Movement

Question 1

What are the five purposes of muscle contractions?

Answer 1

1. Moving the external world
2. Moving yourself around
3. Maintaining equilibrium to prevent movements
4. Communication
5. Active touch

Question 2

What is ‘noise’?

Answer 2

Random variation in motor and sensory signals due to the fact that the action potential sent along neurons is a crude digital signal

Question 3

What is ‘redundancy’?

Answer 3

Goal-directed movements can be carried out in several different ways, using different combinations of movements. Repeated efforts to make the same movement are not identical.
The brain must decide which combination is suitable at that point in time.

Question 4

What are the biological delays?

Answer 4

There are delays in:

1. transduction
2. conduction of action potentials
3. force generation

To produce accurate movement you need to specify when the force reaches its peak.

Question 5

How fast do eye muscles contract?

Answer 5

20ms to reach peak force

Question 6

How fast do limb muscles contract?

Answer 6

30 - 50ms to reach peak force

Question 7

What is ‘non-linearity’?

Answer 7

Mixing individual motor commands does not produce predictable results.

Force depends on length of muscle, load, velocity of shortening, etc.

Question 8

What is ‘non-stationarity’?

Answer 8

Behaviour of motor systems can change over time

Question 9

What is thixotropy?

Answer 9

Muscle contraction depends on history

Question 10

How many motorneurons innervate a muscle?

Answer 10

Hundreds

Question 11

How many synapses does each motorneuron receive?

Answer 11

30,000 approx

Question 12

What is the EPSP of one excitatory synapse?

Answer 12

0.1mV approx

Question 13

What is the spike threshold?

Answer 13

> 10mV depolarisation

Question 14

What are the disadvantages of negative feedback?

Answer 14

Time delays in feedback loop mean that data can be out of date by the time it reaches the CNS so corrective movements that are generated are inappropriate

This leads to instability and oscillations (cold shower phenomenon)

Question 15

What are open-loop systems?

Answer 15

Feedforward systems that utilise anticipatory control where sensory information is used to generate a prediction of what is needed in the future

They need to learn; this is guided by comparing desired and achieved outcomes

Question 16

What is the inverse model?

Answer 16

1. Uses desired outcome as starting point
2. Joints that need to be moved
3. Forces needed to move these joints
4. Motorneuron activity needed to create these forces
5. THUS commands needed to generate movement

Only useful if accurate so it needs to learn

Question 17

How do feedforward models learn?

Answer 17

Comparison of intended motion and achieved motion

Question 18

What is the forward model?

Answer 18

1. Predicts consequences of motor commands before and during movement
2. Receive information on what motor commands are via efference copy and internal feedback
3. Use this information to simulate the response of the motor system and predict the movement
4. Prediction compared to desired result
5. Corrections set up for errors as they are being made

Question 19

What are the centres for feedforward control?

Answer 19

1. Cerebellum

2. Motor cortex