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The Motor Systems

The Motor Systems. What’s 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. Plan.

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The Motor Systems

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  1. The Motor Systems

  2. What’s 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

  3. 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…

  4. Cortical motor areas Basal ganglia Cerebellum Descendingcortical motor paths Descendingbrain stem paths Spinal cord: Intermediate zone Ventral horn MotorSystems Motor systems Muscle

  5. Functional Hierarchy of Motor Paths Cortical motor areas

  6. Functional Hierarchy of Motor Paths Cortical motor areas

  7. Functional Hierarchy of Motor Paths Motor execution: force & direction

  8. Functional Hierarchy of Motor Paths Motor execution: force & direction

  9. Parallel Organization Association & limbic cortex Basal ganglia Cerebellum

  10. Motor Cortical areas Internal capsule from Basal ganglia from Cerebellum

  11. Hierarchical & Parallel Organization of the motor systems • Top down organization of the motor pathways--opposite 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

  12. Organization of Movements • Hierarchical: 3 major types • Reflexes • Postural adjustments • Voluntary movements • …from simple to complex • Diverse & adaptive • Purposeful

  13. Organization of Movements • Hierarchical: 3 major types • Reflexes Spinal cord circuits • Postural adjustments Spinal & Brain stem • Voluntary movements Spinal cord, Brainstem, & cortex Postural adjustments & voluntary movementsdepend more on cerebellar and basal gangliafunction than reflexes Dual purpose: 1) upcoming lectures; 2) context for motor pathways

  14. Reflexes • Stimulus-evoked involuntary muscle contraction • Monosynaptic (+) reflex • Knee-jerk • Jaw-jerk • Simple neural representation (circuit)

  15. Knee Jerk From musclestretch receptors Ventralhorn to muscle

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

  17. Why Disynaptic? • Greater control (neural gate) • Very simple context • More complex response

  18. Intermediatezone Greater control: Spinal Circuits from periphery from highercenters Response inhibitedby inhibition to muscle

  19. Greater control: Spinal Circuits from periphery from highercenters Response blockedby inhibition to muscle

  20. Motor I/O Knee-jerk

  21. Motor I/O Knee-jerk Automaticpostural adjustments • Balance • Limb support • Flexible than reflexes (greater #; each w/control) • Constrained than voluntary

  22. Postural adjustments • Context important • Can reorganize depending on context • Feedback control-reactive • Error correction • Response lags stimulus; sometimes too late; sometimes vicious circle • Feed-forward control-predictive • Response anticipates stimulus • More timely, but depends on practice • Depends on cerebellum,brain stem pathways & spinal cord • More complex neural representation

  23. Voluntary movements • Organized around purposeful acts • Flexible input-output relationships • Limitless • Price to pay: whole brain

  24. Voluntary movements • Organized around purposeful acts • Flexible input-output relationships • Limitless • Price to pay: whole brain • Recruits all motor systems components & much of the association cortex Discuss: • Goal representation • Motor programs

  25. 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

  26. Voluntary movements are organized 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

  27. 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

  28. 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

  29. Cortical motor areas Basal ganglia Descendingcortical motor paths Descendingbrain stem paths Spinal cord: Intermediate zone Ventral horn MotorPathways Motor systems Cerebellum Muscle

  30. Cortical motor areas Descendingcortical motor paths Descendingbrain stem paths Spinal cord: Intermediate zone Ventral horn MotorPathways Motor systems 1° motor cortex Premotor cortex Red nucleusReticular formationVestibular nuclei Superior colliculus Muscle

  31. Indirect Direct Origins of motor paths Motor Pathways Hierarchy Premotorareas Motor Pathways Hierarchy

  32. Motor pathways organized around the motor nuclei

  33. From brain Segmental interneuron Motor neuron Spinal Motor Columns Short Long Propriospinal-- Intersegmental-- neurons Motor columns (motor neurons)

  34. Lateral pathways: limb control Ventral HornOrganization:Proximal - distal rule Medial pathways: trunk control

  35. Lateral pathways: limb control Ventral HornOrganization:Proximal - distal rule Medial pathways: trunk control

  36. Tectum Red nucleus Reticularformation Vestibularnuclei Tectospinaltract Reticulospinaltracts Rubrospinal tract Vestibulospinaltracts Bilateral Contralateral Brain Stem Motor Paths Medial Lateral

  37. 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

  38. Brain stem nuclei Superior colliculus Tectospinal tract Red nucleus Rubrospinal tract Vestibular nuclei Vestibulospinal tracts Reticular formation Reticulospinal tracts

  39. Vestibular & Reticular nuclei Red nucleus Rubro- spinal tract Medialbrain stempaths Bilateral Contralateral Medial Lateral CorticalMotor Paths Pyramidal X LateralCortico-spinaltract Ventralcorticospinaltract

  40. 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

  41. Primary motor Lateral and ventral CST Corticobulbar tract Lateral Brain View

  42. Cortical Motor Areas

  43. Primary motor PMC Lateral and ventral CST Corticobulbar tract Lateral Brain View

  44. SMA Primary motor CMA Lateral and ventral CSTs Corticobulbar tract MidSagittal Brain

  45. 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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