The mossy fibre-granule cell relay of the cerebellum and its inhibitory control by Golgi cells.

Abstract

1. The glomerulus in the cerebellar granular layer is composed of the three elements; the mossy fibre terminal, the granule cell dendrites and the Golgi cell axons. The afferent input to the cerebellar cortex through the glomerulus, the mossy fibre-granule cell relay (M.G.R.), and its inhibitory control by the Golgi cells were studied by recording, a) extracellular field potentials in the granular and molecular layers, b) unitary spikes of granule cells, and c) intracellular postsynaptic potentials in Purkinje cells. 2. Mossy fibres were activated by juxta-fastigial, transfolial, lateral cuneate nucleus and radial nerve stimulation. Stimulation of an adjacent folium (transfolial stimulation) could excite branches of mossy fibres under the stimulating electrode which supply other branches also to the folium under the recording electrode. This technique was utilized to distinguish the response due to mossy fibre activation from those due to the climbing fibre and Purkinje cell axons. 3. These stimulations resulted in, through the M.G.R., a powerful activation of granule cells whose axons (parallel fibres) excited in turn the Purkinje cells and the inhibitory interneurones, including the Golgi cells, in the molecular layer. 4. Field potentials and unitary spikes due to granule cell activity elicited by the stimulation of mossy fibres were markedly depressed for hundreds of milliseconds after the direct stimulation of parallel fibres (LOC stimulation). The postsynaptic potential in Purkinje cells evoked by mossy fibre activation was also depressed by the conditioning LOC stimulation in the same manner. The “spontaneous” background activities recorded from granule cells as unitary spikes and from Purkinje cells as inhibitory synaptic noise were silenced for hundreds of milliseconds after the LOC stimulation. 5. These depressions indicate that the parallel fibre activation evokes an inhibitory action upon M.G.R. On anatomical grounds this inhibition can be mediated only by the Golgi cell, and it is postulated that the inhibitory action is postsynaptic upon the dendrites of granule cells. 6. It is concluded that the Golgi cell inhibition regulates the mossy fibre input to the cerebellar cortex at the M.G.R. by a form of negative feed-back.