the cerebellum

The Cerebellum II: Cellular and Lobular Arrangement

by Sharadsaini on

lobesThe lobules and fissures of the cerebellum are more easily understood if it is imagined that the surface of the cerebellum has been flattened as shown opposite. Using this representation, many of the areas of the cerebellum can be quickly and easily drawn schematically, and their relationship to other cerebellar structures understood.

                                           The medial vermal cerebellum has been subdivided into lobes running down the middle. These are, from dorsal to ventral, the lingula, culmen, declive, folium, tuber, pyramid, uvula, and nodule. The various lobular subdivisions and the main lobes and fissures are also distinguished using this view.

                                            Most of the cerebellar cortex is buried in the folia, and only about 15% is visible. In section, the cerebellar cortex is seen to be uniformly structured throughout, with three clearly defined layers that contain five different types of neurons. The cerebellar cortical layers are, from the surface inwards, the molecular, Purkinje cell (sometimes called piriform), and the granular layers. The medullary layer lies beneath the granular layer.

                                          The molecular layer is relatively sparsely populated with two types of nerve cells: basket cells and outer stellate cells. The axons and dendrites of the outer stellate cells do not leave the molecular layer, and neither do the dendrites of basket cells. These processes run roughly horizontally in the layer, transverse to the long axis of the depth of the folia or infolding. The basket cell bodies are close to those of the Purkinje cells in the next layer, and project fibers that form basket shapes around the cell bodies of the Purkinje cells. Below this layer is the relatively narrow Purkinje cell layer. The Purkinje cells are large Golgi type I neurons; their cell bodies lie in rows along the folia, and their axons project to the intracerebellar nuclei. Some of these Purkinje axons in the archicerebellum project to the brainstem vestibular nuclei. Purkinje dendrites proliferate densely, transverse to the plane of the folia. Immediately below is the relatively wide granular layer, whose cells are very tightly packed and send axons up into the molecular layer, where they branch in Tshapes and run as parallel fibers along the horizontal axis of the folia. Each Purkinje dendritic tree may form synapses with up to half a million parallel fibers that have projected up from the granular layer. Also in the granular layer is a relatively small population of inhibitory Golgi neurons, which project their dendritic trees up into the molecular layer. One Golgi cell may synapse with a row of ten to twelve Purkinje cells, and it appears that Golgi cells do not overlap with respect to the innervation of the Purkinje cells.                       

                                        TCerebellum here are two main types of afferent input to the cerebellum, and both are excitatory. Each Purkinje cell is supplied by one climbing fiber from the contralateral inferior olive . The phylogenetically more ancient archicerebellum and paleocerebellum are served by the correspondingly older accessory olivary nuclear cells. The neocerebellum is supplied with fibers by the newer inferior olive. The second afferent input is through the mossy fibers from many different sources, including the pontine nuclei. These fibers diverge extensively, and one mossy fiber may serve several folia. The mossy fiber axons form multiple rosettes, which synapse with several granular cell dendrites. Inhibitory Golgi axons synapse in these rosettes. It follows, therefore, that since mossy fiber rosettes synapse with granular fibers, which in turn synapse with Purkinje cells, that one mossy fiber can indirectly affect electrical activity in very many Purkinje cells.

Cerebellum

by Sharadsaini on

The hindbrain or rhombencephalon consists of the medulla (myelencephalon), pons (metencephalon), and the cerebellum as its largest structure. The cerebellum consists of two hemispheres joined medially by a relatively narrow vermis, sits in the posterior cranial fossa of the skull beneath the tentorium cerebelli, and is separated from the medulla and pons by the fourth ventricle. The cerebellar cortex has many curved transverse fissures in the form of narrow infoldings called folia. Structurally, the cerebellum is covered by a cortex of gray matter with a medulla of white matter, which holds four intrinsic pairs of nuclei (see below). Observation of the superior surface shows two deep transverse fissures, the primary and the posterior superior fissures. Viewed from the ventral surface, the cerebellum is  divided approximately into superior and  inferior halves by the horizontal fissure. Three pairs of cerebellar peduncles connect the cerebellum to the three lower brain segments. The inferior, middle, and superior cerebellar peduncles connect it to the medulla, pons, and midbrain, respectively.

The superior vermis lies between the hemispheres as a longitudinal ridge; it is more clearly differentiated visually from the hemispheres on the ventral surface, where it is divided by fissures into the nodule, uvula, and pyramid. A stalk extends from the nodule on each side to the flocculus, which forms the flocculonodular lobe. The tonsil is a lobule that lies over the inferior vermis. The inferior medullary velum is exposed if the tonsil is removed.

From an embryological and functional viewpoint, the cerebellum can be divided into three main parts. (i) The archicerebellum, or flocculonodular node, is made of the pairs of flocculi and their peduncular connections. The flocculonodular node is the most ancient part of the cerebellum, present in fish as well as humans, and is connected with the vestibular nuclei and system. It is connected particularly with the dentate nucleus, one of the intrinsic medullary cerebellar nuclei. (ii) The paleocerebellum, or anterior lobe of the cerebellum, lies dorsal to the primary fissure. The lobe also includes the pyramid and uvula of the inferior vermis. The anterior lobe receives inputs via the spinocerebellar tract, originating in stretch receptors, and is the lobe most involved in the control of involuntary muscle tone. This lobe is connected principally to the globose and emboliform nuclei, which project to the red nucleus, and thence to the central tegmental, rubroreticular, rubsospinal, and rubrobulbar efferent pathways. The paleocerebellum evolved in terrestrial vertebrates, which need to use limbs to support the body against the pull of gravity; therefore its connections are mainly spinal, and its functions are concerned with such stereotyped movements such as posture, locomotion, and muscle tone. (iii) The neocerebellum, which, as its name implies, is the phylogenetically newest part of the cerebellum, communicates with the thalamus and motor cortex. This lobe is made up of virtually all the posterior lobe, except for the pyramid and uvula of the vermis. The neocerebellum modulates non-stereotyped, learned behavior such as the learning of manual skills.Cerebellum