The cerebellum is the largest part of the hindbrain. It originates from the dorsal aspect of the brain stem and overlies the fourth ventricle. The cerebellum is connected to the brain stem by three stout pairs of fibre bundles, called the inferior, middle and superior cerebellar peduncles (Fig. 11.1); these join the cerebellum to the medulla, pons and midbrain, respectively. The functions of the cerebellum are entirely motor and it operates at an unconscious level. It controls the maintenance of equilibrium (balance), influences posture and muscle tone, and coordinates movement.
Figure 11.1 Lateral aspect of the brain stem and cerebellum, showing the cerebellar peduncles. Parts of the anterior, posterior and flocculonodular lobes have been removed to display the peduncles more clearly.
External features of the cerebellum
The cerebellum consists of two laterally located hemispheres, joined in the midline by the vermis (Figs 11.2-11.4). The superior surface of the cerebellum lies beneath the tentorium cerebelli and the superior vermis is raised, forming a midline ridge. Conversely, the inferior vermis lies in a deep groove between the hemispheres. The surface of the cerebellum is highly convoluted, the folds, or folia, being oriented approximately transversely. Between the folia lie fissures of varying depths. Some of these fissures are landmarks that are used to divide the cerebellum anatomically into three lobes (Figs 11.2-11.5). On the superior surface, the deep primary fissure separates the relatively small anterior lobe from the much larger posterior lobe. On the underside, the conspicuous posterolateral fissure demarcates the location of small regions of the hemisphere (the flocculus) and vermis (the nodule), which together form the flocculonodular lobe.
Figure 11.2 Superior surface of the cerebellum.
Figure 11.3 Posterior aspect of the cerebellum.
Figure 11.4 Anteroinferior aspect of the cerebellum.
Figure 11.5 Schematic representation of the cerebellum in which the peduncles have been cut and the surface flattened out. The relationships between the anatomical and functional divisions of the cerebellum are shown.
External features of the cerebellum
The cerebellum controls the maintenance of equilibrium, posture and muscle tone and it coordinates movement. It operates at an unconscious level.
The cerebellum is connected to the medulla, pons and midbrain by the inferior, middle and superior cerebellar peduncles, respectively.
The cerebellum consists of a midline vermis and two laterally located hemispheres.
Anatomically, the cerebellum is divided into anterior, posterior and flocculonodular lobes.
Internal structure of the cerebellum
The cerebellum basically consists of an outer layer of grey matter, the cerebellar cortex, and an inner core of white matter. The white matter is made up largely of afferent and efferent fibres that run to and from the cortex and towards which it extends irregular, branch-like projections, referred to in older literature as the arbor vitae (Figs 11.6, 11.7). Buried deep within the white matter are four pairs of cerebellar nuclei (Figs 11.6, 11.8), which have important connections with the cerebellar cortex and with certain nuclei of the brain stem and thalamus.
Figure 11.6 Parasagittal section through the cerebellum. Mulligan’s stain.
Figure 11.7 Coronal section of the brain at the level of the dentate nucleus; myelin stain.
(Section courtesy of the National Museum of Health and Medicine, Armed Forces Institute of Pathology, Washington, DC, USA.)
Figure 11.8 Horizontal section through the cerebellum and brain stem at the level of the fourth ventricle, showing the cerebellar nuclei.
The cerebellar cortex is highly convoluted, forming numerous transversely oriented folia. Within the cortex lie the cell bodies, dendrites and synaptic connections of the vast majority of cerebellar neurones. The cellular organisation of the cortex is the same in all regions (Fig. 11.9). It is divided histologically into three layers:
1. The outer, fibre-rich, molecular layer
2. The intermediate, Purkinje cell layer
3. The inner granular layer, which is dominated by the granule cell.
Figure 11.9 (A–C) Transverse sections of cerebellar folia showing the layers of the cerebellar cortex.
Afferent projections to the cerebellum arise principally from the spinal cord (spinocerebellar fibres), inferior olivary nucleus (olivocerebellar fibres), vestibular nuclei (vestibulocerebellar fibres) and pons (pontocerebellar fibres). Afferent axons mostly terminate in the cerebellar cortex, where they are excitatory to cortical neurones. Fibres enter the cerebellum through one of the cerebellar peduncles and proceed to the cortex as either mossy fibres or climbing fibres, depending upon their origin (Fig. 11.10). All afferents originating elsewhere than the inferior olivary nucleus end as mossy fibres. Mossy fibres branch to supply several folia and end in the granular layer, in synaptic contact with granule cells. The axons of granule cells pass towards the surface of the cortex and enter the molecular layer. Here they bifurcate to produce two parallel fibres that are oriented along the long axis of the folium.
Figure 11.10 The cerebellar cortex. Diagram shows afferent and efferent connections and their relationships to the principal cells of the cerebellar cortex.
The Purkinje cell layer consists of a unicellular layer of the somata of Purkinje neurones. The profuse dendritic arborisations of these cells (see Fig. 2.1B) extend towards the surface of the cortex, into the molecular layer (Fig. 11.10). The arborisations are flattened and oriented at right angles to the long axis of the folium. They are, therefore, traversed by numerous parallel fibres, from which they receive excitatory synaptic input. Inhibitory modulation of intracortical circuitry is provided by numerous other neurones known as Golgi, basket and stellate cells. The axons of Purkinje cells are the only axons to leave the cerebellar cortex. Most of these fibres do not leave the cerebellum entirely but end in the deep cerebellar nuclei. The other type of afferent fibre entering the cerebellar cortex, the climbing fibre, originates from the inferior olivary nucleus of the medulla. These fibres provide relatively discrete excitatory input to Purkinje cells. At the same time, axon collaterals of climbing fibres excite the neurones of the deep cerebellar nuclei. Purkinje cells utilise GABA as their neurotransmitter, which means that the output of the whole of the cerebellar cortex is mediated through the inhibition of cells in the cerebellar nuclei.
Deep within the cerebellar white matter, above the roof of the fourth ventricle, lie four pairs of nuclei. From medial to lateral, they are known as:
dentate nucleus (Figs 11.6, 11.7, 11.8).
The dentate nucleus is by far the largest of the cerebellar nuclei and is the only one that can be discerned clearly with the naked eye (Fig. 11.6). It consists of a thin layer of nerve cells folded into a crinkled bag; as a result, it appears somewhat similar to the inferior olivary nucleus of the medulla, from which it receives afferent fibres. The cerebellar nuclei also receive extracerebellar afferents from the vestibular nuclei, reticular nuclei, pontine nuclei and spinocerebellar tracts, predominantly by means of collaterals of mossy fibres destined for the cerebellar cortex. From within the cerebellum, the nuclei receive dense innervation from the Purkinje cells of the cerebellar cortex itself. The cerebellar nuclei constitute the primary source of efferent fibres from the cerebellum to other parts of the brain. The principal destinations of efferent fibres are the reticular and vestibular nuclei of the medulla and pons, the red nucleus of the midbrain and the ventral lateral nucleus of the thalamus.
Internal structure of the cerebellum
Internally, the cerebellum consists of a surface layer of cortex, highly convoluted to form folia, beneath which lies white matter.
Within the white matter lie cerebellar nuclei (fastigial, globose, emboliform and dentate).
The nuclei are the origin of cerebellar efferent fibres.
Functional anatomy of the cerebellum
The cerebellum is often regarded as consisting of three functional subdivisions, based upon phylogenetic, anatomical and functional considerations (Fig. 11.5).
The archicerebellum, or oldest portion in phylogenetic terms, is equated with the flocculonodular lobe and the associated fastigial nuclei.
The paleocerebellum approximates to the midline vermis and surrounding paravermis, together with the globose and emboliform nuclei.
The neocerebellum comprises the remainder (and vast majority) of the cerebellar hemisphere and the dentate nuclei.
The archicerebellum is primarily concerned with the maintenance of balance (equilibrium). It has extensive connections with the vestibular and reticular nuclei of the brain stem, through the inferior cerebellar peduncles (Fig. 11.11). Vestibular information is carried from the vestibular nuclei to the cortex of the ipsilateral flocculonodular lobe. Cortical efferent (Purkinje cell) fibres project to the fastigial nucleus which, in turn, projects back to the vestibular nuclei and to the reticular formation. A significant proportion of fastigial efferents cross to the contralateral side of the brain stem. The influence of the archicerebellum upon the lower motor system is, therefore, bilateral and principally mediated by means of descending vestibulospinal and reticulospinal projections.
Figure 11.11 Connections of the archicerebellum. Contralateral projections of the fastigial nucleus are not shown.
The paleocerebellum influences muscle tone and posture. Afferents consist principally of dorsal and ventral spinocerebellar tract neurones that carry information from muscle, joint and cutaneous receptors and enter the cerebellum through the inferior and superior cerebellar peduncles, respectively (Fig. 11.12). Fibres terminate largely in the cortex of the ipsilateral vermis and adjacent paravermis. Cerebellar cortical efferents from these areas pass to the globose and emboliform nuclei and also to the fastigial nucleus. The globose and emboliform nuclei project via the superior cerebellar peduncle to the contralateral red nucleus of the midbrain, where they influence the activity of cells giving rise to the descending rubrospinal tract.
Figure 11.12 Connections of the paleocerebellum.
The neocerebellum is concerned with muscular coordination, including the trajectory, speed and force of movements. The principal afferent pathway consists of pontocerebellar fibres (Fig. 11.13). These originate in the pontine nuclei of the basal portion of the pons and cross to the opposite side, entering the cerebellum through its middle peduncle. Pontocerebellar neurones are influenced by widespread regions of the cerebral cortex involved in the planning and execution of movement. Pontocerebellar fibres terminate predominantly in the lateral parts of the cerebellar hemisphere. Output from the neocerebellar cortex is directed to the dentate nucleus. The dentate nucleus, in turn, projects to the contralateral red nucleus and ventral lateral nucleus of the thalamus. The dentate is the largest of the cerebellar nuclei and its efferents form the major part of the superior cerebellar peduncle. The ascending fibres decussate in the caudal midbrain just before reaching the red nucleus. Some relay in the red nucleus with rubrothalamic cells but most bypass the red nucleus and pass directly to the ventral lateral thalamus. The ventral lateral nucleus of the thalamus projects to the cerebral cortex, particularly the motor cortex of the frontal lobe. The neocerebellum thus exerts its coordinating role in movement primarily through an action on cerebral cortical areas, giving rise to descending corticospinal and corticobulbar pathways.
Figure 11.13 Connections of the neocerebellum.
Lesions of the cerebellum
A midline lesion of the cerebellum (such as a tumour) leads to loss of postural control; as a result it is impossible to stand or sit without toppling over, despite preserved coordination of the limbs.
Because of the pattern of ipsilateral and decussated pathways that enter and leave the cerebellum, unilateral lesions of the cerebellar hemisphere cause symptoms on the same side of the body. This is in contrast to cerebral lesions (e.g. in the cerebral cortex, internal capsule or basal ganglia), which give rise to contralateral symptoms.
A unilateral cerebellar hemispheric lesion causes ipsilateral incoordination of the arm (intention tremor) and of the leg, causing an unsteady gait, in the absence of weakness or sensory loss.
Bilateral dysfunction of the cerebellum, caused by alcoholic intoxication, hypothyroidism, inherited cerebellar degeneration/ataxia, multiple sclerosis or paraneoplastic disease, causes slowness and slurring of speech (dysarthria), incoordination of both arms and a staggering, wide-based, unsteady gait (cerebellar ataxia).
Cerebellar lesions also impair coordination of eye movements and the eyes exhibit a to-and-fro motion (nystagmus), greatest in amplitude when gaze is directed to the same side as the lesion. Nystagmus is a very common feature of multiple sclerosis. The combination of nystagmus with dysarthria and intention tremor constitutes ‘Charcot’s triad’, which is highly diagnostic of the disease.
Functional anatomy of the cerebellum
The archicerebellum corresponds to the flocculonodular lobe and fastigial nucleus. Its principal connections are with the vestibular and reticular nuclei of the brain stem and it is concerned with the maintenance of equilibrium.
The paleocerebellum corresponds to the vermis and paravermal area, together with the globose and emboliform nuclei. It receives fibres from the spinocerebellar tracts and projects to the red nucleus of the midbrain.
The neocerebellum corresponds to most of the cerebellar hemisphere and the dentate nucleus. It receives afferents from the pons and projects to the ventral lateral nucleus of the thalamus.
Cerebellar lesions cause incoordination of the upper limbs (intention tremor), lower limbs (cerebellar ataxia), speech (dysarthria) and eyes (nystagmus).