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Systems Neuroscience > Cerebellum: Basic Circuitry > Flashcards

Cerebellum: Basic Circuitry Flashcards

1
Q

After determining which parts of the brain are invovled and after establishing circuitry, what do we need to do?

A

Establish sites of plasticity- candidate sites from circuit details

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2
Q

Why do we want to know the cells where mossy fibre input (from the CS tone) and climbing fibre input (UCS shock or air puff) meet?

A

Synapses on these cells would be candidate sites for the plasticity underlying NMR conditioning

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3
Q

What is the cerebellum?

A

Little brain
Tight folds
Cerebellum has its own cortex

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4
Q

What are the two parts of the cerebellum?

A

Extensive cerebellar cortex
Compact deep nuclei

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5
Q

What does John Eccles 1967 work describe?

A

The structure of each of the cell types in the cerebellum

Their synaptic connections

Their electrophysiology

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6
Q

What is the cerebellar cortical circuitry?

A

Mossy fibres excite granule cells

Granule cell axons (parallel and ascending fibres) excite Purkinje cells

Purkinje cells inhibit cells in cerebellar nuclei

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7
Q

What is the first input to purkinje cells?

A

Mossy fibre inputs

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8
Q

What are mossy fibre inputs?

A

In the case of NMR conditioning these fibres convey information about the tone (CS) to area HVI

Frequency of firing increases with tone intensity

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9
Q

What are granule cells?

A

Mossy fibres synapse with granule cells

The axons of granule cells form parallel fibres, that synapse with the dendrites of Purkinje cells

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10
Q

How many granule cells are there?

A

There are very many granule cells - ~80% of all neurons in the human brain

At least 100 per mossy fibre, hence “expansion recoding”

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11
Q

What are the three layers of the cerebellar cortex?

A

Molecular layer (top)
Purkinje layer (middle)
Granule layer (bottom)

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12
Q

What are purkinje cells?

A

Purkinje cells bodies lie in the middle (purkinje cell) layer

Largest cell in cerebellar cotex with distinctive dendritic field

Sole output cells of the cerebellar cortex- output comes out through axons of purkinje cells and these travel back down through the granular layer into the white matter

Each purkinje cell receives 150,000 parallel fibre synapses

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13
Q

What is in the molecular layer of the cerebellar cortex?

A

Parallel fibres and dendrites of purkinje cells

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14
Q

What is in the granule layer or the cerebellar cortex?

A

Small granular cells

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15
Q

What are golgi cells?

A

Get input from parallel fibres
Project back to the synapses between mossy fibres and granule cells

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16
Q

Are golgi cells inhibitory?

A

Yes

The more parallel fibre input they get the more they reduce it

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17
Q

What is the presumed function of golgi cells?

A

Control expansion recoding

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18
Q

What is expansion recoding?

A

Not quite known fully

Believed to be part of the machinery that ensures that information arising at the synapses is in a suitable form

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19
Q

What are two additional cell types in the molecular layer of the cerebellar cortex?

A

Stellate cells
Basket cells

Both get input from parallel fibres
Both inhibitory

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20
Q

What do basket cells synapse with?

A

Purkinje cell BODY

21
Q

What do stellate cells synapse with?

A

Purkinje cell DENDRITES

22
Q

What is the presumed function of stellate and basket cells?

A

To balance average excitatory drive from parallel fibres

23
Q

What did Heiney et al. (2014) do?

A

Stimulated basket and stellate cells

24
Q

What did Heiney er al. (2014) find?

A

Showed that you can silence the spontaneous firing of purkinje cells by stimulating basket and stellate cells

Showed that you can get control of movement kinematics

25
Q

What are simple spikes?

A

The CS (mossy fibre) pathway causes purkinje cells to fire with simple spikes

These have spontaneous firing rates, usually about 50 spikes/sec but this can reach a maximum rate of >200 spikes/sec

26
Q

What is the second input to purkinje cells?

A

Climbing fibre inputs

27
Q

What are climbing fibre inputs?

A

Climbing fibre inputs come up from the inferior olive

Carry information about the UCS (airpuff/shock)

Typically fire at low frequencies (~1 spike/second)

28
Q

Describe climbing fibres in relation to purkinje cells.

A

Climbing fibres are all wrapped around purkinje cell dendrites
-acts as one enormous synapse

29
Q

Contrast mossy fibre input to climbing fibre input

A

Mossy fibres give rise to parallel fibres with about 150,000 different synapses per purkinje cell

Climbing fibres form one giant synapse

30
Q

What are complex spikes?

A

Produced by climbing fibre input (UCS pathway)

Unusual in shape

Very reliable - whenever climbing fibres fire, purkinje cell also fires

31
Q

What is the effect of complex spikes on output?

A

Little effect on output as they have a low frequency of firing compared to simple spikes

32
Q

What is a candidate site for synaptic plasticity?

A

Cerebellar cortex

Because parallel fibres and climbing fibres both synapse on Purkinje cells of lobule HVI

33
Q

What is the function of climbing fibre input?

A

Possibility that CF input acts to alter efficacy of parallel fibre synapses on Purkinje cells

Idea is that it acts as a teaching or error signal

34
Q

How can we examine if climbing fibre input is a teaching signal that alters the synapses by LTD?

A

Stimulating parallel fibres paired with stimulating climbing fibres

-this explores whether the firing of PF and CF in conjunction can alter the efficacy of the PF synapses on PCs

35
Q

Do we think that CF input (complex spikes) is a teaching signal?

A

Yes

If it is a teaching signal it fits with the fact that it is very relaible and does not affect the output

36
Q

What is another candiate for synaptic plasticity?

A

Deep nuclei

Because mossy fibres (CS) and climbing fibres (UCS) also come together at the deep cerebellar nucleus (in the case of NM conditioning the relevant nucelus is the interpositus nucleus)

37
Q

What is an issue with this second candidate?

A

Much less is known about the electrophysiology here, its not been extensively studied

38
Q

What is a candidate site for synaptic plasticity?

A

Cerebellar cortex

Because parallel fibres and climbing fibres both synapse on Purkinje cells of lobule HVI

39
Q

How could we relate a role in NMR condtioning to general functions of the cerebellum?

A

Clinical and experimental observations of cerebellar damage

Found the cerebellar damage does not cause paralysis but causes inaccurate movements that are slow and uncoordinated

40
Q

What do clinical observations then suggest?

A

Other parts of the brain issue movement commands e.g. cerebral cortex

BUT

The role of the cerebellum is to ensure that they are carried out properly

41
Q

What did Brindley (1964) suggest?

A

Purpose of the cerebellum is to learn motor skills, so when they have been learned a simple or incomplete message from the cerebrum will suffice to provoke their execution

-relates to user-friendliness- automaticity frees use the cerebral cortex

42
Q

What is the general role of mossy fibre inputs?

A

Information about the current state of the body
Information about current motor commands

In NMR- converys infomation about the tone (CS) to area HVI

43
Q

What is the general role of climbing fibre inputs?

A

Difficult to relate firing of CF to specific inputs
BUT
Usually related to sensory signals, such as touch, pain (e.g. airpuff, shock), visual inputs

44
Q

What are cerebellar ‘modules’?

A

Individual areas of the cerebellar cortex

Structure of the cerebellar cortex is very uniform over its entire surface

Different regions have different inputs and outputs, but the same basic organisation

45
Q

What are cerebellar ‘zones’?

A

External wiring is extremely diverse

Receive climbing fibre input from a unique region of the inferior olive and project to a unique region of the deep cerebellar nucleus
This in turn projects to a unique set of neural targets

46
Q

What is the idea of a cerebellar ‘chip’?

A

Basic cortical microcircuit appears to be very similar for the entire cerebellum

But different regions have different external connections

Hence, the idea of a cerebellar ‘chip’ = same basic learning algorithm used for a wide range of tasks

Responsible for not just for learning motor skills, but for cognitive processing, and involved in e.g. dyslexia, autism

47
Q

Why is the idea of a cerebellar ‘chip’ helpful?

A

Means we can borrow ideas from general theories of cerebellar function to understand NMR conditioning

And insights from NMR conditioning about basic cerebellar algorithm can be used to improve general theories

48
Q

Overall, how is eyeblink conditioning related to other ‘cerebellar’ tasks?

A

They are all mediated by the same basic neural circuit

Systems Neuroscience (16 decks)

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