Motor control Flashcards
muscles
composed of elastic fibres that can change length and tension
arranged in antagonist pairs, E.g. biceps and triceps
spinal cord
muscle controlled by motor neurons in spinal cord
AP in motor neuron triggers release of ACh, ntm that makes muscle fibres contract
number and freq of APs and number of muscle fibres determine force muscle can generate
motor neurons originate in spinal cord, exit through ventral root and terminate in muscle fibres
subcortical motor structures
cerebellum
basal ganglia
brainstem
brainstem
12 cranial nerves – reflexes associated with eating, breathing, facial expressions – keep us alive
extrapyramidal tracts – direct pathways from brainstem nuclei, inc substantia nigra, down spinal cord to control posture, muscle tone, movement speed – indirect control
cerebellum
contains more neurons than rest of CNS
controls balance and eye/body coord (but also does more)
lesions result in balance/gait problems, ataxia (fine coord) as well as attentional, planning and language problems
basal ganglia
‘striatum’
five nuclei
critical role in selection and initiation of actions
lesions cause Parkinson’s Disease – problems with certain types of cog function and motor control
cortical motor regions
primary motor cortex (M1)
secondary motor areas
association motor areas
primary motor cortex (M1)
receives input from almost all cortical motor regions
crossed hemispheric control
somatotopic organisation
can see mapping with TMS – can elicit predictable twitches in diff regions using TMS
corticospinal (pyramidal tract) has axons that project directly from the cortex to spinal cord – v. long axons
each cerebral hemisphere controls movement on opposite side of body
secondary motor areas
premotor cortex and supplementary motor area (SMA) – highest parts of hierarchy
planning and control of movement – either sensory guided/internally guided
association motor areas
parietal and prefrontal cortex
Broca’s areas: Speech production
frontal eye fields – eye movements
what are central pattern generators?
neurons in spinal cord called CPGs as they were able to hold representation of entire pattern of movements required to produce complex motor act
key part of hierarchical nature of motor control – enables higher level regions to send v. Simple signal that triggers one of central pattern commands – no need for higher level regions to hold entire representation themselves
evolved to enable actions essential for survival, E.g. running, instincts, reflexes
if specific motor commands controlled by spinal cord, what are motor neurons in brain coding?
Brown and Sherrington (1947)
severed spinal cord of cats and placed them on treadmill
found that without descending commands from cortex/subcortex, cats able to produce rhythmic alternating limb movements required to walk
representation of movement plans
have to plan for movement
do neurons encode trajectory of movement (direction, distance, force)/location of target?
neurons represent movements by encoding location of end-point
Bizzi et al. (1984)
deafferented monkeys trained to point to target in dark room (Bizzi et al. 1984)
deafferented = had all somatosensory feedback signals severed (signals from nerve receptor back to brain)
opposing force applied to arm to hold it in starting position for short time – monkey didn’t know force was applied
if monkey brain encodes trajectory, manip should result in arm falling short of target – monkey wouldn’t adjust force required and so some of force would be removed in dealing with resisting opposing force
if location encoded monkey should still get to target
found that monkey reached target even when opposing force applied
coding of movement direction in primary motor cortex (Georgopoulos et al., 1995)
monkeys moved lever to one of 8 targets arranged in circle
indv neurons in primary motor cortex show preferred direction, i.e. they fire more strongly when movement is in that direction
monkeys moved lever to central location from one of 8 peripheral locations
same neuron preferred movements in same direction, even when target location different
population vectors
neurons preferred multiple directions
each neuron’s response seemed to be tuned to quite broad range of directions
tuning of neurons broad – neurons tend to prefer several directions
hard to predict direction of movement from activity of single neuron
popn vector provides most accurate estimate of planned direction of movement – can be predicted 300ms before initiation of movement
what is a vector?
direction of cell’s preferred direction combined with info about strength of firing
what is a population vector?
sum of indv neuron vectors
brain-machine interfaces (Chapin et al., 1999)
trained rate to press lever for reward
measured multiple neuron responses in motor cortex
neural networks learnt patterns of neuronal activation predicting diff forces exerted on lever
led to complex popn vector responses
switched input to reward delivery system from lever to neuronal popn vector
mice eventually stopped pressing lever as learnt about lack of precise correlation between force exerted and reward
mice continued to produce cortical signals necessary for moving lever
can control from neuronal responses
visuomotor learning and adaptation
motor system able to continually learn new movements and adapt to changing sensory inputs – how?
visuomotor adaptation
patients with lesions in cerebellum, prefrontal cortex and parietal cortex have deficits in learning to move in novel envs
Seidler et al. (2006)
cannot see movements she is making – superimpose visual display
offset between what P sees and what is actually happening
over time adapt movements to mismatch
increased activation across diff motor regions during adaptation phase
what do increases in activation across different motor regions during the adaptation phase mean?
formation and storage of new motor patterns?
prediction error?
increased attention?
what is the effect of tDCS on visuomotor adaptation
anodal and cathodal electrodes
battery powered device that delivers constant electric current
tDCS increases excitability of neurons under anodal electrode
hypothesised to improve learning
what are the effects of transcranial direct current stimulation (tDCS) on visuomotor adaptation: dissociation between cerebellum and M1 (primary motor cortex) (Galea et al., 2011)?
used method to investigate roles of cerebellum and primary motor cortex
tDCS of cerebellum led to faster rate of adaptation
tDCS of motor cortex led to increased retention of adaptation (more error for longer period of time after end of adaptation)
