Week 10: Motor systems Flashcards
How does the motor system work?
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)
What are the 3 governing principles of the motor system?
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)
What are the two main subcortical structures of the motor system?
The basal ganglia and the cerebellum
How do we measure muscle activation?
The potential difference between 2 electrodes placed on the skin
Where does the input to muscles come from?
Alpha motor neurons - causes muscles to contract
How do muscles work?
In agonistic pairs - they act under tension
What is the combination of an alpha motor neuron and muscle fibres referred to as?
Motor units
Describe muscles that we need really fine control over?
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
What about muscles we dont have very high control over?
We have many muscle fibres - producing a lot of force instead of control
Can you have graded force from a motor unit?
NO ALL OR NOTHING
They either fire or they don’t
What are the types of sensory receptors within muscles?
- Golgi tendon organs
2. Muscle spindles
Golgi tendon organs?
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
Muscle spindles?
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
What area mediates simple reflexes?
Circuits in the spinal cord (no brain structures involved/unconscious movement control)
Explain the response of the body during a stretch reflex?
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
Explain a withdrawal reflex
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)
Which hemispheres control each side of the body?
It is actually possible to have ipsilateral control and bilateral control (some cross over some don’t)
What structures are considered to be the ‘proper’ basal ganglia?
Caudate nucleus
Putamen
Globus pallidus
Why is the basal ganglia involved in motor control?
Once behaviours move away from higher order processing and behaviours become more automatic - transfers to the basal ganglia
Damage - problems with learned movements
What are the 3 regions in the cerebellum?
- Neocerebellum - outer layers, role in motor planning
- Spinocerebellum
- Vestibulocerebellum - balance
What is the role of the cerebellum?
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
What if the cerebellum is damaged?
- 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
Primary motor cortex?
Motor commands descend
- Brocas area (speech)
- Frontal eye field (obstacle avoidance, intercepting objects)
Secondary motor areas?
Area 6
- pre-motor cortex (take signals from higher areas)
Involved in the planning of movements and coordinating more complex movements
Motor homunculus?
Different parts of the body are controlled by different regions of the brain
What is the monkey experiment regarding sensory feedback requirements for action?
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
What is endpoint control?
The idea that movements are planned based on the final goal of the movement (not planning how to get there)
What do motor movements code for?
- Movement direction
2. Muscle activity
What is a population vector?
Provides cortical representation of a movement
Can population vectors be used to control external devices? explain the rat experiment
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
How can population vectors be used to control things in the human world?
- can control prosthetic limbs through brain activation patterns
- can control computer games
Explain the effect of the cerebellum on timing (arm positioning/elbow-extension experiment)
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
Basal ganglia and appropriate response initiation?
Holds all responses in check until there is a trigger and it will release the most appropriate response
Regulates movement
The indirect and direct pathways of the basal ganglia?
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
What is the normal pathway of the basal ganglia?
These are normally in balance, allowing for regulation of movement
What happens if the indirect/direct pathways of the basal ganglia aren’t in balance?
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
Prism adaptation experiment?
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
Prism adaptation experiment in those with cerebellum impairments?
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
Role of dopamine in learning motor skills?
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
Using TMS to affect learning?
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
Mirror neurons and expertise?
Skilled dancers observing a dance in which they are experts in - activated neurons in motor cortex
What does movement adaption require?
Requires the same goals but different motor commands
Why does movement adaption occur?
Thought to occur due to error signals that are created through forward models
What are forward models?
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
