Lecture Notes: Spinal Nerve Plexuses

Major Nerve Plexuses

• Definition:
  • Ventral rami from adjacent spinal nerves form a network called a nerve plexus.
  • Plexuses are networks of nerve fibers without associated cell bodies.
  • Peripheral nerves emerging contain fibers from various spinal nerves.
  • Mixed fibers: contain both sensory and motor fibers.
    • Sensory fibers: Axons of sensory neurons enter the CNS through the dorsal root ganglia.
    • Motor fibers: Axons of motor neurons emerge from the anterior horn of the spinal cord.
• Example: Radial nerve: contains sensory fibers for cutaneous sensation and motor fibers for muscle movement in the arm.

Locations of Nerve Plexuses

• Four major plexuses:
  • Two at the cervical level.
  • One at the lumbar level.
  • One at the sacral level.
• Thoracic Region:
  • Spinal nerves T2-T11 are not part of plexuses.
  • Give rise to intercostal nerves.

Description of Plexuses

• Cervical Plexus (C1-C5):
  • Branches into nerves of the posterior neck, head, and the phrenic nerve.
  • Responsible for sensation from head and neck and motor control of respiration.
• Brachial Plexus (C5-T1):
  • Gives rise to the nerves of the arms.
  • Includes the radial, axillary, ulnar, and median nerves.
  • Functions include elbow flexion, shoulder abduction, sensation in shoulder and upper limb.
• Lumbar Plexus (L1-L5):
  • Innervates pelvic region and anterior leg.
  • Major nerves: femoral and obturator nerves.
  • Functions include knee extension and sensation from lower abdomen.
• Sacral Plexus (L4-L5, S1-S4, Co):
  • Major nerve: sciatic nerve (tibial and fibular nerves combined).
  • Functions include plantar flexion and sensation from the sacroiliac joint.
  • Note: Term ‘lumbosacral’ includes lumbar, sacral, and coccygeal plexuses.

Dermatomes and Myotomes

• Dermatome:
  • Area of skin supplied by a single spinal nerve root.
  • Overlap by 50% at edges.
  • Mapped into a dermatome map.
• Myotome:
  • Group of muscles innervated by a single spinal nerve root.
  • Myotome distribution for upper and lower extremities.
  • Examples:
    • C1/C2: Neck flexion/extension
    • L2: Hip flexion
    • S1: Ankle plantar flexion/hip extension

Real-Life Applications

• Dermatome Knowledge:
  • Diagnosing diseases like herpes zoster (shingles).
  • Lumbar radiculopathy: radiating back pain from nerve root compression.
  • Referred pain from visceral organs.
• Myotome Testing:
  • Assists in diagnosing spinal lesions.
  • Looks for muscle weakness to indicate nerve root or disc herniation issues.