Fig. 4.1. The principle of membrane current measurement with a propagating nerve impulse. (A) It is assumed that a propagating wave is initiated at the left and has a uniform velocity at the site where the voltage is measured. To obtain the transmembrane current, Equation 4.1 can be used; implementation will require the measurement of the velocity of propagation so that ²Vm/x² = (1/Q²)²Vm/t² can be evaluated. (B) A portion of the linear core conductor model (assuming the extracellular medium to be bounded) which reflects the physical model above. (Note that because we examine the ionic currents during the propagating nerve impulse, the membrane resistance rm is not constant; hence it is represented by a symbol indicating a variable resistance. To the extent that the ion concentrations may change with time then Em can also be time-varying.) The symbols are explained in the text.