Fig. 8. Action potentials recorded in 4–6 d.p.f. zebrafish inner muscle. (A,B) Although no amount of depolarization of the inner muscle was found to elicit more than one spike, further spiking could be elicited in response to a second depolarization following a brief (>25 ms) return to the `resting' potential in (A) Normal solution and (B) low Cl^– solution. (C,D) The minimal interstimulus interval required to elicit a second spike was greatly diminished by hyperpolarizing the membrane between stimuli. The recovery of the action potential was graded rather than all-or-none. (C) Normal solution, (D) in low Cl^– solution.