Movement the motor system jun 13 2014
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Movement & the Motor System Jun 13, 2014. SENSORIMOTOR CONTROL. From Sensory to Motor. Sensory info -- guide motor system Somatosensory, vestibular, visual Book works from the TOP to BOTTOM Sensory “ up”, motor “ down” Here -- SIMPLE to COMPLEX . Organization. HIERARCHY

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Movement & the Motor System Jun 13, 2014

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Movement & the Motor SystemJun 13, 2014


SENSORIMOTOR CONTROL


From Sensory to Motor

  • Sensory info -- guide motor system

    • Somatosensory, vestibular, visual

  • Book works from the TOP to BOTTOM

    • Sensory “up”, motor “down”

  • Here -- SIMPLE to COMPLEX


Organization

HIERARCHY

  • Analogy to a corporation

    • Workers = muscles

    • Supervisors = spinal cord & brainstem

    • Quality control = basal ganglia & cerebellum

    • Manager = primary cortex

    • President/CEO = higher order cortex


When do you use your motor system?


What is actually “moving” in the “motor system” … ?


MOTOR UNITS


Muscles

  • Skeletal muscle - stability to bony skeleton

  • Move around axes

  • 1000s of muscle fibers

    • “Fast” twitch fibers – speed

    • “Slow” twitch fibers – endurance


Muscles

  • Innervated by motor neurons from spinal cord

    • ACh (neuromuscular junction)

  • Motor unit – motor neuron & muscle fibers

  • Receptors for proprioception

    • Ch. in length (stretch), tension


Muscles

  • Only actively contract - more than one direction depends on antagonistic pairs

    • Contraction/relaxation

    • Ex. biceps & triceps

  • Flexorsbend, extensorsstraighten


SPINAL CIRCUITS


Spinal Cord

  • Via motor neurons

  • Sensory feedback from muscle & somatosensory

  • Can be independent of the cortex

    • Basic reflex circuits

    • Copy of information to cortex

    • Quick, unconscious


Basic Motor Reflexes

Withdrawal reflex

Sensory info  spinal cord  motor response

“Walking” reflex

In response to foot contact, ankle rotation

Infants


Basic Motor Reflexes

Stretch reflex

  • After sudden stretching force

  • Muscle stretch receptor  spinal cord muscle

  • Compensatory contraction

  • Controls external force on body position

    Infant reflexes

  • Sucking, rooting, grasping, startle


Basic Motor Reflexes

  • Requires info to muscle contracting, antagonist, and contralateral

    • Adjust and coordinate both sides


Reflexes

  • Some directed by brainstem

    • Eye movements (saccades, smooth pursuit)

      Complex movement under control of the cortex and descending motor pathways


MOTOR CORTEX


Primary Motor Cortex

Most “movement” requires cortical control

Cortex to motor neurons (spinal cord) & brainstem

Directed movement, interaction with the environment


Primary Motor Cortex

  • Pre-central gyrusof frontal lobe

    • Adjacent to somatosensory area (post-central gyrus)

    • Why is this relevant?

  • Homunculus

    • Fingers, tongue, face – fine motor control


Descending Pathways

  • Several tracts

  • Different tracts/locations  different motor deficits

  • Control of posture/whole body movements vs. limb movements


Descending Pathways

Dorsolateral corticospinal tract

  • Distal muscles (limbs, hands, fingers, etc.)

  • Primary motor cortex medulla (crossover)  spinal cord  muscles


Descending Pathways

Three other tracts…

  • Indirect to spinal cord

  • Synapse w/ cranial nerve motor info

  • Proximal trunk muscles

    1o cortex receives info from secondary motor cortex


When does the motor system need to be suppressed?


Basal Ganglia & Cerebellum


Basal Ganglia

  • Interconnected nuclei

  • Communicates w/ midbrain, rest of cortex

  • Distribution of tone, coordinating muscle groups

    • Shifting weight, posture

  • Cognitive roles

    • Compulsion, motivation, reward


Cerebellum

  • Comm. w/ cortex, basal ganglia, brainstem

  • Corrects movements, motor errors

    • Based out output

  • Rapid movement req. fine control, adapting to changing conditions

  • Also has cognitive roles


Higher Order Motor Control


Secondary Motor Cortex

Premotor cortex

  • Frontal lobe

  • To primary motor

  • Decisions, coordination, programming of motor plans (sequence of events)

    • Order of movements, patterns

    • Imagining action


Secondary Motor Cortex

  • Complex integration of secondary areas

    • Input about target, body position

  • Mirror neurons – observation of motor action

    • Social learning & cooperation

  • Input from association cortex (in prefrontal cortex)


Association Cortex

  • Dorsolateral prefrontal cortex

  • Project to premotor area

  • Control over motor system

    • Directing attention, eye movements

    • Decision to make movements

    • Sensory feedback


Association Cortex

  • General programs stored, adapted to situations

  • Can be developed w/out practice

  • Can be modified w/ practice

    • Group behaviors together

    • Shift control to lower hierarchy (less cortex)


Motor System Dysfunction


Motor Dysfunction

At any point in the hierarchy

  • Muscle weakness, atrophy, paralysis

    • Communication betw. spinal cord & motor unit

  • Descending pathways

    • Motor control, coordination, etc.

    • Cerebellar or basal ganglia disorders


Motor Dysfunction

  • Apraxia – dysfunction in voluntary movement

    • Not due to muscle, comprehension, motivational deficit

  • Reappearance of infant reflexes w/ cortical damage (withdrawal, grasping)

    • Don’t “outgrow” them, inhibited w/ development


Huntington’s Chorea

  • Basal ganglia & cortex

  • Genetic case - autosomal dominant

    • 30-50 years old

    • Protein damages neurons, over-stimulates target cells

    • Death of neurons

  • Severe cognitive, motor dysfunction

  • Jerks, twitches, writhing (dancing movements)


Parkinson’s Disease

  • Death of neurons proj. to basal ganglia

    • Substantianigra

  • Rigidity, tremor, slowed movement; cognitive

  • Starting, stopping voluntary motion

  • Genetic, environmental causes


Parkinson’s Disease

Treatment – replace DA

  • DA can’t cross BBB

  • L-Dopa to replace DA

  • Neurons continue to die

  • Serious side effects


The Frozen Addicts

Awakening the Frozen Addicts (BBC, 1993)

http://youtu.be/QJIMC9d9l2o

MPPP (synthetic opioid)


MPTP

Ingestion of MPTP causes severe, immediate Parkinson’s symptoms…

…What is the mechanism?


MPTP

  • MPTP metabolized to MPP+ by MAO enzyme

  • MPP+ brought into cells – DA reuptake transporter

  • Selective for neurons in substantianigra

    • Neurotoxic


MPTP

  • Implications for control of movement?

    • SN projects to basal ganglia (putamen/caudate)

  • Animal models

    • Primates

    • Some rodents (rats are immune)

  • MPP+ toxicity as a natural cause of Parkinson’s?


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