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.