16. CONTROL OF MOVEMENT

Question 1

List some structures that comprise the motor network

Answer 1

• Motor cortex
• Pre-motor cortex
• Parietal cortex
• Basal ganglia

Question 2

What is the basal ganglia?

Answer 2

• The basal ganglia is a connection/group of interconnected neurones which are involved in motor control

Question 3

What’s the role of the basal ganglia in movement?

Answer 3

• The basal ganglia is involved in selecting the right action for a given situation

Question 4

What are hypokinetic disorders?

Answer 4

• Hypokinetic disorders are characterised by too little movement
• E.g Parkinson’s

Question 5

What are hyperkinetic disorders?

Answer 5

• Hyperkinetic disorders are characterised by too much movement

Question 6

Describe how dopamine levels can cause hyperkinetic or hypokinetic disorders

Answer 6

• Low dopamine = hypokinetic (too little movement)
• High dopamine = hyperkinetic (too much movement)
• Dopamine is proportional to movement levels

Question 7

What does the rate model of movement suggest?

Answer 7

• Proposed by Alexander & Delong
• Rate model suggests that changes in basal ganglia firing control movement
• Sub-thalamic nuclei –> Basal ganglia –I Thalamus & Motor cortex
• The output of the basal ganglia on the thalamus & motor cortex is inhibitory, so increased basal ganglia firing can decrease movement
• Output of subthalamic nuclei is excitatory on basal ganglia

Question 8

How can the rate model explain hypokinetic & hyperkinetic disorders of movement?

Answer 8

• According to the rate model, movement disorders are changes in basal ganglia firing
• Hypokinetic = increased firing of basal ganglia has inhibitory output on thalamus & motor cortex, Inhibition of motor cortex, decreases movement
• Hyperkinetic = decreased firing of the basal ganglia means there’s reduced inhibitory output on the thalamus & motor cortex. Decreased inhibition of motor cortex leads to too much movement

Question 9

What is Parkinson’s disease?

Answer 9

• Parkinson’s disease is a hypokinetic movement disorder clinically characterized by bradykinesia (lack of movement) & muscle rigidity
• Slow movements
• Issues with speed & scale of movement

Question 10

Explain Parkinson’s disease using the rate model

Answer 10

• Parkinson’s disease = too little movement
• Overstimulation of basal ganglia -> Increased firing of basal ganglia -> inhibition of thalamus
• Inhibition of thalamus -> inhibition of motor cortex
• Decreased movement

Question 11

What is Hemiballismus?

Answer 11

• Hemiballismus is a hyperkinetic movement disorder
• It’s characterized by flinging of one side of the body
• Too much unwanted movement
• Can be caused by damage to the sub-thalamic nuclei

Question 12

Explain Hemiballismus using the rate model

Answer 12

• Hemiballismus = too much movement & can be caused by damage to the sub-thalamic nuclei
• Output of sub-thalamic nuclei (STN) is normally excitatory
• If the STN is damaged, there will be decreased firing of the basal ganglia
• Decreased firing of basal ganglia results in reduced inhibition of the thalamus & motor cortex, resulting in excessive unwanted movement

Question 13

How does deep brain simulation provide evidence against the rate model?

Answer 13

• Deep brain stimulation involves shutting off the STN by placing electrodes in sub-thalamic nuclei & connecting it to a pacemaker
• Deep brain stimulation is used as a treatment for movement disorders
• But, the rate model suggests that shutting off the STN would produce unwanted movement. Therefore, this is a limitation of the rate brain model. It is not applicable to everything

Question 14

Instead of the rate model what should be considered when studying movement?

Answer 14

Instead of looking at the rate of firing, the rhythm of neurones interacting should be considered (beta oscillations)

Question 15

Describe what movement is in terms of state changes

Answer 15

• Movement refers to the change from one sensory state to another new sensory state
• The new sensory state needs to be stabilised & the old sensory state needs to be turned off
• An accurate prediction of the new sensory state is needed
• The change is state should be assessed to see if it’s appropriate

Question 16

How are beta levels involved in state changes during movement?

Answer 16

• High beta levels are needed to stabilise a new sensory state
• To change from one state to the next, the current sensory state needs to be turned off
• To turn off the current sensory state, low beta levels are needed

Question 17

How can hyperkinetic & hypokinetic movement disorders occur as a result of beta oscillations?

Answer 17

• Hypokinetic = High beta oscillations mean that the sensory state is too stable & can’t be turned off
• Hyperkinetic = Low beta oscillations mean that the new sensory state can’t be stabilised, constant change between states
• Beta oscillations and dopamine levels are inversely proportional

Question 18

What happens to beta oscillations in Parkinson’s disease?

Answer 18

• In Parkinson’s disease there’s a peak in beta oscillations

Question 19

How do high beta oscillations cause Parkinson’s disease?

Answer 19

• High beta oscillations mean that the current sensory state is too stable.
• Low levels of beta activity is needed to turn off the current state, but high levels lead to an inability to change states
• In Parkinson’s the movement is very slow & small, because the individuals are unable to turn off their current sensory state. Meaning they cannot carry out a new movement

Question 20

Give two ways in which beta oscillations can be lowered to treat Parkinson’s

Answer 20

1. Administer L-DOPA
   - L-DOPA can bind to the dopamine receptors & activate them to help suppress beta oscillations
2. Deep brain stimulation
   - Deep brain stimulation can suppress high levels of beta oscillations

Question 21

What happens to beta oscillations when deep brain stimulation is turned off?

Answer 21

• When deep brain stimulation is turned off, the beta oscillations rise again
• But the peak height for beta oscillations is lower & there’s a delay before the peak returns. The levels of beta oscillations rise slower not immediately

Question 22

What is the consequence of low beta oscillations?

Answer 22

• High beta oscillations are needed to stabilise a state
• Low beta oscillations can lead to excessive, unwanted movement as the states can’t be stabilised. There’s a constant change in state

Question 23

Give an example of a condition characterized by low beta oscillations?

Answer 23

• Tourette’s syndrome
• Low beta oscillations mean that the sensory state is always unstable.
• Excessive movement, because once a new movement is initiated it cannot be stabilised. As it cannot be stabilised, it will be turned off
• Cycle of initiating new movement without being able to stabilise it

Question 24

How can movement disorders caused by low beta oscillations be treated?

Answer 24

• Dopamine receptor antagonists can block the dopamine receptor to reduce the effect of dopamine. Beta levels/activity will increase
• The increase in beta oscillations will allow the sensory state to be stabilized, decreasing the amount of movement

Question 25

What is a cerebellar tremor?

Answer 25

• A cerebellar tremor is caused by damage to the cerebellum

- Issues with sensory feedback leading to overconnection of movement resulting in a tremor

Question 26

How does dopamine affect beta oscillations?

Answer 26

• High dopamine levels can suppress beta oscillations
• High dopamine = lower beta oscillations
• Low dopamine = higher beta oscillations (e.g Parkinson’s)