Term: cerebellar granule cell

Definition: The granule cells of the cerebellar cortex are among the smallest nerve cells in the body. The number of granule cells is enormous, and they are densely packed in the cerebellar cortex of all vertebrates. Rapid Golgi preparations show that the granule cell has an unmistakably characteristic shape, a globular cell body with three or four short, radiating dendrites. These processes are typically sinuous, branching only at their ends, where they produce a gnarled, claw-like, sometimes varicose inflorescence. The dendrites from several granule cells, perhaps as many as six, converge upon the mossy fiber terminal. The axon of the granule cell originates from the cell body, or frequently from the thicker stem of a dendrite, and snakes its way up through the granular layer. In the upper third of the granular layer, axons from neighboring granule cells come together to form thin bundles, which penetrate between the Purkinje cell bodies and ascend into the molecular layer. In this layer each granule cell axon bifurcates like a T, giving rise to a pair of long, thin fibers, 0.1-0.2 micrometers in diameter, running in opposite directions parallel to the longitudinal axis of the folium (Figs. 5 and 53). For this reason they were termed parallel fibers by Ramon y Cajal (1888b).