Download November 12

Brain Control of
Movement
Motor Control Hierarchy
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High level – plans and executes strategy
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Middle level – develops tactics
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Association areas of cortex
Basal ganglia gives the “go” signal
Motor cortex
Cerebellum
Low level – executes movement
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Brain stem, spinal cord
Two Pathways to the Brain
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Two pathways:
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Lateral pathway
Ventromedial pathway
Lateral pathways control fractionated
movement of distal muscles, especially
flexors, under direct cortical control:
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Corticospinal – new, originates in motor cortex
Rubrospinal – old, originates in red nucleus of
midbrain
Ventromedial Pathways
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Four pathways control proximal & axial muscles.
Vestibulospinal – maintains stability of head and
turns it, maintains upright posture & balance
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Tectospinal – orients eyes (fovea) on image
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Input from labyrinth of inner ear
Receives input from superior colliculus
Pontine – resists gravity and maintains posture
Medullary – liberates muscles from anti-gravity
control
Voluntary Movement
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Involves almost all of the cortex.
Goal-directed movement depends on:
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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
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Premotor areas (PMA, SMA) – plan the
motor activity
Primary motor cortex (M1) – initiates motor
activity:
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Basal ganglia loop (near thalamus) gives the
“go” signal
Cerebellar loop – tells the motor cortex how to
carry out the planned activity
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Controls direction, timing and force by activating
populations of motor neurons in learned programs.
Planning Movement
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Goal directed movement involves many
cortical areas that communicate with Area 6
in Frontal lobe.
Area 6 has two parts:
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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
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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.
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Inhibition is released by the substantia nigra,
permitting VL to send a “go” signal to the SMA.
Disorders of Movement
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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
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Caused by degeneration of the substantia
nigra and depletion of dopamine.
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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
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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.
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Chorea – uncontrolled and purposeless movement
with rapid, irregular flow and flicking motions.
Ballism
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Caused by damage or lesions to the basal
ganglia, usually resulting from stroke.
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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
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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
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Creates a detailed sequence of precisely timed
muscle contractions needed to execute
movement.
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Ataxia – uncoordinated, inaccurate movement.
Dyssynergia – decomposition of synergistic
muscle movements.
Dysmetric – imprecise movement, overshooting
or undershooting target.
Alcohol impairs cerebellar functioning.
Cerebellar Motor Loop
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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.
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Cerebellum compares what was intended with
what happened, then modifies circuits to
compensate.