Control of Movement (neuro)

Question 1

Prediction

Answer 1

• movement is not simple, and only works with a lot of prediction
• prediction is key to successful movement
• there are multiple levels to consider movement at level of description/ levels of processing

Question 2

System approach to motor control

Answer 2

• basal ganglia
• parietal cortex
• motor and pre-motor cortex
• central and peripheral motor neurones, muscles
• afferents
• cerebellum

Question 3

Basal Ganglia

Answer 3

• The Basal Ganglia are a collection of heavily interconnected nuclei in the brain which are key to successful motor control.
• A key role appears to be in action selection: i.e. selecting the right action for the given situation.
• Damage to the basal ganglia (by degeneration or injury) is a common cause of movement disorders.

Question 4

Interim summary

Answer 4

• Motor control requires an integrated system.
• This system includes not just the mechanisms of
  movement, but also aspects of motivation, cost, appropriateness, importance etc to move or act at this moment.
• The basal ganglia are a key part of this network and seem to be particularly important in switching from one state to another (when to act) and also the cost of acting (the vigour of movement), and perhaps therefore also which particular action is most appropriate at this time.

Question 5

The rate (Alexander and Delong) model for how the basal ganglia affect movement

Answer 5

• Changes in firing rate (of the output nuclei) determine the
  degree of thalamic inhibition, and therefore the amount of
  movement possible.

Question 6

Parkinson’s disease

Answer 6

• Clinically dominated by a lack of movement: bradykinesia
• Beta suppression with levodopa correlates with reduction in Parkinson’s symptoms
• Replacement dopamine (L-dopa) suppresses LFS in STN LFP & suppression correlates with clinical improvement
• Beta suppression with Deep Brain Stimulation correlates with reduction in Parkinson’s symptoms
• Delayed return of bradykinesia after DBS correlates with delayed return of beta

Question 7

Hyperkinesia

Answer 7

• Hemiballismus is a flinging movement of one side of the body, typically caused by a subthalamic nucleus stroke.

Question 8

Interim summary 2

Answer 8

• The basal ganglia appear to be a central part of motor control.
• Damage or degeneration to the basal ganglia is strongly associated with disorders of movement.
• The simple idea that changes in rate of firing of the basal ganglia determines disorders of movement is not correct.
• At least in Parkinson’s disease, a particular rhythm abnormality appears to disturb control of movement, resulting in slow initiation and poor scaling (size, vigour) of movement.

Question 9

What is needed to be able to move?

Answer 9

• Moving is a change from one (stable) sensory state to another (stable) sensory state.
• In order for this to work you need:
  • To turn down the current sensory state (e.g. turn down the network that is supporting you sitting still).
  • To have an accurate prediction of the new sensory state. (e.g. create a plan/prediction of the new state such as standing up).
  • To have a mechanism to rate how important it is to do this movement rather than to stay in the current state or do another movement.
  • To have a way of stabilising the new sensory state.

Question 10

Movement disorders

Answer 10

• Actual movements in Parkinson’s disease appear normal – it is their initiation, scaling and persistence which is abnormal
• Many movement “disorders” are in fact normal movement occurring at the wrong time: the key abnormality is the inappropriate (sensory) urge to move
• Cerebellar Tremor: Possibly a problem with inappropriate response to sensory feedback that comes into the cerebellum
• In chorea and dystonia, core control of movement seems normal with “noise” added on top - a different level of movement control problem.

Question 11

Summary

Answer 11

• Normal motor control reflects the activity of a widely distributed neural system that integrates environmental information, information about the current state of the body, costs and opportunities of action with a system that can effect action plans through movement.
• Action is in an ever-shifting balance with not acting (or at least not changing the action that is currently happening). It seems that beta oscillations are a marker of the process that keeps the “status quo” and inhibits new action happening.
• Some movement disorders (e.g. Parkinson’s disease,
  Tourette’s syndrome) fit well with a disorder of this “brake” on
  action, with too much present in Parkinson’s and too little in
  Tourette’s.
• Other disorders of movement (e.g. dystonia, chorea, certain
  types of tremor) may arise from other motor control problems,
  for example control of “noise” in the sensorimotor system, or
  problems with generating an appropriate response to sensory
  feedback.