FoundationalConceptsNeuroscience

There are specific terms to describe these actions of drugs. An
agonist is a molecule that binds to a neurotransmitter receptor and
activates it. Thus, for example, a neurotransmitter is an agonist at its
own receptor. An agonist drug molecule would have a shape similar to
the natural neurotransmitter for that receptor, so that it can bind to
the receptor in a fashion similar to the neurotransmitter. Sometimes
the molecular similarity between a neurotransmitter and an agonist
is discernable by comparing the respective molecular structure dia-
grams, but often it is not apparent from such simple diagrams. In the
key-and-lock analogy for a neurotransmitter and its receptor, an ago-
nist is like a second type of key that opens the same lock.
An antagonist is a molecule that binds to a neurotransmitter
receptor and blocks the action of the neurotransmitter at the receptor.
Thus, an antagonist might be similar enough to the neurotransmitter
to bind to the receptor but not similar enough to activate it—it just
sticks there and blocks the binding site so that the neurotransmitter
cannot bind and activate the receptor. In the key-and-lock analogy, an
antagonist is like a key that fits into the lock but doesn’t open it.
One more bit of terminology: any drug or other action that has
a stimulating effect on the sympathetic nervous system is termed
sympathomimetic, from the Greek word mimesis, meaning imitation.
Drugs that have an activating effect on the sympathetic nervous
system are called sympathomimetic drugs. The converse is termed
sympatholytic, from the Greek lysis, to disrupt. Thus, sympatholytic
substances decrease the effects of the sympathetic nervous system
on its target organs. There are analogous terms that apply to effects
on the parasympathetic nervous system—parasympathomimetic and
parasympatholytic substances act to increase or decrease, respectively,
the effects of the parasympathetic system on its target organs.
Because the sympathetic and parasympathetic systems are always