Presynaptic currents in frog motor endings.

Membrane currents were recorded from non-myelinated frog endings by external electrodes. Changes in shape of the signals recorded at varying distances from the myelin end could be explained by assuming a non uniform distribution of Na and K channels along the presynaptic terminal. Specific channel blocking agents revealed that Na channels are present in highest density in the first half of each terminal branch and at almost undetectable levels near the extreme end, while K channels show a more widespread distribution with higher density at medial parts. Suppression of K conductance revealed Ca current which was seen as outward current near the myelin end.