Spinal Nerves: Organization and Structure

• Spinal nerves, part of the Peripheral Nervous System (PNS), connect the Central Nervous System (CNS) (brain and spinal cord) to muscles, sensory receptors, and glands.
• There are 31 pairs of spinal nerves arising from the spinal cord that supply all parts of the body except the head.

Spinal Nerve Organization

• Spinal nerves are named based on their origin point.
• There are 8 cervical nerves (C1-C8).
• There are 12 thoracic nerves (T1-T12).
• There are 5 lumbar nerves (L1-L5).
• There are 5 sacral nerves (S1-S5).
• There is 1 coccygeal nerve (Co).
• The cervical and lumbar enlargements contain nerves serving the upper and lower limbs.

Spinal Nerve Structure

• Each spinal nerve forms from the union of two roots.
  • The posterior (dorsal) root carries sensory (afferent) fibers from the body to the spinal cord.
  • The anterior (ventral) root carries motor (efferent) fibers from the spinal cord to muscles.
• Spinal nerves divide into two branches upon exiting the spinal cord through the intervertebral foramen.
  • The Dorsal Ramus supplies the posterior trunk.
  • The Ventral Ramus supplies the limbs, lateral, and anterior trunk.

Spinal Plexuses

• Most ventral rami (except T2–T12) form networks called plexuses.
• The Cervical Plexus (C1-C5) supplies the neck, back of the head, and shoulders.
  • It includes the phrenic nerve, which controls the diaphragm for breathing.
• The Brachial Plexus (C5-T1) supplies the shoulders, arms, forearms, wrists, and hands.
  • Major nerves include:
    • Axillary nerve controls the deltoid muscle and teres minor.
    • Musculocutaneous nerve controls the biceps and anterior arm.
    • Radial nerve controls the triceps, posterior arm, and forearm.
    • Median nerve controls the anterior forearm and some hand muscles.
    • Ulnar nerve controls the medial forearm and most of the hand.
• The Lumbar Plexus (L1-L4) innervates the thigh, abdominal wall, and psoas muscle.
  • Major nerves include:
    • Femoral nerve controls the anterior thigh, hip flexors, and knee extension.
    • Obturator nerve controls the medial thigh and adductor muscles.
• The Sacral Plexus (L4-S4) serves the buttocks, lower limbs, pelvic structures, and perineum.
  • Major nerves include:
    • Sciatic nerve (the largest nerve in the body) supplies the posterior thigh.
    • Tibial nerve controls the posterior leg and foot muscles.
    • Common fibular nerve controls the anterior and lateral leg and dorsum of the foot.
    • Pudendal nerve controls the perineum and pelvic floor muscles.

Thoracic Nerves (T1-T12)

• Thoracic nerves T2-T12 do not form a plexus unlike nerves in other regions.
• Instead, they form intercostal nerves that supply the muscles and skin of the thorax and abdomen.

Dermatomes and Myotomes

• Dermatomes: Skin areas supplied by a single spinal nerve, important for diagnosing nerve injuries.
• Myotomes: Muscle groups innervated by a single spinal nerve are used for spinal nerve assessment.

Clinical Importance

• Damage to one spinal nerve does not completely paralyze a muscle because most muscles receive innervation from multiple nerves.

• Spinal nerve assessment involves testing specific muscle actions like C5 for deltoid movement, and L3 & L4 for knee extension.

• Spinal nerves connect the CNS to the body, with 31 pairs divided into cervical, thoracic, lumbar, sacral, and coccygeal regions.

• Spinal nerve branches include dorsal (posterior) and ventral (anterior) rami.

• Plexuses (cervical, brachial, lumbar, sacral) allow nerve mixing for motor control redundancy.

• Dermatomes and myotomes are used clinically to diagnose spinal injuries.