Neurobiology of Motor Control

Question 1

What are muscles?

Answer 1

• composed of elastic fibres that can change length and tension
• arranged in antagonist pairs

Question 2

What is the spinal cord?

Answer 2

• muscles are controlled by motor neurons in the spinal cord
• action potential in a motor neuron triggers release of acetylcholine (neurotransmitter that makes muscle fibres contract)
• number and frequency of action potentials and the number of muscle fibres determine the force the muscle can generate

Question 3

What are the subcortical motor structures?

Answer 3

• brainstem
• cerebellum
• basal ganglia

Question 4

What is the brainstem?

Answer 4

• 12 cranial nerves (reflexes of eating, breathing, facial expressions)
• extrapyramidal tracts (direct pathways from brainstem nuclei to spinal cord to control posture, muscle tone and movement speed)

Question 5

What is the cerebellum?

Answer 5

• contains more neurons than rest of CNS combined
• controls balance and eye/body coordination
• lesions result in balance/gait problems, ataxia, attentional/planning/language problems

Question 6

What is the basal ganglia?

Answer 6

• 5 nuclei: caudate, putamen, globus pallidus, subthalamic nucleus, substantia nigra
• critical role in selection and initiation of actions
• parkinson’s disease is due to loss of dopamine neurons in the basal ganglia
• dual role for the basal ganglia: dual gating role for basal ganglia in cognition and movement (rely in part to the same neuronal circuits)

Question 7

What are the cortical motor regions?

Answer 7

• primary motor cortex: receives input from almost all cortical motor regions
• secondary motor areas: premotor cortex and supplementary motor area responsible for planning and control of movement
• association motor areas: parietal and prefrontal cortex

Question 8

What are central pattern generators?

Answer 8

• biological neural circuits that produce rhythmic motor outputs in the absence of rhythmic input
• evolved to enable actions essential for survival

Question 9

How is movement direction in primary motor cortex coded?

Georgopoulos et al, 1995

Answer 9

• monkeys moved a lever to one of 8 targets arranged in a circle
• individual neurons in primary motor cortex show a preferred direction (i.e. fire most strongly when movement is in that direction)
• monkeys moved a lever to central location from one of 8 peripheral locations
• the same neuron preferred movements in the same direction even when the target location was now different
• evidence that neurons in primary motor cortex encode the direction of movement

Question 10

What are population vectors?

Answer 10

• sum of individual neuron vectors (provides most accurate estimate of planned direction)
• a vector is the direction of cell’s preferred direction combined with information on strength of firing
• turning of neurons is broad (prefer several directions)
• hard to predict movement

Question 11

What are brain-machine interfaces?

Chapin et al, 1999

Answer 11

• trained rats to press lever for reward and measured multiple neuron responses in motor cortex
• neural networks learnt patterns of neuronal activation predicting different forces exerted on lever
• switched input to reward delivery system from lever to neuronal population vector (so it directly controls the reward system)
• mice eventually stopped pressing the lever as they learnt about the lack of precise correlation between force exerted and reward
• mice continued to produce cortical signals necessary for moving the lever (still producing the signals that are associated with the pressing responses which allow them to receive the reward)
• can use these kinds of processes for prosthesis (teach a pattern classifier to learn the patterns of neuronal activation associated with certain movement)

Question 12

What is visuomotor adaptation?

Seidler et al, 2006

Answer 12

• participant has under surface so unable to see it, on surface there’s visual display that reproduces any movement they make with their arm
• can be accurate to the direction they move their hand or inaccurate visual feedback of different directions,
• in the first 4 trials, when they believe they aren’t hitting the target correctly their actual trajectory shows they move to the target and then adjust so the visual feedback tells them they hit it
• in the final 4 trials they make more accurate movements in the correct direction, they adapt to the erroneous (incorrect) feedback
• patients with lesions in cerebellum, prefrontal cortex and parietal cortex have deficits in learning to move in novel environments

Question 13

What is the effect of transcranial direct current stimulation (tDCS) on visuomotor adaptation?

Answer 13

• tDCS increases excitability of neurons under anodal electrode
• hypothesised to improve learning
• dissociation between cerebellum and M1 (Galea et al, 2011): faster adaptation with cerebellum tDCS, slower de-adaptation with M1 tDCS, cerebellum is important for learning new mapping
• primary motor cortex is important for consolidating newly learnt mapping (de-adaptation is encouraging to hold onto new maps when should actually be unlearning them)

Question 14

What is the effect of tDCS on the cerebellum on forward models?
(Miall et al, 2007)

Answer 14

• participants completed a task where they would their arm out to the side and then when they hear a tone would have to move towards the target in front of them instead
• movement trajectory shows that when there’s no TMS people are more accurate (closer to the target) when there is TMS over the cerebellum their reaching movements aren’t as accurate, TMS on the cerebellum was disrupting the forward model, disrupting the ability to predict the sensory consequences of a motor command
• when there’s no forward model the ability to predict trajectory becomes inaccurate
• forward model generated by cerebellum uses information about future positions of limb to compute the trajectory required to hit the target