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.