Descending Pathways

Descending Pathways

1. UMNs control LMN

   • UMN originate in the cerebral cortex or brainstem & descend to LMN which produce motor activity

2. UMN descent

   • Medial (anterior tracts): control axial muscles
   • Lateral (lateral column): distal muscles & precise movement

3. Medial descending pathways

   • Reticulospinal pathway
     • Receive input from almost all pathways; control bilateral postural muscles, all four limbs & reflexive head & movements to sound & signals
       • UMNs originate in reticular formation of brainstem, synapse in spinal cord & innervate many regions
   • Vestibulospinal pathway
     • Receive info from CNVIII about head & neck position. Medial: bilaterally control neck & upper back muscles. Laterally: ipsilaterally control postural muscles & maintain COG
     • UMNs arise in medulla & synapse with LMN in SC
   • Tectospinal pathway
     • Visual & auditory inputs; subconscious control of head, eye, back & upper limb position according to stimulus
     • UMNs arise from midbrain, decussate immediately & synapse with LMNs in brainstem or SC
   • Medial/anterior corticospinal pathway
     • Input from motor cortex; innervate axial & gross motor movements
     • UMNs descend through internal pathway, brainstem & anterior column of SC; decussates in target SC segment & synapses with LMN in ventral horn

4. Lateral descending pathways

   • Rubrospinal pathway
     • Receive input from cerebellum & motor cortex; small role in humans but can assist in upper limb control and tone if lateral CST is damaged
     • UMNs arise from red nucleus in the midbrain, decussate, descend in SC (cervical region only) & synapse with contralateral LMNs
   • Lateral corticospinal pathway
     • Main tract for control of VOLUNTARY limb movements; responsible for fractionation of movements; sometimes called the direct pathway
       • Arise from primary & secondary motor cortices & control contralateral movements
     • UMN located in primary motor cortex (grey matter), through internal capsule into medullary pyramids where it decussates but DOES NOT SYNAPSE
       • Descends in lateral column (white matter) & into segment of SC in ventral horn - exits SC via ventral root

5. Pyramidal pathway (aka direct pathway)

   • UMNs extend from cortex to brainstem or SC
   • Cross over at levels of the pyramids of the medulla
     • Extrapyramidal tracts: control involuntary movement
   • Pyramidal tracts: control voluntary movement