Fine structure of granular layer in turtle cerebellum with emphasis on large glomeruli

The pond turtle has an unfolded, helmet shaped cerebellum. Granular, Purkinje, and molecular layers are well defined. The light and electron microscopic structure of the granular layer is described with emphasis on morphological aspects of interest for microelectrode studies. The mossy fibers run rostrocaudally. They form many en passant swellings provided with a neurofibrillary core and shaped as slender ovoids, which constitute the central glomerular axon (CGA) of paradigmatically simple glomeruli. The terminal swellings have a more complicated outline and form complex glomeruli. The glomeruli range from 10 to 60 μm and from 7 to 20 μm along the major and minor axes, respectively. Many of these glomeruli are twice as big as those in the cat cerebellum. The surface of the largest CGAs equals the neuromuscular appositional surface of small motor end plates. About 15% of this surface has synaptic specialization. The number of granule cell dendrites participating in the largest glomerulus is higher than 150. The granule cell dendrites are longer than in mammals, but it is estimated that synaptic inputs would be attenuated by only 20% if they spread electrotonically to the soma. The dendritic tips are both simpler and larger than in mammals. They are filled with special mitochondria, having peculiar cristae and numerous matrix dense granules. Synaptic junctions between granule cell dendrites and the central glomerular axons are larger than in mammals, occurring as single, large patches or as series of 3 to 5 short junctions. These and other features are suggestive of a powerful synaptic link. Synapses between cerebellar afferents and Golgi cells exist. However, most of the boutons synapsing on Golgi cells belong to ascending granule cell axons.