A Threshold Equation for Action Potential Initiation
Figure 10
Experimental difficulties in the measurement of Na activation curves.
A, Estimation of the activation slope ka from simulated voltage-clamp data as a function of the inactivation time constant τh. The model was of a membrane with only Na channels, and activation and inactivation curves were Boltzmann functions (see Materials and Methods). The activation slope was measured by a Boltzmann fit in the hyperpolarized region (<−40 mV). The activation slope ka was 6 mV in the model (dashed line), but the measurement overestimated it when the inactivation time constant was very close to the activation time constant. B, Na activation curve measured in vitro (dots, digitized from [86]) and Boltzmann fits over the entire voltage range (dashed curve) and over the hyperpolarized range (V<−40 mV, solid curve).