Question

The size of the pupil decreases due to contraction of
A. Radial muscles
B. Circular muscles
C. Both circular and radial muscles
D. Nictitating membrane

Answer 1

Hint: Pupil dilation and constriction is under sympathetic and parasympathetic control respectively which in turn control the functioning of radial and circular muscles which dilate or constrict the pupil.

Complete answer: Pupil allows light to strike the retina and is located at the center of the iris of the eye. Pupil and iris together control the quantity of light entering the iris. At times of low light pupil dilates to improve the vision whereas in high light conditions pupil constricts to decrease the amount of light entering the eye. Iris consists of two types of smooth muscles: radial and circular muscles. Sphincter pupillae are the circular muscles and dilator pupillae are the radial muscles. when sphincter pupillae or circular muscles contract the size of the pupil decreases. As a result the amount of light reaching the retina also decreases.
Hence, option B is correct
Nictitating membranes are absent in humans. It is the third eyelid in some animals and helps in moistening and protecting the eye during vision.

Additional information:
The physiological response changing the size of the pupil is called the pupillary response. There are two types of responses :
1. Constriction response: Contraction of the pupil using circular muscles controlled by the parasympathetic nervous system. It is caused by antihypertensive drugs or some other drugs eg opiates
2. Dilation response: Widening of the pupil using radial muscles controlled by the sympathetic nervous system. It is caused by adrenaline or hallucinogens.

Note: Increase and decrease in the size of the pupil is directly due to contractions of radial and circular muscles respectively and size of pupil controls the amount of light entering the eye. hence iris and pupil together are the deciding factor for the amount of light reaching the retina.