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Understanding Spinal Nerve Plexuses

May 3, 2025

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.