Fig. 3. K⁺ microelectrode voltage difference as a function of distance from a K⁺ source. Experimental measurements (open circles) were made by vibrating the microelectrode through an amplitude of 100µm at each distance. Theoretical values (filled circles) were calculated according to the following equation: V=S[(-U)/(C_Br²+Ur)]/2.3, where V is the change (mV) over the vibration excursion, S is the slope of the electrode, r is the distance from the source, r is the amplitude of the vibration, C_B is the background activity of K⁺ and U is an empirical constant. The inset shows the calculation of the empirical constant U. Static measurements were made at a series of distances from the source, and the millivolt outputs were then converted to activity values. A plot of these activity (C) values versus the inverse of the distance from the K⁺ source (1/r) yields a line with a slope of U, according to the equation: C=C_B+U/r, where C_B is the background activity of K⁺ (20 mmoll^-1) and U (µmolcm^-2) defines the diffusion characteristics of the gradient source (r²=0.969, P<0.0001). Using the method of Piñeros et al. (Piñeros et al., 1998), electrode efficiency (85%) was calculated as the ratio of the slope of the experimental data (dashed line) to the slope of the theoretical data (solid line).