Descending Motor Pathways in the Spinal Cord

Types of Descending Pathways

• Divided into:
  • Pyramidal tracts
  • Extrapyramidal tracts

Pyramidal Tracts

• Travel through: Medullary pyramids of the medulla oblongata
• Originate from: Cerebral cortex
• Function: Voluntary control of muscles of the body and face

Components of Pyramidal Tracts

1. Corticospinal Tracts

   • Control body muscles
   • Pathway:
     • Cell bodies in cerebral cortex
     • Axons pass through internal capsule, crus cerebri in midbrain, pons, medulla oblongata
     • 75% decussate to opposite side (lateral corticospinal tract)
     • 25% remain ipsilateral (anterior corticospinal tract)
     • Synapse with lower motor neurons in ventral horns

2. Corticobulbar Tracts

   • Control head, face, neck muscles
   • Pathway:
     • Originate from cerebral cortex
     • Axons pass through internal capsule
     • Synapse with cranial nerve nuclei in brainstem
   • Unique Feature: Bilateral innervation (exceptions: lower third of face with cranial nerve VII, hypoglossal nerve)

Extrapyramidal Tracts

• Do not pass through: Medullary pyramids
• Function: Involuntary control, modulation of muscles (e.g., muscle tone, balance)
• Originate from: Brainstem

Components of Extrapyramidal Tracts

1. Rubrospinal Tract

   • Origin: Red nucleus in midbrain
   • Function: Fine motor control, primarily upper limbs
   • Innervation: Contralateral

2. Reticulospinal Tracts

   • Medial (Pontine) Reticulospinal Tract
     • Origin: Pontine reticular formation
     • Function: Facilitates voluntary movement, excites extensor muscles, increases muscle tone
   • Lateral (Medullary) Reticulospinal Tract
     • Origin: Medullary reticular formation
     • Function: Inhibits voluntary movement, decreases muscle tone, regulates automatic breathing

3. Vestibulospinal Tract

   • Origin: Vestibular nuclei
   • Function: Balance, posture
   • Innervation: Ipsilateral

4. Tectospinal Tract

   • Origin: Superior colliculus in midbrain
   • Function: Coordination of head and eye movements
   • Innervation: Contralateral

Characteristics of Descending Pathways

• No second neuron in CNS
• Signal Pathway: Cell body in cerebral cortex or brainstem, travels down spinal cord/brainstem to synapse with lower motor neuron

Lesions in Descending Pathways

Pyramidal Tract Lesions

• Potential damage sites: Spinal cord, internal capsule
• Symptoms of Lesions:
  • Hypertonicity (increased muscle tone)
  • Hyperreflexia (exaggerated reflexes)
  • Weakness
  • Clonus (involuntary muscle contractions)
  • Babinski's Sign (upward movement of big toe on stimulation)
  • Hoffmann's Sign (flicking middle finger causes thumb flexion/adduction)

Extrapyramidal Tract Lesions

• Symptoms:
  • Parkinsonism (rigidity, bradykinesia, tremors, postural deficits)
  • Chorea
  • Athetosis
  • Dystonia