Figure 6.

Noise analysis: the number of Ca²⁺ channels and size of single-channel currents. A, The Ca²⁺ current I–V relationship for amphibian papilla hair cells after leak subtraction, series resistance compensation (80–90%), and liquid junction potential correction. The Ca²⁺current (I_Ca) begins to activate at approximately −70 mV (inset) and peaks at approximately −30 mV (n = 5 cells). B, The hair cell was held at −90 mV, briefly hyperpolarized to −110 mV to relieve any steady-state inactivation, and then depolarized to +30 mV for 10 ms to open all Ca²⁺ channels. A Ca²⁺ tail current (bottom trace) was elicited by repolarizing the hair cell from +30 to −90 mV (top trace) in the presence of the L-type Ca²⁺ channel agonist BayK 8644 (5 μm) and 2 mm external Ca²⁺. The decay of the tail current between the two dashed lines was used for noise analysis. C, Here, we show 100 superimposed traces (in gray) that were obtained with the same protocol as in B. The mean (in blue) and variance (in red) were calculated on a point-by-point basis. D, The variance was plotted against the mean (in black). The single-channel current (i = 0.44 pA) and the number of Ca²⁺ channels (N_Ca = 2372) were estimated by fitting the data to a parabolic function (in red) (see Materials and Methods).