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2.11. Cerebellum
Overview
As discussed in Section 2.1., the cerebellum is a hindbrain structure. Embryologically, it develops from the rhombencephalon, then the metencephalon, which then separates into the pons and cerebellum.
The cerebellum’s main role is to modify motor signals that come from the motor cortex so that the contractions of the body’s muscles are smooth, coordinated and as intended. The cerebellum does this ‘refinement’ by integrating sound, vision, proprioception and balance into the descending motor signals; the cerebellum then sends the motor signals back up to the primary cortex, and from here they will travel down the spinal cord and to the muscles (shown in Figure 2.11.2.).
The cerebellum’s main role is to modify motor signals that come from the motor cortex so that the contractions of the body’s muscles are smooth, coordinated and as intended. The cerebellum does this ‘refinement’ by integrating sound, vision, proprioception and balance into the descending motor signals; the cerebellum then sends the motor signals back up to the primary cortex, and from here they will travel down the spinal cord and to the muscles (shown in Figure 2.11.2.).
Anatomy
Anatomy
The cerebellum has three anatomical divisions, called lobes (these are shown in Figure 2.11.1).
The cerebellum can also be said to have three different anatomical divisions:
On top of this… functionally, the cerebellum has three divisions:
The cerebellum has three anatomical divisions, called lobes (these are shown in Figure 2.11.1).
- Anterior lobe
- Posterior lobe
- Flocculonodular lobe
The cerebellum can also be said to have three different anatomical divisions:
- Vermis (the median portion)
- Paravermis (the medial portions)
- Lateral zones (the most lateral portions – mainly made up of the cerebellar hemispheres)
On top of this… functionally, the cerebellum has three divisions:
- Vestibulocerebellum – coordinates balance and eye movements
- Spinocerebellum – coordinates trunk and proximal limb movements (vermis) and distal limb movements (paravermis)
- Cerebrocerebellum (also called the neocerebellum) – involved in coordinating and planning voluntary movements.
Physiology
Physiology
The cerebellar pathways are complex, but they can be simplified into three sequential stages which you can read about below.
Stage 1
There are two fibre types that enter the cerebellum (i.e. are afferent fibres) – these are the climbing and mossy fibres that carry proprioceptive, vestibular/balance, auditory and visual information into the cerebellar cortex (by the way, the cerebellar cortex is totally different to the cerebral cortex!) Both the climbing and mossy fibres are excitatory, and they have multiple connections, the first of which arises just as the fibres enter the cerebellar cortex and the connection is with the deep cerebellar nuclei. So, the climbing and mossy fibres excite the deep cerebellar nuclei which send signals to the motor cortices.
Stage 2
Remember I said that the climbing and mossy fibres have multiple connections? Well, the second set of connections is with inhibitory Purkinje cells in the cerebellar cortex. These Purkinje cells then go on to have connections with the deep cerebellar nuclei, which they inhibit. This stage 2 pathway takes a little longer to operate than the stage 1 pathway mentioned earlier. So, the combined effects of stages 1 and 2 are initial excitation followed by inhibition. Now onto stage 3…
Stage 3
Another type of cell in the cerebellar cortex, the Golgi cell, inhibits the mossy fibres going to the Purkinje cells. When this happens, the Purkinje cells are inhibited from inhibiting the deep cerebellar nuclei (this is called disinhibition, meaning the deep cerebellar nuclei can send out excitatory signals). This stage 3 pathway takes even longer to operate than the stage 1 and 2 pathways. So, the combined effect of all three pathways is now excitation, then inhibition, then disinhibition (aka excitation).
So, in conclusion, most of the output from the cerebellum is excitatory to the motor cortices.
The cerebellar pathways are complex, but they can be simplified into three sequential stages which you can read about below.
Stage 1
There are two fibre types that enter the cerebellum (i.e. are afferent fibres) – these are the climbing and mossy fibres that carry proprioceptive, vestibular/balance, auditory and visual information into the cerebellar cortex (by the way, the cerebellar cortex is totally different to the cerebral cortex!) Both the climbing and mossy fibres are excitatory, and they have multiple connections, the first of which arises just as the fibres enter the cerebellar cortex and the connection is with the deep cerebellar nuclei. So, the climbing and mossy fibres excite the deep cerebellar nuclei which send signals to the motor cortices.
Stage 2
Remember I said that the climbing and mossy fibres have multiple connections? Well, the second set of connections is with inhibitory Purkinje cells in the cerebellar cortex. These Purkinje cells then go on to have connections with the deep cerebellar nuclei, which they inhibit. This stage 2 pathway takes a little longer to operate than the stage 1 pathway mentioned earlier. So, the combined effects of stages 1 and 2 are initial excitation followed by inhibition. Now onto stage 3…
Stage 3
Another type of cell in the cerebellar cortex, the Golgi cell, inhibits the mossy fibres going to the Purkinje cells. When this happens, the Purkinje cells are inhibited from inhibiting the deep cerebellar nuclei (this is called disinhibition, meaning the deep cerebellar nuclei can send out excitatory signals). This stage 3 pathway takes even longer to operate than the stage 1 and 2 pathways. So, the combined effect of all three pathways is now excitation, then inhibition, then disinhibition (aka excitation).
So, in conclusion, most of the output from the cerebellum is excitatory to the motor cortices.
Clinical Top Tip:
Cerebellar Ataxia
Cerebellar ataxia is a collection of abnormal movement signs and symptoms that develop when the cerebellum is not working properly, for example, due to a tumour, bleed in the back part of the floor of the skull (called the posterior cranial fossa), problem alcohol use or multiple sclerosis. Cerebellar ataxia has three primary symptoms: unsteady gait, poor muscle coordination and uncontrolled eye movements. Dysarthria can also be a symptom of cerebellar ataxia – the term ‘dysarthria’ means individuals have difficulty controlling the muscles that they use to speak and swallow.
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