Damage to which nerve causes dilation of pupils?

The dilation of pupils, also known as mydriasis, is primarily controlled by the oculomotor nerve. This nerve is responsible for several eye functions, including the constriction of the pupil through the sphincter muscle of the iris and the movement of the eye. When the oculomotor nerve is damaged or compromised, the balance between the constrictor and dilator muscles of the iris can be affected, leading to the inability to constrict the pupil properly. This results in pupil dilation because the opposing action of the dilator muscle—the sympathetic innervation—becomes more pronounced without the antagonistic effect of the constrictor muscle.

In contrast, the optic nerve is primarily responsible for vision and does not directly control pupil size. The facial nerve influences facial expressions and some functions of taste, and the trigeminal nerve is mainly involved with sensation in the face and motor functions such as biting and chewing. Hence, the action of pupil dilation is specifically linked to the dysfunction or damage of the oculomotor nerve.