The Nerve Impulse Seen from Outside
Dexter M. Easton July 2000 ©

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Topic 2. Nerve membranes can be viewed as electrical circuits

In Fig. 2, the circuit of Fig. 1A has been superimposed on a greatly enlarged diagrammatic section through the cell membrane. In this context, the diagram serves (much simplified) as an equivalent circuit of a tiny segment of the nerve membrane. Electrical capacitance is an important feature, especially in controlling the time course of events, but it will be ignored at this point. In the neuron, the current flows not in wires but in the salt solution. In the solution, as in the wires, the direction of current is the direction that positive charges move.

 
 
Figure 2. Simple equivalent electrical circuit representing neuron membrane.

 
 
Ve, the voltage drop across Re, in the solution external to the nerve fiber, will be the main focus of attention in this lesson.

The membrane battery, EM, on which Ve depends, exists only so long as there is separation of + and – charges across the membrane. The plus charges are mainly on K+ and Na+ ions; the negative charges are carried on Cl ions and on negatively charged organic molecules. In an electrical circuit such as house wiring, or in an electronic device such as a computer, emphasis is put on the negative charges (electrons) moving in wires. In nerve, we see current mainly as Na+ and K+ ions traveling in aqueous solution. The cations move toward a region of negativity. Anions such as Cl move in the opposite direction.

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