Biophysical characteristics of the <i>Odontella sinensis</i> action potential.

<p>A) The upper panel shows a current clamp recording illustrating typical action potentials elicited by a brief 1 nA current injection pulse. The average threshold voltage for eliciting and action potential was −68 mV (SE±3, n = 12) with peak depolarisation reaching +21 mV (SE±2, n = 12 ) before the repolarising phase. The time scale bar represents 50 ms. The middle panel shows a representative family of whole cell currents recorded in response to a series of voltage clamp depolarisations from a holding potential of −100 mV to +30 mV (indicated by the inset). Scale bars represent 10 nA and 5 ms and dotted line represents 0 nA. The lower panel consists of an average current voltage curve of the peak inward current elicited by membrane depolarisations. The current peaks at −22.3 mV (SE±1.7, n = 38) and reverses at +20.0 mV (SE±1.9, n = 38). Standard error bars for both current and voltage are indicated. B) Voltage dependent inactivation. The upper panel illustrates a typical experiment where the cell was treated with a progressively depolarised pre-pulse before eliciting a peak inward current with a depolarisation stimulus of −30 mV (protocol indicated by inset). The time scale bar represents 10 ms and the current scale bar represents 10 nA. Capacity transients of the current responses are masked from the traces. Lower panel shows a plot of the average normalised peak current against pre-pulse voltage which is described by a Boltzmann curve (n = 12). The voltage at which half of the channels are inactivated (V_inact) is −69 mV. C) Recovery from inactivation. The upper panel illustrates a typical experiment where the inward current is elicited with a depolarising voltage clamp pulse followed by a second identical stimulation with varying delay (voltage clamp protocol indicated by inset). The first pulse results in transient inactivation of Na⁺ channels which progressively recover from this state of inactivation as the delay between the first and second pulse increases. The lower panel is a plot of the second evoked peak current expressed as a percentage of the first peak plotted against the time interval (averaged from 16 experiments). The resulting curve is fitted by a single exponential giving a time constant of 6.74 ms for recovery from inactivation. Capacity transients of the current responses to the second stimulus are masked from the traces. The current scale bar represents 10 nA and the time scale bar presents 10 ms.