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Understanding Spinal Cord and Nerves

May 6, 2025

Spinal Cord and Spinal Nerves

Spinal Cord Function

  • Conduction:
    • Afferent: Sensory input from body to brain.
    • Efferent: Motor commands from brain to body.
  • Link: Structural and functional link between brain and body.
  • Neural Integration: Minimal; most processing occurs in the brain.
  • Reflexes: Fast responses to stimuli that do not involve the brain.

Spinal Cord Gross Anatomy

  • Extends from the brain’s medulla through the vertebral canal.
  • Four parts: cervical, thoracic, lumbar, and sacral.
  • Ends at L1 vertebra with the conus medullaris; cauda equina extends inferiorly.
  • Two enlargements:
    • Cervical Enlargement: Innervates upper limbs.
    • Lumbar Enlargement: Innervates lower limbs.

Spinal Nerve Identification and Gross Anatomy

  • Nerve: Cable-like bundle of axons.
  • Connective Tissue Wrappings:
    • Epineurium: Around the entire nerve.
    • Perineurium: Around fascicles.
    • Endoneurium: Around individual axons.
  • 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Spinal Roots and Spinal Nerve

  • Dorsal/Posterior Root: Afferent sensory; cell body in dorsal root ganglion.
  • Ventral/Anterior Root: Efferent motor; cell body in ventral horn.

Protection and Support of the Spinal Cord

  • Protected by bone, meninges, and cerebrospinal fluid.
  • Meninges:
    • Dura Mater: Tough, outer layer; stabilizes spinal cord.
    • Arachnoid Mater: Web-like layer; contains CSF.
    • Pia Mater: Delicate layer adhering to spinal cord.
    • Filum Terminale: Anchors spinal cord to coccyx.

Clinical View: Lumbar Puncture

  • Used to obtain CSF for diagnosis, performed below L1, typically at or below L4.

Distribution of Gray Matter

  • Contains: Neuron cell bodies, dendrites, unmyelinated axons.
  • Horns:
    • Posterior Horns: Sensory neurons and interneurons.
    • Anterior Horns: Somatic motor neurons.
    • Lateral Horns: Autonomic motor neurons (T1-L2).

Gray Commissure

  • Connects left and right gray matter, surrounds central canal.

Nuclei

  • Groups of Cell Bodies:
    • Sensory Nuclei: Somatic and visceral sensory.
    • Motor Nuclei: Somatic (anterior) and autonomic (lateral).

Distribution of White Matter

  • Composed of: Myelinated axons.
  • Regions:
    • Posterior Funiculus: Sensory tracts.
    • Lateral Funiculus: Sensory and motor tracts.
    • Anterior Funiculus: Sensory and motor tracts.

Overview of Conduction Pathways

  • Pathways:
    • Sensory: Ascends to brain.
    • Motor: Descends from brain.
  • Characteristics: Paired, decussate, involve chains of neurons.

Sensory Pathways

  • Receptors: Somatic (tactile, proprioceptors) and visceral.

Posterior Funiculus-Medial Lemniscal Pathway

  • Proprioception, touch, pressure, vibration.
  • Chain: Primary, secondary, tertiary neurons.

Anterolateral Pathway

  • Crude touch, pressure, pain, temperature.
  • Chain: Primary, secondary, tertiary neurons.

Spinocerebellar Pathway

  • Proprioception.
  • Chain: Primary, secondary neurons.

Motor Pathways

  • Control skeletal muscles.
  • Neurons:
    • Upper Motor Neuron: Brain.
    • Lower Motor Neuron: Spinal cord.

Direct (Pyramidal) Pathway

  • Upper Motor Neurons: Primary motor cortex to lower motor neurons.

Indirect Pathway

  • Lateral: Flexor limb muscles.
  • Medial: Muscle tone, movements of head, neck, trunk.

Clinical View: Treating Spinal Cord Injuries

  • Steroids, antibiotics, potential for neural stem cell treatment.

General Distribution of Spinal Nerves

  • Splits After Intervertebral Foramen:
    • Posterior Ramus: Back muscles and skin.
    • Anterior Ramus: Trunk, limbs.
    • Rami Communicantes: Autonomic fibers.

Dermatomes

  • Skin segment supplied by a single spinal nerve.
  • Clinical Uses: Localize nerve damage, referred pain.

Clinical View: Shingles

  • Reactivation of chickenpox; affects dermatomes.

Nerve Plexuses

  • Four Main: Cervical, brachial, lumbar, sacral.

Cervical Plexuses

  • Innervates: Neck, diaphragm.

Brachial Plexuses

  • Innervates: Upper limbs.
  • Injuries: Affect arm function, sensation.

Lumbar Plexuses

  • Femoral and Obturator Nerves: Thigh muscles.

Sacral Plexuses

  • Sciatic Nerve: Longest nerve; splits into tibial and fibular nerves.

Clinical View: Sacral Plexus Injuries

  • Sciatica, other neurological issues.

Characteristics of Reflexes

  • Rapid, involuntary responses.
  • Reflex Arc: Pathway from stimulus to response.

Classifying Spinal Reflexes

  • Types: Spinal/cranial, somatic/visceral, mono/polysynaptic, ipsilateral/contralateral, innate/acquired.

Common Spinal Reflexes

  • Stretch, Golgi Tendon, Withdrawal, Crossed-Extensor.

Reflex Testing

  • Clinical Utility: Diagnosing nerve or spinal damage.