The Nervous System: Central Nervous System

Involuntary Movement Through Reflexes

  • Reflex Arc: Neural pathway for a reflex.
    • Components: Sensory receptor \rightarrow Afferent neuron \rightarrow Integration center (CNS) \rightarrow Efferent neuron(s) \rightarrow Effector organ (muscle, gland).
  • Types of Reflexes: Classified as cranial/spinal, somatic/autonomic, innate/conditioned, and monosynaptic/polysynaptic.
  • Muscle Spindle Stretch Reflex: Monosynaptic (stretch) and polysynaptic (reciprocal inhibition of flexors), ipsilateral, somatic, innate, spinal.
  • Withdrawal and Crossed-Extensor Reflexes: Polysynaptic, involves stimulation of flexor/extensor and reciprocal inhibition of opposing muscles.
  • Pupillary Light Reflex: Cranial (CN II, III), autonomic, innate, polysynaptic. Regulates iris and pupil size.

Voluntary Motor Control

  • Voluntary Motor Action: Involves idea development, motor command programming, muscle activation, and feedback.
  • Innervation of Skeletal Muscle: Lower motor neuron originates in ventral horn/cranial nerve nuclei, forms neuromuscular junction (always excitatory).
  • Input to Motor Neurons: From afferent neurons, pyramidal tract neurons, and extrapyramidal tract neurons.
  • Pyramidal Tracts (Direct Pathway):
    • Function: Fine control of voluntary movement, especially distal extremities.
    • Lateral Tract: Upper motor neurons from primary motor cortex \rightarrow decussates in medulla \rightarrow contralateral control of distal extremities.
    • Anterior Tract: Upper motor neurons from primary motor cortex \rightarrow decussates in spinal cord \rightarrow contralateral control of axial muscles.
  • Extrapyramidal Tracts (Indirect Pathway):
    • Function: Support voluntary movement of proximal extremities, indirect input to motor neurons.
    • Rubrospinal Tract: Originates in red nuclei (midbrain) \rightarrow decussates in midbrain \rightarrow contralateral flexion of upper extremities.
    • Vestibulospinal Tract: Originates in vestibular nuclei \rightarrow some decussation in midbrain \rightarrow bilateral control of head, neck, lumbar muscles for posture/balance.
    • Tectospinal Tract: Originates in superior colliculi (midbrain) \rightarrow decussates in midbrain \rightarrow contralateral head/extraocular muscle movement for tracking.
    • Reticulospinal Tract: Originates in reticular formation \rightarrow mostly ipsilateral (some medullary decussation) \rightarrow pontine (medial) stimulates leg extensors; medullary (lateral) inhibits leg extensors for posture.
  • Motor Coordination:
    • Cerebellum: Provides feedback, integrates for fine-tuning, contributes to muscle tone and memory.
    • Basal Nuclei: Provides feedback, controls automatic learned motions, selects purposeful movements, excitatory/inhibitory regulation.
  • Disorders of Basal Nuclei:
    • Huntington's Chorea: Degenerative, loss of pathway from basal nuclei to thalamus, results in exaggerated involuntary motions.
    • Parkinson's Disease: Degenerative, lack of dopamine in substantia nigra, results in rigidity, tremors at rest, difficulty with movementinitiation/stopping.