The peripheral nervous
system is a channel for the relay of sensory and
motor impulses between the central nervous
system on the one hand and the body surface,
skeletal muscles, and internal organs on the
other hand. It is composed of (1) spinal nerves,
(2)
cranial nerves, and (3) certain parts of the
autonomic nervous system. As in the central
nervous system, peripheral nervous pathways are
made up of
neurons (that is, nerve cell bodies and
their
axons and
dendrites) as well as the points at which
one neuron communicates with the next (that is,
the synapse). The structures commonly known as
nerves (or by such names as roots, rami, trunks,
and branches) are actually composed of orderly
arrangements of the axonal and dendritic
processes of many nerve cell bodies.
The
cell bodies of peripheral neurons are often
found grouped into clusters called
ganglia. Based on the type of nerve cell
bodies found in ganglia, they may be classified
as either
sensory or motor. Sensory ganglia are found
as oval swellings on the dorsal roots of spinal
nerves, and they are also found on the roots of
certain cranial nerves. The sensory neurons
making up these ganglia are unipolar. Shaped
much like a golf ball on a tee, they have round
or slightly oval cell bodies with concentrically
located nuclei, and they give rise to a single
fiber that undergoes a T-shaped bifurcation, one
branch going to the periphery and the other
entering the brain or spinal cord. There are no
synaptic contacts between neurons in a sensory
ganglion.
Motor ganglia are
associated with neurons of the autonomic nervous
system. Many of these are found in the
sympathetic trunks, two long chains of ganglia
lying along each side of the vertebral column
from the base of the skull to the coccyx; these
are referred to as paravertebral. Other motor
ganglia (called prevertebral) are found near
internal organs innervated by their projecting
fibers, while still others (called terminal
ganglia) are found on the surfaces or within the
walls of the target organs themselves. Motor
ganglia contain multipolar cell bodies, which
have irregular shapes and eccentrically located
nuclei and which project several dendritic and
axonal processes. Preganglionic fibers
originating from the brain or spinal cord enter
motor (autonomic) ganglia, where they synapse on
multipolar cell bodies. These postganglionic
cells, in turn, send their processes to visceral
structures.
Sensory input from the
body surface, from joint, tendon, and muscle
receptors, and from internal organs passes
centrally through the dorsal roots of the spinal
cord. fibers from motor cells in the spinal cord
exit via the ventral roots and course to their
peripheral targets (autonomic ganglia or
skeletal muscle). The spinal nerve is formed by
the joining of dorsal and ventral roots, and it
is the basic structural and functional unit on
which the peripheral nervous system is built.
In
humans there are 31 pairs of spinal nerves. In
descending order from the most rostral end of
the spinal cord, there are 8 cervical
(designated C1–C8), 12 thoracic (T1–T12), 5
lumbar (L1–L5), 5 sacral (S1–S5), and 1
coccygeal (Coc1). Each spinal nerve exits the
vertebral canal through an opening called the
intervertebral foramen. The first spinal nerve
(C1) exits between the skull and the first
cervical vertebra; consequently, spinal nerves
C1–C7 exit above the correspondingly numbered
vertebrae. Spinal nerve C8, however, exits
between the 7th cervical and first thoracic
vertebrae, so that, beginning with T1, all other
spinal nerves exit below their corresponding
vertebrae.
Just
outside the intervertebral foramen, two
branches, known as the gray and white rami
communicates, connect the spinal nerve with the
sympathetic trunk. These rami, along with the
sympathetic trunk and more distal ganglia, are
concerned with the innervation of organs. In
addition, small meningeal branches leave the
spinal nerve and gray ramus and reenter the
vertebral canal, where they innervate
dura and blood vessels.
More peripherally, the spinal nerve divides into
ventral and dorsal rami. All dorsal rami (with
the exception of those from C1, S4, S5, and
Coc1) have medial and lateral branches, which
innervate deep back muscles and overlying skin.
The medial and lateral branches of the dorsal
rami of spinal nerves C2–C8 supply both muscles
and skin of the neck. Those of T1–T6 are mostly
cutaneous (that is, supplying only the skin),
while those from T7–T12 are mainly muscular.
Dorsal rami from L1–L3 have both sensory and
motor fibers, while those from L4–L5 are only
muscular. Dorsal rami of S1–S3 may also be
divided into medial and lateral branches,
serving deep muscles of the lower back as well
as cutaneous areas of the lower buttocks and
perianal area. Undivided dorsal rami from S4,
S5, and Coc1 also send cutaneous branches to the
gluteal and perianal regions.
Ventral rami of the spinal nerves carry sensory
and motor fibers for the innervation of the
muscles, joints, and skin of the lateral and
ventral body walls and the extremities (see
below Plexuses of the ventral rami). Both dorsal
and ventral rami also contain autonomic fibers.
Because spinal nerves
contain both sensory fibers (from the dorsal
roots) and motor fibers (from the ventral
roots), they are known as mixed nerves. When
individual fibers of a spinal nerve are
identified by their specific function, they may
be categorized as one of four types: (1) general
somatic afferent, (2) general visceral afferent,
(3) general somatic efferent, and (4) general
visceral efferent. The term somatic refers to
the body wall (broadly defined to include
skeletal muscles as well as the surface of the
skin), and visceral refers to structures
composed of smooth muscle, cardiac muscle, or
glandular epithelium (or a combination of
these). Efferent fibers carry motor information
to skeletal muscle and to autonomic ganglia (and
then to visceral structures), and afferent
fibers carry sensory information from them.
General somatic afferent receptors are sensitive
to pain, thermal sensation, touch and pressure,
and changes in the position of the body. (Pain
and temperature sensation coming from the
surface of the body are called exteroceptive,
while sensory information arising from tendons,
muscles, or joint capsules are called
proprioceptive.) General visceral afferent
receptors are found in organs of the thorax,
abdomen, and pelvis; their fibers convey, for
example, pain information from the digestive
tract. Both types of afferent fiber project
centrally from cell bodies in dorsal-root
ganglia.
General somatic efferent fibers originate from
large ventral-horn cells and distribute to
skeletal muscles in the body wall and in the
extremities. General visceral efferent fibers
also arise from cell bodies located within the
spinal cord, but they exit only at thoracic and
upper lumbar levels or at sacral levels (more
specifically, at levels T1–L2 and S2–S4). fibers
from T1–L2 enter the sympathetic trunk, where
they either form synaptic contacts within a
ganglion, ascend or descend within the trunk, or
exit the trunk and proceed to ganglia situated
closer to their target organs. fibers from
S2–S4, on the other hand, leave the cord as the
pelvic nerve and proceed to terminal ganglia
located in the target organs. Postganglionic
fibers arising from ganglia in the sympathetic
trunk rejoin the spinal nerves and distribute to
blood vessels, sweat glands, and arrector pili
muscles, while postganglionic fibers arising
from prevertebral and terminal ganglia innervate
viscera of the thorax, abdomen, and pelvis.
All
plexuses arising from the ventral rami of spinal
nerves contain sensory, motor, and autonomic
fibers. The plexuses are the cervical, brachial,
lumbar, sacral, and coccygeal.
Cervical
levels C1–C4 are the main contributors to the
cervical plexus; in addition, small branches
link C1 and C2 with the vagus nerve, C1 and C2
with the hypoglossal nerve, and C2–C4 with the
accessory nerve. Sensory branches of the
cervical plexus are the lesser occipital nerve
(to scalp behind the ear), the great auricular
nerve (to the ear and to the skin over the
mastoid and parotid areas), transverse cervical
cutaneous nerves (to lateral and ventral neck
surfaces), and supraclavicular nerves (along the
clavicle, shoulder, and upper chest). Motor
branches serve muscles that stabilize and flex
the neck, muscles that stabilize the hyoid bone
(to assist in actions like swallowing), and
muscles that elevate the upper ribs.
Originating from C4,
with small contributions from C3 and C5, are the
phrenic nerves, which carry sensory information
from parts of the pleura and pericardium and
motor impulses to muscles of the diaphragm.
Injury to the phrenic nerves would paralyze the
diaphragm and render breathing difficult or
impossible.
Cervical levels C5–C8
and thoracic level T1 contribute to the
formation of the brachial plexus; small
fascicles also arrive from C4 and T2. Spinal
nerves from these levels converge to form
superior (C5 and C6), middle (C7), and inferior
(C8 and T1) trunks, which in turn split into
anterior and posterior divisions. The divisions
then form cords (posterior, lateral, and
medial), which provide motor, sensory, and
autonomic fibers to the shoulder and upper
extremity.
Nerves to shoulder and pectoral muscles include
the dorsal scapular (to the rhomboid muscles),
suprascapular (to supraspinatus and
infraspinatus), medial and lateral pectoral (to
pectoralis minor and major), long thoracic (to
serratus anterior), thoracodorsal (to latissimus
dorsi), and subscapular (to teres major and
subscapular). The axillary nerve carries motor
fibers to the deltoid and teres minor muscles as
well as sensory fibers to the lateral surface of
the shoulder and upper arm. The biceps,
brachialis, and coracobrachialis muscles, as
well as the lateral surface of the forearm, are
served by the musculocutaneous nerve.
The three major nerves of the arm, forearm, and
hand are the radial, median, and ulnar. The
radial nerve innervates the triceps, anconeus,
and brachioradialis muscles, eight extensors of
the wrist and digits, and one abductor of the
hand; it is also sensory to part of the hand.
The median nerve branches in the forearm to
serve the palmaris longus, two pronator muscles,
four flexor muscles, thenar muscles, and
lumbrical muscles; most of these serve the wrist
and hand. The ulnar nerve serves two flexor
muscles and a variety of small muscles of the
wrist and hand.
Cutaneous innervation of the upper extremity
originates, via the brachial plexus, from spinal
cord levels C3–T2. The shoulder is served by
supraclavicular branches (C3, C4) of the
cervical plexus, while the anterior and lateral
aspects of the arm and forearm have sensory
innervation via the axillary (C5, C6), nerve as
well as the dorsal (C5, C6), lateral (C5, C6),
and medial (C8, T1) antebrachial cutaneous
nerves. These same nerves have branches that
wrap around to serve portions of the posterior
and medial surfaces of the extremity. The palm
of the hand is served by the median (C6–C8) and
ulnar (C8, T1) nerves. The ulnar nerve also
wraps around to serve medial areas of the
dorsum, or back, of the hand. A line drawn down
the midline of the ring finger represents the
junction of the ulnar-radial distribution on the
back of the hand and the ulnar-median
distribution on the palm. A small part of the
thumb and the distal thirds of the index,
middle, and lateral surface of the ring finger
are served by the median nerve. The inner aspect
of the arm and adjacent armpit is served by
intercostobrachial and posterior and medial
brachial cutaneous nerves (T1–T2).
Spinal nerves from
lumbar levels L1–L4 contribute to the formation
of the lumbar plexus, which, along with the
sacral plexus, provides motor, sensory, and
autonomic fibers to gluteal and inguinal regions
and to the lower extremity. Lumbar roots are
organized into dorsal and ventral divisions.
Minor cutaneous and muscular branches of the
lumbar plexus include the iliohypogastric,
genitofemoral, and ilioinguinal (projecting to
the lower abdomen and to inguinal and genital
regions) and the lateral femoral cutaneous nerve
(to skin on the lateral thigh). Two major
branches are the obturator and femoral nerves.
The obturator enters the thigh through the
obturator foramen; motor branches proceed to the
obturator internus and gracilis muscles as well
as the adductor muscles, while sensory branches
supply the articular capsule of the knee joint.
An accessory obturator nerve supplies the
pectineus muscle of the thigh and is sensory to
the hip joint.
The sartorius muscle and medial and anterior
surfaces of the thigh are served by branches of
the anterior division of the femoral nerve. The
posterior division of the femoral nerve provides
sensory fibers to the inner surface of the leg
(saphenous nerve), to the quadriceps muscles
(muscular branches), the hip and knee joints,
and the articularis genu muscle.
The
ventral rami of L5 and S1–S3 form the sacral
plexus, with contributions from L4 and S4.
Branches from this plexus innervate gluteal
muscles, muscles forming the internal surface of
the pelvic basin (including those forming the
levator ani), and muscles that run between the
femur and pelvis to stabilize the hip joint
(such as the obturator, piriformis, and
quadratus femoris muscles). These muscles lend
their names to the nerves that innervate them.
Cutaneous branches from the plexus serve the
buttocks, perineum, and posterior surface of the
thigh.
The major nerve of the sacral plexus, and the
largest in the body, is the sciatic. Formed by
the joining of ventral and dorsal divisions of
the plexus, it passes through the greater
sciatic foramen and descends in back of the
thigh. There, sciatic branches innervate the
biceps femoris, semitendinosus and
semimembranosus muscles, and part of the
adductor magnus muscle. In the popliteal fossa
(just above the knee) the sciatic nerve divides
into the tibial nerve and the common fibular (or
peroneal) nerve. The tibial nerve (from the
dorsal division) continues distally in the calf
and innervates the gastrocnemius muscle, deep
leg muscles such as the popliteus, soleus, and
tibialis posterior, and the flexor muscles,
lumbrical muscles, and other muscles of the
ankle and plantar aspects of the foot. The
peroneal nerve, from the ventral division,
passes to the anterior surface of the leg and
innervates the tibialis anterior, the fibularis
muscles, and extensor muscles that elevate the
foot and fan the toes. Cutaneous branches from
the tibial and common fibular nerves serve the
outer sides of the leg and the top and bottom of
the foot and toes.
The ventral rami of S4,
S5, and the 1st coccygeal nerve form the
coccygeal plexus, from which small anococcygeal
nerves arise to innervate the skin over the
coccyx (tailbone) and around the anus. |