Figure 2.
Inactivation of a Na+ conductance can produce shunt peaking. (a) Literal circuit model of drone bee photoreceptor. Membrane capacitance, C, light conductance, glight, voltage-dependent Na+ conductance gNa, and voltage-independent K+ conductance, gK. Light current, Na+ current and K+ current reversal potentials are EL, ENA and EK, respectively. Leak conductance, gleak, with reversal potential of light current, sets dark-adapted resting potential of photoreceptor. (b) Phenomenological RrLC circuit of the photoreceptor describing small signal behaviour of membrane to current injected around steady-state voltage, V0. The voltage-dependent conductance gNa is modelled as a resistance, R, in parallel with two phenomenological branches, representing the effect of activation (LNa,m, rNa,m) and inactivation (LNa,h, rNa,h), respectively. (c) Impedance of the model drone photoreceptor membrane (blue, dark grey in print) and a modified membrane model with 10 times faster inactivation (red, mid grey in print). (d) Relative GBWP of drone photoreceptor as function of activation and inactivation time constants of the voltage-dependent Na+ conductance. Thick black line marks the border between band-pass membranes and low-pass membranes. (Online version in colour.)