The responses of pacemaker and nonpacemaker Aplysia neurons to voltage clamp commands of less than 200 msec duration are essentially identical. For moderate depolarizing commands there is an early inward transient current followed by a late outward current and an outward tail current when the membrane is clamped back to resting potential. On long (1–2 sec) commands in pacemakers there is a marked sag in the late current and an inward tail current. Etail, the potential of the membrane at which there is no net current flow under the conditions prevailing at the end of the clamp, shifts from about -9.0 mv on short commands to +5.0 mv on long commands. In contrast there is no marked sag of the late current or inward tail current on long commands in nonpacemakers, and Etail is near -9.0 mv for both short and long commands. The current sag and shift in Etail can be ascribed to a decreased conductance (presumably to K+) at the end of the long as compared to the short command in half of the pacemaker neurons. In the remaining cells the essential difference from nonpacemakers appears to be either a greater restricted extracellular space or a more active potential-dependent electrogenic Na+ pump in pacemakers.

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