Nerve impulses are electrical impulses that travel along the axons of nerve cells (neurons) carrying various information. Propagation of electricity is possible due to differences in electrical potential between the inside of neuron and what is outside. This difference is due to different constitution of positive and negative ions when comparing the inside of neuron and its surroundings;
Resting potential is the time when no impulse is propagated. During the resting potential the inside of neuron is negatively charged (around -70 mV), while the outside matter is positive. It might be easier to remember this if you notice that "inside" contains an "n" in it - n for "negative".
Axons contain potassium (K+) and sodium (Na+) channels. Both of these ions are present inside neurons as well as around them. It is essentially movement of Na+ and K+ through the channels that is responsible for electrical impulses conduction. There is more Na+ outside than inside – converse is true for K+.
An impulse that travels along an axon is called action potential. Action potential has 3 stages:
Restoration or refractory period follows afterwards, when the resting potential is established anew and no new impulse can be propagated. I will now explain each stage in detail:
At rest, Na+ channels are closed. When impulse arrives at the neuron, it causes them to open.
There is more of Na+ outside, so they enter the axon through their channels due to simple diffusion. Moreover, the ions are positively charged so they are attracted to the negative inside of axon. We thus say that there is Na+ influx (entry to the cell) and that Na+ follows electrochemical gradient.
Because we have positive charge (Na+ ions) flowing inside, axon becomes depolarized: from negative, it turns positive. When the potential changed from ~ - 70 mV to ~ +30 mV Na+ channels close and no more Na+ moves in.
At ~+30 mV K+ channels open. There is less of K+ outside. Also, the outside is currently negative due to Na+ influx. K+ moves out due to electrochemical gradient, just like sodium did, but in the opposite direction – outside. The movement outside is called efflux.
Because positive charge (K+) is now leaving the cell, the inside becomes once more negative. Thus this step is called repolarization.
There is a lot of K+ ions flowing outside which causes the inside of cell become even more negative than it was at rest. This phase is called hyperpolarization or undershoot.
The difference in potential between the inside and outside of neuron is now correct: inside is once more negative and outside positive. However, K+ and Na+ are not in the correct place – this makes it impossible to convey another impulse. To reestablish resting potential we need to move Na+ outside and K+ in. This happens during the restoration phase.
For the restoration we need sodium/potassium pump. This pump is powered by ATP so this is an example of active transport. It moves 3 Na+ ions outside and 2 K + in.
In the end, resting potential is reestablished: there is more of Na+ outside, more K+ inside so both the charges and ions are correctly allocated and the new impulse can be sent along the axon.