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Abstract 1. A hyperpolarizing pulse applied through an external microelectrode to a nerve terminal during the falling phase of its action potential can suppress transmitter release; a depolarizing pulse applied during the same period can potentiate the release. 2. The facilitating action of calcium on transmitter release was studied by synchronizing the arrival of the nerve impulse with a rapid increment of the external calcium concentration. Using ionophoretic pulses of calcium, an effect could be produced with very little delay; sometimes the calcium pulse was effective when it preceded the arrival of the nerve impulse by only 1 to 2 ms.
Katz et al. (Tue,) studied this question.