motor control Flashcards
describe the structure of muscles?
composed of elastic fibers that can change length and tension
antagonist pairs
what are muscles controlled by?
and how?
motor neurons in the spinal cord
action potential in these neurons triggers release of acetylcholine which makes muscle fibres contract
what do the 12 cranial nerves do?
control essential reflexes which keep us alive
what are extrapyramidal tracts?
and hence what is the function of the brainstem?
direct pathways from brainstem nuclei down spinal cord to control posture, muscle tone and movement speed
so this is the role of the brainstem in motor control
what are the brainstem and cerebellum and basal ganglia examples of?
subcortical motor structures
describe the structure and function of the cerebellum?
what do lesions lead to?
more neurons than rest of CNS combined
controls balance and coordination
lesions lead to ataxia (loss of fine coordination)
what is the structure and function of the basal ganglia?
what do lesions cause?
role in selection and initiation of actions
lesions cause Parkinsons disease
contains 5 nuclei
examples of cortical motor regions
primary motor cortex
secondary motor areas
association motor areas
describe the function and organisation of the primary motor cortex?
what do lesions produce?
regulates activity of spinal motor neurons
somatotopic organisation - different regions represent different body parts
lesions produce hemiplegia (loss of contralesional (other side of body) voluntary movement)
describe the function and structure of secondary motor areas?
what do lesions cause?
contain premotor cortex and supplementary motor area
involved in planning and control of movement (top of hierarchy)
lesions result in apraxia (can’t link simple gestures to create actions)
describe association motor areas?
what it contains?
what are the functions of these sections?
parietal - representing space and attention - lesions produce apraxia
and prefrontal cortex - contains Broca’s area and frontal eye fields for eye movement
what did Sherrington do?
method?
what he found?
conclusions drawn?
severed spinal cord of cats and placed them on a treadmill
found cats able to produce limb movements to walk despite no commands from cortex or subcortex
so neurons able to hold representation of entire pattern movements required to produce complex motor act
what did Bizzi do?
method?
how conclusions generated?
trained monkeys who had severed somatosensory signals to reach target
applied opposing force to arm but monkey’s couldn’t feel it
if brain encodes trajectory, opposing force to arm should result in arm falling short of target as wouldn’t adjust to force
if location encoded then monkey should get target
monkey reached target so neurons represent movement by encoding location of end point
what did Georgopoulos do?
and what did they find?
what do these findings suggest about encoding of neurons?
found neurons in primary motor cortex show preferred direction both moving lever in and out (fire most strongly when movement in that direction)
actually preferred multiple directions as neuronal response tuned to broad range of directions
shows neurons also encoded with direction as location different
what is a vector?
direction of cell’s preferred direction combined with info about strength of firing
what is a population vector?
sum of individual neuron vectors
provide most accurate estimate of planned direction of movement
what did Chapin do regarding brain-machine interfaces?
what he did?
what he found?
trained rats to press lever for reward
measured neuron responses in motor cortex
neural networks learnt patterns of neuronal activation predicting different force exerted on lever. led to complex population vector response
changed it so that mice gained reward if displayed cortical signals necessary to move lever without actually moving it
how can you study visuomotor adaptation in a lab?
what was found about patients with lesions in motor areas?
hand hidden
mismatch between where subject moves hand and where they see it move
over time adapt movements to this mismatch
see increased activation across many different motor regions during visuomotor adaptation phase
lesions in motor areas performed poorly suggesting motor regions are critical
whats are the effects of transcranial direct current stimulation (tDCS) on visuomotor adaptation?
what is tDCS?
cerebellum and motor cortex?
deadaptation?
tDCS - currents applied to scalp which change excitability of neurons
tDCS of cerebellum - faster rate of adaptation
tDCS of motor cortex - increased retention of adaptation (Galea)
deadaptation (no mismatched feedback) occured but hindered if motor cortex stimulated (double dissociation between cerebellum and motor cortex)
what have tDCS studies shown about the role of the cerebellum?
- important for learning new mapping by generating forward models (prediction of sensory consequences of motor command) as errors used to correct future predictions
what have tDCS studies shown about the role of the primary motor cortex?
important for consolidating newly learnt mapping
describe studies into effect of TMS on cerebellum?
hear beep then move arm toward visual target
TMS of cerebellum changed movement trajectory
deactivating cerebellum = deactivating forward model
as provides prediction of where hand will be when movement is initiated and adjusts motor command accordingly
what is the role of the basal ganglia in motor control?
movement initiation and inhibition
complex network of inhibitory (indirect pathway) and excitatory (direct pathway) connections within basal ganglia and connecting it to other structures
pathways act in mutual antagonism to balance the system
describe Parkinsons disease?
symptoms?
loss of dopamine neurons in the basal ganglia
symptoms:
hypokinesia: reduction in volunatry movement
bradykinesia: slow movement
tremor
describe treatment of Parkinsons?
levodopa - dopamine precursor (converted into dopamine after entering CNS)
deep brain stimulation
describe cognitive function in parkinsons?
what does this suggest about the role of the basal ganglia?
have impairments in cognitive function
so basal ganglia has a dual role in cognition and action as cognitive and motor control rely on same neuronal circuits
what is the explanation for Sherrington’s findings that cortex commands not necessary for movement?
central pattern generators - neurons in spinal cord being able to hold representation of pattern of movements required to produce a complex act
high region of hierarchy sending signal to trigger central pattern commands so no need for high level regions to hold representations
what is a central patttern generator?
biological neural circuits that produce rhythmic outputs in the absense of rhythmic input
evolved to enable actions essential for survival
what question regarding neurons does Bizzi’s research tackle?
whether neurons encode trajectory (direction, distance and force) of movement or location of the target
found they encode location
what are the compoents of the motor control system hierarchy?
premotor and parietal cortex (top) primary motor cortex basal ganglia cerebellum brainstem spinal cord
ends in output signals to muscles (bottom)
what is the role of the spinal cord in motor control?
the neurons in the spinal cord control muscles
by generating action potentials to make muscles contract/relax
