Fig. 5. Temperature dependence of the muscle fiber resting potential in the presence and absence of serotonin (5-HT). Symbols represent the change in resting potential with respect to the value measured at 2°C measured in the same eight muscle fibers as a function of temperature in the absence (solid symbols) and presence (open symbols) of 5-HT. Data are normalized in both cases to the values of the resting membrane potential recorded at 2°C. Addition of 5-HT (100 nmol l^–1) at 2°C produced a mean change in resting membrane potential of +4.75 mV; the data measured in the presence of 5-HT is offset accordingly. Arrows indicate the temperatures at which IJPs reverse polarity (T_rev) under control conditions (6.6°C) and in the presence of 5-HT (13.2°C). At each temperature N=8 except as follows: for 10°C N=6, for 12°C N=4, for 14°C N=2.