Cerebellar Cortex and Nuclei

The Purkinje cell (PC) layer is a monolayer composed entirely of PCs, a 100 µm-thick sheet of 15 million neurons situated between the molecular and granular layers. The PC is the defining neuron of the cerebellum. It is among the largest cells in the nervous system, with a pear-shaped soma (35 × 70 µm) and a fanlike appearance of its dendritic tree. The proximal dendrite divides into two major dendrites that branch multiple times to form a flattened plate (400 × 20 µm) in the parasagittal plane oriented perpendicular to the long axis of the folium. Each PC has over 150,000 spines, with a density 25 times higher on distal dendrites, where parallel fibers (PFs) synapse, than on proximal dendrites, where climbing fibers (CFs) synapse. The PC is the only neuron with axons leaving the cerebellar cortex. The axon descends through a constricted region surrounded by the pinceau of basket cell axon terminals, acquires a myelin sheath, and descends to the deep cerebellar nuclei or vestibular nuclei. Recurrent collaterals course back toward the molecular layer, inhibiting interneurons as well as the soma and proximal dendrites of neighboring PCs.

The molecular layer is 300 µm thick. It contains granule cell axons and PFs, dendritic arborizations of PCs and Golgi interneurons, and cell bodies of basket, stellate, and supporting glial cells.

Parallel fibers are formed when the granule cell axon ascends through the PC layer into the molecular layer and branches in the shape of a T to form the PF, one of the thinnest vertebrate axons. It travels parallel to the long axis of the folium for 1 to 3 mm in the rat and cat, and possibly 6 to 8 mm in primates.

The basket cell lies in the lower third of the molecular layer just above the PCs. Its dendrites extend up into the molecular layer in a fan-shaped field 30 µm wide in the parasagittal plane (the same plane as the PC dendritic tree), giving off relatively few branches, interdigitating with the dendritic fields of the PCs, and contacted by the PFs. Its axon courses in the parasagittal plane among the lower dendrites of 9 or 10 PCs. It emits a succession of descending branches that envelope the PC somata in an axonal sheath with numerous synaptic contacts, giving the basket cell its name. Terminal axonal branches surround the initial segment of the PC axon in a dense fiber plexus with the appearance of an old paintbrush (French, pinceau). This axo-axonic complex is unique in the mammalian nervous system. Sparse ascending collaterals from the basket cell axon synapse on secondary and tertiary PC dendrites.

Stellate cells are small, 5 to 10 µm in diameter, with short, profusely branching dendrites contacted by parallel fibers and axons that terminate on PC dendrites. Superficial stellate cells in the upper molecular layer have short axons oriented in the parasagittal plane. Deep stellate cells in the middle part of the molecular layer have long axons up to 450 µm in the parasagittal plane, providing ascending and descending collaterals early in their course, but they rarely enter the pericellular PC plexus and do not participate in the pinceau.

The granular layer is 200 µm to 300 µm deep and contains granule, Golgi, Lugaro, and unipolar brush cells. Granule cells number about 50 billion, 3,000 per single PC. They have minimal cytoplasm, are among the smallest neurons in the brain (6-8 µm diameter), and are the most numerous. Their density renders the granule cell layer a deep blue on stains such as Nissl, which label nuclear material. The granule cell has three to five clawlike branched dendrites that participate in the granule cell glomerulus, pale islands between the granule cells containing a complex articulation between terminal rosettes of mossy fiber afferents, arborizations of granule cell dendrites, and Golgi cell axons. The granule cell axon ascends into the molecular layer where its PFs provide excitatory input to the PCs.

Golgi cells are irregularly rounded or polygonal inhibitory interneurons numbering approximately 1 per 1.5 PCs. In contrast to the PC, basket, and stellate cells, the Golgi cell dendritic tree has a three-dimensional configuration. Large Golgi cells, 10 to 24 µm in diameter, lie in the upper half of the granular layer, their dendrites arising as one or two main trunks with subsidiary branches that ascend to the outer zone of the molecular layer. Smaller Golgi cells, 9 to 18 µm, are in the depths of the granular layer, with dendrites that radiate out from the soma. One to three axons emerge from the Golgi cell body or from proximal dendrites and divide repeatedly, resulting in a multitude of fine branches that form an elaborate, dense plexus extending throughout the granular layer and participating in the granule cell glomerulus.

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