The propagation of the action potential moves a signal from one end of an axon to the other. But how is the signal then passed to the next neuron, so that the message can continue traveling toward its destination? For many years, no one saw this as a prob-lem. Descartes, for example, believed that reflexes were formed from a long, continuous strand of nervous tissue—in essence, one neuron. According to this view, the incoming sensory information triggers a response at one end of this neuron. The signal is then propagated down the length of the cell and eventually triggers a response at the end of the same neuron. But this view (and several variations of it) soon faced a major prob-lem: If myelinated neurons can send a signal at 120 meters per second, then how long should it take someone to withdraw their hand if they happen to touch a hot stove? The sensation of heat would have to travel from fingertips to brain and then get back out to the arm muscles to produce the movement—in all, a distance of less than 2 meters. Based on a signal speed of 120 meters per second, we can predict that the person will respond in less than one-hundredth of a second. But in fact, the response is likely to be 20 times slower than that (roughly 200 milliseconds).
By the end of the nineteenth century, therefore, most researchers were convinced that the neuronal transmission must involve some intervening steps and that these steps slow things down. Today we know that this conjecture was correct; the lines of neuronal communication depend on a succession of neurons, not just on one long neuron that somehow reaches from the sensory input all the way to the muscles that produce the response. Within this succession of neurons there’s a small gap between adjacent neurons, so the neural signal has to move down a neuron’s axon, jump across the gap and then trigger the next neuron’s response, move down its axon, and so on. This gap between neurons is called the synapse.
Transmission across the synapse does slow down the neuronal signal. But it’s a tiny price to pay, because this setup yields a huge advantage: Each neuron receives informa-tion from (i.e., has synapses with) many other neurons, and this allows the “receiving” neuron to integrate information from many sources. Among other benefits, this pattern of many neurons feeding into one makes it possible for several weak signals to add together, eliciting a response that any one of the initial signals could not trigger on its own. In addition, communication at the synapse is adjustable:
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