54 CHAPTER 2
There is one neurotransmitter that is not taken back into the vesicles, however.
Because ACh is responsible for muscle activity, and muscle activity needs to happen rap-
idly and continue happening, it’s not possible to wait around for the “sucking up” pro-
cess to occur. Instead, an enzyme* specifically designed to break apart ACh clears the
synaptic gap very quickly (a process called enzymatic degradation.) There are enzymes
that break down other neurotransmitters as well.I think I understand the synapse and neurotransmitters now, but
how do I relate that to the real world?Knowing how and why drugs affect us can help us understand why a doctor might
prescribe a particular drug or why certain drugs are dangerous and should be avoided.
Because the chemical molecules of various drugs, if similar enough in shape to the neu-
rotransmitters, can fit into the receptor sites on the receiving neurons just like the neu-
rotransmitters do, drugs can act as agonists or antagonists. Drugs acting as agonists, for
example, can mimic or enhance the effects of neurotransmitters on the receptor sites of
the next cell. This can result in an increase or decrease in the activity of the receiving cell,
depending on what the effect of the original neurotransmitter (excitatory or inhibitory)
was going to be. So if the original neurotransmitter was excitatory, the effect of the ago-
nist will be to increase that excitation. If it was inhibitory, the effect of the agonist will be
to increase that inhibition. Another deciding factor is the nervous system location of the
neurons that use a specific neurotransmitter.
For example, some antianxiety medications, such as diazepam (Valium®), are
classified as benzodiazepines ( to Learning Objective 15.10.) and are agonists
for GABA, the primary inhibitory neurotransmitter in the brain. Areas of the brain
that you will learn about later that play a role in controlling anxiety, agitation, and
fear include the amygdala, orbitofrontal cortex, and the insula (LeDoux & Damasio,
2013; Zilles & Amunts, 2012). By increasing the inhibitory (calming) action of GABA,
the benzodiazepines directly calm these specific brain areas (Julien et al., 2011; Preston
et al., 2008).
Other drugs act as antagonists, blocking or reducing a cell’s response to the action
of other chemicals or neurotransmitters. Although an antagonist might sound like it has
only an inhibitory effect, it is important to remember that if the neurotransmitter that the
antagonist affects is inhibitory itself, the result will actually be an increase in the activity
of the cell that would normally have been inhibited; the antagonist blocks the inhibitory
effect.
Last, some drugs yield their agonistic or antagonistic effects by impacting the
amount of neurotransmitter in the synapse. They do so by interfering with the regu-
lar reuptake or enzymatic degradation process. Remember that the neurotransmitter
serotonin helps regulate and adjust people’s moods, but in some people the nor-
mal process of adjustment is not working properly. Some of the drugs used to treat
depression are called SSRIs (selective serotonin reuptake inhibitors). SSRIs block the
reuptake of serotonin, leaving more serotonin available in the synapse to bind with
receptor sites. Over several weeks, the individual ’s mood improves. Although the
reason for this improvement is not as simple as once believed (i.e., low levels of sero-
tonin = low levels of mood) or fully understood, SSRIs are effective for depression,
anxiety, and obsessive- compulsive disorder (Hyman & Cohen, 2013; Julien et al., 2011;
Stahl, 2013).
This section covered the neuron and how neurons communicate. The next section
looks at the bigger picture—the nervous system itself.enzymatic degradation
process by which the structure of a
neurotransmitter is altered so it can
no longer act on a receptor.
*enzyme: a complex protein that is manufactured by cells.