Lecture Notes: The Spinal Cord

Overview

• Second lecture of the third unit focusing on the spinal cord.
• Previously covered nervous tissue.
• Upcoming focus on the anatomy and physiology of the spinal cord.
• Emphasis on spinal reflexes and reflex arcs.

Anatomy of the Spinal Cord

• Dimensions:
  • About 18 inches long.
  • Ends around vertebrae L1 or L2.
  • Diameter ~ 0.5 inches.
  • Bilateral symmetry with posterior median sulcus and anterior median fissure.
• Regions and Structures:
  • Starts at the medulla oblongata.
  • Ends at the conus medullaris, tapering into the cauda equina.
  • Enlargements: Cervical (shoulders), Lumbar (pelvis, lower limbs).
  • 31 spinal cord segments aligned with vertebrae.
  • Dorsal and ventral roots combine to form spinal nerves.
• Gray and White Matter:
  • Gray matter: Neuron cell bodies.
  • White matter: Columns of nerve fibers.

Meninges of the Spinal Cord

• Three-layer structure:
  • Dura Mater: Outer tough layer, epidural space contains adipose tissue.
  • Arachnoid Mater: Middle layer, subarachnoid space contains cerebrospinal fluid (CSF).
  • Pia Mater: Innermost layer, collagen and elastic fibers.
• Clinical Relevance:
  • Meningitis: Infection of the meninges.
  • Epidural anesthesia and spinal taps involve these spaces.

Physiology: Reflexes

• Definition: Automatic responses to stimuli.
• Components of Reflex Arc:
  • Arrival of stimulus.
  • Activation of sensory neuron.
  • Information processing.
  • Activation of motor neuron.
  • Response of effector (e.g., muscle contraction).
• Types of Reflexes:
  • Development: Innate vs. acquired.
  • Response: Somatic (skeletal muscle) vs. visceral (organ systems).
  • Complexity: Monosynaptic (simple) vs. polysynaptic (complex).
  • Site of Processing: Cranial vs. spinal.

Monosynaptic and Polysynaptic Reflexes

• Monosynaptic Reflexes:
  • Example: Stretch reflex (knee-jerk).
  • Direct connection between a sensory neuron and a motor neuron.
• Polysynaptic Reflexes:
  • Example: Withdrawal reflex.
  • Involves interneurons, more than one synapse.
  • Reciprocal inhibition allows the flexor reflex to function.

Clinical Focus: Amyotrophic Lateral Sclerosis (ALS)

• Characteristics: Degeneration of motor neurons, leads to muscle atrophy.
• Causes:
  • 90% sporadic cases, 10% familial.
  • 2% linked to mutations in SOD1 gene.
• Symptoms: Weakness, muscle atrophy, difficulty breathing.
• Treatment: Limited; Riluzole provides modest survival extension.

Conclusion

• Next focus will shift to the brain.
• Final lecture for this unit will cover the brain anatomy and function.