The Synapse
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:
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.