Internal acoustic meatus

Internal acoustic meatus refers to a small bony canal that opens on to the posterior surface of the petrous part of temporal bone, inside the posterior cranial fossa. It allows for the passage of three important structures, namely the vestibulocochlear nerve, facial nerve and the labyrinthine artery.

The vestibulocochlear or the eighth cranial nerve is a special sensory nerve that originates from the junction of pons and medulla oblongata and passes through the internal acoustic meatus to innervate the inner ear or labyrinth, within the petrous part of the temporal bone. The nerve comprises of vestibular and cochlear components, which innervate the vestibular and cochlear parts of the inner ear, respectively. The vestibular part comprises of utricle, saccule and semicircular canals, and is linked with balance regulation, while the cochlear part is concerned with hearing.

The facial nerve also originates from the brainstem and passes through internal acoustic meatus, however, it has a totally different destination. It courses through a bony facial canal within the petrous bone and exits on the side of the base of the skull via the stylomastoid foramen. Within the bony facial canal, the facial nerve gives off three neuronal branches, including the greater petrosal nerve (for the lacrimal gland), the nerve to stapedius muscle and the chorda tympani nerve.

The labyrinthine artery (a.k.a. internal auditory artery) also accompanies vestibulocochlear and facial nerves. It is a branch of the vertebrobasilar circulatory system and supplies vascular supply to the inner ear.