Nerve impulses are signals carried along nerve fibers

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

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Topic 7. Diffusion potential is exemplified by the "Nernst potential"

The magnitude of the potential difference generated by the restrained diffusion of a particular ion species depends on the concentration gradient of that ion. If the concentrations are the same across the membrane, on both sides, then the difference is zero; if the concentration on one side of the membrane is near zero, then the difference is very large. The concentration ratio determines the voltage difference. The relation for K⁺ is shown in Fig. 7. A particular voltage difference corresponding to a particular "balance" or equilibrium of K⁺ distribution across the membrane is called the equilibrium potential, E_K.

Figure 7. The Nernst potential.

For the neurone, the relatively greater concentration of K⁺ inside tends to make the outside positive with respect to the inside, as the ions move outward away from their (negative) counter-ions, and thus to leave the inside relatively negative. The magnitude of the resulting voltage difference (E_K) depends on the ratio of K⁺ concentations, inside to outside, as revealed in Fig. 7.

Start with the concentration of K⁺ outside the same as that on the inside of the membrane. Then decrease the outside K⁺ concentration toward zero (Fig. 7, a). The E_K, positive outside, with respect to the inside, increases toward a large value. The equation for this relation is the familiar exponential decay function (Fig. 7):

(K⁺_out) = (K⁺_in)×exp(-CE_K), where C is a proportionality constant.

Rotated 90^o clockwise (Fig. 7, b), the graph, E_K as a function of K⁺ concentration, displays E_K as negative inside with respect to outside as zero. A logarithmic plot of this function brings it into conformity with conventional practice, illustrating the Nernst equation:

E_K = –(1/C)×ln[(K⁺_in)/(K⁺_out)]

The Na⁺ distribution has a similar result, but with the voltage oppositely directed. You can see that the voltage difference determined by the K⁺ gradient could be decreased by the oppositely directed Na⁺ gradient. The Na⁺ gradient generates a Na Nernst potential of about 50 mV, inside positive to the outside, in opposition to the K⁺ Nernst potential of about –100 mV, inside negative to the outside.

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