S3: The Cerebellum

Question 1

List the advisory centres for the motor cortex in the brain

Answer 1

• The basal ganglia.

- The cerebellum.

Question 2

Describe the basal ganglia and cerebellum as subcortical motor controllers

Answer 2

• The basal ganglia and cerebellum regulate planning and execution of voluntary movement.
• They don’t send commands but they advise the motor cortex.
• Both receive cortical inputs and both project to the motor cortex (via thalamus).
• Both needed for accurate smooth movement and posture - the motor cortex is not enough.
• Lesions in the basal ganglia and cerebellum lead to motor symptoms (positive and negative) which highlights their importance in movement.

Question 3

Describe the volume of neurones in the cerebellum and the afferent and efferent fibres of it.

Answer 3

The cerebellum is 10% of our brain volume but has more than 50% of our brain neurones. CBM has expanded during evolution increasing with different species going up the evolutionary tract.

• There are 40 x more input axons than output so more afferent tracts going into the cerebellar peduncles than efferents.
• The efferent outputs are usually motor orientated and they tend to go to motor areas e.g. motor and premotor cortex.
• However, the efferents do not go to the spinal cord.

Question 4

What are the three types of symptoms associated with cerebellar lesions?

Answer 4

1. Hypotonia: low muscle tone always (muscles are healthy though) and uncoordinated muscle contraction.
2. Postural ataxia: Can’t keep stable posture e.g. sway and fall especially with eye closed.
3. Intention (‘ataxia’) tremor: Overshoot, oscillations of voluntary movements which is linked to the execution of tasks.

Question 5

What is the function of the cerebellum?

Answer 5

1. It helps improve performance in future: crucial for motor learning of complex motor tasks e.g. keyboard typing improves with more practice and time.
2. Rapid on line refinement of rapid ‘ballistic’ movement - fast, adaptive, accurate successful movements e.g. tennis player hitting ball with racket.

Question 6

Describe theory on mechanism of function: CBM as a feedback comparator

Answer 6

• The motor cortex (in the cerebral cortex) sends down a set if motor commands for each movement, down to the pyramidal neurones in motor cortex to activate a set of spinal alpha motor neurones to contract a set of muscles.
• As muscles contract (and make the movement), the receptors (spindle afferents) in muscles are sending back sensory information to CNS through the spinal cord.
• It is thought that as the set of motor commands go out, they are also sent to the cerebellum and compared to sensory feedback from the muscles (compare what is desired to what actually happened).
• On the basis of the different, CBM sends a adaptive signal back to the cerebral cortex (motor cortex) to change the firing of neurones in the motor cortex. A feedback comparison control system.
• However, this mechanism is not what happens in real life as it is too slow for rapid movement e.g. Tennis hitting ball. Each synaptic delay is about 10 milliseconds.
• This theory is ok for motor learning but too slow for one line control.

Question 7

Describe theory on mechanism of function: CBM as a feed-forward comparator

Answer 7

• Before the movement (motor commands) even goes out from the cerebral cortex (motor cortex), it is being compared with a internal model of the whole body system.
• This is a lot of information but the cerebellum contains many nerve cells. So the cerebellum with its huge amount of neurones, contains an internal map of the whole body.
• This means that a set of commands going out from the motor cortex, before they can even take effect, they are being compared in the cerebellum.
• The cerebellum is advising on how the movement will go, what the movement will be like etc. This occurs rapidly.
• This theory is our most accurate.

Question 8

What are the three anatomical/functional domains of the cerebellum?

Answer 8

• Spino-cerebellum: modulates descending motor systems in brainstem (via reticular formation vestibular nuclei). This system is more primitive.
• Vestibulo-cerebellum: regulates balance and eye movements (via vestibular nuclei in brainstem).
• Cerebro/cortico-cerebellum: high level of planning of movement, regulates cortical motor programs (via thalamus to cortex).

Question 9

Describe the cerebro-cerebellar pathway

Answer 9

• The pyramidal neurone sending its long, thick projection fibres down its tract to a-motor neurones in spinal cord. It branches on the way and sends a collateral fibre to a relay in the pons which then synapses to a neurone that projects to the cerebellum.
• Each set of movement commands going down has its own unique fingerprint that will be mapped into the cerebellum (set of stimuli).
• From the cerebellum purkinje cells will feedback through relay through deep cerebellar nuclei, back up through the synapse in the thalamus to the cortical area (probably to some intermediate and then motor cortex).
• This is the cortico-cerebella cortical loop. This is how the cerebellum gives it feedback to cortex to adapt a skilled movement.

Question 10

What is the somatotopic map?

Answer 10

Somatotopic mapping of the body onto the cerebellum. There is a similar mapping on the cerebral hemispheres but not as accurate as cerebellum.

Question 11

Describe the cellular structure of the cerebellum

Answer 11

The cerebellar matrix is highly regular (ordered) structure compared to other parts of the brain. This is a basic circuit. It has purkinje cells which have huge neurones and granule cells which are very numerous.

Question 12

What is the output of the CBM?

Answer 12

• Out of the cerebellum is via purkinje cells which are GABAergic and inhibitory.
• They fire the deep cerebellar nuclei (which is excitatory) and synapse with other neurones that take the output to either the thalamus to the cerebral cortex or the vestibular nucleus for the reticular formation.

Question 13

Describe structure of purkinje neurones

Answer 13

• The soma for purkinje neurones is 50 micrometres in diameter (very big for a cell).
• 2D dendritic fields (it is in a layer).
• Ordered array of layers.
• They surround the granule cells.

Question 14

What are the two excitatory inputs to a purkinje cell?

Answer 14

1. Climbing fibres which are from the cerebral cortex and they go to the inferior olive. There is one per purkinje cell (p cell) but about 10 purkinje cells per climbing fibre. They enwrap the purkinje cells so if the climbing fibres fire one AP, the purkinje cell always fires.
2. Parallel fibres from granule cells. Moss fibres from the pons/brainstem synapse onto parallel fibres. There are 1 million parallel fibres that synapse per purkinje cell and many p-cells per parallel fibre. Each parallel fibre has one synapse and each purkinje cell will not always fire when parallel fibre fires AP as its input is much less.

Question 15

Describe cells affecting cerebellar function and cerebellar learning

Answer 15

• Cerebellum gets a collateral copy of AP (motor commands from motor cortex before movement occurs) and they activate granule cells. They will synapse purkinje cells (which may not all fire).
• Learning –> limbic system gives reward for correct movement which activates the climbing fibres to fire –> purkinje cells fire and synapses strengthened + climbing fibres get strengthened at the same time (because the climbing fibre sends an emotional reward) –> set of movement is adapted as cerebellum remembers. This system evolves as an individual grows.
• Climbing fibres are very potent input to P cell and produce long term changes in responsiveness to parallel fibre inputs. They promote synaptic strengthening in successful circuits.