Construct steady-state inactivation (h∞) curve from two-pulse voltage clamp protocol

[JCorson]

Overview

Implement a two-pulse voltage clamp analysis to measure steady-state inactivation (h∞) as a function of conditioning prepulse voltage, and overlay a Boltzmann fit.

Motivation

The activation G-V curve (#177) shows how channels open with depolarisation. The steady-state inactivation (h∞) curve shows how channels are inactivated at rest at different voltages. Together they are the canonical pair for characterising voltage-gated channels, and their voltage overlap defines the window current — a clinically important feature in cardiac and neuronal channelopathies.

Proposed behaviour

• The user runs a voltage clamp Step protocol with multiple sweeps, each using a different conditioning prepulse voltage followed by a fixed test pulse.
• From the peak current at the test pulse in each sweep, compute normalised availability: h∞(V) = I_peak(V) / I_peak_max.
• Fit a Boltzmann curve: h∞(V) = 1 / (1 + exp((V − V_half) / k)) and report V_half and slope factor k.
• Plot h∞ vs. prepulse voltage, overlaid with the fit curve.

Implementation notes

• Distinct from the standard step protocol — the two-pulse structure may need a new voltage clamp protocol type, or could be approximated with the existing Step protocol if the conditioning pulse is treated as part of the step sequence.
• The Boltzmann fit and normalisation logic should be shared with Compute and plot normalised conductance vs. voltage (g-V curve) #177's activation curve fitting.
• V_half and k should be reported in a metrics panel.