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Brain Control of Movement . Motor Control Hierarchy. High level – plans and executes strategy Association areas of cortex Basal ganglia gives the “go” signal Middle level – develops tactics Motor cortex Cerebellum Low level – executes movement Brain stem, spinal cord.

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motor control hierarchy
Motor Control Hierarchy
  • High level – plans and executes strategy
    • Association areas of cortex
    • Basal ganglia gives the “go” signal
  • Middle level – develops tactics
    • Motor cortex
    • Cerebellum
  • Low level – executes movement
    • Brain stem, spinal cord
two pathways to the brain
Two Pathways to the Brain
  • Two pathways:
    • Lateral pathway
    • Ventromedial pathway
  • Lateral pathways control fractionated movement of distal muscles, especially flexors, under direct cortical control:
    • Corticospinal – new, originates in motor cortex
    • Rubrospinal – old, originates in red nucleus of midbrain
ventromedial pathways
Ventromedial Pathways
  • Four pathways control proximal & axial muscles.
  • Vestibulospinal – maintains stability of head and turns it, maintains upright posture & balance
    • Input from labyrinth of inner ear
  • Tectospinal – orients eyes (fovea) on image
    • Receives input from superior colliculus
  • Pontine – resists gravity and maintains posture
  • Medullary – liberates muscles from anti-gravity control
voluntary movement
Voluntary Movement
  • Involves almost all of the cortex.
  • Goal-directed movement depends on:
    • Knowing where the body is in space.
    • Knowing where it wants to go.
    • Selection of a plan to get it there.
  • Once a plan is devised, it must be kept in memory until it can be executed.
  • Instructions to implement the plan must be issued.
  • These functions are localized to different areas.
parts of the motor system
Parts of the Motor System
  • Premotor areas (PMA, SMA) – plan the motor activity
  • Primary motor cortex (M1) – initiates motor activity:
    • Basal ganglia loop (near thalamus) gives the “go” signal
    • Cerebellar loop – tells the motor cortex how to carry out the planned activity
      • Controls direction, timing and force by activating populations of motor neurons in learned programs.
planning movement
Planning Movement
  • Goal directed movement involves many cortical areas that communicate with Area 6 in Frontal lobe.
  • Area 6 has two parts:
    • PMA (premotor area)
    • SMA (supplemental motor area) – lesions produce apraxia (impaired complex acts)
  • Area 6 plans an action and stays active until it is executed (“go” signal).
the go signal
The “Go” Signal
  • Area 6 receives a “Go” signal from the thalamus (VLo).
  • Input to the thalamus comes from the basal ganglia deep in subcortical areas.
  • A circuit through the basal ganglia inhibits excitation of the SMA by VL.
    • Inhibition is released by the substantia nigra, permitting VL to send a “go” signal to the SMA.
disorders of movement
Disorders of Movement
  • Hypokinesia – a lack of movement caused by increased inhibition of the thalamus by the basal ganglia.
  • Hyperkinesia – too much movement caused by decreased basal ganglia input, removing inhibition of the thalamus.
  • Bradykinesia – slowness of movement.
  • Akinesia – difficulty initiating movement.
parkinson s disease
Parkinson’s Disease
  • Caused by degeneration of the substantia nigra and depletion of dopamine.
    • Impairs the “go” signal circuit to VLo & SMA.
  • Symptoms are bradykinesia, akinesia, increased muscle tone (rigidity), tremors of hands and jaw, especially at rest.
  • Treated by the drug L-Dopa, the precursor to dopamine.
huntington s disease
Huntington’s Disease
  • Hereditary, progressive, lethal syndrome caused by loss of neurons in the basal ganglia, cortex, and elsewhere.
  • Symptoms are hyperkinesia, dyskinesia (abnormal movement), dementia (impaired cognition), and personality disorders.
    • Chorea – uncontrolled and purposeless movement with rapid, irregular flow and flicking motions.
ballism
Ballism
  • Caused by damage or lesions to the basal ganglia, usually resulting from stroke.
    • Loss of excitation of the global pallidus (normally inhibiting VLo) results in too much excitation of SMA.
  • Symptoms are violent, flinging movements of the extremities.
coding the direction of movement
Coding the Direction of Movement
  • Motor cortex (M1) neurons fire at different rates depending on the desired direction.
  • Firing rates are averaged across populations of M1 neurons.
  • When contributing neurons are inhibited, resultant direction changes.
  • Cerebellum controls sequence.
cerebellum
Cerebellum
  • Creates a detailed sequence of precisely timed muscle contractions needed to execute movement.
    • Ataxia – uncoordinated, inaccurate movement.
    • Dyssynergia – decomposition of synergistic muscle movements.
    • Dysmetric – imprecise movement, overshooting or undershooting target.
  • Alcohol impairs cerebellar functioning.
cerebellar motor loop
Cerebellar Motor Loop
  • Cerebellum supplies input to the motor cortex via the pons (pontine nuclei) and area VLc of the thalamus.
  • Feedback from the sensory cortex guides activity of the cerebellum to create and store learned programs of movement.
    • Cerebellum compares what was intended with what happened, then modifies circuits to compensate.