Electricity and the brain

Part 3 of 3 : Electric currents that create the action potential

The negative voltage of the neuron attracts the positive charges on the outside, but they can not enter the neuron because the neuronal membrane prevents them. That is when another group of proteins steps in. These proteins are sensitive to the voltage of the neuron (voltage-dependent ion channels). They are specific for a single atom, have a structure that reminds us of a tube with a door that opens when a certain potential is reached. Thus, when a neuron receives a stimulus (from the 5 senses or another neuron), there is a fluctuation of the neuron’s resting potential (Figure 4). If a specific potential is reached, the voltage-dependent proteins door opens and lets in the positive charges. This movement of charges is an electrical current that can be measured in amperes, usually on the order of picoampere (10^-12 ampere).

When we think of brain electricity, there are two very important voltage-dependent channels: sodium channels (Na) and potassium channels (K). When the neuron is depolarized so as to achieve a certain potential (threshold potential), these channels open and let their specific atoms (ions) go through (Figure 4). Note that sodium channels have faster kinetics (open faster) than potassium channels ; they will open first, sodium will quickly enter the neuron and there is then an increase of positive charges inside the neuron, up until the sodium channels close. At that time, the voltage in the neuron rises to about 40millivolts (mV). Then the potassium channels come into play. They allow potassium ions to leave the neuron to eliminate the positive charges inside the neuron and allow the neuron to come back to a negative voltage. This very fast sequence of opening and closing is called the action potential. This action potential will develop in the neuron cell body and spread along the axon like a wave in water until it reaches the end where it can communicate with another neuron, allow muscle contraction, allow the release of hormones, etc..

Eric Trudel 

McGill University