Hodgkin–Huxley: Pulse Threshold Current
Fixed-width current pulse on a squid-axon membrane. Step I until an action potential appears and report the threshold current.
University / research· 26 min·Related simulator: Biophysics, Fluids & GeoscienceHodgkin–Huxley Action Potential
Goal
Determine the minimal pulse current I_th that elicits a spike (peak V jumps from near rest to tens of millivolts).
Equipment
- HH membrane (squid axon)
- Current-pulse stimulator
- Voltage scope
Experiment
Theory
Hodgkin–Huxley sodium activation is regenerative: below threshold the membrane returns toward rest; above it a full action potential fires. For a long pulse I_th approaches the rheobase. This bench uses a fixed 8 ms pulse. It never prints a threshold value — you diagnose the spike from the voltage trace and peak V.
Procedure
- Pulse width is fixed at 8 ms and known. You only change the pulse current I.
- Watch the live scope and peak V. Subthreshold responses stay near rest; a spike jumps peak V to about +30 mV.
- Step I in 0.05 µA/cm². I_th is the smallest I that fires. The bench does not label “AP / no AP”.
- Record that threshold current. Repeat the hunt at least 6 times (you may start from different I).
- Take the mean of the I column and compare with the reference.
Conclusion
The mean threshold current agrees with the hidden I_th for this pulse width. Main uncertainties: discrete current steps and judging the all-or-none jump by eye.