Which cranial nerve is primarily responsible for the sensory function of the external ear?

The trigeminal nerve (CN V) is the cranial nerve primarily responsible for the sensory function of the external ear. It has three major branches, with the mandibular branch contributing to the sensation of the auricle and the external auditory canal. This means that any sensory information such as touch, pain, or temperature from the outer ear structures is transmitted through this nerve.

In contrast, other cranial nerves mentioned in the options have different primary functions related to the ear. While the facial nerve (CN VII) plays a role in the motor function of the muscles of facial expression and has a small sensory function related to taste, it does not primarily handle sensory information from the external ear. The glossopharyngeal nerve (CN IX) primarily provides sensory innervation to the posterior one-third of the tongue and parts of the pharynx, and although it has some involvement in the ear, it is not the main nerve for the external ear. The vagus nerve (CN X) does have branches that contribute to sensation in the ear but is not the primary sensory nerve for that area. Thus, the trigeminal nerve is the most significant in terms of sensory function related to the external ear.