Motor Control

Question 1

MOTOR CONTROL ANATOMY

Answer 1

basal ganglia -> premotor/supplementary motor cortex/parietal cortex (-> cerebellum) -> primary motor cortex -> brainstem -> spina cord -> output signals (to muscles)

Question 2

MUSCLES

Answer 2

• composed of elastic fibres; change length/tension
• arranged in antagonist pairs (ie. biceps/triceps)
  bicep contracts -> tricep relaxes -> flexion
  tricep contracts -> bicep relaxes -> extension

Question 3

SPINAL CORD

Answer 3

• muscles controlled by motor neurons in spinal cord
• action potential in motor neuron triggers acetycholine release (neurotransmitter; contracts muscle fibres)
• number/frequency of action potentials/muscle fibre number determine generated muscle force

Question 4

BRAINSTEM

Answer 4

• subcortical motor structure
• 12 cranial nerves = reflexes associated w/eating/breathing/facial expressions
• extrapyramidal tracts = direct pathways from brainstem nuclei incl. substantia nigra, down spinal cord -> control posture/muscle tone/movement speed

Question 5

CEREBELLUM

Answer 5

• subcortical motor structure
• contains more neurons than rest of combined CNS
• controls balance/eye & body coordination
• lesions = balance/gait problems; ataxia (fine coordination)/attentional/planning/language problems

Question 6

BASAL GANGLIA

Answer 6

• subcortical motor structure
• 5 nuclei:
  1. CAUDATE
  2. PUTAMEN
  3. GLOBUS PALLIDUS
  4. SUBTHALAMIC NUCLEUS
  5. SUBSTANTIA NIGRA
• critical role in action selection/initiation
• Parkinson’s

Question 7

PRIMARY MOTOR CORTEX (M1)

Answer 7

• cortical motor region
• receives input from almost all cortical motor regions
• crossed hemispheric control
• somatotopic organisation
• lesions = hemiplegia (voluntary movement loss on contralesional side of body)

Question 8

SECONDARY MOTOR AREAS

Answer 8

• cortical motor region
• premotor cortex/supplementary motor area (SMA)
• planning/movement control
• lesions = patients produce simple gestures BUT cannot link then into meaningful actions (ie. brushing hair)

Question 9

ASSOCIATION MOTOR AREAS

Answer 9

• cortical motor region
• parietal/prefrontal cortex
• parietal = critical for representing space/attention/sensorimotor integration
• lesions = apraxia (skilled action loss)
• Broca’s area = speech production
• frontal eye fields = eye movements

Question 10

CENTRAL PATTERN GENERATORS

Answer 10

• (probs) evolved to enable actions essential for survival (ie. running)

Question 11

MOVEMENT DIRECTION CODING IN M1

Answer 11

GEORGOPOULOS et al (1995)

• monkeys moved lever to 1/8 targets arranged in circle
• individual M1 neurons show preferred direction (ie. fire most strongly when movement in that direction)
• monkeys moved lever to central location from 1/8 peripheral locations
• same neuron preferred movement in same direction even w/dif target location

Question 12

POPULATION VECTORS

Answer 12

• vector = cell’s preferred direction + firing strength info
• neuron tuning = broad; neurons prefer several directions
• hard to predict movement direction from single neuron activity
• population vector = individual neuron vectors sum
• population vector provides most accurate estimate of planned movement direction
• movement direction predicted from pop vector 300ms prior movement initiation

Question 13

BRAIN-MACHINE INTERFACES

Answer 13

CHAPIN et al (1999)

• trained rate press reward lever
• measured multiple neuron motor cortex responses
• neural networks (ie. backpropagation) learnt neuronal activation patterns predicting dif forces exerted on lever
• switched input -> reward delivery system, lever -> neuronal pop vector
• mice eventually stopped pressing lever; learnt precise correlation lack between force exerted/reward
• mice continued cortical signal production necessary for lever movement

Question 14

VISUOMOTOR ADAPTATION

Answer 14

actual movement trajectory -> target; centre -> observed movement trajectory

• ^ activations across motor regions w/visuomotor adaptation BUT meaning? (new motor pattern formation/storage; prediction error; increased attention)
• patients w/cerebellum/prefrontal cortex/parietal cortex lesions = novel environment movement learning deficits

Question 15

TDCS (TRANSCRANIAL DIRECT CURRENT STIMULATION) X VISUOMOTOR ADAPTATION

Answer 15

• tDCS ^ neuron excitability under anodal electrode

- hypothesised to improve learning

Question 16

TDCS X VISUOMOTOR ADAPTATION: CEREBELLUM/M1 DISSOCIATION

Answer 16

GALEA et al (2011)
- faster adaptation w/cerebellum tDCS
- slower de-adaptation w/M1 tDCS
CEREBELLUM
- important for learning new mapping
- involved in forward model generation
- time lag between motor command/movement initiation generation
- cerebellum generates sensory consequence prediction of motor command
- such predictions (forward models) = essential in visuomotor adaptation; errors used to correct future predictions
M1
- important for consolidating newly learnt mapping

Question 17

MOVEMENT TRAJECTORY

Answer 17

• “… average directional errors in TMS condition consistent w/reaching movements planned/initiated from estimated hand position 138ms out of date…”
• forward model generated by cerebellum uses info about future limb position to compute trajectory required to hit target

Question 18

BASAL GANGLIA X MOTOR CONTROL

Answer 18

• basal ganglia plays key role in movement initiation
• mutually antagonistic pathways
• direct pathway = ^ excitation; movement initiation
• indirect pathway = ^ inhibition; movement inhibition

Question 19

PARKINSON’S DISEASE

Answer 19

• dopamine neuron loss in basal ganglia
• symptoms include:
  1. HYPOKINESIA = voluntary movement reduction
  2. BRADYKINESIA = slow movement
  3. TREMOR
• first line treatment = dopamine precursor Levodopa (L-dopa); crosses blood-brain barrier; enters CNS; converted to dopamine
• DBS (deep brain stimulation) investigated

Question 20

BASAL GANGLIA DUAL ROLE

Answer 20

• inflexible cognitive/motor function in PD
• dual gating role for basal ganglia in cog/movement; allows new thoughts/movement occurrence
• motor/cog control rely to some extent on same neuronal circuits

Question 21

SUMMARY

Answer 21

• many dif brain regions involved in motor control
• movement direction coding by pop vectors; use in brain-machine interfaces
• visuomotor adaptation/dif roles for cerebellum (forward models)/M1 (learning consolidation)
• Parkinson’s = dual effects on motor/cog function; linked w/basal ganglia role