Essentials of Anatomy and Physiology

vagus nerves (vagusmeans “wanderer”) branch exten-
sively to the larynx, heart, stomach and intestines, and
bronchial tubes.
The functions of the cranial nerves are summarized
in Table 8–4.

THE AUTONOMIC

NERVOUS SYSTEM

The autonomic nervous system (ANS) is actually
part of the peripheral nervous system in that it consists
of motor portions of some cranial and spinal nerves.
Because its functioning is so specialized, however, the
autonomic nervous system is usually discussed as a
separate entity, as we will do here.
Making up the autonomic nervous system are vis-
ceral motor neuronsto smooth muscle, cardiac mus-
cle, and glands. These are the visceral effectors;
muscle will either contract or relax, and glands will
either increase or decrease their secretions.
The ANS has two divisions: sympathetic and

parasympathetic. Often, they function in opposition

to each other, as you will see. The activity of both divi-

sions is integrated by the hypothalamus, which

ensures that the visceral effectors will respond appro-

priately to the situation.

AUTONOMIC PATHWAYS

An autonomic nerve pathway from the central nervous

system to a visceral effector consists of two motor

neurons that synapse in a ganglion outside the CNS

(Fig. 8–12). The first neuron is called the pregan-

glionic neuron, from the CNS to the ganglion. The

second neuron is called the postganglionic neuron,

from the ganglion to the visceral effector. The ganglia

are actually the cell bodies of the postganglionic

neurons.

SYMPATHETIC DIVISION

Another name for the sympathetic division is thora-

columbar division, which tells us where the sympa-

thetic preganglionic neurons originate. Their cell

The Nervous System 187

BOX8–8 LUMBAR PUNCTURE

Box Figure 8–B Cerebrospinal fluid from a patient with

meningitis. The bacteria are streptococci, found in pairs. The

large cells are WBCs. (500) (From Sacher, RA, and

McPherson, RA: Widmann’s Clinical Interpretation of

Laboratory Tests, ed. 11. FA Davis, Philadelphia, 2000, Plate

52, with permission.)

Alumbar puncture(spinal tap) is a diagnostic

procedure that involves the removal of cere-

brospinal fluid to determine its pressure and con-

stituents. As the name tells us, the removal, using a

syringe, is made in the lumbar area. Because the

spinal cord ends between the 1st and 2nd lumbar

vertebrae, the needle is usually inserted between

the 4th and 5th lumbar vertebrae. The meningeal

sac containing cerebrospinal fluid extends to the

end of the lumbar vertebrae, permitting access to

the cerebrospinal fluid with little chance of damag-

ing the spinal cord.

Cerebrospinal fluid is a circulating fluid and has

a normal pressure of 70 to 200 mmH 2 O. An abnor-

mal pressure usually indicates an obstruction in cir-

culation, which may be caused by infection, a

tumor, or mechanical injury. Other diagnostic

tests would be needed to determine the precise

cause.

Perhaps the most common reason for a lumbar

puncture is suspected meningitis, which may be

caused by several kinds of bacteria. If the patient

does have meningitis, the cerebrospinal fluid will be

cloudy rather than clear and will be examined for

the presence of bacteria and many white blood

cells. A few WBCs in CSF is normal, because WBCs

are found in all tissue fluid.

Another abnormal constituent of cerebrospinal

fluid is red blood cells. Their presence indicates

bleeding somewhere in the central nervous system.

There may be many causes, and again, further test-

ing would be necessary.