**Fig. 4.10.** The equivalent circuit of the Hodgkin-Huxley model. The voltage sources show the polarity of the positive value. The calculated Nernst voltages of sodium, potassium, and chloride designate the value of corresponding voltage sources. With the normal extracellular medium, *V*_{Na} has a positive value (Equation 4.7) while *V*_{K} and *V*_{L} have negative values (Equations 4.8 and 4.9). During an action impulse, *G*_{Na} and *G*_{K} vary as a function of transmembrane voltage and time.