In the brain, there are cells called "neurons." It is said that about 10 billion to 100 billion cells are in a brain. Many arms are growing from one neuron. With the arms, one neuron makes about 1,000 joint with other neurons.

This joint is called "synapse." The neuron transmits information through the change of electric potential like an electronic circuit. The information sending neurons are called "presynaptic cells," and the information receiving neurons are called "postsynaptic cells."

Fig: Synapse and cells before and after it

Synapses are not directly connected. Information goes from presynaptic cells to postsynaptic cells in one way. If you can compare it with a digital electronic circuit, it is similar to a state in which a conductor connects from the output pin of one microcomputer and the input pin of another microcomputer.

When a signal arrives at the presynaptic cell, this cell releases a special substance "neurotransmitter" into the synapse intervals. On the surface of the postsynaptic cell has a substance "receptor" like a sensor. This receptor is responsive to neurotransmitters. When the neurotransmitter binds to the receptor, the potential of the postsynaptic cell rises.

Fig: An enlarged view of synapses

In this explanation, it may be misunderstood that a signal is always transmitted to the postsynaptic cell when a signal arrives at presynaptic cells. It is not so. If the signal arrives once to the presynaptic cell, the potential of the postsynaptic cell changes only slightly, and it returns to the original potential without transmitting a new signal to the next cell.

Fig: Only one weak potential change occurs in one signal

Then, when is the new signal transmitted? For example, when signals arrive in succession from multiple cells in a short time, the changes in the potentials generated by the respective signals are summed up and increased.

Fig: A large potential change occurs when a signal comes in successively

When the potential rises to a certain extent, another switch of the postsynaptic cell is activated, and it rises further by about 100 mV at a stroke. This rapid rise in potential is called "firing," and when this happens, neurons transmit a signal to the further cells. "A certain extent" of potential depends on cells and circumstances, but it is typically about 15 mV. (*12) In the figure, by a single stimulus, the potential was changed only by about 6 mV, by three overlapping stimuli, the potential exceeds 15 mV.

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Footnotes:

* 11 There is another transmission path from postsynaptic cells to the presynaptic cells, but this is not important to the story, so I omit it.

* 12 mV (millivolts) is the unit of voltage. Since the voltage of the dry cell is approximately 1.5 V = 1500 mV, 15 mV is one-hundredth of the voltage of the dry cell.