16. CONTROL OF MOVEMENT Flashcards
List some structures that comprise the motor network
- Motor cortex
- Pre-motor cortex
- Parietal cortex
- Basal ganglia
What is the basal ganglia?
- The basal ganglia is a connection/group of interconnected neurones which are involved in motor control
What’s the role of the basal ganglia in movement?
- The basal ganglia is involved in selecting the right action for a given situation
What are hypokinetic disorders?
- Hypokinetic disorders are characterised by too little movement
- E.g Parkinson’s
What are hyperkinetic disorders?
- Hyperkinetic disorders are characterised by too much movement
Describe how dopamine levels can cause hyperkinetic or hypokinetic disorders
- Low dopamine = hypokinetic (too little movement)
- High dopamine = hyperkinetic (too much movement)
- Dopamine is proportional to movement levels
What does the rate model of movement suggest?
- 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
How can the rate model explain hypokinetic & hyperkinetic disorders of movement?
- 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
What is Parkinson’s disease?
- 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
Explain Parkinson’s disease using the rate model
- 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
What is Hemiballismus?
- 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
Explain Hemiballismus using the rate model
- 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
How does deep brain simulation provide evidence against the rate model?
- 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
Instead of the rate model what should be considered when studying movement?
Instead of looking at the rate of firing, the rhythm of neurones interacting should be considered (beta oscillations)
Describe what movement is in terms of state changes
- 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
How are beta levels involved in state changes during movement?
- 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
How can hyperkinetic & hypokinetic movement disorders occur as a result of beta oscillations?
- 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
What happens to beta oscillations in Parkinson’s disease?
- In Parkinson’s disease there’s a peak in beta oscillations
How do high beta oscillations cause Parkinson’s disease?
- 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
Give two ways in which beta oscillations can be lowered to treat Parkinson’s
- Administer L-DOPA
- L-DOPA can bind to the dopamine receptors & activate them to help suppress beta oscillations - Deep brain stimulation
- Deep brain stimulation can suppress high levels of beta oscillations
What happens to beta oscillations when deep brain stimulation is turned off?
- 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
What is the consequence of low beta oscillations?
- 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
Give an example of a condition characterized by low beta oscillations?
- 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
How can movement disorders caused by low beta oscillations be treated?
- 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
What is a cerebellar tremor?
- A cerebellar tremor is caused by damage to the cerebellum
- Issues with sensory feedback leading to overconnection of movement resulting in a tremor
How does dopamine affect beta oscillations?
- High dopamine levels can suppress beta oscillations
- High dopamine = lower beta oscillations
- Low dopamine = higher beta oscillations (e.g Parkinson’s)
