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What’s the motor system?
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Parts of CNS and PNS specialized for control of
limb, trunk, and eye movements |
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Also holds us together |
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From simple reflexes (knee jerk) to
voluntary movements (96mph fast ball) |
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Remarkable: Muscles only contract |
Plan
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Components of the motor systems |
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Focus on spinal control of limbs and
trunk |
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Same principles apply to to head
control via brain stem |
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Basic principles of movement control |
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What is helpful for understanding basic
motor system organization |
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Motor programs for voluntary movement |
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Descending motor pathways |
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Note about motor system’s bad rep… |
Motor systems
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Hierarchical & Parallel
Organization of the motor systems
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Top down organization of the motor
pathways--opposite that of sensory paths |
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Subcortical motor centers--cerebellum
& basal ganglia--access cortical motor areas via the thalamus (not just
sensory) |
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Organization of multiple subcortical
and cortical motor circuits-reminiscent of parallel sensory pathways |
Organization of Movements
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Hierarchical: 3 major types |
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Reflexes |
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Postural adjustments |
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Voluntary movements |
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…from simple to complex |
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Diverse & adaptive |
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Purposeful |
Organization of Movements
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Hierarchical: 3 major types |
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Reflexes Spinal cord circuits |
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Postural adjustments Spinal &
Brain stem |
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Voluntary movements Spinal cord, Brain
stem, & cortex |
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Reflexes
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Stimulus-evoked involuntary muscle
contraction |
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Monosynaptic (+) reflex |
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Knee-jerk |
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Jaw-jerk |
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Simple neural representation (circuit) |
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Knee Jerk
Reflexes
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Stimulus-evoked involuntary motor
muscle contraction |
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Monosynaptic (+) reflex |
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Knee-jerk |
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Jaw-jerk |
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Disynaptic reflex (+) |
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withdrawal |
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Why Disynaptic?
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Greater control (neural gate) |
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Very simple context |
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More complex response |
Spinal Circuits
Spinal Circuits
Motor I/O
Motor I/O
Postural adjustments
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Context important |
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Can reorganize depending on context |
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Feedback control-reactive |
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Error correction |
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Response lags stimulus; sometimes too
late; sometimes vicious circle |
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Feed-forward control-predictive |
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Response anticipates stimulus |
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More timely, but depends on practice |
Voluntary movements
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Organized around purposeful acts |
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Flexible input-output relationships |
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Limitless |
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Price to pay: whole brain |
Voluntary movements
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Organized around purposeful acts |
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Flexible input-output relationships |
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Limitless |
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Price to pay: whole brain |
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Recruits all motor systems components
& much of the association cortex |
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The goal of voluntary
movements is represented… somewhere
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Motor equivalence |
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Individual motor actions share
important characteristics even when performed in different ways |
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Abstract representation; effector
independent |
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Hand writing |
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Soccer |
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Goal representation |
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??Association & Premotor cortex |
Voluntary movements are
organized by motor programs
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Translate goal into action |
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Formation of a movement representation,
or
motor program |
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??Premotor cortex --> Primary motor
cortex |
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Program |
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To produce the desired goal, which
muscles should contract and when |
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2 Key movement characteristics that are
programmed |
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Spatial (hand path; joint angles) Kinematic
program |
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Force Dynamic program |
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Kinematic & Dynamic
Programs
in Reaching
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Reach to target--(Sensation to Action) |
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Visual cortex-->Association
cortex-->Premotor-->1° motor |
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Distinct kinematic and dynamic programs |
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Reach up |
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Against gravity |
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More force to achieve goal |
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Reach down |
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Gravity assists |
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Less force to achieve goal |
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Flexible control |
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Summary
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Motor behavior hierarchy |
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Reflexes |
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Postural adjustments |
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Voluntary movements |
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Internal/neural representations |
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Reflexes simple; invariant |
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Postural adjustments |
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Voluntary movements complex; flexible |
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Voluntary movements |
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Goal representation |
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Kinematic and dynamic programs |
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No wonder why voluntary movement
recruit entire
motor system |
Motor systems
Motor systems
Motor Pathways Hierarchy
Motor pathways organized
around the motor nuclei
Spinal Motor Columns
Ventral
Horn
Organization:
Proximal - distal rule
Ventral
Horn
Organization:
Proximal - distal rule
Brain Stem Motor Paths
Brain Stem Pathways
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Lateral |
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Rubrospinal tract: distal limb control;
crude |
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Medial |
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Tectospinal tract: eye-head
coordination |
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Reticulospinal tract: automatic
postural adjustments and movements (hip; shoulder) |
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Vestibulospinal tract: balance (axial
muscles); automatic postural adjustments |
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Cortical
Motor
Paths
Cortical motor paths
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Lateral corticospinal tract |
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Limb control mostly |
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Ventral corticospinal tract |
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Proximal muscle control; mostly upper
body |
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For cranial muscle control:
Corticobulbar tract |
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with medial and lateral components |
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Why bother study the motor
pathways?
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Anatomical substrates: How it works |
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Multiple parallel paths & diversity
of spinal connections |
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Damage to 1° motor cortex and pre-motor
cortex projections recover some lost functions |
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Damage to cortex and brain stem paths
recover some lost functions |
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With spinal cord injury. loss of
monosynaptic connections and alternate paths via segmental and intersegmental
interneurons can recover some lost functions |
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