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The brain is made up of two types of cells. Glial cells act as support cells for neurons, the cells responsible for sending signals in the brain. Action potentials — electrical signals that travel across neurons — are the means for receiving, analyzing, and conveying information in the brain. They have an amplitude of about 100 millivolt (mV) and last for around 1 millisecond (ms).
A neuron is made up of four distinct parts. The cell body contains the nucleus and other cell structures. Dendrites branch out from the cell body like the branches on a tree, and receive information from other neurons. The axon is a long extension on one side of the cell body, similar to the trunk of the tree, and it ends in the presynaptic terminals.
This type of cell is polarized, meaning the electrical charge inside the neuron is different from the charge outside the cell. Dendrites receive signals from other neurons that can change the charge inside the cell. A neuron at rest is more negatively charged than the surrounding area. Excitatory postsynaptic potentials bring the charge closer to zero, and inhibitory postsynaptic potentials make the charge even more negative.
At the axon hillock, all of these potentials are averaged in one of two ways: across time or across space. The further away from the axon hillock a potential is, the less effect it has. The longer a potential lasts, the more effect it has on the axon hillock.
If the averaged charge from the postsynaptic potentials reaches a certain threshold, an action potential is generated. Postsynaptic potentials can be different sizes depending on the signals received by the dendrites, but the action potential operates on an all-or-none principle, meaning there is no gradient — either there is one or there is not.
The action potential is an electrical signal that travels down the neuron’s axon. The axon is coated in a myelin sheath, which, similar to the insulation on an electrical wire, allows the signal to travel faster. It carries the electrical signal to the presynaptic terminals, which then communicate to another neuron.
Between two neurons there is a gap called a synapse. When the presynaptic terminal on a neuron receives a signal from an action potential, it sends chemicals called neurotransmitters into the synapse. These chemicals are absorbed by another neuron. Neurotransmitters are the mechanism for sending signals between neurons.
A college professor of mine told me that a neuron firing is like flushing a toilet. I can't remember all the similarities, but I know that one related to the refractory period. Once you flush a toilet, you can't flush it again right away, and a neuron can't fire twice in rapid succession, either.
Some other similarities were that both are all or none (you can't half flush a toilet, and a neuron can't half fire) and both go in only one direction (fortunately, in the case of a toilet!).
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