129. Descending Pathways – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Descending pathways carry motor commands from the brain to the spinal cord, controlling voluntary movement, posture, muscle tone, and balance. The lateral system mainly controls skilled movements of distal muscles, whereas the medial system regulates posture and proximal muscle activity.
• Descending pathways are functionally classified into:
  • Lateral system: Lateral corticospinal and rubrospinal tracts. They terminate on lateral motor neuron pools and control fine, skilled movements, especially of the hand and fingers.
  • Medial system: Anterior corticospinal, reticulospinal, vestibulospinal, and tectospinal tracts. They terminate on medial motor neuron pools and regulate posture, muscle tone, balance, and movements of the trunk, head, neck, and proximal limbs.
• The corticospinal tract is the principal pathway for voluntary motor control.
  • It originates mainly from the primary motor cortex (Area 4), with additional fibers from premotor, supplementary motor, and sensory cortices.
  • Fibers pass through the corona radiata → internal capsule → cerebral peduncle → pons → medullary pyramids.
  • About 80% of fibers cross in the pyramidal decussation to form the lateral corticospinal tract, while 20% continue as the anterior corticospinal tract.
  • The lateral tract controls distal limb muscles, whereas the anterior tract controls axial and proximal muscles involved in posture.
• Internal capsule is the most common site of corticospinal tract lesions because fibers are densely packed. Lesions above the pyramidal decussation produce contralateral hemiplegia with upper motor neuron (UMN) signs.
• Features of UMN paralysis include spasticity, hypertonia, exaggerated deep tendon reflexes, positive Babinski sign, loss of superficial reflexes, and minimal muscle atrophy. These occur because descending inhibitory control over spinal reflexes is lost, increasing gamma motor neuron activity and muscle spindle sensitivity.
• Rubrospinal tract originates from the red nucleus, crosses immediately, descends in the lateral funiculus, facilitates flexor muscles, and assists the corticospinal tract in skilled upper limb movements.
• Vestibulospinal tracts arise from the vestibular nuclei.
  • Lateral vestibulospinal tract activates antigravity extensor muscles and maintains posture and balance.
  • Medial vestibulospinal tract regulates head and neck posture during head movements. Excessive activity contributes to decerebrate rigidity.
• Reticulospinal tracts originate from the reticular formation.
  • Pontine reticulospinal tract facilitates extensor muscles and maintains posture.
  • Medullary reticulospinal tract inhibits extensor activity, preventing excessive muscle tone.
  • Together, they regulate muscle tone, posture, and automatic movements.
• The tectospinal tract arises from the superior colliculus and mediates reflex head and neck movements toward visual stimuli. Monoaminergic pathways (raphespinal and ceruleospinal tracts) modulate pain transmission and spinal motor activity.
• Lower motor neuron (LMN) paralysis is characterized by flaccid paralysis, hypotonia, marked muscle wasting, fasciculations, reduced or absent tendon reflexes, and normal plantar response. Sensory loss may accompany peripheral nerve lesions.
• Patterns of paralysis include monoplegia, hemiplegia, paraplegia, quadriplegia (tetraplegia), and isolated paralysis, depending on the site of the lesion.