Miniature endplate potentials recorded from skeletal muscle fibers were initially described in frogs (Fatt and Katz, '50) and subsequently in rats (Liley, '56; Li, '58), cats (Boyd and Martin, '55) and guinea pigs (Brooks, '56). These miniature potentials are thought to be generated by spontaneous liberation of quanta of transmitter substance from motor nerve endings (Fatt and Katz, '52; del Castillo and Katz, '54, '55). A synchronous occurrence of miniature endplate potentials represents a synchronous release of quanta of the transmitter. This gives rise to endplate potentials and spike discharges.
Recently, similar small potentials recorded from neurones in the cerebral cortex were reported (Li, '59). It was suggested that these small potentials recorded from the cortex are due to random bombardment of impulses through presynaptic fiber terminals and that the mechanisms of impulse transmission across the neuromuscular junction and in the cerebral cortex are probably the same. However, this investigation (Li, '59) was concerned with the "spontaneous" occurrence of small potentials and did not extend to a study of their responses to presynaptic stimulation.
In the present investigation evidence was sought for such small potentials elicited by presynaptic volleys. The experiments were carried out with intracellular electrodes recording from cat's sornatosensory cortex while electrical stimulation was applied to the contralateral superficial radial nerve. It will be shown that the postsynaptic potentials thus produced consisted of many small potentials. Further, the postsynaptic potentials, which were evoked through polysynaptic pathways, were frequently found to persist for as long as 100 msec. These observations