1. Under appropriate conditions stimulation of the neuromuscular junction by a single impulse or a brief train causes subsequent impulses to release increased amounts of transmitter. It has been proposed that this facilitation of transmitter release is due to calcium which remains on release sites following the conditioning stimuli. This residual calcium hypothesis is examined in frog cutaneous pectoris muscle in the present study. 2. The second component of facilitation is shown to depend on the amount of calcium present during tetanization as required by the residual calcium hypothesis. 3. The quantitative behaviour of the facilitation accompanying tetani of a number of frequencies and durations is found to be consistent with the residual calcium hypothesis. 4. Quantitative analysis of facilitation leads to a description of the time course of removal of calcium from release sites which is discussed.
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