example, in increased heart rate and respiration. For this reason, the
sympathetic system is sometimes simplistically referred to as the
“fight or flight” system, highlighting its role in preparing the body
to respond to a crisis. Conversely, in times of rest and recuperation,
parasympathetic activity dominates. Several of the prominent effects
of sympathetic and parasympathetic activity are listed here:
Sympathetic Parasympathetic
increases heart rate decreases heart rate
dilates lung airways constricts lung airways
dilates pupils of eyes constricts pupils of eyes
inhibits salivation stimulates salivation
inhibits bladder from voiding — stimulates bladder to void
decreases intestinal motility stimulates intestinal motilityThe sympathetic and parasympathetic systems use different neu-
rotransmitter molecules. When the sympathetic nerve fibers make
connections with target tissues—such as the heart, lungs, intestines,
bladder, or iris of the eye—the signaling molecule (neurotransmitter)
used at those connections is norepinephrine. For the parasympathetic
neural connections with target tissues, acetylcholine is the neuro-
transmitter. In both cases, the receptors are GPCRs.
Chemical synapses are often sites of action for the effects of drugs
upon the nervous system. Here by drug I mean a chemical that in
small amounts has a significant effect on the functioning of an organ-
ism. How might a drug molecule have an effect at a chemical synapse?
One possibility is for the drug molecule to bind to a neurotransmitter
receptor and either activate the receptor or block it.