Resting Potential of the Nerve Membrane

Neurons are specially built to be excitable cells, and the nerve impulse itself is a travelling wave of bioelectrical (electrochemical) disturbance moving along a neuron, generated by shifting electrical charge across the neuronal membrane. Every neuron's plasma membrane carries a voltage difference between its outer and inner faces, even though the total number of ions is roughly the same on both sides -- what differs is which ion predominates where. The external tissue fluid contains both Na+ and K+, but Na+ and Cl- predominate there, whereas inside the fibre K+ predominates, alongside large, negatively charged protein molecules and nucleic acids that cannot cross the membrane.

This arrangement, called the polarised (resting) state, is actively maintained: although some Na+ and K+ constantly leaks across the membrane, the Na+-K+ pump continuously forces Na+ back out and K+ back in against their concentration and electrochemical gradients, using ATP -- specifically pumping 3 Na+ ions out for every 2 K+ ions pumped in, which is why the pump is described as electrogenic. The net result of this unequal ion distribution is a steady voltage difference (the resting potential) of about -50 to -100 millivolts, averaging around -70 mV, produced mainly because the resting membrane is far more permeable to K+ than to Na+ -- slightly more K+ leaks out than Na+ leaks in, on top of the fixed negative charge of the trapped intracellular proteins and nucleic acids. Beyon …