420
UNIT THREE
body part reach it in the same nerve, even though the f bers
originate From diFFerent spinal nerves
(f g. 11.32)
.
Cervical Plexuses
The
cervical plexuses
lie deep in the neck on either side.
They are Formed by the anterior branches oF the f
rst Four
cervical nerves. ±ibers From these plexuses supply the mus-
cles and skin oF the neck. In addition, f bers From the third,
Fourth, and f Fth cervical nerves pass into the right and leFt
phrenic
(Fren
ik)
nerves,
which conduct motor impulses to
the muscle f bers oF the diaphragm.
Brachial Plexuses
The anterior branches oF the lower Four cervical nerves and
the f rst thoracic nerve give rise to
brachial plexuses.
These
networks oF nerve f bers are deep in the shoulders between the
neck and the axillae (armpits). The major branches emerging
From the brachial plexuses include the Following
(f g. 11.33)
:
1.
Musculocutaneous nerves
supply muscles oF the arms on
the anterior sides and the skin oF the Forearms.
2.
Ulnar nerves
supply muscles oF the Forearms and hands
and the skin oF the hands.
3.
Median nerves
supply muscles oF the Forearms and
muscles and skin oF the hands.
oFten useFul in localizing the sites oF injuries to dorsal roots
or to the spinal cord.
The
ventral root
(anterior, or motor, root) oF each spinal
nerve consists oF axons From the motor neurons whose cell
bodies lie within the gray matter oF the cord. A ventral root
and a dorsal root unite to Form a spinal nerve, which extends
outward From the vertebral canal through an
intervertebral
foramen.
Just beyond its Foramen, each spinal nerve branches.
One oF these parts, the small
meningeal branch,
reenters the
vertebral canal through the intervertebral Foramen and sup-
plies the meninges and blood vessels oF the cord, as well as
the intervertebral ligaments and the vertebrae.
As
f gure 11.31
shows, a
dorsal branch
(dorsal ramus) oF
each spinal nerve turns posteriorly and innervates the muscles
and skin oF the back. The main portion oF the nerve, the
ven-
tral branch
(ventral ramus), continues Forward to supply mus-
cles and skin on the Front and sides oF the trunk and limbs.
The spinal nerves in the thoracic and lumbar regions have a
Fourth branch, or
visceral branch,
which is part oF the auto-
nomic nervous system.
Except in the thoracic region, anterior branches oF the spi-
nal nerves combine to Form complex networks called
plexuses
instead oF continuing directly to the peripheral body parts. In
a plexus, the f bers oF various spinal nerves are sorted and
recombined, so f bers associated with a particular peripheral
Posterior
median sulcus
Posterior
horn
Lateral horn
Anterior
horn
Central
canal
Anterior
median
fissure
Ventral root
(a)
(b)
Dorsal root
Dorsal root
ganglion
Dorsal branch
of spinal nerve (dorsal ramus)
Ventral branch
of spinal nerve (ventral ramus)
Paravertebral
ganglion
Visceral branch
of spinal nerve
Dorsal root
Ventral branch
of spinal nerve
Visceral branch
of spinal nerve
Dorsal branch
of spinal nerve
Ventral
root
Paravertebral
ganglion
Spinal nerve
FIGURE 11.31
Spinal nerve. (
a
) Each spinal nerve has a dorsal and ventral branch. (
b
) The thoracic and lumbar spinal nerves also have a visceral branch.
previous page 450 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 452 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off