Fig. 1.

Intrinsic frequency response profile of N cells to sinusoidal current injections. (A) Fluorescent labeling of a recorded N cell was used to identify the cell location post-hoc by comparison to a schematic drawing of the known location of leech sensory neurons. (B) Square pulse current injection just below (left) and above (right) the action potential threshold. Action potential waveform shows the N cell characteristics long after hyperpolarization. (C) Voltage responses to different sinusoidal stimulation frequencies: left, 0.5 Hz; middle, 2 Hz; right, 10 Hz. (D) Number of supra-threshold responses summed over the 10 sinusoidal cycles as a function of stimulation frequency. Each symbol represents the response of a single cell (n=9). (E) Action potential firing rate from which the stimulation rate was subtracted is plotted as a logarithmic function of the stimulation frequency. The dotted zero line indicates the same firing rate as the number of stimulation cycles, hence faithful firing. The solid line represents the zero action potential line, where no supra-threshold response was elicited at any time during stimulation. Symbols are as in D. Inset, magnified low stimulation frequencies in a linear graph.