Peripheral Nervous System Part B

Spinal Nerves

• 31 pairs of mixed nerves arising from the spinal cord
• Supply all parts of the body except the head.
• Named according to their point of issue:
  • 8 cervical (C1-C8)
  • 12 thoracic (T1-T12)
  • 5 Lumbar (L1-L5)
  • 5 Sacral (S1-S5)
  • 1 Coccygeal (C0)
• Connect to the spinal cord via two medial roots
• Each root forms a series of rootlets that attach to the spinal cord
• Ventral roots arise from the anterior horn and contain motor (efferent) fibers
• Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Nerve Plexuses

• Ventral rami, except those of T2-T12, form interlacing nerve networks called plexuses
• Found in the cervical, brachial, lumbar, and sacral regions
• Each resulting branch of a plexus contains fibers from several spinal nerves

Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall

• The back is innervated by dorsal rami
• The thorax is innervated by ventral rami T1-T12 as intercostal nerves
• Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

Cervical Plexus

• Formed by ventral rami of C1-C4
• Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders
• Most important nerve is the phrenic nerve
• The phrenic nerve is the major motor and sensory nerve of the diaphragm

Brachial Plexus

• Formed by C5-C8 and T1 (C4 and T2 may also contribute)
• Gives rise to nerves that innervate the upper limb
• Four major branches:
  • Roots – five ventral rami (C5-T1)
  • Trunks – upper, middle, and lower, which form divisions
  • Divisions – anterior and posterior serve the front and back of the limb
  • Cords – lateral, medial, and posterior fiber bundles

Brachial Plexus: Nerves

• Axillary – innervates the deltoid and teres minor
• Musculocutaneous – sends fibers to the biceps brachii and brachialis
• Median – branches to most of the flexor muscles of the arm
• Ulnar – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus
• Radial – innervates essentially all extensor muscles

Lumbar Plexus

• Arises from L1-L4 and innervates the thigh, abdominal wall, and related muscles
• Major nerves are the femoral and the obturator

Sacral Plexus

• Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and related sacral areas
• Major nerve is the sciatic, the longest and thickest nerve of the body
• The sciatic is actually composed of two nerves: the tibial and the common fibular nerves

Dermatomes

• A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve
• All spinal nerves except C1 participate in dermatomes

Test your knowledge on spinal nerves and their networks with this quiz. Explore the anatomy and functions of the 31 pairs of spinal nerves, their connections, and the various plexuses formed by the ventral rami. This quiz is ideal for biology and anatomy students.