Week 10: Motor systems

Question 1

How does the motor system work?

Answer 1

Top down process
High level association areas that determine the plans, goals and desired outcomes of a particular movement

These then interact with lower levels in the system that execute the motor commands and provide feedback to determine whether the desired outcome was achieved (feedback loops between each level of the system - so that we know if we need to alter the commands based on outcomes)

Question 2

What are the 3 governing principles of the motor system?

Answer 2

Have a hierarchical and parallel organisation

Sensory input guides output (eg. using information from receptors in the muscles)

Nature and locus of control change with learning (Things that typically require conscious management become automatic as we learn)

Question 3

What are the two main subcortical structures of the motor system?

Answer 3

The basal ganglia and the cerebellum

Question 4

How do we measure muscle activation?

Answer 4

The potential difference between 2 electrodes placed on the skin

Question 5

Where does the input to muscles come from?

Answer 5

Alpha motor neurons - causes muscles to contract

Question 6

How do muscles work?

Answer 6

In agonistic pairs - they act under tension

Question 7

What is the combination of an alpha motor neuron and muscle fibres referred to as?

Answer 7

Motor units

Question 8

Describe muscles that we need really fine control over?

Answer 8

They will have very few muscle fibres in a muscle unit - we need a lot of alpha motor neurons and cortical area designated to these muscles

Question 9

What about muscles we dont have very high control over?

Answer 9

We have many muscle fibres - producing a lot of force instead of control

Question 10

Can you have graded force from a motor unit?

Answer 10

NO ALL OR NOTHING

They either fire or they don’t

Question 11

What are the types of sensory receptors within muscles?

Answer 11

1. Golgi tendon organs

2. Muscle spindles

Question 12

Golgi tendon organs?

Answer 12

Embedded in tendon that connects muscle to bone

Senses force or tension in muscle itself

If didn’t have, muscles could contract to strongly - they prevent the muscle from ripping off the bone

Give feedback re: produced force

Question 13

Muscle spindles?

Answer 13

Embedded in muscle tissue -
Detect changes in muscle length (stretch receptors)

Have their own muscle control system (intrafusal muscle fibres) - if they didn’t have these, wouldn’t be tension in muscle, would lose their ability to sense when muscles relax - need these to be able to give feedback

Question 14

What area mediates simple reflexes?

Answer 14

Circuits in the spinal cord (no brain structures involved/unconscious movement control)

Question 15

Explain the response of the body during a stretch reflex?

Answer 15

This is monosynaptic

Body at rest
After eg. tap on knee occurs there is an increase in firing rates
Synapse onto alpha motor neuron of quadriceps

Question 16

Explain a withdrawal reflex

Answer 16

This is not monosynaptic

Eg. stab finger - there is an increase in firing rates from the finger

• Excitatory connection in the bicep muscles to pull away
• Inhibitory spinal interneurons are excited, inhibiting triceps as want to turn of muscles that would have a deleterious effect

These combined (contraction of biceps and relaxation of triceps cause a rapid flexion of the elbow joint)

Question 17

Which hemispheres control each side of the body?

Answer 17

It is actually possible to have ipsilateral control and bilateral control (some cross over some don’t)

Question 18

What structures are considered to be the ‘proper’ basal ganglia?

Answer 18

Caudate nucleus
Putamen
Globus pallidus

Question 19

Why is the basal ganglia involved in motor control?

Answer 19

Once behaviours move away from higher order processing and behaviours become more automatic - transfers to the basal ganglia

Damage - problems with learned movements

Question 20

What are the 3 regions in the cerebellum?

Answer 20

1. Neocerebellum - outer layers, role in motor planning
2. Spinocerebellum
3. Vestibulocerebellum - balance

Question 21

What is the role of the cerebellum?

Answer 21

Acts as a comparator - takes information from the descending signals from upper level regions and compares them to the feedback from somatosensory and vestibular systems - error signals produced to be able to modify future movements

Question 22

What if the cerebellum is damaged?

Answer 22

• Problems adapting to novel situations if you cannot make these comparisons (working out how to move in a novel environment)
• Movements can be too large or too small - cant adapt to reduce this error in subsequent errors
• balance and eye movement problems

Question 23

Primary motor cortex?

Answer 23

Motor commands descend

• Brocas area (speech)
• Frontal eye field (obstacle avoidance, intercepting objects)

Question 24

Secondary motor areas?

Answer 24

Area 6
- pre-motor cortex (take signals from higher areas)
Involved in the planning of movements and coordinating more complex movements

Question 25

Motor homunculus?

Answer 25

Different parts of the body are controlled by different regions of the brain

Question 26

What is the monkey experiment regarding sensory feedback requirements for action?

Answer 26

Dorsal route is severed in a single side of the body - no sensory feedback from this side. The monkey here chooses not to use the limbs that have no sensory feedback

However, if you severe the dorsal route on the other side as well, you have no sensory feedback from either of the limbs and the monkey starts to use both limbs again

In the first scenario the brain is choosing not to use the limbs but they can be used! sensory feedback is not required for action

Question 27

What is endpoint control?

Answer 27

The idea that movements are planned based on the final goal of the movement (not planning how to get there)

Question 28

What do motor movements code for?

Answer 28

1. Movement direction

2. Muscle activity

Question 29

What is a population vector?

Answer 29

Provides cortical representation of a movement

Question 30

Can population vectors be used to control external devices? explain the rat experiment

Answer 30

Rat presses on a lever to control a robotic arm to receive a sugary drink reward

Recordings of the activations in the primary motor neurons mean that can actually disconnect the lever from the robotic arm to control the level directly from brain activation and still receive reward.

Rat thinks about movement causing the neurons to fire as they were when doing the movement

Question 31

How can population vectors be used to control things in the human world?

Answer 31

• can control prosthetic limbs through brain activation patterns
• can control computer games

Question 32

Explain the effect of the cerebellum on timing (arm positioning/elbow-extension experiment)

Answer 32

Target appears and need to move arm to match.

Normal patients:
Done through a burst in the agonist muscle followed by a burst in the antagonist muscles to decelerate arm into intended target position - no undershoot or overshoot in this case

Cerebellum deficits patients:
Get stereotypical wanderings around the target because cerebellum regulates the timing of the bursts in the muscles - produce too much timing in the agonist muscle to begin with which causes overshoot, the burst in the antagonist muscle then causes them to come back to an undershot again.. multiple bursts until reach the target - do get there but it isn’t a smooth pursuit

Question 33

Basal ganglia and appropriate response initiation?

Answer 33

Holds all responses in check until there is a trigger and it will release the most appropriate response

Regulates movement

Question 34

The indirect and direct pathways of the basal ganglia?

Answer 34

Direct: is the accelerator as the direct pathway inhibits the basal ganglia - meaning it doesnt have as much control as it normally does

Indirect: is the brakes! excites the basal ganglia and therefore it has its normal control, inhibiting output

Question 35

What is the normal pathway of the basal ganglia?

Answer 35

These are normally in balance, allowing for regulation of movement

Question 36

What happens if the indirect/direct pathways of the basal ganglia aren’t in balance?

Answer 36

Can be out of balance in both directions

Things such as parkinsons disease arise: lack of movement - thalamus reducing less drive onto the motor cortex

Huntingtons disease: lot more uncontrolled movement - reducing braking

Question 37

Prism adaptation experiment?

Answer 37

Reaching for a target

There is then a visual shift and hand movements are displaced relative to where you thought they were - with learning via feedback or comparative function (cerebellum) you can improve (learn)

When visual rotation is then removed and vision lines up with what you would expect there are initial errors because motor commands have adapted to the change and again need to relearn

Question 38

Prism adaptation experiment in those with cerebellum impairments?

Answer 38

Before a rotation is imposed, you get small errors compared to normal controls where you see a reduction in error

when there is a visual shift, they continue to make very large errors (60-70degrees) because they are unable to update motor commands to compensate for environmental changes

when changed back to normal, there is very little error and no relearning needs to occur because they didn’t adapt to the shift

Question 39

Role of dopamine in learning motor skills?

Answer 39

Destroyed dopaminergic neurons in rats - these rats were not able to learn.

As soon as the rats got a dopamine injection, they then began to learn

When the infusion ended, because learning has already occurred and has been consolidated they are able to move at this new found success rate so dopamine is just required for the initial learning

Question 40

Using TMS to affect learning?

Answer 40

TMS can be used to increase or decrease brain region activity

If the motor cortex is depressed through TMS, you don’t get as much learning
If you wait 6 hours after initial learning and then stimulate, you don’t get a reduction in performance because the motor cortex is now not needed

locus of control has changed and the movement has been consolidated into a different region

Question 41

Mirror neurons and expertise?

Answer 41

Skilled dancers observing a dance in which they are experts in - activated neurons in motor cortex

Question 42

What does movement adaption require?

Answer 42

Requires the same goals but different motor commands

Question 43

Why does movement adaption occur?

Answer 43

Thought to occur due to error signals that are created through forward models

Question 44

What are forward models?

Answer 44

Networks in the brain that predict the sensory consequences of a movement

When prediction doesn’t match the actual state, an error signal is created

This error signal acts as a training signal to update the forward model