Intracellular recordings of caudate neuronal responses evoked by temporally combined stimulations of cortex, thalamus, and substantia nigra were made in the cat. Excitatory postsynaptic potentials (EPSPs) which temporally coincided were additive. EPSPs which coincided with an inhibitory postsynaptic potential (IPSP) previously evoked from the same stimulus site were enhanced. The cortical stimulus was prepotent in the sense that EPSPs evoked from thalamic or nigral stimulation were inhibited by the cortical IPSP. The cortical EPSP was enhanced if it was evoked during a nigral or thalamic IPSP. These results are discussed in the context of recent reports concerning the fine structure of synaptic contacts of input fibers to the caudate nucleus.
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