Fig. 7. Action potentials recorded in 4–6 d.p.f. zebrafish inner and outer muscle. (A,B) Action potentials were recorded from inner muscle fibres under current-clamp conditions in response to the injection of depolarizing current when the fibre was previously held at –70 mV (N=15). However, when the same depolarizing current was injected after holding the fibre at –45 mV, the evoked spike was considerably attenuated. Increasing the amplitude of the depolarizing current did not restore spike shape (data not shown). (A) Normal solution, (B) low Cl^– in the pipette-filling solution (N=5). (C,D) Applying 10 ms depolarizing pulses of increasing amplitude from a `resting' membrane potential of around –70 mV never evoked more than one spike (N=10) in (C) Normal solution and (D) with low Cl^– in the pipette-filling solution (N=5). (E,F) In contrast to inner muscle fibres, depolarization of outer muscle fibres never elicited an action potential (N=5) in (E) Normal solution and (F) with low Cl^– in the pipette-filling solution (N=5).