A number of papers have dealt with changes in threshold produced by a variety of drugs and ions. For example, at the nodes of Ranvier, a moderate increase in potassium concentration raises the threshold voltage, whereas removal of external calcium lowers it (Hashimura and Wright, '58). On the contrary, Jenerick and Gerard ('53) and Jenerick ('56) observed in muscle fibers a lowering of the threshold voltage and the critical depolarization for firing with increasing external potassium concentration. The critical depolarization in muscle fibers was found to increase with rising and to decrease with lalliiig calcium coiiceiitration ( lshiko and Sato, '57). In lobster axons, rheobase at first decreases in Ca-free medium, and then rises (Adelman, '56).
While investigating the mechanism of anodal restoration (Narahashi, '62, '64), it was found that threshold potential undergoes a remarkable change during conduction block and anodal restoration. By threshold potential is meant the membrane potential at which firing occurs. The change in threshold potential is particularly interesting in K-rich solutions and in low-Ca low-Mg solutions, in view of observations on ionic currents in voltageclamped nerve fibers under these ionic conditions (Adelman and Moore, '61; Frankenhaeuser, '57, '62; Frankenhaeuser and Hodgkin, '57; Shanes, Freygang, Grundfest, and Amatniek, '59; Weidmann, '55). The present paper reports a study of the change in threshold potential in anodally restored axons with special reference to anode break excitation.
Methods
Giant axons in the circumoesophageal or thoracic connectives of the lobster,