The Motor Systems Whats the motor system? Parts of CNS and PNS - - PDF document

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The Motor Systems Whats the motor system? Parts of CNS and PNS - - PDF document

Oct 13, 2023 257 likes •433 views

The Motor Systems 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)

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The Motor Systems

What’s the motor system?

  • Parts of CNS and PNS specialized for control
  • f limb, trunk, and eye movements
  • Also holds us together
  • From simple reflexes (knee jerk) to voluntary

movements (96mph fast ball)

  • Remarkable: Muscles only contract

Plan

  • Components of the motor systems

– Focus on spinal control of limbs and trunk – Same principles apply to to head control via brain stem

  • Basic principles of movement control

– What is helpful for understanding basic motor system organization

  • Motor programs for voluntary movement
  • Descending motor pathways
  • Note about motor system’s bad rep…
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Motor Systems

Cortical motor areas Basal ganglia Cerebellum Descending brain stem paths Descending cortical motor paths Spinal cord: Intermediate zone Ventral horn Muscle

Functional Hierarchy of Motor Paths

Cortical motor areas

Functional Hierarchy of Motor Paths

Cortical motor areas

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Functional Hierarchy of Motor Paths

Motor execution: force & direction

Functional Hierarchy of Motor Paths

Motor execution: force & direction

Parallel Organization

Basal ganglia Cerebellum

Association & limbic cortex

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from Cerebellum Internal capsule Motor Cortical areas from Basal ganglia

Hierarchical & Parallel Organization

  • f the motor systems
  • Top down organization of the motor pathways--
  • pposite that of sensory paths
  • Subcortical motor centers--cerebellum & basal

ganglia--access cortical motor areas via the thalamus (not just sensory)

  • Organization of multiple subcortical and cortical

motor circuits-reminiscent of parallel sensory pathways

Organization of Movements

  • Hierarchical: 3 major types

– Reflexes – Postural adjustments – Voluntary movements

  • …from simple to complex
  • Diverse & adaptive

– Purposeful

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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, & cortex

Postural adjustments & voluntary movements depend more on cerebellar and basal ganglia function than reflexes Dual purpose: 1) upcoming lectures; 2) context for motor pathways

Reflexes

  • Stimulus-evoked involuntary muscle

contraction

  • Monosynaptic (+) reflex

– Knee

  • jerk

– Jaw

  • jerk
  • Simple neural representation (circuit)

Knee Jerk

From muscle stretch receptors to muscle

Ventral horn

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Reflexes

  • Stimulus-evoked involuntary motor

muscle contraction

  • Monosynaptic (+) reflex

– Knee

  • jerk

– Jaw

  • jerk
  • Disynaptic reflex (+)

– withdrawal

Why Disynaptic?

  • Greater control (neural gate)

– Very simple context

  • More complex response

from periphery from higher centers to muscle Response inhibited by inhibition

Greater control:

Intermediate zone

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from periphery from higher centers to muscle Response blocked by inhibition

Greater control:

Motor I/O

Knee-jerk

Motor I/O

Knee-jerk Automatic postural adjustments Flexible than reflexes (greater #; each w/control) Constrained than voluntary

  • Balance
  • Limb support
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Postural adjustments

  • Context important

– Can reorganize depending on context

  • Feedback control
  • r

eactive

– Error correction – Response lags stimulus; sometimes too late; sometimes vicious circle

  • Feed
  • forward control
  • p

redictive

– Response anticipates stimulus – More timely, but depends on practice

  • Depends on cerebellum, brain stem

pathways & spinal cord

  • More complex neural representation

Voluntary movements

  • Organized around purposeful acts
  • Flexible input
  • utput relationships

– Limitless – Price to pay: whole brain

Voluntary movements

  • Organized around purposeful acts
  • Flexible input
  • utput relationships

– Limitless – Price to pay: whole brain

  • Recruits all motor systems components &

much of the association cortex Discuss:

  • Goal representation
  • Motor programs
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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
  • ??Association & Premotor cortex

Voluntary movements are

  • rganized by motor programs
  • Translate goal into action

– Formation of a movement representation, or motor program

  • ??Premotor cortex --> Primary motor cortex
  • Program

– To produce the desired goal, which muscles should contract and when

  • 2 Key movement characteristics that are

programmed

– Spatial (hand path; joint angles) Kinematic program – Force Dynamic program

Kinematic & Dynamic Programs in Reaching

  • Reach to target--(Sensation to Action)

– Visual cortex-->Association cortex-->Premotor-->1° motor

  • Distinct kinematic and dynamic programs

– Reach up

  • Against gravity
  • More force to achieve goal

– Reach down

  • Gravity assists
  • Less force to achieve goal

– Flexible control

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Summary

  • Motor behavior hierarchy

– Reflexes – Postural adjustments – Voluntary movements

  • Internal/neural representations

– Reflexes simple; invariant – Postural adjustments – Voluntary movements complex; flexible

  • Voluntary movements

– Goal representation – Kinematic and dynamic programs – No wonder why voluntary movement recruit entire motor system

Motor Pathways

Cortical motor areas Basal ganglia Cerebellum Descending brain stem paths Descending cortical motor paths Spinal cord: Intermediate zone Ventral horn Muscle

Motor Pathways

Cortical motor areas Descending brain stem paths Descending cortical motor paths Spinal cord: Intermediate zone Ventral horn Muscle 1° motor cortex Premotor cortex Red nucleus Reticular formation Vestibular nuclei Superior colliculus

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Motor Pathways Hierarchy

Direct Indirect

Premotor areas

Origins of motor paths

Motor pathways organized around the motor nuclei

Motor columns (motor neurons) Propriospinal-- Intersegmental-- neurons

Spinal Motor Columns

Short Long

From brain Segmental interneuron Motor neuron

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Ventral Horn Organization: Proximal

  • distal

rule

Lateral pathways: limb control Medial pathways: trunk control

Ventral Horn Organization: Proximal

  • distal

rule

Lateral pathways: limb control Medial pathways: trunk control

Brain Stem Motor Paths

Red nucleus Rubrospinal tract Tectum Tectospinal tract Reticular formation Reticulospinal tracts Vestibulospinal tracts Vestibular nuclei Medial Lateral Bilateral Contralateral

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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

Brain stem nuclei

Red nucleus Rubrospinal tract Vestibular nuclei Vestibulospinal tracts Reticular formation Reticulospinal tracts Superior colliculus Tectospinal tract Ventral corticospinal tract Lateral Cortico- spinal tract

Pyramidal X Red nucleus Rubro- spinal tract Vestibular & Reticular nuclei Medial brain stem paths

Medial Lateral

Cortical Motor Paths

Bilateral Contralateral

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Cortical motor paths

  • Lateral corticospinal tract

– Limb control mostly

  • Ventral corticospinal tract

– Proximal muscle control; mostly upper body

  • For cranial muscle control:

Corticobulbar tract

– with medial and lateral components

Primary motor Lateral and ventral CST Corticobulbar tract

Cortical Motor Areas

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PMC Lateral and ventral CST Corticobulbar tract Primary motor Primary motor SMA CMA Lateral and ventral CSTs Corticobulbar tract

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 – 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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