Fig. 1.

Elevating [Ca²⁺]_iabove threshold triggers a regenerative [Ca²⁺]_i response. A large DRG neuron was voltage-clamped at −56 mV in the presence of 5 mmcaffeine. The membrane was depolarized to −20 mV for 36 sec (trace 1), 39 sec (trace 2), or 68 sec (trace 3). If the [Ca²⁺]_i rose above threshold (horizontal arrow), a regenerative [Ca²⁺]_i response developed (traces 2 and 3, but not1). The dotted lines indicate repolarization. The inset provides an example of the all-or-none generation of action potentials. Depolarizing current pulses (700 pA) were applied to a DRG neuron held in whole-cell current clamp. The dotted lines indicate termination of the pulses. The threshold for triggering an action potential is indicated by the horizontal arrow.