The major difference between passive and active nerve membranes is that the transmembrane proteins of active membrane increase their conductances in response to signals. The signals can be electrical (sodium channels open and allow an inrush of sodium ions when the transmembrane potential, Vm, reaches the threshold voltage for depolarization, V9, e.g., +10 mV above the resting potential). Signals can also be chemical, where neurotransmitters or second messenger molecules bind with specific ion-channel gates, opening them. The abrupt increase in specific ionic conductances causes ion currents to pass through the membrane in directions determined by the combined factors of the transmembrane potential and the concentration gradient of the ion across the membrane.
The specific ionic conductance is defined in the case of sodium by gNa - V(Vm - VNa) S/cm2 1.2-20
Vm (t) is the actual transmembrane potential, and VNa is the Nernst potential for sodium ions, defined by:
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