The Release of Transmitter by Presynaptic Impulses

Abstract

It is not possible directly to demonstrate that a nerve impulse evokes an EPP by causing the release of a hundred or more packets of ACh within a fraction of a millisecond, for there is an almost synchronous superposition of or more packets of ACh within a fraction of a millisecond, for there is an almost synchronous superposition of the constituent miniature EPPs. However, if the magnesium concentration of the extracellular fluid is increased from the normal level of 1 mM to about 1OmM, the EPP becomes very small and fluctuates in amplitude, from zero in a step-like manner (del Castillo and Katz 1954b, 1956b; Boyd and Martin 1956a; Liley 1956b). Decrease of calcium acts similarly and cumulatively. Comparison between the random min. EPPs and the EPPs in Fig. 25A—C suggests that this fluctuation is attributable to variations in the number of packets released by a nerve impulse, 0, 1, 2 or more, and the quantal composition of the EPPs can actually be recognized when cooling increases the temporal dispersion (Fig. 25D, E). Statistical analysis using Poisson’s theorem gives a precise confirmation of this quantal composition of the EPPs, and shows that the quanta are identical with those randomly released to give the min. EPPs. As illustrated in Fig. 9A, B extracellular recording of min. EPPs is much more selective than intracellular; hence the quantal fluctuations of the extracellular EPPs are much more in evidence than with the intracellular EPPs. Extracellular recording thus is of special advantage in revealing quantal composition when the intracellularly recorded EPPs have a rather high quantal content (del Castillo and Katz 1956a).