The cerebellum divides into three lobes, each divided into lobules. The principal lobes are the ANTERIOR LOBE, which is separated by the PRIMARY FISSURE from the POSTERIOR LOBE and, separated by the POSTEROLATERAL FISSURE, the FLOCCULONODULAR LOBE. (These are also known respectively as the PALAEOCEREBELLUM, NEOCEREBELLUM and ARCHICEREBELLUM.) The lobes contain SOMATOTOPIC maps of the entire body, as well as neurons responsive to sensory input. They are organized, one behind the other, on an anterior-posterior axis, and are also divided into three longwise. Running along the centre (that is, down the midline from anterior to posterior) is a strip called the CEREBELLAR VERMIS; the tissues to either side of this are known as the CEREBELLAR HEMISPHERES, one on the right, one on the left. Each area, right and left hemisphere and the vermis, has different connections. These are best described in terms of function, as is outlined in the table on p. 146.
The internal organization of the cerebellum is highly structured (making it of considerable interest to modellers of NEURAL NETWORKS). The cerebellar cortex has three layers: the outer layer is the MOLECULAR LAYER, below which is the PURKINJE CELL layer, below which is the GRANULAR LAYER. The molecular layer is composed of axons from GRANULE CELLS (in the granular layer) which collateralize, sending PARALLEL FIBRES running horizontally through the molecular layer. The dendrites of Purkinje cells (from the layer below) also extend into the molecular layer, interacting with the parallel fibres. Also present in the molecular layer are STELLATE CELLS and BASKET CELLS, which
The functional divisions of the cerebellum and their connections
The Purkinje cell layer contains the impressive Purkinje cells, arranged in a single layer. These are large neurons, with cell bodies 50–80 μm wide and exceptionally extensive dendritic fields that extend into the molecular layer. The axons run down through the white matter to the deep cerebellar nuclei (where they form synapses and use GABA as their neurotransmitter): this is the only output of the cerebellar cortex. The Purkinje cell layer has two main inputs: one from MOSSY FIBRES, the other from CLIMBING FIBRES. These are discriminated both anatomically and electrophysiologically. The mossy fibres come from the granule cell layer (including the CEREBELLAR GLOMERULI) which provide the parallel fibres in the molecular layer, with which Purkinje cell dendrites make contact. Some 200000 parallel fibres contact each Purkinje cell. The climbing fibres come from outside the cerebellum—from the INFERIOR OLIVE. Axons from the inferior olive come directly through to the Purkinje cells, each fibre making multiple synaptic contacts with approximately ten Purkinje cells. Electro physiologically, mossy fibre activation produces very powerful and complex action potentials (see ACTION POTENTIAL) in Purkinje cells. Climbing fibres induce straightforward action potentials. The third layer of the cerebellar cortex is the granular layer, packed with small granule cells and the cerebellar glomeruli, complexes formed by granule cells and mossy fibres. This layer is astonishingly dense: there are more neurons in the human granule cell layer of the cerebellum than there are in the cerebral cortex.
This gives a brief idea of the organization of the cerebellum, introducing the main components and briefly explaining how they interact. However, this is a most complex structure and readers are encouraged to seek further information. The chapter in Kandel, Schwartz & Jessell (1991) is an excellent introduction.
Reference
Kandel E.R., Schwartz J.H. & Jessell T.M. (2000) Principles of Neural Science, 4th edn, McGraw-Hill, New York.
This is the complete article, containing 838 words
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