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.
Structure of the Spinal
Nerves
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.
Functional Components
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.
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