Motor Control Flashcards
MOTOR CONTROL ANATOMY
basal ganglia -> premotor/supplementary motor cortex/parietal cortex (-> cerebellum) -> primary motor cortex -> brainstem -> spina cord -> output signals (to muscles)
MUSCLES
- composed of elastic fibres; change length/tension
- arranged in antagonist pairs (ie. biceps/triceps)
bicep contracts -> tricep relaxes -> flexion
tricep contracts -> bicep relaxes -> extension
SPINAL CORD
- 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
BRAINSTEM
- 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
CEREBELLUM
- 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
BASAL GANGLIA
- 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
PRIMARY MOTOR CORTEX (M1)
- 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)
SECONDARY MOTOR AREAS
- 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)
ASSOCIATION MOTOR AREAS
- 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
CENTRAL PATTERN GENERATORS
- (probs) evolved to enable actions essential for survival (ie. running)
MOVEMENT DIRECTION CODING IN M1
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
POPULATION VECTORS
- 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
BRAIN-MACHINE INTERFACES
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
VISUOMOTOR ADAPTATION
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
TDCS (TRANSCRANIAL DIRECT CURRENT STIMULATION) X VISUOMOTOR ADAPTATION
- tDCS ^ neuron excitability under anodal electrode
- hypothesised to improve learning
TDCS X VISUOMOTOR ADAPTATION: CEREBELLUM/M1 DISSOCIATION
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
MOVEMENT TRAJECTORY
- “… 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
BASAL GANGLIA X MOTOR CONTROL
- basal ganglia plays key role in movement initiation
- mutually antagonistic pathways
- direct pathway = ^ excitation; movement initiation
- indirect pathway = ^ inhibition; movement inhibition
PARKINSON’S DISEASE
- 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
BASAL GANGLIA DUAL ROLE
- 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
SUMMARY
- 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
