How to conceptualize the action potential?

Question

In my AP Biology class, we were taught that action potentials are not electrical impulses in the same way current travels through a wire. Rather, we were taught that action potentials are changing concentration gradients of sodium and potassium ions.

However, when I looked into modeling action potentials I saw capacitances, inductances, and currents--properties associated with electrical circuits.

My question: action potentials--are they electrical currents in the classical sense? If so, what is the charge that is flowing (is sodium and potassium ions?), and how is the overall charge neutrality of the body maintained?

Answer 1

First of all, electric current is defined as movement of charges, $I=\frac{dQ}{dt}$. In electronics that you see around that is not very useful definition, because electrons move much slower than signals, that is changing electric field. Speed of electric field propagation reaches speed of light, whereas electrons move at 1%-30% or $c$.

In biology, especially regarding action potential, only movement of charges is important, not propagation of electric field, probably because conductance is much smaller than in copper wires. that is signal propagates not through interaction between charges, but via intermediate agent/amplifier -- ion channels.

Charge in body is more or less constant, because action potentials are cyclic: after depolarization membrane comes back to -70mV or something. There is no flow of charge out of the body.

Answer 2

Short answer
Action potentials are mediated by electric currents and can be modeled by electronic circuits.

Background
An electric current is the flow of charge. Therefore, action potentials are mediated by current flow. However, action potentials are mediated by the flow of ions across the membrane (Fig. 1), whereas current flow in electric circuits is typically mediated by the flow of electrons (Fig. 2).

^{Fig. 1. Action potential propagation. Source: Chen, Carnegie Mellon.}

The action potential propagates in the direction of the dendrite. However, the underlying current flow is perpendicular to the axonal membrane and is mediated by ion channels. These ion channels are activated through the sensing of the electrical field. For example, voltage-operated Na⁺ channels are activated by a small depolarizing potential, while voltage-gated potassium channels open during the later phase of the action potential by a large depolarization.

Whatever the differences, an axon can be well modeled by a parallel resistor-capacitor circuit powered by the membrane potential. The electrochemical properties of the cell membrane makes it equivalent to a circuit with a set of resistors (which are voltage-dependent), batteries (whose voltage depends on ion concentration differences), and a capacitor all connected in parallel (see Fig. 2).

^{Fig. 2. Electronic circuit representation of the axon. Source: Yale University}.

As to your question on charge balance; neutrality is maintained, because Na⁺ and K⁺ flow are going in opposite directions and neutralize the net flux of charge.