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NORMAL HUMAN NERVOUS SYSTEM 17to dump their supply of neurotransmitter into the synapse that separates the nervecell from its neighbors.Neurotransmitter/Receptor Binding. At this point, the neurotransmitter chemicalis free in the synapse (extracellular fluid) and drifts (diffuses) in all directions. Someof the neurotransmitter molecules float across the synapse and bind to receptors onthe surface of the adjacent nerve cell. Each neurotransmitter has its own uniquethree-dimensional shape and binds with certain receptors but not others. The bindingbetween a neurotransmitter and a receptor is similar to fitting a key into a lock.When the neurotransmitter binds the receptor, the signal has been passed to theneighboring nerve cell. This is the process of neurotransmission.In many cases, the neurotransmitter sends an excitatory signal to its neighbor. Inother words, it tells the neighboring nerve cell to wake up and get busy. As a result,an action potential (i.e., nerve impulse) is more likely to be fired in that neuron.Other neurotransmitters act as inhibitory messengers that reduce the possibility thatthe neighboring cell will fire an action potential. In other words, an inhibitory neurotransmittertells the neighboring neuron to take a break and get some rest. Thisactually makes it more difficult (at least for a while) to create an action potential inthe neighboring cell. The excitable neighbor is therefore calmed down by the inhibitoryneurotransmitter. When the neurotransmitter binds to a receptor on the cellmembrane surface of the adjacent nerve cell, a second messenger often carries itssignal inside the neighboring nerve cell. This concludes the process called signaltransduction. What happens after the second messenger system inside the nerve cellhas been activated is now the subject of much research. Although they are poorlyunderstood, the work of these second messengers may eventually help explain thedelayed effects of many psychiatric medications.Stopping Neurotransmission. Turning off the neurotransmitter signal once ithas been released into the synapse is critical to successful communication betweennerve cells. This is of paramount importance because unbridled stimulation can beharmful to nerve cells. For example, one of the problems in the minutes and hoursfollowing a stroke is that nerve cells near the stroke area can literally be stimulatedto death. In fact, some of the new medications used to minimize damage to the brainafter a stroke act by literally calming the cells in the brain. Thus, signal terminationis a critically important aspect of neurotransmission.As we noted earlier, when the neurotransmitter is released from the axon terminalinto the synapse, it is free to diffuse across the synapse to bind the receptors on theneighboring nerve cell. However, other fates may await the neurotransmitter onceit’s released into the synapse. In general, these other processes act to terminateneurotransmission by preventing the neurotransmitter from reaching the receptor onthe adjacent nerve cell. There are, in fact, five distinct mechanisms for terminatingthe neurotransmitter signal once it has been released into the synapse.1. Diffusion. Instead of drifting to the opposite side of the synapse, the neurotransmittermolecule can also drift outside the synapse altogether. In this free