Fig. 3. (A) Subthreshold response and all-or-none spike recorded from a caudal heart muscle fibre in response to electrical stimulation by a suction electrode. Two traces are superimposed. External solution, artificial seawater (ASW). The dotted line indicates 0 mV. (Bi-iii) Reversible suppression by externally applied d-tubocurarine (d-TC; 20 mol l^-1) of subthreshold responses evoked by a train of electrical stimuli by a suction electrode. The dotted lines indicate 0 mV. The recordings in A and in B were obtained from different individual caudal heart muscle fibres.