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)
what are the effects of tDCS on visuomotor adaptation: Dissociation between cerebellum and M1?
cerebellum imp for learning new mapping
involved in generation of forward models
time lag between generation of motor commands and movement initiation
cerebellum generates prediction of sensory consequences of motor command
such predictions (forward models) essential in visuomotor adaptation- errors used to correct future predictions
primary motor cortex (M1) imp for consolidating newly learnt mapping – less flexible role and more instructive role – passing on motor plans to spinal cord motor neurons
cerebellum and M! may play diff roles in visuomotor adaptation
what are the effects of TMS of cerebellum on forward models? (Miall et al., 2007)
got subjects to move arm to right and then when tone occurred to make movement to visual target
task requires prediction of where hand will be in future because there’s a delay between hearing tone and actually initiating forward movement
if subject just generated motor command when heard tine they would miss target because motor command (trajectory) would be out of date and inaccurate by time motor command actually initiated
movement trajectory
‘average directional errors in TMS condition consistent with reaching movements being planned and initiated from estimated hand position 138ms out of date’
deactivate cerebellum – deactivate forward movement
forward model generated by cerebellum uses info about future position of limb to compute trajectory required to hit target
what is the role of the basal ganglia in motor control?
play key role in movement initiation
complex network of inhibitory and excitatory connections within basal ganglia and connecting BG to other structures
input from substantia nigra into putamen and travels wither via direct route to globus pallidus/via indirect route via substantia nigra
pathways mutually antagonistic
direct pathway leads to increased excitation – movement initiation
indirect pathway leads to increased inhibition – movement inhibition
enables system to achieve balance – activation of direct pathway can lead to too much movement which can be balanced out by inhibitory activation of indirect pathway
Parkinson’s disease
loss of dopamine neurons in basal ganglia
main symptoms:
hypokinesia – reduction in voluntary movement
bradykinesia – slow movement
tremor
how can Parkinson’s disease be treated?
first line treatment = Levodopa (‘L-dopa’) – dopamine precursor
crosses blood-brain barrier, enters CNS and converted to dopamine
stops working after a while
side-effects
more recently, researchers investigated effects of deep brain stimulation (DBS)
some success
cognitive function of basal ganglia
impairments also seen in cog function
Ps trained to press left/red things and squares and right for green things and triangles
then in test phase see pairs of stimuli and have to press left/right – told to respond to either colour/shape
on some trials, respond to same dimension and on others switches (Hayes et al., 1998)
dual role for basal ganglia
inflexible cog function as well as motor function in PD
dual gating role for basal ganglia in cognition and movement – allowing new thoughts and movements to occur
motor control and cog control rely
how is motor control carried out?
motor control carried out by muscles, composed of elastic fibres that can change length and tension and act antagonistically – so when biceps contract, triceps relaxes which enables flexion of elbow, and if triceps contracts and biceps contracts this enables extension
what are the five nuclei in the Basal Ganglia
Caudate, putamen, globus, pallidus, subthalamic nucleus and substantia nigra
what does the motor cortex regulate?
activity of spinal motor neurons
somatotopic organisation
diff regions represent diff parts of body – representation – larger areas dedicated to parts of body involved in movement such as hands and face
what do lesions to M1 produce?
hemiplegia, loss of voluntary movements on contralesional side of body
what do lesions to secondary motor areas result in?
patients can produce simple gestures but cannot link them into meaningful actions, E.g. brushing hair
what do lesions in association motor areas produce?
apraxia, loss of skilled action – also more general problems with attention, spatial attention (neglect)
parietal cortex
Critical for representing space, attention, sensorimotor integration
