Final answer:
The facial nerve innervates the orbicularis oculi muscle, which controls the ability to close the eyelids tightly. This is tested using the corneal reflex and by evaluating the strength of the facial nerve during a physical exam.
Step-by-step explanation:
The nerve that innervates the muscle for tight closure of the eyelids is the facial nerve. This nerve is responsible for controlling the muscles of facial expression, including the orbicularis oculi muscle, which is the muscle that allows for the tight closure of the eyelids. A specialized reflex, known as the corneal reflex or the eye blink reflex, involves this muscle. When the cornea is stimulated, the sensory information travels through the trigeminal nerve, while the motor response to close the eyelids via the orbicularis oculi travels through the facial nerve.
In a clinical exam, an examiner may test facial nerve function by asking a patient to squeeze their eyes shut tightly while the examiner attempts to pry them open. This test evaluates the strength and function of the facial nerve and orbicularis oculi muscle.