Spinal and Supra-Spinal Motor Pathways Lecture PDF

Spinal and supra-spinal motor pathways Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Describe the major motor pathways of the central nervous system Explain the descending neural drive to lower motor neurons for volitional movements Understan...

Spinal and supra-spinal motor pathways Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Describe the major motor pathways of the central nervous system Explain the descending neural drive to lower motor neurons for volitional movements Understand hierarchical control of the motor system Major steps (control points) in pathway of Voluntary Movement Neural Muscular Motor cortex – 3 areas within frontal lobe Primary motor cortex or M1 Posterior portion of frontal lobe Projects long axons to spinal cord Some directly synapse onto αmotorneurons Others influence motor output indirectly through interneurons Homunculus representation Premotor areas Projects axons to M1 and spinal cord More complex actions ( selection motor plans) E.g. direction or velocity of movement Homunculus and the corona radiata lateral Homunculus Somatotopic arrangement Distorted map of the motor functions Movements not muscles Corona radiata Contained within internal capsule Connects cortical regions of motor cortex with brainstem and spinal cord Upper motor neurons 2 key motor tracts Cortico-spinal Connects to spinal motor neurons Cortico-bulbar Connects to cranial motor neurons medial Upper motor neuron pathway Motor tracts (bundles of axons) Pathways of motor fibres Origin and endpoint in the name E.g., corticospinal Lateral pathways (proximal and distal muscles of arms and legs) Lateral corticospinal tract Medial pathways (axial/trunk muscles) Anterior corticospinal Projection onto motor neurons Direct vs indirect Organization within spinal cord Medial (proximal muscles) – lateral (distal muscles) Anterior (flexors) – posterior (extensors) Many other tracts, all involved in the processing and modulation of motor pathways – acting through interneurons Corticospinal tract Motor cortex to spinal motor neurons Corona radiata projections Pyramidal tracts Majority of fibres (80-90%) decussate at the medullary pyramids Contra-lateral fibres - lateral corticospinal tract Remaining 10-20% form the anterior corticospinal tract Cross contra-laterally at the spinal segment they terminate Some of these fibres make DIRECT synapses with spinal motor neurons Critical for fine control of distal hand muscles Many other motor tracts Many project bilaterally onto spinal circuits More coordinated actions e.g., axial trunk muscles for balance Proximity of tracts to spinal motor neuron organization Lateral pathways Anterior-medial pathways Sub-cortical motor systems – basal ganglia and cerebellum Side loops of the motor system Influence the output of upper motor neurons, not direct modulation of spinal motor neurons Higher-order cognitive function and sensory integration Basal Ganglia Important for action initiation Lots of input from higher cortical regions Collection of 5 nuclei Caudate nucleus and putamen form striatum Globus pallidus, subthalamic nucleus, and substantia nigra Input travels to striatum and output comes from globus pallidus and substantia nigra Critical role in initiating voluntary movements Modulates motor cortex Cerebellum Negative feedback correction Detect difference in intended and actual movements Corrects for “motor error” based on current state of the system Feed-forward mechanism of adjustment Requires learning through trial and error Critical for balance and coordinated movements Large degree of sensory-motor processing Damage often leads to ataxia Challenge to study in humans Deep structures Feedforward correction How it all works together Example – central pattern generators Spinal circuit and higher order commands Internal circuits in spinal cord (interneuron loops) modulated/activated by higher order cortical centres Enable flexibility of movement and force output Highest levels of hierarchy issue the commands, lower level circuitry create specific muscle activation patterns BUT – some upper motor neurons (e.g., corticospinal) project directly to α-motor neurons Dextrous movements of fingers/hands Directional tuning of movement direction Upper motor neurons in M1 display preferential activation for movement direction Example – same neuron being recorded (left and right figure) But preferred direction is TOWARDS the monkey Need many cells (populations) to understand movement intention microelectrode microelectrode Some take home thoughts Direct and indirect pathways Upper motor neurons projecting onto spinal circuits i.e., connect to spinal motor neurons via interneurons Some upper motor neurons project DIRECTLY onto spinal motor neurons Lateral cortico-spinal tract – controlling digits of hand Flexibility Not rigid cords/tracts that directly control specific muscles Spinal cord included! Not just a rigid highway from higher-order centres Very complex – not well understood overall Even less so in humans NHP vs humans