Essentials of Anatomy and Physiology

injury. In length, the spinal cord extends from the
foramen magnum of the occipital bone to the disc
between the first and second lumbar vertebrae.
A cross-section of the spinal cord is shown in Fig.
8–3; refer to it as you read the following. The internal
gray matteris shaped like the letter H; gray matter
consists of the cell bodies of motor neurons and
interneurons. The external white matteris made of
myelinated axons and dendrites of interneurons.
These nerve fibers are grouped into nerve tracts based
on their functions. Ascending tracts(such as the dor-
sal columns and spinothalamic tracts) carry sensory

impulses to the brain. Descending tracts(such as the

corticospinal and rubrospinal tracts) carry motor

impulses away from the brain. Lastly, find the central

canal; this contains cerebrospinal fluidand is con-

tinuous with cavities in the brain called ventricles.

SPINAL NERVES

There are 31 pairs of spinal nerves, those that emerge

from the spinal cord. The nerves are named according

to their respective vertebrae: 8 cervical pairs, 12 tho-

racic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 very

small coccygeal pair. These are shown in Fig. 8–4;

notice that each nerve is designated by a letter and a

number. The 8th cervical nerve is C8, the 1st thoracic

nerve is T1, and so on.

In general, the cervical nerves supply the back of

the head, neck, shoulders, arms, and diaphragm (the

phrenic nerves). The first thoracic nerve also con-

tributes to nerves in the arms. The remaining thoracic

nerves supply the trunk of the body. The lumbar and

sacral nerves supply the hips, pelvic cavity, and legs.

Notice that the lumbar and sacral nerves hang below

the end of the spinal cord (in order to reach their

proper openings to exit from the vertebral canal); this

is called the cauda equina, literally, the “horse’s tail.”

Some of the important peripheral nerves and their

destinations are listed in Table 8–3.

Each spinal nerve has two roots, which are neurons

entering or leaving the spinal cord (see Fig. 8–3). The

dorsal rootis made of sensory neurons that carry

impulses into the spinal cord. The dorsal root gan-

glionis an enlarged part of the dorsal root that con-

tains the cell bodies of the sensory neurons. The term

ganglionmeans a group of cell bodies outside the

CNS. These cell bodies are within the vertebral canal

and are thereby protected from injury (see Box 8–2:

Shingles).

The ventral rootis the motor root; it is made of

the axons of motor neurons carrying impulses from

the spinal cord to muscles or glands. The cell bodies

of these motor neurons, as mentioned previously, are

in the gray matter of the spinal cord. When the two

nerve roots merge, the spinal nerve thus formed is a

mixed nerve.

SPINAL CORD REFLEXES

When you hear the term reflex, you may think of an

action that “just happens,” and in part this is so. A

reflexis an involuntary response to a stimulus, that is,

172 The Nervous System

Table 8–2 THE NERVE IMPULSE

State or Event Description

Polarization

(the neuron

is not carrying

an electrical

impulse)

Depolarization

(generated

by a stimulus)

Propagation of

the impulse

from point

of stimulus

Repolarization

(immediately

follows

depolarization)

• Neuron membrane has a ()
  charge outside and a () charge
  inside.
  •Naions are more abundant out-
  side the cell.
  •Kions and negative ions are
  more abundant inside the cell.
  Sodium and potassium pumps
  maintain these ion concentrations.


• Neuron membrane becomes very
  permeable to Naions, which
  rush into the cell.


• The neuron membrane then has a
  () charge outside and a ()
  charge inside.


• Depolarization of part of the
  membrane makes adjacent mem-
  brane very permeable to Naions,
  and subsequent depolarization,
  which similarly affects the next
  part of the membrane, and so on.


• The depolarization continues
  along the membrane of the neu-
  ron to the end of the axon.


• Neuron membrane becomes very
  permeable to Kions, which rush
  out of the cell. This restores the
  () charge outside and ()
  charge inside the membrane.


• The Naions are returned outside
  and the Kions are returned
  inside by the sodium and potas-
  sium pumps.


• The neuron is now able to
  respond to another stimulus and
  generate another impulse.