The cerebellum accounts for about 10% of the total weight of the brain and occupies the posterior cranial fossa. It consists of two hemispheres united in the midline by the vermis (Fig. 7.28). Three peduncles connect each hemisphere to the three parts of the brainstem (Fig. 7.20). The superior peduncle enters the midbrain, the middle peduncle consists of the transverse fibres of the pons and the inferior peduncle connects with the medulla. The ventral surface of the vermis lies on the superior medullary velum and the roof of the medullary part of the fourth ventricle.
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Figure 7.28 Cerebellum: superior aspect. |
The cerebellum is bounded posteriorly by a convex border that lies below the attached margin of the tentorium cerebelli. From this border the superior surface slopes concavely upwards, in conformity with the shape of the tentorium. The posteroinferior surface is boldly convex below the posterior border and occupies the concavity of the occipital bone.
The surface of the cerebellum is indented by fine slitlike sulci, between which lie more or less parallel folds or folia. In the main the folia and sulci lie transversely from side to side across the whole extent of the cerebellum.
A well-marked groove, the horizontal fissure (of no functional significance), extends around the posterolateral border of each hemisphere and divides the cerebellum into superior and inferior halves. On the superior surface, is a much shallower groove, the primary fissure. The hemispheres consist of a small anterior lobe on the superior surface in front of the primary fissure and a large posterior lobe comprising the rest of the hemisphere behind the primary fissure (Fig. 7.28). The tonsil is a roughly spherical lobule on the inferior aspect of the posterior lobe. The tonsil may be displaced down through the foramen magnum in conditions of severe raised intracranial pressure or in congenital craniovertebral and hindbrain malformations.
The vermis consists of superior and inferior parts. The superior vermis forms a ridge between the hemispheres (anterior lobe) on the superior surface. Its anterior part, the lingula, lies in contact with the superior medullary velum (Fig. 7.29). On the inferior surface of the posterior lobe there is a deep groove, the vallecula, between the hemispheres. The inferior vermis lies in the groove and consists of the tuber vermis, pyramid, uvula and nodule. The nodule lies on the roof of the fourth ventricle and projecting laterally from each side of the nodule is a slender band of white matter whose bulbous extremity, capped with grey matter, can be seen from in front, lying in the angle between cerebellum and pons. This is the flocculus; the choroid plexus of the fourth ventricle projects just beside it (Fig. 7.18). The two flocculi and the nodule form the flocculonodular lobe.
In functional terms the cerebellum is divided into a corpus cerebelli (which has afferents from the spinal cord and trigeminal nuclei, and inputs from the pontine nuclei) and a flocculonodular lobe (which, with the lingula, has connections with the vestibular nuclei). Lesions of the latter part lead to disturbances of equilibrium with no alteration of spinal reflexes. The anterior lobe and the pyramid are the main recipients of spinal and trigeminal afferents. Lesions of this part cause disturbances of postural mechanisms with increased muscle reflexes. Cerebropontine connections are relayed by the pontine nuclei via the middle peduncle to the posterior lobe, tuber vermis and uvula. Lesions of these regions result in hypotonia, diminished or pendulum muscle jerks, intention tremor, clumsy movements and nystagmus (oscillatory movement of the eye).
The essential function of the cerebellum is the coordination of movement. Cerebellar lesions do not cause paralysis, but disturbances of movement and balance.
Like the cerebrum, the cerebellum is surfaced with a cortex of grey matter, with the white matter internal. Unlike the cerebral cortex, the cerebellar cortex consists of three layers. Embedded within the white matter are four pairs of nuclei. The most lateral, the dentate nucleus, is the largest. It forms a crenated crescent, resembling the inferior olivary nucleus in the medulla, open towards the superior peduncle. Its main connections are cerebropontocerebellar, and its efferent fibres leave the hilum and pass to the contralateral red nucleus, thalamus and cerebral cortex.
Cerebellar peduncles and connections
The superior and middle peduncles are simple; the inferior peduncle contains a great mixture of fibres.
The superior peduncle contains efferent fibres, passing from the dentate nucleus to the red nucleus, thalamus and cortex of the opposite side. It also contains the afferent anterior spinocerebellar tract, and tectocerebellar fibres from the midbrain.
The middle peduncle contains afferent fibres from the pontine nuclei of the opposite side.
The inferior peduncle contains the efferent cerebellovestibular tract and the afferent vestibulocerebellar tract, both connected to the vestibular nuclei of the same side. It also contains the afferent posterior spinocerebellar and cuneocerebellar tracts (the latter consisting of proprioception fibres from the upper limb), and the olivocerebellar tract from the olivary nuclei of the opposite side.
Blood supply
Two arteries supply the large convex posteroinferior surface and one artery supplies the small upper surface of each cerebellar hemisphere. They anastomose with each other on the cerebellar surface, but their perforating branches into the cerebellum are, as elsewhere in the nervous system, end arteries.
The posterior inferior cerebellar artery is one of the most tortuous arteries in the body, and is the largest branch of the vertebral artery. It arises ventrally from the vertebral artery, near the lower end of the olive, and spirals back around the medulla below the hypoglossal rootlets and then between the rootlets of the glossopharyngeal and vagus nerves. It supplies the choroid plexus of the fourth ventricle and is distributed to the inferior vermis and the back of the cerebellar hemispheres. It supplies, in passing, the adjacent part of the medulla as described on page 481.
The anterior inferior cerebellar artery arises from the basilar artery at the lower part of the pons and passes back on the inferior surface of the cerebellar hemisphere, supplying this surface and the adjacent flocculus. It may give rise to the labyrinthine artery if it has not arisen from the basilar.
The superior cerebellar artery arises near the termination of the basilar and passes laterally to wind around the cerebral peduncle below the fourth nerve. It is distributed over the superior surface of the cerebellum.
Venous drainage is from the surface of the cerebellum into the nearest available venous sinus of the dura mater. Thus the superior and posterior surfaces drain into the straight and transverse sinuses, inferior surfaces into the inferior petrosal, sigmoid and occipital sinuses.
