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An action potential is an electrical signal that travels through a brain cell, or neuron, to send signals to other cells. One of the main events of an action potential is the rising phase, when the neuron changes to be very positively charged. After the rising phase, repolarization happens, returning the neuron to its original charge. Finally, there is a hyperpolarization stage where the neuron is more negatively charged than before.
A neuron is made up of three basic parts. The cell body of the neuron is where the metabolic events occur and the nucleus is held. Coming off the cell body like branches on a tree are dendrites, thin protrusions that extend to other neurons to receive signals. At the other end of the cell body is the axon, a long extension similar to the trunk on a tree. The events of an action potential take place along the axon.
Dendrites of the neuron receive signals from other neurons in the brain. These electrical signals can be positive or negative. All of the electrical signals the neuron receives are added up, or summed, at the axon hillock, a small mound between the axon and the cell body. If there is enough of a positive charge at the axon hillock, an action potential will be generated.
The first of the events of an action potential is the rising phase. During this phase, a typical neuron goes from having a negative charge of -70 millivolts (mV) to a positive charge of +50 mV. This change is caused by the opening of sodium channels in the neuron, which open to allow positively charged sodium ions into the cell, changing its overall charge.
A repolarization event comes after the rising phase, and in this event the cell returns to its resting charge of -70 mV. The cell becomes more negatively charged because the sodium channels close, keeping the positively charged sodium ions from entering the cell. In this phase, potassium channels open up; this means that the potassium ions that are positively charged can leave the cell, contributing to its negative charge.
More potassium ions escape than is necessary to bring the cell to its resting potential, so for a short time the cell enters the hyperpolarization stage. During the last of the events of an action potential, hyperpolarization, the cell actually becomes even more negatively charged than when it is at rest. This makes it difficult for an action potential to be generated during this stage.