "insulation" conduct nerve impulses faster than the smaller fibers that lack the myelin coating.

In moving through the network of nerve fibers, an impulse travels by a sort of chain reaction. One nerve cell triggers the next cell along the route. At the end of the nerve fiber, the impulse jumps to the next by crossing a junction, called a synapse. Here at the synapse, the end of the axon branches like the limbs of a tree. The microscopic branches nearly touch the similarly branched endings of the dendrites on the next nerve cell. While the exact nature of the nerve impulse is not known, it apparently is a combination of electrical and chemical action. The chemical changes are produced along nerve cells when an impulse is transmitted. Then the nerve cells seem to provide their own source of electricity and are able to recharge themselves. The central nervous system includes the brain and the spinal cord. Connected to the central nervous system is the peripheral nervous system, which includes the 12 cranial nerves running to sense organs, the heart, and other internal organs. The peripheral system also includes the 31 pairs of spinal nerves, both sensory and motor, that reach to skeletal muscles throughout the body.

A cross section of the spinal cord appears as a rounded mass of fibers and other tissues. There is a notch on one side and a faint line running from the notch to the opposite side. The color of the spinal cord is white, except for a gray-colored "H" in the center. The gray is the color of the nerve cell bodies and fibers that lack a fatty coating. The white fibers get their coloring from the myelin insulation around them.

In several parts of the body, nerve fibers branch out and connect with other nerve bundles. Such a group of branching and interconnecting nerves is called a plexus. One of the best known of these is the solar plexus located in the back of the stomach. The autonomic nervous system is separated into two sets, the sympathetic and the parasympathetic. One set of nerves causes activity among the smooth muscles of the body; the other reverses the action. The sympathetic nerves constrict the blood vessels and speed up the heartbeat; the parasympathetic nerves dilate the blood vessels and slow the heartbeat. These are automatic actions, involving the involuntary muscles; hence, the name autonomic.

The cranial nerves are attached to the central nervous system through the base of the brain. Most of the dozen pairs of nerves in the system are both sensory and motor. The cell bodies of the cranial nerve motor fibers are within the brain, but the sensory nerve cell bodies are located in ganglia outside the brain.

The cranial nerves include the olfactory, which runs to receptors in the nasal cavity and provides us with our sense of smell, and the optic, which runs to the retina of the eye. Three other cranial nerves control the muscles that move the eyeballs. Still another, the acoustic, carries impulses from the sound receptors in the ear. Cranial nerves also include fibers running to the tongue, muscles of the face, jaw, and neck, as well as the vagus, which reaches the heart, lungs, larynx, stomach, intestines, liver, pancreas, spleen, and kidneys.

The cerebrum grows much faster than the other parts of the brain. It folds back upon itself a number of times, producing the convolutions that give the human brain the appearance of a huge gray walnut. The convolutions enable the body to carry an immense amount of surface area in a small space. The gray matter that gives the cerebrum its color is the cortex, a barklike covering, and it contains nerve cells with axons that extend into a layer of white matter beneath.

The furrows and folds of the convolutions may seem haphazard, but they divide each of the hemispheres into distinct regions. One deep furrow starting near the middle of each hemisphere and running downward and forward is called the central sulcus. This is an important dividing line between sensory and motor nerve areas.

The lateral sulcus, which starts at the bottom and cuts upward and backward, separates two small regions assigned to nerves of auditory communication. On the upper surface, near the motor region, is the speech control center. On the lower side of the furrow, toward the sensory region, is the hearing area. The region of the cerebrum associated with vision is at the far back in the occipital lobe. Here, apparently, are projected the bits of color and light information received by the millions of rods and cones in the retinas of the eyes. Millions of fibers from the sensory area have been traced to the cortex where they seem to be plugged into associated areas. The associated areas apparently are the "memory cores" of the human brain.

Knowledge of the sensory and motor areas of the brain helps locate the source of problems involving the nervous system. For example, nerve fibers from the right side of the body cross over in the brain stem to the left side of the brain. Similarly, nerve fibers from the left side of the body cross over to the right side of the brain. Thus, when a person becomes paralyzed on the right side, doctors can assume safely there has been damage to the motor area on the left side of the brain.

One region of the cerebrum that has defied mapping is the so-called silent area of the frontal lobes. This portion of the brain, behind the forehead, seems to be related to emotions and moral traits rather than simple sensory-motor functions.

We mentioned earlier that electrical currents in nerve fibers could be measured and traced. The brain not only shows evidence of electrical activity, but it produces small current changes known as brain waves. The brain waves may appear at a frequency of about 10 per second, but they vary in frequency and size. Each person is said to have a brain wave pattern that is as distinctive as his or her handwriting. The use of the electroencephalogram, which records the electrical activities of the brain, is valuable in diagnosis.

When the brain developed with the spinal cord from a common tube, several hollow regions remained. These cavities are filled with a liquid called cerebrospinal fluid. The fluid circulates and serves as a watery cushion or shock absorber. It also carries to the spinal cord and brain some of the proteins, carbohydrates, and other food substances. The fluid is produced continuously and reabsorbed into the blood vessels in the brain.

Because of their common origin, the brain and spinal cord have the same set of protective membranes, although these vary slightly in different areas. The innermost of the three layers, the pia mater, fits over the convolutions of the brain like a tight, delicate glove. It carries many fine blood vessels that nourish the brain. Outside the pia mater is a membrane called the arachnoid. It resembles a filmy spider web. Then the outer layer, the dura mater, protects the central nervous system from being damaged by contact with the bony surfaces of the skull and spine and serves as a support for these.

At the base of the cerebrum, where it joins the spinal cord, are three structures known as the pons, the medulla, and the cerebellum. The pons looks like a bridge made of coarse fibers. The fibers connect the medulla with the higher centers of the brain. The medulla, just below the pons, is an important switching center for nerve impulses going to and from the higher brain centers. It's also headquarters for many vital body functions. Body temperature, heart rate, breathing, swallowing, and the size of openings in blood vessels are controlled by the medulla through the autonomic nervous system.

The cerebellum, second largest of the brain divisions, is divided into ribbed hemispheres. Like the cerebrum, much of the cerebellar areas have been mapped. The front and back areas control muscle tone. An area behind the back lobe is concerned with equilibrium. The two main hemisphere areas control coordination of voluntary movements. Thus, the cerebellum is important for such activities as walking, dancing, playing ball, or even for such routine tasks as tying a shoelace or lifting a glass of water to your lips. If the cerebellum is damaged, many activities requiring coordination could be lost. Even talking distinctly would be difficult because the cerebellum helps coordinate the muscles of the vocal cords.

This article and accompanying test were prepared by BAPR member Nancy Patterson of Los Angeles, California. The article, ``The Nerves,'' is reprinted by permission of the American Medical Association from The Human Machine, copyright 1979.

Test for "The Nerves - CEU Exam"

Now it is time for the test on what you have read. Most of the answers to the questions will be found in the article. However, for some questions it will be necessary to consult a medical dictionary.

1. The membrane lying between the dura mater and the pia mater is the

A. neurilemma
B. corte
C. medulla
D. arachnoid

2. Which is not a cranial nerve?

A. trigeminal
B. glossopharyngeal
C. brachial
D. oculomotor

3. The pupil of the eye would be dilated by the

A. sympathetic nerves
B. sensory nerve cells
C. parasympathetic nerves
D. myelin sheath

4. A sulcus is a

A. reflex arc
B. associative neuron
C. deep furrow
D. nissl body

5. Emotions seem to be centered in the

A. temporal lobe
B. frontal lobe
C. parietal lobe
D. occipital lobe

6. Which of the following is a meninx?

A. pia mater
B. lateral sulcus
C. lateral ventricle
D. medulla oblongata

7. The main relay center for sensory impulses to the cerebral cortex is the

A. trochlear nerve
B. thalamus
C. trigeminal nerve
D. neurilemma

8. The pituitary gland is located in the

A. spinal ganglion
B. foramen magnum
C. sella turcica
D. sympathetic trunk

9. The cardiac muscle and smooth muscle tissue is regulated by the

A. central nervous system
B. pericardial sac
C. cardiac sphincter
D. autonomic nervous system

10. The brain is also called the

A. corpus
B. cerebellum
C. encephalon
D. occiput

11. In particular, a "sulcus" separates the

A. gyri
B. axillary nodes
C. fourth cranial nerve
D. dendrites

12. The heart rate and body temperature are regulated in the

A. cerebrum
B. medulla
C. pons varolii
D. thrombus

13. Multiple sclerosis is a disease in which there are patches of

A. nissl bodies
B. branching synapses
C. pangenesis
D. demyelination

14. A ganglion is

A. a cystic tumor
B. a lateral ventricle
C. an abductor muscle
D. an extensor tendon

15. Which cranial nerve does not affect the eye?

A. trochlear
B. oculomotor
C. abducens
D. trigeminal

16. The neurilemma is also known as

A. Schwann's membrane
B. Roentgen's sheath
C. the hyaline membrane
D. the vagus membrane

17. Amplitude measures on an EEG would be about

A. 1 to 10 microvolts
B. 10 to 100 microvolts
C. 100 to 1,000 microvolts
D. 1,000 to 10,000 microvolts

18. The spinal cord enters the cranium through the

A. jugular foramen
B. foramen ovale
C. foramen magnum
D. carotid foramen

19. The pituitary gland is also known as the

A. hypophysis
B. hypothenar
C. anterior capsule
D. lateral ganglion

20. The spinal nerves are

A. sensory only
B. motor only
C. both sensory and motor
D. neither sensory nor motor