You are walking down the street when a strange dog begins to chase you. You manage to duck into a store and notice in a nearby hanging mirror that your pupils are very dilated. Which of the following most likely accounts for this?
A) The parasympathetic nervous system was stimulated by the scare and it, in turn, stimulated the circular smooth muscles of the iris to contract.
B) The parasympathetic nervous system was stimulated by the scare and it, in turn, stimulated the radial smooth muscles of the iris to contract.
C) The sympathetic nervous system was stimulated by the scare and it, in turn, stimulated the radial smooth muscles of the iris to contract.
D) The sympathetic nervous system was stimulated by the scare and it, in turn, stimulated the circular smooth muscles of the iris to contract.

Question

Quizplus · Accepted Answer

The dilation of the pupils is controlled by the iris smooth muscles, which are innervated by both the sympathetic and parasympathetic nervous systems. In situations of fear or stress, the sympathetic nervous system is usually activated, leading to the dilation of the pupils. The radial smooth muscles, controlled by the sympathetic nervous system, contract and expand the pupillary diameter, allowing more light to enter the eye and improving the visual awareness. Therefore, it is likely that the dog's chase has triggered a sympathetic response, causing the dilation of the pupils. Choice C correctly identifies this process.

You Are Walking Down the Street When a Strange Dog