Cerebellum Flashcards
What are the three layers of the cerebellar cortex
1) Molecular layer
2) Purkinje cell layer
3) Granule cell layer
What do mossy fibres synapse onto?
The bottom layer containing granule cells - 1 mossy fibre synapses with about 50 granule cells
They also synpapse onto parallel fibres
Where do parallel fibres synapse?
Purkinje cells in the middle layer of the cortex
Where do granule cells send their axons?
To the molecular layer where they split into two to produce parallel fibres
What are purkinje cells?
They are the sole output of the cerebellum
Largest cells in the cerebral cortex
They have a flat but large dendritic field
Can be visualised after injection of a specialised fluid
How many parallel fibre snpases can a purkinje cell receive?
150,000
What is a climbing fibre?
Each purkinje cell receives input from a single climbing fibre
They are axons of cells in the inferior olive, at the base of the brainstem
They form about 1000 synapses with each purkinje cell
What are purkinje cell simple spikes?
These fire spontaneously as ‘simple’ spikes at around 50 spikes/sec
What can increase the spiking rate of purkinje cells?
Parallel-fibre input can increase firing rate to 200 spikes /sec
What are the features of complex spikes?
They are produced by climbing fibre input
They are very reliable and efficient
Low frequency of firing compared with simple spikes, so little effect on output
What is the effect of cerebellar damage/ alcohol on the cerebellum?
Does not cause paralysis but can cause movements to be inaccurate, slow and uncoordinated
What did Brindley suggest is the role of the cerebellum in 1964?
To learn motor skills - this ‘frees up’ the cerebral cortex
Which fibres convey the error signal?
Climbing fibres
What is the the decorrelated learning rule?
A synaptic weight between parallel fibres and purkinje fibres is changed according to the correlation between the parallel fibres signal and the error signal conveyed by the climbing fibre eg
- if there is a positive correlation weight is reduced and vise versa
Learning stops when there is no longer correlation between any parallel-fibre signal and climbing fibre signal
How does the long term depression occur in the cerebellum?
Synapses between parallel fibres and purkinje cells become depressed with the error signal idea - Signals are positively correlated with climbing-fibre signals
Describe the vestibulo-occular reflex
When the head rotates the eyes move in the opposite direction to keep the visual world stable
It is important as it would be a problem if we didn’t have this reflex because vision would be blurred and lost = failure to detect prey or mate
When do we use the vestibulo-occular reflex?
When we work, walk etc
What area of the cerebral cortex is involved in the vestibulo-occular reflex?
The flocculus - inactivation of this area would prevent the feflex
Other than movement what else is the cerebellum thought to now be involved in?
Sensory prediction
Active sensing
Emotional and cognitive processing
Disorders such as autism, dyslexia and schizophrenia
What is the chip metaphor?
Corresponds to the idea of a cerebellar chip
Each functional subregion of the cerebellar cortex implements the same basic algorithm but it is plugged into its own unique external circuitry
Explains in an outline how the cerebellum could be involved in so many different functions
Why is it ineffective to tickle yourself?
You know in advance what is going to happen
Imaging evidence (Blakemore et al 1999) suggests that the attenuating signal comes from the cerebellum
This is consistent with the cerebellum predicting sensory effects of your own movements
What type of head movements does the cerebellum respond to?
Passive head movements eg does not respond to active head movements
This is useful in a wide range of circumstances and is important in distinguishing your own movement from that of something else which is important for survival
