Which receptors are located on the radial muscle of the iris?

The radial muscle of the iris, also known as the dilator pupillae, is primarily innervated by alpha-1 adrenergic receptors. When these receptors are activated by norepinephrine, they cause contraction of the radial muscle, leading to pupillary dilation (mydriasis).

This physiological response plays a crucial role in adjusting the amount of light entering the eye; during low light conditions, dilation of the pupil allows more light to pass through, enhancing visibility.

The other receptors mentioned—such as alpha-2 and beta receptors—are associated with different types of physiological responses. Alpha-2 receptors are mainly involved in inhibitory processes and are not primarily responsible for radial muscle contraction. Beta receptors are generally associated with functions in the heart and smooth muscle relaxation, and are not found in significant quantities in the iris's radial muscle. Thus, the predominance of alpha-1 receptors in the radial muscle makes this the accurate answer.