Figure 7.1. Muscles in the iris of the eye. The pupillary opening, through
which light passes, is in the center. Sphincter muscles constrict the opening;
radial muscles dilate it.For some kinds of eye examinations, the examining clinician
may put drops into the eye to dilate the pupil. When drops are put
directly into the eye, the target of their action is very close by, and
the effects are fully experienced within a few minutes. Dilating the
pupil allows the clinician to take a close look inside the eyeball and
examine the retina. From our discussion, we conclude that either a
sympathomimetic drug or a parasympatholytic drug can be used to
accomplish the needed dilation of the pupil. Suppose the clinician
applies some drops of a solution containing a sympathomimetic drug.
Within minutes the drug enhances activity at the connections of the
sympathetic nervous system with the radial muscles of the iris, the
muscles contract, and the pupil opens—mission accomplished.
Now the clinician must shine a bright light into the eye in order
to see inside the eyeball and examine the retina. However, when the
bright light shines on the eye, it triggers a rapid constriction of the
pupil to protect the visual system from being overstimulated. This
is what happens when you walk outside on a sunny day after having
been inside a darkened room, like a movie theater. This light-induced
constriction of the pupil is achieved via parasympathetic neural input
to the sphincter muscles of the iris. Thus, in order to carry out the eye
examination, the clinician must use not a sympathomimetic drug but
a parasympatholytic one. Both result in pupil dilation, but only the
parasympatholytic drug blocks the muscles that constrict the pupil in
response to light.
Historically, in ophthalmology, a drug called atropine was used
for this purpose. (More on atropine in Chapter 8.) These days eye