Electrical responses evoked in different regions of the cerebellar cortex of cat by stimulating various cerebello-petal pathways have been analyzed for their component postsynaptic potentials (p.s.p.'s). The principal analytical tools of the present work were pharmacological agents; the selective inactivator of depolarizing (excitatory) axodendritic synapses, γ-aminobutyric acid (GABA, or C4); the homologous C6 and C8 ω-amino acids, which inactivate selectively the hyperpolarizing (inhibitory) axodendritic synapses; and the general inactivator of inhibitory synapses, strychnine. Some experiments employed the analytical possibilities of activity cycles.

The potentials evoked in one cerebellar region by different exciting pathways may differ markedly in their responses to drugs or may show different types of activity cycle. Also, the potentials evoked in various cortical regions by one cerebello-petal pathway are acted upon differently by the testing drugs. These differences are believed to be due to involvement of different proportions of excitatory and inhibitory, axosomatic and axodendritic p.s.p.'s. The analyses of a number of different responses confirm an earlier conclusion, that the cerebellar cortex is relatively lacking in inhibitory axodendritic p.s.p.'s in comparison with the cerebral cortex. Only the cortex of the paramedian lobule appears to be endowed with a considerable proportion of inhibitory p.s.p.'s, a finding which correlates with other data.

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