Motor control

Question 1

Acting not thinking

Answer 1

1. everything that happens in our brain is geared towards action
2. even thoughts are ultimately there to help generate action
3. motor control is therefore at the forefront of psychology
4. motor control is where all the internal processes converge on our ability to interact meaningfully with the world around us

Question 2

overview of some of the motor control systems in the brain

Answer 2

1. system is organised hierarchically
2. at the bottom of the hierarchy is the spinal cord - provides direct control of the muscles via motor neurons (Axons can be up to 1m in length) and interneurons
3. top of the hierarchy is the premotor and parietal cortex which are responsible for action plans and goals
4. between these levels are the motor cortex and subcortical brain regions which translate the motor plans and goals into specific actions

Question 3

muscles

Answer 3

1. composed of elastic fibres that can change length and tension
2. arranged in antagonistic pairs - enables flexion

Question 4

spinal cord

Answer 4

1. muscles are controlled by motor neurons in the spinal cord
2. motor neurons originate in spinal cord, exit through the ventral root and terminate in muscle fibres
3. action potential in a motor neuron triggers the release of acetylcholine, NT that makes muscle fibres contract
4. number and frequency of action potentials and the number of muscle fibres determine the force the muscle can generate

Question 5

subcortical motor structures: brainstem

Answer 5

1. evolutionarily old part of the brain
2. 12 cranial nerves control essential reflexes for keeping us alive
3. extrapyramidal tracts - send direct pathways down the spinal cord, from brainstem nuclei, to exert indirect control over posture, muscle tone and movement speed

Question 6

subcortical motor structures: cerebellum

Answer 6

1. contains more neurons than rest of CNS combined
2. controls balance and eye/body coordination
3. lesions result in balance/gait problems, ataxia (fine coordination) as well as attentional, planning and language problems

Question 7

subcortical motor structures: basal ganglia

Answer 7

1. key node in the subcortical motor control system
2. contains 5 nuclei - caudate. putamen, globus pallidus, subthalamic, substantia nigra
3. critical role in the selection and initiation of actions
4. lesions cause parkinsons

Question 8

cortical motor regions: primary motor cortex

Answer 8

1. motor cortex regulates the activity of spinal motor neurons
2. corticospinal (pyramidal tract) has axons that project directly from the cortex to the spinal cord
3. received input from almost all cortical motor regions
4. each cerebral hemisphere controls movement on the opposite side of the body
5. somatotopic organisation - different regions represent different body parts
6. can elicit predictable twitches in different regions using TMS
7. lesions to M1 produce hemiplegia - loss of voluntary movements on contralesional side of body

Question 9

cortical motor regions: secondary motor areas

Answer 9

1. premotor cortex and supplementary motor area are highest parts in the hierarchy, involved in planning and the control of movement, either sensory guided or internally guided
2. lesions result in apraxia, which affects movement of limbs and also speech, patients can produce simple gestures but cannot link them into meaningful actions

Question 10

cortical motor regions: association motor areas

Answer 10

1. association motor areas include the parietal and prefrontal cortex
2. parietal - critical for representing space, attention, sensorimotor integration
3. lesions of parietal cortex produce apraxia, but also more general problems with attention
4. Broca’s area is also in PFC
5. frontal eye field - eye movements

Question 11

central pattern generators

Answer 11

1. shows how various regions enable motor control
2. sherrington - completely severed the spinal cord of cats and placed them on a treadmill
3. found that even without any descending commands from the cortex or subcortex, cats were able to produce the rhythmic alternating limb movements required to walk
4. neurons in the spinal cord were called central pattern generators because they were able to hold a representation of the entire pattern of movement required to produce a complex motor act
5. key part of the hierarchical nature of motor control - enables higher level regions to send a very simple signal that triggers one of these central pattern commands, no need for higher level regions to hold the entire representation themselves
6. CPGs probably evolved to enable actions essential for survival
7. if higher level neurons in the brain does not represent specific action plans then what do they dor

Question 12

representation of movement plans

Answer 12

higher level regions encode the plans for performing a movement but how

Question 13

coding of movement direction in primary motor cortex

Answer 13

1. neurons can also represent the direction of a movement
2. Georgopoulos et al - monkeys moved a lever to one of 8 targets arranged in a circle, individual neurons in primary motor cortex show a preferred 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

Question 14

population vectors

Answer 14

1. more complex than that
2. neurons didn’t just prefer a single direction
3. preferred multiple directions
4. each neuron’s response seemed to be tuned to quite a broad range of directions
5. concept of a population vector has been hugely influential
6. the idea is that each neuron has a preferred direction, which combined with the strength of firing gives what we call a vector
7. neurons tend to prefer several directions such that its hard to predict direction of movement from any single neurons
8. however using the population vector (sum of the individual neuron vectors to predict direction of movement

Question 15

brain machine interfaces

Answer 15

1. Chapin et al - trained rats to press a lever for reward and measured multiple neuron responses in motor cortex
2. neural networks were allowed to learn the patterns of neuronal activation predicting the different forces exerted on the lever
3. led to complex population vector responses, showing output of 40-odd neurons during different stages of lever pressing
4. then switched input to reward delivery system from lever to neuronal population vector
5. mice eventually stopped pressing the lever as they learnt about the lack of precise correlation between the force exerted and the reward
6. mice continued to produce cortical signals necessary to move the lever

Question 16

visuomotor learning and adaptation

Answer 16

1. motor system is able to continually learn new movements and adapt to changing sensory inputs
2. visuomotor adaptation can be studied in lab
3. increased activation across many different motor regions during the adaptation phase

Question 17

visuomotor adaptation

Answer 17

1. increased activations across motor regions with visuomotor adaptation but what do these increases mean
2. patients with lesions in cerebellum, prefrontal cortex and parietal cortex have deficits in learning to move in novel environments

Question 18

effects of transcranial direct current stimulation on visuomotor adaptation

Answer 18

1. electrical currents are applied to the scalp producing changes in the excitability of neurons under the electrode
2. Galea et al used this method to investigate the different roles of the cerebellum and the primary motor cortex
3. tdcs of the cerebellum led to a faster rate of adaptation
4. tdcs of the motor cortex led to increased retention of adaptation (more error for a longer period of time after the end of adaptation)
5. suggests cerebellum and M1 may play different roles in visuomotor adaptation - cerebellum involved in learning new mapping, one theory is that it does this by generative forward models
6. whereas m1 seems to be important for consolidating newly learnt mapping, plays a less flexible role and more of an instructive role, passing on motor plans to spinal cord motor neurons

Question 19

effects of transcranial direct current stimulation of the cerebellum on forward models

Answer 19

1. Miall et al - subjects asked to move their arm to the right and when a tone occurred to make a movement to a visual target
2. task requires a prediction of where your hand will be in the future because there is a delay between hearing the tone and actually initiating the forward movement
3. if the subject just generated a motor command when they heard the tone they would miss the target because the motor command (trajectory) would be out of date and inaccurate by the time the motor command was actually initiated

Question 20

movement trajectory

Answer 20

1. found that normally the subjects were pretty accurate
2. however with TMS the path of the hand matched what would be expected if the command had been issued from a point 138ms earlier than when the movement was initiated
3. demonstrates that the cerebellum must provide a prediction of where the hand will be when the movement is initiated and adjusts the motor command accordingly