C h a p t e r 3 4
Organization and Control of Neural Function
839
vessels. Supporting structures of the spinal cord are dis-
cussed further in Chapter 36.
Early in fetal life, the spinal cord extends the entire
length of the vertebral column and the spinal nerves exit
through the intervertebral foramina (openings) near their
level of origin. Because the vertebral column and spinal
dura grow at a faster rate than the spinal cord, a dispar-
ity develops between each succeeding cord segment and
the exit of its dorsal and ventral nerve roots through the
corresponding intervertebral foramina. In the newborn,
the cord terminates at the level of L2 or L3. In the adult,
the cord usually terminates in the inferior border of L1,
and the arachnoid mater and its enclosed subarachnoid
space, which is filled with CSF, do not close down on
the filum terminale until they reach the level of S2. This
results in the formation of a pocket of CSF, the
dural
cisterna spinalis
, which extends from approximately L2
to S2. Because this area contains an abundant supply
of CSF and the spinal cord does not extend this far, the
area often is used for sampling the CSF. A procedure
called a
spinal tap
, or puncture, can be done by inserting
a special needle into the dural sac at L3 or L4. The spi-
nal roots, which are covered with pia mater, are in little
danger of trauma from the needle used for this purpose.
Spinal Nerves
The peripheral nerves that carry information to and
from the spinal cord are called
spinal nerves
. There are
31 left-right pairs of spinal nerves (8 cervical, 12 tho-
racic, 5 lumbar, 5 sacral, and 1 coccygeal); each pair is
named for the segment of the spinal cord from which
it exits. Because the first cervical spinal nerve exits the
spinal cord just above the first cervical vertebra (C1),
this nerve is given the number of the bony vertebra just
below it (see Fig. 34-11A). However, the numbering is
changed for all lower levels. An extra cervical nerve, the
C8 nerve, exits above the T1 vertebra, and each of the
subsequent nerves are numbered for the vertebra just
above its point of exit.
Each spinal cord segment communicates with its cor-
responding body segment through the paired segmental
spinal nerves. Each spinal nerve, accompanied by the
blood vessels supplying the spinal cord, enters the spinal
canal through an intervertebral foramen, where it divides
into two branches, or roots. One branch enters the dor-
solateral surface of the cord (i.e., dorsal root), carrying
the axons of afferent neurons into the CNS. The other
branch leaves the ventrolateral surface of the cord (i.e.,
ventral root), carrying the axons of efferent neurons into
the periphery. These two branches or roots fuse at the
intervertebral foramen, forming the mixed spinal nerve—
“mixed” because it has both afferent and efferent axons.
After emerging from the vertebral column, the spi-
nal nerve divides into two branches or
rami
(singular,
ramus
): a small dorsal primary ramus and a larger ven-
tral primary ramus (Fig. 34-13). The thoracic and upper
lumbar spinal nerves also lead to a third branch, the
ramus communicans, which contains sympathetic axons
supplying the blood vessels, the genitourinary system,
and the gastrointestinal system. The dorsal ramus
contains sensory fibers from the skin and motor fibers
to muscles of the back. The ventral primary ramus con-
tains motor fibers that innervate the skeletal muscles of
the anterior body wall and the legs and arms.
Spinal nerves do not go directly to skin and muscle
fibers; instead, they form complicated nerve networks
called
plexuses
(see Fig. 34-11A). A plexus is a site of
intermixing nerve branches. Many spinal nerves enter
a plexus and connect with other spinal nerves before
exiting from the plexus. Nerves emerging from a plexus
form progressively smaller branches that supply the skin
and muscles of the various parts of the body. The PNS
contains four major plexuses: the cervical, brachial,
lumbar, and sacral plexuses.
Spinal Reflexes
A reflex is a highly predictable relationship between
a stimulus and a motor response. A spinal reflex is a
response mediated by cells in the spinal cord, bypass-
ing any conscious effort from the brain. A reflex may
involve neurons at a single spinal cord segment (i.e., seg-
mental reflexes), several or many reflexes (i.e., segmental
reflexes), or structures in the brain (i.e., suprasegmental
reflexes).
A spinal reflex involves a sensory receptor, an affer-
ent pathway, an integrating center in the spinal cord,
an efferent pathway, and an effector muscle or gland.
A classic example is the knee-jerk or patellar reflex, in
which the leg jerks when the kneecap is briskly tapped
with a reflex hammer. The reflex reaction is initiated
by sensory receptors at the site of the stimulus, and
relayed to the spinal cord via afferent sensory neurons.
This happens in a fraction of a second, allowing people
to jerk away before the brain is even aware of a prob-
lem. The integrating center in the spinal cord, in turn,
Pia mater
Dorsal
Ventral
Subarachnoid space
Dorsal root
Arachnoid
Dura mater
Meningeal branch
Dorsal primary ramus
Ventral primary ramus
Rami
communicans
Ventral root
Third thoracic
vertebra
Denticulate
ligament
FIGURE 34-13.
Cross-section of vertebral column at the level
of the third thoracic vertebra, showing the meninges, the spinal
cord, and the origin of a spinal nerve and its branches or rami.
