Block diagram representation of the multiscale model 2.1 to 2.3, in voltage clamp. Note the additional voltage $Vf$⁠, accounting for the dynamics of the fast ion channels. While this voltage will be merged with $V$ in the final multiscale model, $τf$ is necessary for the identification of $Iion$ (section 3). This is a parallel Wiener system with the linear systems on the left feeding into the nonlinearity on the right.