So, this is going to be a little bit technical, but do try to follow along.
It's very, very important. Now, let's take a closer look at the action
potential, or as it's sometimes called, the nerve impulse. The action potential
is an electrical impulse that travels along the axon. That is surrounded by a
semipermeable membrane that allows only certain ions to cross through it. Ions
are chemically charged particles that are found inside and outside the cells.
In its resting state, the fluid outside the axon contains a higher
concentration of positive ions than the inside of the axon. Which has many
negatively charged ions. This resting potential is measured at negative 70
millivolts, or thousandths of a volt. So, neurons do not stay in this resting
state. Rather, they can change and do change when an impulse increases the
positive charge inside the neuron to a certain threshold. The neuron becomes
locally depolarized to a charge of positive 40 millivolts and fires an action
potential or a nerve impulse. The action potential travels along the axon to
signal to other neurons. More specifically during an action potential, sodium
ions, that are positive charges, enter the axon. After the action potential
occurs, potassium ions cross outside the cells. And then, the neuron must
transport these ions to return to it's resting state. This results in a
refractory period wherein the neuron cannot generate another action potential.
This entire process from start to finish lasts only about five milliseconds, or
five 1000 of a second. So, one important point to make is that every time a
neuron fires it fires to the same degree or amount goes up to positive 40
millivolts no more, no less. This is known as the all or none principle.
Stronger responses involve simply more neurons firing not individual neurons
firing more strongly. So, if you want to find out more about some of these
technical details, you can find that information on some links that we'll post
on the web.
