Cerebellum

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Chapter 12

Cerebellum

Basic anatomy

The cerebellum, often refered to as the ‘little brain’, makes up only 10% of the total brain volume but contains more than 50% of the total number of neurons. It is composed of an outer layered cortical region and an inner subcortical mass of cells which form the deep cerebellar nuclei (DCN). The cerebellum is topographically arranged and has ipsilateral control of movement, that is, the left hemisphere of the cerebellum controls the left side of the body. The cerebellum is divided by two deep transverse fissures into three lobes, anterior, posterior and flocculonodular lobe. On the upper surface there are three functional regions separated anatomically by two longtitudinal stripes. The central vermis (medial zone) and two paravermal regions or hemispheres. The paravermal regions are divided into the lateral and intermediate zones.

The cerebellum receives information from many sources. Figure 12.1 shows the main afferent connections. Note that the vestibular and spinal inputs to the cerebellum remain ipsilateral as the information conveyed from these sources is also ipsilateral. However, the inputs from the cerebral cortex and the inferior olive (which contralateral) need to decussate (cross-over) before entering the cerebellum.

Figure 12.1 The afferent connections of the cerebellum.

Efferent connections

The efferent connections from the cerebellum are shown in Figure 12.2. Note that the pathway to the vestibular nuclei is a direct projection, however all other efferent pathways exit the cerebellum by one of the DCN.

Figure 12.2 The efferent connections of the cerebellum.

The axons from the dentate nucleus must again decussate to ensure the cerebral cortex receives information related to the appropriate side of the body. Output is ultimately destined to influence the the descending tracts which will innervate the relevant muscles for movement.

The intrinsic circuitry within the cerebellum

Somatosensory information remains topographically arranged within the cerebellum although less organized than that of the cerebral cortex. Histologically, the cerebellum is organized into five cell layers which form a modulatory circuit throughout the cerebellar cortex. This circuitry is highly complex and beyond the scope of this book. However, in brief, the cells involved are:

Mossy fibres, which transmit information into the cerebellum.

Granule cells, which form parallel fibres and make thousands of contacts with a Purkinje cell. This system also has a collateral which is excitatory to the DCN.

Purkinje cells, which transmit information out of the cerebellum. These cells are inhibitory at the DCN.

Climbing fibres, which form the inferior olivary nucleus. One climbing fibre has many excitatory contacts with one Purkinje cell and is excitatory to the DCN.

Basket cells, stellate cells, Golgi cells, which are found only within the cerebellar cortex and are inhibitory in output.

In brief, the cerebellar circuitry forms a loop between the input neurons (mossy fibres and granule cells/parallel fibres and climbing fibres) and the output neurons (Purkinje cells) (Fig. 12.3) – the final output being via the DCN. The Golgi, stellate and basket cells work as interneurons to control/modulate the flow of information through this loop with their inhibitory contacts on the Purkinje cell. Also, the whole circuit has various other connections which allow it to be self-modulating. The final output is decided by the balance of excitation and inhibition at the DCN (granule cell and climbing fibres excitatory and Purkinje cell inhibitory).

Figure 12.3 Summary of the intrinsic circuitry of the cerebellum.

Function of the cerebellum

The three regions of the cerebellum have similar intrinsic circuitry (Fig. 12.3) but different functions which are a consequence of the origins and destinations of the input and output neurons, respectively. In brief, the cerebrocerebellum loop provides the cerebellum with knowledge about the intended plan for movement and the vestibulocerebellum and spinocerebellum loops monitor the actual position and motion of the body during the movement. Any mismatch/error is corrected and the movement is refined in terms of both timing and magnitude to meet the original goal. The correction is managed directly or indirectly via various descending tracts.

Cerebrocerebellum

This loop is specifically involved in the regulating, planning and timing of movement. Its connections with the superior colliculus also give it a role in visually guided coordination of voluntary movement. In addition, it has a further role in regulating distal limb movment and speech, with damage to the region presenting as limb ataxia (S3.18, 26) and dysarthria (S3.16), respectively.

Vestibulocerebellum

This loop is important in the regulation of posture and balance (S3.18, 32) via axial muscle control and vestibular reflexes. It also governs the eye movements associated with equilibrium (vestibulo-ocular reflex) (S2.10).