Which cranial nerve is responsible for narrowing the pupil and focusing the lens?

The oculomotor nerve is primarily responsible for controlling most of the eye's movements, as well as regulating the functions of the pupil and lens. This cranial nerve innervates the sphincter pupillae muscles, which are essential for constricting the pupil in response to light (a function known as miosis) and helping to focus on close objects by adjusting the lens curvature (accommodation). By facilitating these processes, the oculomotor nerve plays a critical role in visual acuity and the ability to adapt to varying light conditions and distances.

In contrast, the optic nerve is involved in transmitting visual information from the retina to the brain, but it does not control eye movement or pupil size. The trigeminal nerve primarily serves sensory functions for the face and controls muscles involved in chewing. The facial nerve controls muscles of facial expression and does not directly contribute to pupil diameter or lens focus.