FoundationalConceptsNeuroscience

active and have opposite effects, enhancing the effects of one has
essentially the same consequences as decreasing the effects of the
other. Thus, a sympathomimetic drug might produce very similar
effects on the body as a parasympatholytic substance. And a parasym-
pathomimetic drug might produce effects similar to a sympatholytic
one. While their bodily effects might be comparable, the underlying
cellular and molecular mechanisms would be different. This may or
may not be of consequence, depending upon the exact nature of the
systems being affected.
Consider the following example. Muscles in the iris of the eye
make connections with the autonomic nervous system, and these
connections are used to regulate the size of the pupillary opening. The
sympathetic nervous system activates muscles—called radial muscles
—that pull the iris back, opening or dilating the pupil (Fig. 7.1). The
parasympathetic nervous system activates muscles—called circular
sphincter muscles—that pull the iris closed, constricting the size
of the pupil. Thus, the pupil of the eye is dilated by either sympath-
omimetic or parasympatholytic drugs and constricted by either sym-
patholytic or parasympathomimetic drugs.