Spinal Cord & Spinal Nerves

General Structure

  • Spinal cord links brain & PNS.
  • Extends from foramen magnum to 2nd lumbar vertebra.
  • Sections: cervical, thoracic, lumbar, sacral.
  • Enlargements:
    • Cervical: Nerves to upper limbs.
    • Lumbar: Nerves to lower limbs.
  • Additional structures:
    • Conus Medullaris: Cone-like tapering point.
    • Cauda Equina: Nerves resembling a horse's tail.

Meninges

  • Protective connective tissue around CNS.
  • Three layers:
    • Dura Mater: Tough, outermost layer.
      • Epidural space: Between dura mater and periosteum; site for anesthetics.
    • Arachnoid Mater: Fragile, web-like middle layer.
      • Sub-dural space: Between dura mater & arachnoid mater.
    • Pia Mater: Innermost layer against brain/spinal cord.
      • Sub-arachnoid space: Between arachnoid & pia mater; contains CSF.

Cross Section

  • White Matter: Myelinated neurons.
  • Gray Matter: Non-myelinated neurons.
  • Posterior (Dorsal) Root/Horn: Sensory neurons.
  • Anterior (Ventral) Root/Horn: Motor neurons.
    • Somatic: Skeletal muscle (voluntary).
    • Autonomic: Cardiac muscle (involuntary).

Reflexes

  • Reflex: Automatic response to stimulus without conscious thought.
  • Reflex Arc: Information flow.
    • Components: Sensory receptor, sensory neuron, interneuron, motor neuron, effector organ.

Types of Spinal Cord Reflexes

  • Stretch Reflex: Muscle contracts in response to stretching (e.g., patellar reflex).
  • Golgi-Tendon Reflex: Protects tendons from excessive force.
  • Flexor/Withdrawal Reflex: Moves body part away from stimulus.
    • Reciprocal Innervation: Coordinates flexor and extensor muscles.
  • Crossed Extensor Reflex: Counterbalance during withdrawal (e.g., stepping on a tack).

Spinal Cord Pathways

  • Ascending Tracts: Sensory information to brain.
  • Descending Tracts: Motor information from brain.

Structure of Peripheral Nerve

  • Layers of collagen fibers for protection.
    • Epineurium: Wraps entire nerve.
    • Perineurium: Wraps bundles of fibers.
    • Endoneurium: Wraps individual nerve fiber.

Spinal Nerves

  • 31 pairs.
  • Cervical (C1-C8), Thoracic (T1-T12), Lumbar (L1-L5), Sacral (S1-S5), Coccygeal.
  • Dermatome: Area of skin innervated by each spinal nerve pair.

Nerve Plexus

  • Network of nerves innervating a body region.
  • Major plexi:
    • Cervical Plexus (C1-C5): Innervates neck, shoulders.
      • Phrenic Nerve: Innervates diaphragm.
    • Brachial Plexus (C5-T1): Innervates upper limb.
      • Nerves: Axillary, Radial, Musculo-cutaneous, Median, Ulnar.
    • Lumbar Plexus (L1-L4)
    • Sacral Plexus (L4-S4): Lumbo-Sacral Plexus.
      • Nerves: Obturator, Femoral, Sciatic (Tibial and Fibular branches).