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The Peripheral Nervous System and Reflex Activity:
Part C
Spinal Nerves
• 31 pairs of mixed nerves named according to their point of
issue from the spinal cord
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8 cervical (C1–C8)
12 thoracic (T1–T12)
5 Lumbar (L1–L5)
5 Sacral (S1–S5)
1 Coccygeal (C0)
Spinal Nerves: Roots
• Each spinal nerve connects to the spinal cord via two roots
• Ventral roots
• Contain motor (efferent) fibers from the ventral horn motor
neurons
• Fibers innervate skeletal muscles)
Spinal Nerves: Roots
• Dorsal roots
• Contain sensory (afferent) fibers from sensory neurons in the
dorsal root ganglia
• Conduct impulses from peripheral receptors
• Dorsal and ventral roots unite to form spinal nerves, which then
emerge from the vertebral column via the intervertebral
foramina
Spinal Nerves: Rami
• Each spinal nerve branches into mixed rami
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Dorsal ramus
Larger ventral ramus
Meningeal branch
Rami communicantes (autonomic pathways) join to the ventral
rami in the thoracic region
Spinal Nerves: Rami
• All ventral rami except T2–T12 form interlacing nerve networks
called plexuses (cervical, brachial, lumbar, and sacral)
• The back is innervated by dorsal rami via several branches
• Ventral rami of T2–T12 as intercostal nerves supply muscles of
the ribs, anterolateral thorax, and abdominal wall
Cervical Plexus
• Formed by ventral rami of C1–C4
• Innervates skin and muscles of the neck, ear, back of head,
and shoulders
• Phrenic nerve
• Major motor and sensory nerve of the diaphragm (receives fibers
from C3–C5)
Brachial Plexus
• Formed by ventral rami of C5–C8 and T1 (and often C4 and T2)
• It gives rise to the nerves that innervate the upper limb
• Major branches of this plexus:
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Roots—five ventral rami (C5–T1)
Trunks—upper, middle, and lower
Divisions—anterior and posterior
Cords—lateral, medial, and posterior
Brachial Plexus: Nerves
• Axillary—innervates the deltoid, teres minor, and skin and joint capsule of
the shoulder
• Musculocutaneous—innervates the biceps brachii and brachialis and skin
of lateral forearm
• Median—innervates the skin, most flexors and pronators in the forearm,
and some intrinsic muscles of the hand
• Ulnar—supplies the flexor carpi ulnaris, part of the flexor digitorum
profundus, most intrinsic muscles of the hand, and skin of medial aspect
of hand
• Radial—innervates essentially all extensor muscles, supinators, and
posterior skin of limb
Lumbar Plexus
• Arises from L1–L4
• Innervates the thigh, abdominal wall, and psoas muscle
• Femoral nerve—innervates quadriceps and skin of anterior
thigh and medial surface of leg
• Obturator nerve—passes through obturator foramen to
innervate adductor muscles
Sacral Plexus
• Arises from L4–S4
• Serves the buttock, lower limb, pelvic structures, and perineum
• Sciatic nerve
• Longest and thickest nerve of the body
• Innervates the hamstring muscles, adductor magnus, and most muscles in
the leg and foot
• Composed of two nerves: tibial and common fibular
Innervation of Skin
• Dermatome: the area of skin innervated by the cutaneous
branches of a single spinal nerve
• All spinal nerves except C1 participate in dermatomes
• Most dermatomes overlap, so destruction of a single spinal
nerve will not cause complete numbness
Innervation of Joints
• Hilton’s law: Any nerve serving a muscle that produces
movement at a joint also innervates the joint and the skin over
the joint