Cerebellum and fourth ventricle

Cerebellum and Fourth Ventricle

Cerebellum

The cerebellum (L. cerebellum = little brain) is the largest part of the hindbrain and second largest part of the brain as a whole. It weighs about 150 g. It is located in the posterior cranial fossa underneath the tentorium cerebelli and behind the pons and medulla oblongata. It is separated from the pons and medulla by a cavity of the fourth ventricle (Fig. 10.1). Its surface bears numerous fissures separating narrow folia which are mostly transverse.

Fig. 10.1 Median sagittal section through brainstem and cerebellum.

The cerebellum is connected to the brainstem by these three pairs of large fibre tracts called cerebellar peduncles.

The three primary functions of the cerebellum are:

1. Maintenance of posture.

2. Maintenance of muscle tone.

3. Coordination of voluntary motor activity.

In addition to above, it also adjusts coordination of skilful volitional movements by perfect timing among the contracting groups of agonist and antagonist muscles. This is achieved through the use of somatic sensory information (proprioceptive sensations from muscles and joints) in modulating the motor output from the cerebrum and brainstem (see comparator function of cerebellum on page 119). Sherrington regarded the cerebellum as the head ganglion of the proprioceptive system.

The cerebellar disease manifests the following triad of motor dysfunctions:

• Dysequilibrium, i.e. loss of balance characterized by gate and trunkal ataxia.

• Hypotonia, i.e. loss of the resistance normally offered by muscles on palpation.

• Dyssynergia, i.e. loss of coordinated muscular activity.

External Features

The external features of the cerebellum comprise three parts, two surfaces, two notches, and three well-marked fissures (Figs 10.2–10.4).

Fig. 10.2 Superior view of the cerebellum. Note the presence of fissures and folia on the surface of the cerebellum.

Fig. 10.3 Schematic diagram to show the various subdivisions of the cerebellum as seen on the superior surface. (L = lingula, CL = central lobule, C = culmen, D = declive, F = folium, AL = ala, QL = quadrate lobule, LS = lobulus simplex, SSL = superior semilunar lobule.)

Fig. 10.4 Schematic diagram to show the various subdivisions of the cerebellum as seen on the inferior surface. (N = nodule, U = uvula, P = pyramid, T = tuber, BI = biventral lobule, ISL = inferior semilunar lobule.)

Parts

The cerebellum consists of two large lateral hemispherical lobes, the cerebellar hemispheres which are united to each other by a narrow median worm-like portion, called vermis. The superior and inferior aspects of vermis are termed superior and inferior vermis respectively. The ridge-like superior vermis is continuous on either side with the superior surface of cerebellar hemisphere imperceptively. The inferior vermis is more clearly demarcated from the hemispheres in the floor of vallecula cerebelli.

Surfaces

The superior surface of the cerebellum is convex. The two cerebellar hemispheres are continuous with each other on this surface. The inferior surface presents a deep median notch called vallecula which separates the two cerebellar hemispheres. The floor of the vallecula is formed by inferior vermis and is limited on each side by sulcus valleculae.

Notches

The anterior aspect of cerebellum is marked by a wide shallow anterior cerebellar notch which accommodates pons and medulla. The posterior cerebellar notch is deep and narrow, and lodges the falx cerebelli.

Fissures

• The horizontal fissure is most conspicuous and runs along the lateral and posterior margins of the cerebellum. It marks the junction between the superior and inferior surfaces of the cerebellum.

• The posterolateral fissure lies on the inferior surface of the cerebellum and separates the flocculonodular lobe from the rest of the cerebellum (corpus cerebelli).

• The V-shaped fissura prima on the superior surface cuts the superior vermis at the junction of its anterior two-third and posterior one-third. It divides the corpus cerebelli into anterior and posterior (middle) lobes.

N.B. There are several other fissures which subdivide the vermis and cerebellar hemispheres into lobules and given fanciful names (Table 10.1).

Table 10.1

Various subdivisions (lobules) of vermis and cerebellar hemisphere

Lobes	Subdivisions of vermis	Subdivisions of cerebellar hemisphere
Anterior lobe	Lingula Central lobule Culmen	No lateral projection Alae Quadrangular lobule
Posterior lobe	Declive Folium Tuber Pyramid Uvula	Lobulus simplex Superior semilunar lobule Inferior semilunar lobule Biventral lobule Tonsil
Flocculonodular lobe	Nodule	Flocculus

It is advisable for the student not to burden his memory with all these names, and should remember only those which have a functional or descriptive value.

Subdivisions of Cerebellum

Anatomical subdivisions

Anatomically the cerebellum is divided into three lobes (Fig. 10.5), viz. anterior, posterior and flocculonodular.

Fig. 10.5 Morphological and functional subdivisions of the cerebellum. The organ is being opened out (schematically) to show both superior and inferior surfaces together. The parts seen above the horizontal fissure form the superior surface and those below the fissure inferior surface of the cerebellum. (CL = culmen, C = central lobule, D = declive, F = folium, T = tuber, P = pyramid, U = uvula, AL = ala, QL = quadrate lobule, LS = lobulus simplex, SSL = superior semilunar lobule, ISL = inferior semilunar lobule, BL = biventral lobule.)

• The anterior lobe lies on the superior surface anterior to the fissura prima.

• The posterior/middle lobe lies between the fissure prima on the superior surface and posterolateral fissure on the inferior surface.

• The flocculonodular lobe is smallest of all and lies on the inferior surface in front of the posterolateral fissure.

The subdivisions (lobules) of the vermis and cerebellar hemispheres which constitute these lobes are listed in Table 10.1.

Morphological subdivisions (Fig. 10.5)

Based on phylogenetic and functional criteria the cerebellum is divided into three parts, archicerebellum, paleocer-ebellum, and neocerebellum.

Archicerebellum (vestibular cerebellum)

Phylogenetically it is the oldest part of the cerebellum. The fishes and lower amphibians possess only this component of the cerebellum. It consists of flocculonodular lobe and lingula.

The archicerebellum is chiefly vestibular in connections and concerned with the maintenance of equilibrium, tone and posture of trunk muscles.

Paleocerebellum (spinal cerebellum)

Phylogenetically it is next part of the cerebellum to appear in terrestrial vertebrates with the appearance of limbs, viz. reptiles and birds. It consists of anterior lobe (except lin-gula) and pyramid and uvula of inferior vermis.

The paleocerebellum is chiefly spinocerebellar in connections and concerned with the tone, posture and crude movements of the limbs.

Neocerebellum (cerebral cerebellum)

Phylogenetically it is the most recent part of the cerebellum to develop. It develops in primates in association with the enlargement of the telencephalon and the cerebral cortex. It is prominent in higher mammals. It is made up of middle lobe, the largest part of the cerebellum (except the pyramid and the uvula of inferior vermis).

The neocerebellum is chiefly corticoponto-cerebellar in connections and is concerned with smooth performance of skilled voluntary movements.

The features of three morphological subdivisions of the cerebellum are enumerated in Table 10.2.

Table 10.2

Components, nuclei, connections and functions of three morphological subdivisions of the cerebellum

Internal Structure

The cerebellum is made up of a thin surface layer of grey matter, the cerebellar cortex and a central core of white matter. Embedded within the central core of white matter are masses of grey matter called intracerebellar nuclei.

The cerebellar cortex is folded in such a way that the surface of cerebellum presents a series of parallel transverse fissures and intervening narrow leaf-like bands called folia. Each folium consists of a slender branched lamina of central core of white matter covered by a thin layer of grey matter. The central core of white matter being arranged in the form of the branching pattern of a tree, is called arbor vitae cerebelli (arbor vitae = tree of life).

Grey matter

The grey matter of the cerebellum is represented by: (a) the cerebellar cortex, and (b) the intracerebellar nuclei.

Structure of cerebellar cortex (Fig. 10.6)

The structure of the cerebellar cortex is uniform throughout (homotypical).

Fig. 10.6 Structure of cerebellar cortex, showing intrinsic neurons and their processes. A single folium is shown cut longitudinally (right) and transversely (left). Note that cerebellar glomerulus is a synaptic complex involving four types of neurites: the mossy fibre terminal, the dendrites of granule and Golgi cells, and the Golgi cell axon terminal.

Cerebellar cortex consists of three distinct layers: (a) an outer molecular layer, (b) an intermediate Purkinje cell layer, and (c) an inner granular layer.

Molecular (plexiform) layer

Molecular layer mainly consists of numerous dendritic arborizations of Purkinje cells and relatively few nerve cells which are widely spaced.

The nerve cells are of two types: (a) the basket cells, and (b) the stellate cells.

The basket cells are small in size with little cytoplasm and extensive processes. The longest of these processes (axon) of each cell assumes a transverse course parallel to the cortical surface and at right angle to the longitudinal axis of the folia. The transverse axons, synapse through numerous basket-like nets of collaterals with dendrites of many (about 500) Purkinje cells.

The stellate cells with characteristically short processes are scattered near the surface. Their axons arborise with dendritic spines of the Purkinje cells.

Purkinje cell layer

Purkinje cells layer consists of a single row of large flask-shaped cells, the Purkinje cells. A dendrite arises from the neck of the flask, passes upwards into the molecular layer, where it undergo profuse branching to form an elaborate dendritic tree. The dendrites of Purkinje cells synapse with: (a) the collaterals from the basket cells, (b) the axons of the granule cells (parallel fibres), and (c) the climbing fibres. The axons of the Purkinje cells arise from their deeper poles and pass through the granular layer into the white matter, where they relay into the intracerebellar nuclei (except those from the flocculonodular lobe which pass directly to the vestibular nuclei).

The outgoing Purkinje axons constitute the sole output from the cerebellar cortex and exert an inhibitory influence on the intracerebellar nuclei.

Granular layer

The inner granular layer consists of numerous closely packed small granule cells. This layer also contains few large Golgi cells.

Each granule cell gives rise to four or five short dendrites, which make claw-like endings which synapse with the terminals of the mossy fibres. The axon of each granule cell passes into the molecular layer where it bifurcates at a T junction, and its branches run parallel to the long axis of the cerebellar folium. These fibres are known as parallel fibres. They run at right angle to the dendritic processes of the Purkinje cells and make synaptic contacts with them.

The Golgi cells are large and prominent but scanty. Their den-drites ramify in the molecular layer.

Intrinsic neurons of the cerebellar cortex

There are five types of intrinsic neurons in the cerebellar cortex, viz. (a) Purkinje cells, (b) granule cells, (c) stellate cells, (d) basket cells, and (e) Golgi cells.

All the intrinsic neurons of cerebellar cortex are inhibitory except granule cells. Such a collection of inhibitory neurons is not found anywhere else in the CNS except in the cerebellum.

Intracerebellar nuclei

The intracerebellar nuclei (also called central nuclei) are masses of grey matter embedded in the white matter of the cerebellum. On each side of the midline they are four in number. From lateral to medial side these are: (a) dentate nucleus, (b) emboliform nucleus, (c) globose nucleus, and (d) fastigial nucleus (Fig. 10.7).

Fig. 10.7 Horizontal section of the cerebellum showing intracerebellar nuclei (also called central nuclei of the cerebellum).

The dentate nucleus is the most prominent of the intracerebellar nuclei and largest in primates, especially in humans. It is the nucleus of neocerebellum and therefore receives afferent fibres from it. In sections, it has a shape, like a crumpled-bag with its hilum facing anter-omedially. The interior of the nucleus is filled with white matter made up of efferent fibres that leave the nucleus through the hilum, forming most of the superior cerebellar peduncle. These fibres include dentorubral and dentothalamic fibres relaying in the red nucleus and ventral lateral nucleus of the thalamus respectively. Fibres from red nucleus and thalamus project to the spinal cord and cerebral cortex respectively.

The emboliform nucleus is oval in shape and situated medial to the dentate nucleus, partially covering its hilum. It is the nucleus of paleocerebellum, hence receives afferent fibres from it and gives fibres to the red nucleus via superior cerebellar peduncle. The red nucleus projects to the spinal cord through rubrospinal tract, which facilitates the flexor muscle tone.

The globose nucleus is rounded in shape and lies between the emboliform and fastigial nuclei. It has similar connections to that of emboliform nucleus. The globose and emboliform nuclei together are sometimes referred to as nucleus interpositus.

The fastigial nucleus lies near the midline in the vermis and close to the roof of the fourth ventricle. It is smaller than the dentate but larger than the emboliform or globose nuclei. It is nucleus of archi-cerebellum, hence receives afferent fibres from flocculonodular lobe (archicerebellum) and conveys efferent fibres to the vestibular and reticular nuclei. The fastigial connections influence the extensor muscle tone.

White matter

The white matter of cerebellum is made up of three types of fibres: (a) intrinsic, (b) afferent, and (c) efferent.

The intrinsic fibres remain confined within the cerebellum. They connect different regions of the cerebellum either in the same hemisphere or of the two cerebellar hemispheres. Through afferent and efferent fibres the cerebellum is connected with the other parts of the CNS.

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Tags: Textbook of Clinical Neuroanatomy

Jan 2, 2017 | Posted by admin in NEUROLOGY | Comments Off

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