The cerebellar cortex is a uniform structure. It is divided into three distinct layers, the molecular layer, the Purkinje
cell layer, and the granular layer. The cerebellar cortex contains five types of neurons: (1) the Purkinje, (2) granule, (3) Golgi, (4) stellate, and (5) basket cell. The molecular layer is the outermost layer and contains primarily the axons of the granule cells, termed parallel fibers, as well as dendrites of the Purkinje and Golgi cells and two types of interneurons (stellate and basket cells). The large pear-shaped cell bodies of the Purkinje cells are aligned side by side in a single layer, known as the Purkinje cell layer. The extensive dendritic tree of the Purkinje cell extends into the molecular layer in a single plane. The granular layer is the innermost layer and contains primarily densely packed, small granule cells as well as a few larger interneurons (Golgi cells).
The mossy fiber afferents terminate in the granular cell layer. Mossy fiber contacts with dendrites of the granule and Golgi cell are known as cerebellar glomeruli. The mossy fibers alter the activity of the Purkinje cell via the parallel fibers, which are the axons of the granule cells. Each Purkinje cell receives inputs from numerous granule cells, and each granule cell collects inputs from several mossy fibers. The climbing fibers connect directly with the dendrites of the Purkinje cell. Each Purkinje cell receives input from a single climbing fiber, and one climbing fiber contacts 1 to 10 Purkinje cells.
The Purkinje cell is the only output neuron of the cerebellar cortex. Purkinje cells have an inhibitory action on the cerebellar nuclear neurons. Both mossy and climbing fiber afferents have an excitatory action on the Purkinje cell. The excitatory input is modulated by inhibitory interneurons (stellate and basket cells in the molecular layer, and Golgi cells in the granular layer).
Oscarssony proposed that the cerebellar cortex consists of numerous structural-functional units with distinct efferent and climbing fiber afferent connections, known as microzones. y Ito introduced the concept of corticonuclearmicrocomplexes. He defined a microcomplex as a cortical microzone and an associated group of nuclear cells that regulates a single function of motor control.
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