CHEMISTRY OF THE BRAIN: MOLECULES, MOOD, AND MENTAL ILLNESS - Chemistry: Notable Research and Discoveries - page 80

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ready for the next event, most of the released neurotrans-
mitter molecules should be gone, otherwise the postsynaptic
effect may linger. In a few cases, this happens, but in most
synapses, there is a mechanism that clears the “field” of any
residual neurotransmitter. For a few neurotransmitters such
as acetylcholine, enzymes lurking in the membranes of the cells
break apart the neurotransmitter molecule and prevent further
activation of the receptors. Most neurotransmitters, though,
are carried back into the presynaptic membrane and recycled
for future use. This process is called reuptake.
Students sometimes wonder why there are so many dif-
ferent neurotransmitters. Would only a few suffice instead?
The reason nervous systems employ such a bewildering di-
versity is apparently to obtain a multitude of different effects.
Receptors are the means by which the postsynaptic target
is affected, and having a wide variety of receptors, activated
by different molecules, enables synaptic transmission to ac-
complish a number of different tasks. Peptides may modu-
late a neuron's properties and have long-term affects, while
receptors for other neurotransmitters are more fast-acting.
And, although the figure on page 79 illustrates the common
case in which synaptic transmission occurs from a neuron
to a specific target, in other cases, the neurotransmitter
may have a wider release, affecting a nonspecific group of
neurons. This is true with neurotransmitters such as dopa-
mine. One neurotransmitter, nitric oxide, can even act as a
retrograde messenger, traveling from the postsynaptic cell
to the presynaptic cell.
transmission mechanisms. Although chemical transmission seems to
be the most important mechanism, there are synapses in the brain that
are mediated not by chemicals but by narrow openings that connect
the cells. Transmission across these synapses, which are known as gap
junctions, is electrical—the currents flow from one cell to another. Gap