Resting potential, action potentials and myelin

Last time

Before we start: last time, in The endocrine system compared with the nervous system, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A signal made of ions

A nerve impulse is not electricity flowing down a wire. It is a wave of sodium and potassium ions crossing the membrane, one patch after another. The wave travels at up to a hundred metres a second, and what makes it fast is a fatty sheath. This lesson explains the wave first, then the sheath.

The resting potential

When a neurone is not firing, the inside of the cell is negative compared with the outside. The difference is the resting potential, about minus seventy millivolts. The sodium potassium pump uses A T P to move three sodium ions out and two potassium ions in. So there are more positive ions outside. The membrane is also much more permeable to potassium than to sodium, through leak channels. Potassium ions diffuse out, making the inside more negative.

The action potential

A stimulus opens a few sodium channels, so sodium ions enter and the membrane depolarises. If it reaches the threshold, about minus fifty-five millivolts, voltage gated sodium channels open. More sodium ions rush in, in positive feedback. The potential rises to about plus forty millivolts. This is depolarisation. Then the sodium channels close and voltage gated potassium channels open. Potassium ions leave, and the potential falls back. This is repolarisation, and it overshoots slightly, causing hyperpolarisation. The pump and leak channels then restore the resting potential.