Chapter 13
The Peripheral Nervous System and Reflex Activity
503
13
A spinal nerve is quite short (only 1–2 cm). Almost im-
mediately afer emerging From its Foramen, it divides into
a small
dorsal ramus
(ra
9
mus; “branch”), a larger
ventral
ramus
, and a tiny
meningeal branch
(me˘
-nin
9
je-al) that
reenters the vertebral canal to innervate the meninges and
blood vessels within. Each ramus, like the spinal nerve itselF,
is mixed.
Special rami called
rami communicantes
, which contain au-
tonomic (visceral) nerve fibers, attach to the base oF the ventral
rami oF the thoracic spinal nerves (±igure 13.8).
Innervation of Specific Body Regions
Te spinal nerve rami and their main branches supply the entire
somatic region oF the body (skeletal muscles and skin) From the
neck down. Te dorsal rami supply the posterior body trunk. Te
thicker ventral rami supply the rest oF the trunk and the limbs.
²o be clear, let’s review the difference between roots and rami:
Roots lie medial to and Form the spinal nerves. Each root is
strictly sensory or motor.
Rami lie distal to and are lateral branches oF the spinal nerves.
Like spinal nerves, rami carry both sensory and motor fibers.
BeFore we get into the specifics oF how the body is inner-
vated, it is important For you to understand some points about
the ventral rami oF the spinal nerves. Except For ²
2
–²
12
, all ven-
tral rami branch and join one another lateral to the vertebral
column, Forming complicated interlacing nerve networks called
nerve plexuses
(±igure 13.7). Nerve plexuses occur in the cervi-
cal, brachial, lumbar, and sacral regions and primarily serve the
limbs. Notice that
only ventral rami form plexuses
.
Within a plexus, fibers From the various ventral rami criss-
cross one another and become redistributed so that (1) each re-
sulting branch oF the plexus contains fibers From several spinal
nerves and (2) fibers From each ventral ramus travel to the body
periphery via several routes. As a result, each muscle in a limb
receives its nerve supply From more than one spinal nerve. An
advantage oF this fiber regrouping is that damage to one spinal
segment or root cannot completely paralyze any limb muscle.
In the rest oF this section, we summarize the major groups
oF skeletal muscles served. ±or more specific inFormation on
muscle innervations, see ²ables 10.1–10.17.
Cervical Plexus and the Neck
Buried deep in the neck under the sternocleidomastoid mus-
cle, the ventral rami oF the first Four cervical nerves Form the
looping
cervical plexus
(Figure 13.9)
. Most oF its branches
are
cutaneous nerves
that supply only the skin
(Table 13.3)
.
Hypoglossal
nerve (XII)
C
1
C
2
C
3
C
4
C
5
Segmental
branches
Lesser occipital
nerve
Greater auricular
nerve
Ansa cervicalis
Phrenic nerve
Supraclavicular
nerves
Accessory nerve (XI)
Transverse
cervical nerve
Ventral
rami:
Ventral rami
Figure 13.9
The cervical plexus.
The nerves colored gray
connect to the plexus but do not belong to it. See Table 13.3 for
structures served. (Posterior view.)
Table 13.3
Branches of the Cervical Plexus
(See Figure 13.9)
NERVES
VENTRAL RAMI
STRUCTURES SERVED
Cutaneous Branches (Superficial)
Lesser occipital
C
2
(C
3
)
Skin on posterolateral aspect of head and neck
Greater auricular
C
2
, C
3
Skin of ear, skin over parotid gland
Transverse cervical
C
2
, C
3
Skin on anterior and lateral aspect of neck
Supraclavicular (medial, intermediate, and lateral)
C
3
, C
4
Skin of shoulder and clavicular region
Motor Branches (Deep)
Ansa cervicalis (superior and inferior roots)
C
1
–C
3
Infrahyoid muscles of neck (omohyoid, sternohyoid, and sternothyroid)
Segmental and other muscular branches
C
1
–C
5
Deep muscles of neck (geniohyoid and thyrohyoid) and portions of
scalenes, levator scapulae, trapezius, and sternocleidomastoid muscles
Phrenic
C
3
–C
5
Diaphragm (sole motor nerve supply)
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