AFTERPOTENTIALS IN NONMYELINATED NERVE-FIBERS

1. The sucrose-gap technique was employed to examine the different types of afterpotentials that follow, in desheathed rabbit vagus nerves, a single action potential (AP) elicited by a short (0.4 ms) supramaximal depolarizing pulse. 2. A fast and a low hyperpolarizing afterpotential (fHAP and sHAP) as well as a depolarizing afterpotential (DAP) followed a single spike. Both the fHAP and the sHAP showed a dependence on the K+ electrochemical gradient, indicating that they are due to an outwardly oriented current of K+ ions. 3. The fHAP was sensitive to low concentrations of tetraethyl-ammonium (TEA; 1 mM) and a 4-aminopyridine (4-AP; 10-mu-M) and to millimolar concentrations of Ba2+. We conclude that the fHAP reflects the tail of the delayed rectifier K+ current. 4. The sHAP contained a Ca2+-sensitive component that showed a requirement for voltage-dependent Ca2+ entry during the AP. This component was completely blocked by low concentration of TEA (1 mM) and by Cd2+ (1 mM), but unaffected by 4-AP. These observations suggest that it reflects a current flowing through Ca2+-activated K+ channels. The remaining, apparently Ca2+-insensitive, component was insensitive to 4-AP and could be blocked by TEA only at concentrations > 50 mM. 5. The DAP usually appeared when the external concentration of K+ was increased to above approximately 8 mM, but sometimes it was clearly visible even at lower [K+]o. The DAP was TEA insensitive and entirely Ca2+ dependent. This latter property is inconsistent with the widely accepted hypothesis according to which the DAP reflect the accumulation of K+ in the extracellular space during the AP. 6. The origins of both the Ca2+-insensitive component of the sHAP and the DAP are not clear. However, in view of the fact that the sucrose-gap technique records not only the membrane potential of the nerve fibers but also of the surrounding glia, there is the possibility that these afterpotentials reflect changes in the electrical properties of the satellite Schwann cells.