Motor Pathways

The Motor/Pyramidal system 

  • The cerebral cortex acts to produce a motor action through fiber tracts 

  • The motor impulses originate from: 

    • Precentral gyrus from upper motor neurons which stimulate 

    • Lower motor neurons located in the ventral horn of the spinal cord 

    • It’s a two neuron system 

  • The cerebellum and basal ganglia influence lower neurons through both descending pathways or modulation of the cerebral cortex 

 

  1. Stimulation of the primary motor cortex results in UMNs motor impulses being sent to LMNs 

  2. Motor neurons in the cerebellum or basal ganglia also send impulses to LMNs 

  3. UMN axons pass without interruption to LMNs in the spinal cord on the contralateral side of the body 

  4. LMNs then send their axons through spinal nerves directly to individual muscles 

 

Somatic motor pathways 

  • Precentral gyrus: primary motor cortex contains cell of bodies of UMN 

  • Lower motor neuron cell bodies found in the brainstem (cranial nerves) or anterior horn of spinal cord 

 

Motor Regions 

  • Precentral gyrus (primary motor area): 

    • Controls: 

      • Speed of movement 

      • Force of muscle contraction 

      • Direction of movement 

  • Premotor area (anterior to precentral gyrus):  

    • Planning of movements 

 

Descending Corticospinal Tract 

  • UMN fibers run in the descending tract to the LMNs 

  • Corticospinal tract in the primary descending tract 

    • Crosses over at junction of brainstem and spinal cord 

    • Synapses with LMN in ventral horn of spinal cord 

 

Motor neuron injury 

  

UMN defect 

LMN defect 

Spastic paralysis 

Flaccid paralysis 

No muscle atrophy 

Significant atrophy 

Fasciculations and fibrillations not present 

Fasciculations and fibrillations present 

Hyperreflexia 

hyporeflexia 

Babinski reflex present 

Babinski reflex not present 

 

Basal ganglia 

  • Complex collection of strongly interconnected cell bodies within the brain 

  • Important role in initiating voluntary movements and control postural adjustments  

  • Consists of strongly interconnected nuclei that influence motor function via feedback to cortex 

 

Cerebellum 

  • - Affects voluntary muscle contractions & Influences timing and force of contraction of voluntary muscle 
    - Influences motor function via feedback loop to cortex and spinal cord 
    - Site of motor learning 
     
    --Posture and balance 
    --Sequential movements (eating, dressing, writing) 
    --Rapidly alternating repetitive movements 
    --Smooth pursuit movements 
    --Properties of movement like acceleration, velocity, trajectory