Cerebellum and fourth ventricle

10


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.



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:



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:




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).









Fissures






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.





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







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).



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




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.





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).



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.


Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Cerebellum and fourth ventricle
Premium Wordpress Themes by UFO Themes