Lecture Notes: Chapter 13 Part 2 - Spinal Cord and Reflexes

Overview

• Continuation of Chapter 13 focusing on spinal cord reflexes and descending motor tracts.
• Part 1 covered ascending sensory tracts and spinal cord structure.
• This session covers the descending motor pathways and spinal reflexes.

Key Topics

• Descending Motor Tracts
  • Carry motor commands from cerebral cortex to skeletal muscles.
  • Known as somatic efferent pathways.
  • Involve both upper and lower motor neurons.

Upper and Lower Motor Neurons

• Upper Motor Neurons (UMN):

  • Arise from the cerebral cortex.
  • Travel down through the brain (midbrain, medulla oblongata) to brainstem.
  • Synapse in the spinal cord at the ventral gray horn.

• Lower Motor Neurons (LMN):

  • Soma located in the ventral gray horn of the spinal cord.
  • Axons exit through the ventral root to form spinal nerves that innervate skeletal muscles.

Corticospinal Tracts

• Anterior Corticospinal Tract:
  • Responsible for 15% of neurons that do not decussate at the medullary pyramids.
• Lateral Corticospinal Tract:
  • 85% of neurons that decussate at the medullary pyramids.
• Corticobulbar Tract:
  • Connects upper motor neurons directly to the cranial nerve nuclei in the brainstem.

Direct vs Indirect Pathways

• Direct Pathways:

  • Corticospinal tracts that carry motor commands directly from cerebral cortex to spinal cord or cranial nerve nuclei.

• Indirect Pathways (Medial and Lateral Pathways):

  • Involve multiple synapses in basal nuclei, red nucleus, and reticular formation before reaching the lower motor neurons.
  • Provide complex regulation of motor commands and coordination.

Indirect Pathways Details

• Vestibulospinal Tract:

  • Originates from the vestibular apparatus.
  • Controls balance and equilibrium reflexes.

• Tectospinal Tract:

  • Originates from colliculi in midbrain.
  • Reflexive head and neck movements in response to visual or auditory stimuli.

• Reticulospinal Tract:

  • From reticular formation.
  • Subconscious regulation of posture and balance.

• Rubrospinal Tract:

  • From red nucleus.
  • Coordinates distal upper limb movements.

Spinal Cord Reflexes

Reflex Arcs

• Monosynaptic Reflex Arc:

  • Involves a single synapse between a sensory and a motor neuron.
  • Example: Stretch reflex triggered by muscle spindle.

• Polysynaptic Reflex Arc:

  • Involves one or more interneurons.
  • Example: Golgi tendon reflex, withdrawal reflex.

Specific Reflexes

• Stretch Reflex:

  • Activated by muscle spindle detecting unexpected stretch.
  • Results in muscle contraction.

• Golgi Tendon Reflex:

  • Protects muscles from excessive tension by inhibiting contraction.

• Withdrawal Reflex:

  • Responds to painful stimulus by contracting flexor muscles and inhibiting extensors.

• Crossed Extensor Reflex:

  • Complements the withdrawal reflex by extending the opposite limb to maintain balance.

General Concepts

• Interneuron Functionality:
  • Critical for complex reflexes involving reciprocal inhibition and contralateral innervation.
• EPSP and IPSP:
  • Excitatory and inhibitory postsynaptic potentials that affect motor neuron activation.

Conclusion

• This chapter provides a basis for understanding motor pathways and reflex physiology.
• Sets the stage for exploring brain functions in subsequent chapters.