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CHAPTER # 13(c)
THE PERIPHERAL
NERVOUS SYSTEM &
REFLEX ACTIVITY
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Spinal Nerves
• 31 pairs of mixed nerves named according to
their point of issue from the spinal cord
• 8 cervical (C1–C8)
• 12 thoracic (T1–T12)
• 5 Lumbar (L1–L5)
• 5 Sacral (S1–S5)
• 1 Coccygeal (C0)
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Cervical plexus
Brachial plexus
Cervical
enlargement
Intercostal
nerves
Cervical
nerves
C1 – C8
Thoracic
nerves
T1 – T12
Lumbar
enlargement
Lumbar plexus
Sacral plexus
Cauda equina
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Lumbar
nerves
L1 – L5
Sacral nerves
S1 – S5
Coccygeal nerve Co1
Figure 13.6
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)
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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
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Gray matter
White matter
Ventral root
Dorsal root
Dorsal root
ganglion
Dorsal ramus
of spinal nerve
Ventral ramus
of spinal nerve
Spinal nerve
Dorsal and
ventral rootlets
of spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Anterior view showing spinal cord, associated nerves, and vertebrae.
The dorsal and ventral roots arise medially as rootlets and join
laterally to form the spinal nerve.
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Figure 13.7 (a)
Spinal Nerves: Rami
• Each spinal nerve branches into mixed rami
• Dorsal ramus
• Larger ventral ramus
• Meningeal branch
• Rami communicantes (autonomic pathways)
join to the ventral rami in the thoracic region
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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
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Dorsal ramus
Ventral ramus
Spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Intercostal nerve
Dorsal root
ganglion
Dorsal root
Ventral root
Branches of intercostal
nerve
• Lateral cutaneous
• Anterior cutaneous
Sternum
(b) Cross section of thorax showing the main roots and
branches of a spinal nerve.
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Figure 13.7 (b)
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)
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Ventral rami
Segmental
branches
Hypoglossal
nerve (XII)
Lesser occipital
nerve
Greater auricular
nerve
Transverse
cervical nerve
Ansa cervicalis
Ventral
rami:
C1
C2
C3
C4
Accessory nerve (XI)
Phrenic nerve
C5
Supraclavicular
nerves
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Figure 13.8
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Table 13.3
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:
• Roots—five ventral rami (C5–T1)
• Trunks—upper, middle, and lower
• Divisions—anterior and posterior
• Cords—lateral, medial, and posterior
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Roots (ventral rami):
C4
C5
Dorsal scapular
Nerve to
subclavius
Suprascapular
Cords
C6
Posterior
divisions
C7
Lateral
C8
Posterior
T1
Upper
Middle
Trunks
Lower
Long thoracic
Medial pectoral
Lateral pectoral
Medial
Axillary
Musculocutaneous
Radial
Upper subscapular
Median
Ulnar
Medial cutaneous
nerves of the arm
and forearm
Lower subscapular
Thoracodorsal
(a) Roots (rami C5 – T1), trunks, divisions, and cords
Anterior
divisions
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Posterior
divisions
Trunks
Roots
Figure 13.9 (a)
Anterior
divisions
Posterior
divisions
Major terminal
branches
(peripheral nerves)
Musculocutaneous
Median
Ulnar
Radial
Axillary
Trunks
Cords
Roots
Divisions
Anterior
Lateral
Medial
Posterior
Anterior
Posterior
Posterior
Anterior
Posterior
Trunks
Upper
Roots
(ventral
rami)
C5
C6
Middle
C7
C8
Lower
T1
(d) Flowchart summarizing relationships within the
brachial plexus
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Figure 13.9 (d)
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
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Axillary
nerve
Anterior
divisions
Posterior
divisions
Trunks
Roots
Humerus
Radial nerve
Musculocutaneous nerve
Ulna
Radius
Ulnar nerve
Median nerve
Radial nerve (superficial branch)
Dorsal branch of ulnar nerve
Superficial branch of ulnar nerve
Digital branch of ulnar nerve
Muscular branch
Median nerve
Digital branch
(c) The major nerves of the upper limb
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Figure 13.9 (c)
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Table 13.4
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
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Ventral rami
Iliohypogastric
Ilioinguinal
Genitofemoral
Lateral femoral
cutaneous
Obturator
Femoral
Lumbosacral
trunk
Ventral
rami:
Iliohypogastric
L1
Ilioinguinal
Femoral
Lateral femoral
L2
cutaneous
Obturator
L3
Anterior femoral
cutaneous
Saphenous
L4
L5
(a) Ventral rami and major branches
of the lumbar plexus
(b) Distribution of the major nerves from
the lumbar plexus to the lower limb
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Figure 13.10
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Table 13.5
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
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Ventral rami
Ventral rami:
L4
Superior
gluteal
Lumbosacral
trunk
Inferior
gluteal
Common
fibular
Tibial
Posterior
femoral
cutaneous
Pudendal
Sciatic
L5
S1
S2
S3
S4
S5
Co1
Ventral rami and major branches
of the sacral plexus
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Figure 13.11 (a)
Superior gluteal
Inferior gluteal
Pudendal
Sciatic
Posterior femoral
cutaneous
Common fibular
Tibial
Sural (cut)
Deep fibular
Superficial fibular
Plantar branches
(b) Distribution of the major nerves from
the sacral plexus to the lower limb
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Figure 13.11 (b)
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Table 13.6
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
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C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
C2
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T2
C5
C6
C6
C7
L1
C8
L2
T12
S2
S3
T2
C5
C6
L1
C8
L2
S1
L4
S2
S3
S4
S5
C6
C7
C6
C7
C8
C8
L2
S2
S1
L1
L3
L5
L4
T11
T12
L1
L3
L5
C7
C6
S1 S2
L3
C5
L2
L5
L4
L3
L5
L5
L4
S1
Anterior
view
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S1
(b) Posterior
view
L4
L5
L4
L5
S1
Figure 13.12
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
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