Motor Neurons

Motor Neurons

1. Lower Motor Neurons

   • responsible for the generation of force by the muscles they innervate
   • Motor Unit
     • a single LMN and all of the muscle fibers it innervates
     • Motor Neuron Pool
       • collection of LMNs that innervate a single muscle
   • respond to inputs from three sources: dorsal root ganglia, spinal interneurons, projections from higher brain centers
   • Central Pattern Generator
     • example is flexion and extension of the quadriceps muscle while walking: +when some cells are active, others are inhibited... these cells lie in the ventral horn on the same side of the spinal cord (include the flexor and extensor motor neurons as well as interneurons) +descending inputs from higher levels provide continuous input to excitatory interneurons
     • neuronal network capable of generating a rhythmic pattern of motor activity
       • normally controlled by higher centers in the brain stem and cortex

2. Descending Pathways

   • Dorsolateral system
     • control of distal musculature
       • +involved in finer motor control of distal muscles (i.e. finger movements)
       • axons terminate in only one or two spinal segments, allowing precise control over muscle groups
     • Lateral Corticospinal Tract
       • +especially important for control of distal musculature and steering the extremities and manipulating the environment 
     • Rubrospinal Tract
   • Ventromedial system
     • control of axial and postural musculature
       • distribute collaterals to many segments along their distribution in the spinal cord
         • +allows for the coordination of intersegmental movements  +well-suited for the control of groups of muscles involved in posture and balance
     • tecto- and vestibulospinal tracts
     • pontine and medullary reticulospinal tracts
       • PRST enhances the anti-gravity reflexes of spinal cord
         • +excites upper limb flexors and lower limb extensors
       • MRST inhibits anti-gravity reflexes of spinal cord
         • +inhibits upper limb flexors and lower limb extensors

3. Primary Motor Cortex

   • +also called area 4 of brodmann and motor I
   • Upper Motor Neurons
   • provides the descending signals to execute movements
     • corticospinal neurons project directly to LMNs in spinal cord