Clinical Neuroscience Week 4 - The Spinal Cord

Spinal Cord Anatomy and Function

• 31 pairs of spinal nerves
• Spinal cord ends at approximately L1/L2 in adults
• Cervical and lumbar regions of grey matter are enlarged compared to thoracic
• 3-order neural pathway: information travels through 3 stops to reach destination
• Cervical nerves exit superior (above) to their corresponding vertebral body. There are 8 cervical nerves and 7 cervical vertebrae.
• Thoracic, lumbar, and sacral nerves exit inferior (below) to their corresponding vertebral body.
• Cauda equina: the remaining spinal nerve fibers below L1/L2, located within the thecal sac.

Spinal Cord - Internal Structure

• Gray matter:
  • Dorsal horn: Sensory information enters the spinal cord.
  • Lateral horn: Only in the thoracic region; houses sympathetic information relayed to organs.
  • Gray commissure: Connects the two sides of the spinal cord.
  • Ventral horn: Motor outputs and lower motor neuron cell bodies.
• White matter:
  • Dorsal column: Sensory information.
  • Lateral column: Motor and autonomic functions.
  • Anterior column: Motor outputs.

Spinal Cord - Sensory and Motor Information

• Sensory (afferent) information enters through dorsal horn, motor (efferent) from ventral horn.
• Information travels from the periphery (sensory nerves) to the spinal cord, then to nerve roots and ultimately to the nerves themselves

White Matter Pathways

• Anterolateral system:
  • Lateral spinothalamic tract
  • Anterior spinothalamic tract
• Spinocerebellar tracts: transmit sensory information to the cerebellum
• Dorsal column-medial lemniscus system:
  • Fasciculus gracilis
  • Fasciculus cuneatus

Spinal Cord Longitudinal Organization

• Regional differences in spinal cord structure: Cervical and lumbar regions have more gray matter due to increased need for arm/leg function.
• Thoracic region has lateral horns for containing sympathetic nervous system.
• White matter decreases as tracts disperse during descent from cervical to sacral regions.

Spinal Cord Blood Supply

• Anterior spinal artery: Supplies the anterior two-thirds of the spinal cord.
• Posterior spinal arteries: Supply the posterior one-third.
• Anterior spinal artery injuries often lead to motor deficits; posterior artery stroke impacts sensory tracts more.

Spinal Cord Movement Coordination and Reflexes

• Reflexes: Basic, withdrawal, and reciprocal inhibition reflexes.
• Stepping pattern generators: Spinal circuits that generate rhythmic movements like walking.
• Descending systems (Upper Motor Neurons): Plan, initiate, and direct voluntary movements.
• Stretch reflex (monosynaptic): Quick response to tendon tap (e.g. knee-jerk), involves muscle spindles and alpha motor neurons. -Involves reciprocal inhibition: Relaxation of the opposing muscle.

Reciprocal Inhibition

• Antagonist muscle opposition is inhibited through inhibitory interneurons (e.g., during a stretch reflex).
• Muscle synergies: Coordinated action of functionally linked muscles.
• Lesions or diseases, such as amyotrophic lateral sclerosis (ALS), can lead to abnormal synergies and movement patterns.

Flexor Withdrawal Reflex

• Protective reflex involving flexion of a limb in response to a noxious stimulus.
• Involves multiple spinal segments (polysynaptic).
• Persistence even after spinal cord injury.

Stepping Pattern Generators

• Adaptable networks of spinal interneurons generating alternating flexion/extension of limbs.
• Responsible for rhythmic, stereotypical movements (walking).
• Spinal cord can generate rhythmic movement patterns in the absence of higher-level brain input, although cortex is needed for complex behaviors like balance coordination and preventing unwanted movements.

Peripheral Nervous System

• Upper Motor Neurons (UMN) vs. Lower Motor Neurons (LMN):
  • UMN lesions cause global weakness, hypertonia, hyperreflexia.
  • LMN lesions result in focal weakness, hypotonia, hyporeflexia, atrophy.
• Peripheral nerves:
  • Located outside brain and spinal cord.
  • Contain myelinated (Schwann cells) and unmyelinated axons.
  • Lesions lead to focal weakness or sensory loss (compared to diffuse loss associated with CNS lesions).
• Peripheral nerve classification:
  • Size and myelination of axon determine conduction velocity.
  • Carry sensory, motor, or both functions (somatic or autonomic).

Dermatomes

• Area of skin innervated by axons entering the spinal cord through a single dorsal root.
• Help specify location and extent of nerve damage, specifically after spinal cord or traumatic vertebral column injuries.

Myotomes

• Specific spinal nerves responsible for specific muscle actions.

Amyotrophic Lateral Sclerosis (ALS)

• Progressive neurodegenerative disease, unknown cause.
• Loss of muscle strength/motor neuron control.
• Symptoms of both UMN and LMN disease.
• Fatal, with few living more than a decade post-diagnosis.
• Supportive treatments, some medications.

Spinal Cord Regeneration

• Peripheral nerves: Can regenerate, with Schwann cells promoting regrowth.
• Central nervous system (CNS): Regeneration is limited due to glial scarring, lack of suitable growth factors.
• CNS has poor ability to regenerate when injured, in contrast to the peripheral nervous system.