Question 81 of 100

(demo test)

The iris holds 2 muscles. The first is the iris sphincter muscle. It forms a ring that encircles the pupil. When it contracts, the pupil gets smaller in bright light. The second is the iris dilator muscle. Its fibers run like spokes from the pupil out to the edge of the iris. When they contract, the pupil widens in dim light. Those two muscles working against each other set the pupil size.

Knowing both muscles helps you understand dilating drops. A mydriatic drug makes the pupil larger. It works by stimulating the iris dilator muscle. Phenylephrine is a common mydriatic. A cycloplegic drop is the one that paralyzes the sphincter, so the pupil dilates passively. A miotic drug does the opposite. It makes the sphincter muscle contract, so the pupil gets smaller and less light enters the eye. Pilocarpine is a common miotic. Both drug types act on the same two muscles, just in opposite directions. The pupil itself is just the round opening in the middle of the iris. It looks black because the inside of the eye is dark.

Remember the pair as a squeeze muscle and a spoke muscle. The sphincter squeezes the pupil closed, and the dilator pulls it open. Every drop you instill for dilation acts on one or both of them. Pupil size also reports on nerve health. That is why pupil testing comes before any dilating drop goes in. Once a drop is in, the reading is no longer reliable. Being able to explain the iris and dilating drops in plain words is a core part of the COA patient services and education content area.

Incorrect Answers

• B. Four would be too many. The textbook names only a sphincter and a dilator inside the iris.
• C. Five would count muscles that do not exist. Only the sphincter and the dilator lie inside the iris; the ciliary muscle sits behind it.
• D. The iris is not a passive curtain. It has its own working muscle fibers that change pupil size.

Testing Pearls

• The sphincter encircles the pupil and constricts it in bright light.
• The dilator runs in a spoke pattern and widens the pupil in dim light.
• Mydriatics dilate the pupil by stimulating the iris dilator muscle.
• Miotics constrict by making the sphincter contract.
• Mydriatic and cycloplegic bottles carry a red cap under the standard color code.

References

1. American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 2, Anatomy and Physiology of the Eye.
2. American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 6, Basics of Ophthalmic Pharmacology.