Motor Pathways

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The Motor Systems
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Whats the motor system?

• Parts of CNS and PNS specialized for control of
  limb, trunk, and eye movements
• Also holds us together
• From simple reflexes (knee jerk) to voluntary
  movements (96mph fast ball)
• Remarkable Muscles only contract

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Plan

• Components of the motor systems
• Basic principle of movement control
• What is helpful for understanding basic motor
  system organization
• Motor programs
• Descending motor pathways

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MotorSystems
Motor systems
Muscle
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Functional Hierarchy of Motor Paths
Motor execution force
direction
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Parallel Organization
Basal ganglia
Cerebellum
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(No Transcript)
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Organization of Movements

• Diverse adaptive
• Hierarchical 3 major types
• Reflexes
• Postural adjustments
• Voluntary movements

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Organization of Movements

• Hierarchical 3 major types
• Reflexes Spinal cord circuits
• Postural adjustments Spinal Brain stem
• Voluntary movements Spinal cord, Brain stem,
  and cortex

Postural adjustments voluntary movementsdepend
more on cerebellar and basal gangliafunction
than reflexes
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Reflexes

• Stimulus-evoked involuntary muscle contraction
• Monosynaptic () reflex
• Knee-jerk
• Jaw-jerk

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Knee Jerk
From musclestretch receptors
to muscle
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Reflexes

• Stimulus-evoked involuntary motor muscle
  contraction
• Monosynaptic () reflex
• Knee-jerk
• Jaw-jerk
• Disynaptic reflex ()
• withdrawal

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Why Disynaptic?

• Greater control (gate)
• Very simple context
• More complex response

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Spinal Circuits
Greater control
from periphery
from highercenters
Response inhibitedby inhibition
to muscle
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Spinal Circuits
Greater control
from periphery
from highercenters
Response blockedby inhibition
to muscle
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Motor I/O
Knee-jerk
Automaticpostural adjustments
Balance Limb support

• Flexible than reflexes
• Constrained than voluntary

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Postural adjustments

• Context important
• Maintain balance--supported v/s unsupported
• Feedback control-reactive
• Error correction
• Response lags stimulus sometimes too late
• Feed-forward control-predictive
• Response anticipates stimulus
• More timely, but depends on internal models
• Practice, learning

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Voluntary movements

• Organized around purposeful acts
• Flexible input-output relationships
• Limitless
• Price to pay whole brain

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Motor I/O
Knee-jerk
Automaticpostural adjustments
Voluntarymovement
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Voluntary movements

• Organized around purposeful acts
• Flexible input-output relationships
• Limitless
• Price to pay whole brain

• Discuss
• Goal representation
• Motor program

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The goal of voluntary movements is represented
somewhere

• Motor equivalence
• Individual motor actions share important
  characteristics even when performed in different
  ways
• Abstract representation effector independent
• Hand writing
• Soccer
• Goal representation

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Voluntary movements are organized by motor
programs

• Translate goal into action
• Formation of a movement representation, or motor
  program
• Program
• To produce the desired goal, which muscles should
  contract and when
• 2 Key movement characteristics
• Spatial (hand path joint angles) Kinematic plan
• Forces/loads Dynamic plan
• All accomplished by contracting muscles

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Kinematic Dynamic Plans

• Reach to target
• Straight hand path
• Reach up
• Against gravity
• More force to achieve goal
• Reach down
• Gravity assists
• Less force to achieve goal

• Kinematic and dynamic representations of upcoming
  movement

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Summary

• Motor behavior hierarchy
• Reflexes
• Postural adjustments
• Voluntary movements
• Internal representations
• Reflexes and postural adjustments--simple
  invariant
• Voluntary movements--complex flexible
• Voluntary movements
• Kinematic and dynamic representations
• Goal representation

• Motor hierarchy
• spinal, brain stem, cortical motor circuits
• Descending motor pathways

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MotorSystems
Motor systems
1 motor cortex Premotor cortex
Red nucleusReticular formationVestibular
nuclei Superior colliculus
Muscle
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Motor Pathways Hierarchy
Premotorareas
Motor Pathways Hierarchy
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Spinal Motor Columns
Short
Long
Propriospinal-- Intersegmental-- neurons
Motor columns (motor neurons)
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Ventral HornOrganizationProximal - distal rule
Lateral pathways limb control
Medial pathways trunk control
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Brain Stem Motor Paths
Medial
Lateral
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Brain Stem Pathways

• Lateral
• Rubrospinal tract distal limb control crude
• Medial
• Tectospinal tract eye-head coordination
• Reticulospinal tract automatic postural
  adjustments and movements (hip shoulder)
• Vestibulospinal tract balance (axial muscles)
  automatic postural adjustments

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Brain stem nuclei
Superior colliculus Tectospinal tract
Red nucleus Rubrospinal tract
Vestibular nuclei Vestibulospinal tracts
Reticular formation Reticulospinal tracts
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CorticalMotor Paths
Medial
Lateral
Pyramidal X
LateralCortico-spinaltract
Ventralcorticospinaltract
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Cortical Motor Areas
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Primary motor
PMC
Lateral and ventral CST Corticobulbar tract
Lateral Brain View
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SMA
Primary motor
CMA
Lateral and ventral CSTs Corticobulbar tract
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Why bother study the motor pathways?

• Anatomical substrates How it works
• Multiple parallel paths diversity of spinal
  connections
• Damage to 1 motor cortex and pre-motor cortex
  projections recover some lost functions
• Damage to cortex and brain stem paths recover
  some lost functions
• Loss of direct connections and alternate paths
  can recover some lost functions