Synaptic electrogenesis in eel electroplaques.

Whether evoked by neural or by chemical stimulation, the synaptic membrane of eel electroplaques contributes a depolarizing electrogenesis that is due to an increased conductance for Na and K. The reversal potential (E(S)) is the same for the two modes of synaptic activation. It is inside-positive by about 30-60 mv, or about midway between the emf's of the ionic batteries for Na (E(Na)) and K(E(K)). The total conductance contributed by synaptic activity (G(S)) varied over a fivefold range, but the individual ionic branches, G(SSNa), and G(SSK), change nearly equally so that the ratio G(SSNa):G(SSK) is near unity. G(SSK) increases independently of the presence or absence of Na in the bathing medium, and independently of the presence or absence of the electrically excitable G(K) channels. When activated, the synaptic membrane appears to be slightly permeable to Ca and Mg. When the membrane is depolarized into inside positivity the conductance of the synaptic components decreases and approaches zero for large inside-positive values. Thus, the synaptic components become electrically excitable when the potential across the membrane becomes inside-positive, responding as do the nonsynaptic components, with depolarizing inactivation.