Motor control

Question 1

muscles

Answer 1

composed of elastic fibres that can change length and tension

arranged in antagonist pairs, E.g. biceps and triceps

Question 2

spinal cord

Answer 2

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

Question 3

subcortical motor structures

Answer 3

cerebellum

basal ganglia

brainstem

Question 4

brainstem

Answer 4

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

Question 5

cerebellum

Answer 5

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

Question 6

basal ganglia

Answer 6

‘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

Question 7

cortical motor regions

Answer 7

primary motor cortex (M1)

secondary motor areas

association motor areas

Question 8

primary motor cortex (M1)

Answer 8

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

Question 9

secondary motor areas

Answer 9

premotor cortex and supplementary motor area (SMA) – highest parts of hierarchy

planning and control of movement – either sensory guided/internally guided

Question 10

association motor areas

Answer 10

parietal and prefrontal cortex

Broca’s areas: Speech production

frontal eye fields – eye movements

Question 11

what are central pattern generators?

Answer 11

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?

Question 12

Brown and Sherrington (1947)

Answer 12

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

Question 13

representation of movement plans

Answer 13

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

Question 14

Bizzi et al. (1984)

Answer 14

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

Question 15

coding of movement direction in primary motor cortex (Georgopoulos et al., 1995)

Answer 15

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

Question 16

population vectors

Answer 16

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

Question 17

what is a vector?

Answer 17

direction of cell’s preferred direction combined with info about strength of firing

Question 18

what is a population vector?

Answer 18

sum of indv neuron vectors

Question 19

brain-machine interfaces (Chapin et al., 1999)

Answer 19

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

Question 20

visuomotor learning and adaptation

Answer 20

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

Question 21

Seidler et al. (2006)

Answer 21

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

Question 22

what do increases in activation across different motor regions during the adaptation phase mean?

Answer 22

formation and storage of new motor patterns?

prediction error?

increased attention?

Question 23

what is the effect of tDCS on visuomotor adaptation

Answer 23

anodal and cathodal electrodes

battery powered device that delivers constant electric current

tDCS increases excitability of neurons under anodal electrode

hypothesised to improve learning

Question 24

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)?

Answer 24

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)

Question 25

what are the effects of tDCS on visuomotor adaptation: Dissociation between cerebellum and M1?

Answer 25

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

Question 26

what are the effects of TMS of cerebellum on forward models? (Miall et al., 2007)

Answer 26

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

Question 27

movement trajectory

Answer 27

‘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

Question 28

what is the role of the basal ganglia in motor control?

Answer 28

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

Question 29

Parkinson’s disease

Answer 29

loss of dopamine neurons in basal ganglia

main symptoms:

hypokinesia – reduction in voluntary movement

bradykinesia – slow movement

tremor

Question 30

how can Parkinson’s disease be treated?

Answer 30

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

Question 31

cognitive function of basal ganglia

Answer 31

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)

Question 32

dual role for basal ganglia

Answer 32

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

Question 33

how is motor control carried out?

Answer 33

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

Question 34

what are the five nuclei in the Basal Ganglia

Answer 34

Caudate, putamen, globus, pallidus, subthalamic nucleus and substantia nigra

Question 35

what does the motor cortex regulate?

Answer 35

activity of spinal motor neurons

Question 36

somatotopic organisation

Answer 36

diff regions represent diff parts of body – representation – larger areas dedicated to parts of body involved in movement such as hands and face

Question 37

what do lesions to M1 produce?

Answer 37

hemiplegia, loss of voluntary movements on contralesional side of body

Question 38

what do lesions to secondary motor areas result in?

Answer 38

patients can produce simple gestures but cannot link them into meaningful actions, E.g. brushing hair

Question 39

what do lesions in association motor areas produce?

Answer 39

apraxia, loss of skilled action – also more general problems with attention, spatial attention (neglect)

Question 40

parietal cortex

Answer 40

Critical for representing space, attention, sensorimotor integration