Descending Pathways and Motor Tracts

Overview of Descending Pathways

• Descending pathways carry motor signals down the spinal cord.
• Divided into two main types:
  • Pyramidal Tracts
  • Extrapyramidal Tracts

Pyramidal Tracts

• Responsible for voluntary control of muscles (body and face).
• Originate in the cerebral cortex.
• Travel through the medullary pyramids of the medulla oblongata.

Components of Pyramidal Tracts

1. Corticospinal Tracts

   • Control body movements.
   • Pathway: Cerebral cortex → Internal capsule → Crust cerebri (midbrain) → Pons → Medulla oblongata.
   • Around 75% fibers decussate (cross) to the opposite side (lateral corticospinal tract).
   • 25% remain ipsilateral (anterior corticospinal tract) and decussate at cervical/thoracic levels.

2. Corticobulbar Tracts

   • Control voluntary movements of head, face, and neck.
   • Pathway: Cerebral cortex → Internal capsule → Synapse at lower motor neurons in brain stem.
   • Innervate bilaterally (except cranial nerve VII and hypoglossal nerve).
   • Stroke in this area may cause drooping in lower third of the face.

Extrapyramidal Tracts

• Do not pass through the pyramids.
• Responsible for involuntary control (e.g., muscle tone, balance).
• Originate in the brain stem.

Types of Extrapyramidal Tracts

1. Rubrospinal Tract

   • Originates: Red nucleus in midbrain.
   • Responsible for fine motor control, primarily affecting upper limbs.
   • Contralateral innervation.

2. Reticulospinal Tracts

   • Medial/Pontine Reticulospinal Tract: Originates from pontine reticular formation, facilitates voluntary movement.
   • Lateral/Medullary Reticulospinal Tract: Originates from medullary reticular formation, inhibits voluntary movement and decreases muscle tone.

3. Vestibulospinal Tract

   • Originates: Vestibular nuclei, receive inputs from cerebellum.
   • Involved in maintaining balance and posture.
   • Ipsilateral innervation.

4. Tectospinal Tract

   • Originates: Superior colliculus of midbrain.
   • Coordinates head and eye movements.
   • Provides contralateral innervation.

Lesions and Clinical Manifestations

• Pyramidal Tract Lesions: Result in deficits on the contralateral side of the body due to decussation.

  • Symptoms:
    • Hypertonicity (increased muscle tone)
    • Hyperreflexia (exaggerated reflexes)
    • Muscle weakness
    • Clonus (involuntary muscle contractions)
    • Babinski's sign: Dorsiflexion of the big toe upon lateral foot stimulation.
    • Hoffmann's sign: Finger flick test for upper limbs.

• Extrapyramidal Tract Damage: Can lead to Parkinsonism (rigidity, bradykinesia, tremors, postural deficits), chorea, athetosis, and dystonia.

Important Considerations

• Lesions can occur anywhere along the spinal cord or internal capsule.
• Internal capsule lesions impact both body and facial motor functions.
• They are a common site for strokes and are susceptible during hemorrhagic events.