Lecture Notes: Spinal Cord and Reflexes

Overview

• Follow-up to part one of Chapter 13.
• Focus on descending motor tracts and spinal cord reflexes.

Descending Motor Tracts

Motor Commands Pathways

• Descending pathways carry motor commands from the cerebral cortex to skeletal muscles.
• Somatic Efferent Pathways: Part of the peripheral nervous system.

Neurons Involved

• Upper Motor Neurons:
  • Originate from the cerebral cortex.
  • Descend through the brain, brainstem (midbrain, medulla oblongata), and spinal cord.
• Lower Motor Neurons:
  • Soma located in ventral gray horn of the spinal cord.
  • Axons exit via the ventral root to skeletal muscle.

Corticospinal Pathway

• Three Tracts:
  • Anterior corticospinal tract.
  • Lateral corticospinal tract.
  • Corticobulbar tract.
• Corticobulbar Tract:
  • Involves cranial nerves.
  • Upper motor neurons terminate in cranial nerve nuclei.
• Corticospinal Tracts:
  • Anterior and lateral tracts.
  • Decussation at the medullary pyramids (85% decussate = lateral, 15% don't = anterior).

Direct and Indirect Pathways

Direct Pathways

• Corticospinal pathways: direct connection from cortex to lower motor neuron.

Indirect Pathways

• Medial and lateral pathways.
• Involves additional brain structures (e.g., basal nuclei, red nucleus, substantia nigra).
• Examples: Rubrospinal tract, Reticulospinal tract.

Reflexes

Reflex Arc Basics

• Reflex: involuntary response to a stimulus.
• Components: sensory input, central processing, motor output.

Types of Reflexes

Monosynaptic Reflex (Stretch Reflex)

• Simplest reflex arc with a single synapse.
• Example: Patellar reflex.
• Muscle Spindle: Sensory receptor in muscle, senses stretch.
• Fastest reflex due to single synapse.

Polysynaptic Reflex (Golgi Tendon Reflex)

• Prevents muscle from excessive force.
• Involves inhibitory interneurons.
• Golgi Tendon Organ: Sensory receptor within tendon, detects tension.
• Inhibits motor neuron to prevent muscle damage.

Withdrawal Reflex

• Reaction to painful stimuli (e.g., touching a hot object).
• Involves reciprocal inhibition to prevent antagonist muscle contraction.

Crossed Extensor Reflex

• Complements withdrawal reflex in the legs.
• Involves contralateral extensor contraction for balance.

Conclusion

• Understanding motor pathways and reflexes highlights the complex interplay in the nervous system.
• Sets the stage for learning about brain interconnections in the next chapter.