What physiological effect does the activation of M3 receptors typically include?

The activation of M3 receptors is primarily associated with functions that involve smooth muscle contraction and glandular secretion. One of the characteristic physiological effects linked to M3 receptor activation is the constriction of the pupils, also known as miosis. This effect occurs because M3 receptors are present in the iris sphincter muscle, and their stimulation leads to muscle contraction, resulting in a smaller pupil diameter.

M3 receptors also play a role in facilitating other parasympathetic responses, such as bronchoconstriction in the lungs, increased secretion of saliva, and enhancement of digestion by promoting gland secretions in the gastrointestinal tract. However, in the context of this question, the specific and defining action of pupil constriction highlights the role of M3 receptors in modulating ocular function through parasympathetic stimulation.