Internal acoustic meatus

Location and structure

The structure is located in the petrous part, or pyramid, of the temporal bone (pars petrosa). On the posterior surface of the pyramid is the internal acoustic opening (porus acusticus internus), which leads into the internal acoustic meatus (meatus acusticus internus). The canal runs from medial to lateral.

From the cranial cavity side, the internal acoustic meatus connects the posterior cranial fossa (fossa cranii posterior) with the inner ear. The canal of the facial nerve (canalis nervi facialis) originates at the fundus of the meatus.

What passes through the internal acoustic meatus?

The cranial nerves and labyrinthine vessels pass through the internal acoustic meatus:

• Facial nerve (nervus facialis, cranial nerve VII) — motor fibers of the facial nerve innervate the facial expression muscles.
• Intermediate nerve (nervus intermedius) — runs together with the facial nerve; carries sensory, taste, and secretory fibers.
• Vestibulocochlear nerve (nervus vestibulocochlearis, cranial nerve VIII) — macroscopically divided into vestibular and cochlear roots.
• Vestibular ganglion (ganglion vestibulare) — located in the internal acoustic meatus; contains bipolar neurons.
• Labyrinthine artery (arteria labyrinthi) — most commonly a branch of the basilar artery, less frequently of the anterior inferior cerebellar artery.
• Labyrinthine veins — pass through the internal acoustic meatus alongside the labyrinthine arteries.

Topography of neural pathways

The motor fibers of the facial nerve emerge from the brainstem at the cerebellopontine angle (angulus pontocerebellaris), traverse the subarachnoid space together with the intermediate nerve and vestibulocochlear nerve, and enter the internal acoustic meatus. Within the internal acoustic meatus, the facial and intermediate nerves separate from nerve VIII.

Further on, the facial and intermediate nerves deviate laterally and enter the facial canal. The labyrinthine segment of the facial canal runs perpendicular to the longitudinal axis of the pyramid anteriorly to the genu.

The cochlear nerve is formed by the central processes of neurons of the spiral ganglion (ganglion spirale). It passes through the internal acoustic meatus in close proximity to the vestibular nerve, then crosses the subarachnoid space in the cerebellopontine angle and enters the brainstem directly below the inferior cerebellar peduncle.

Blood supply to labyrinthine structures

Labyrinthine artery (a. labyrinthi) enters the internal acoustic meatus (meatus acusticus internus), where it divides into cochlear and vestibular branches.

• Cochlear branch (r. cochlearis) runs along the axis of the cochlea, gives off branches to the spiral canal of the cochlea and the osseous spiral lamina, and supplies the basilar membrane, spiral organ, and the periosteum of the osseous labyrinth.
• Vestibular branches (rr. vestibulares) supply the membranous vestibule, nourishing the utricle (utriculus), saccule (sacculus), and the ampullae of the semicircular ducts.

Clinical significance: topographic diagnostics

When the facial nerve is damaged within the internal acoustic meatus along with cranial nerve VIII, a Level 1 symptom complex develops:

• peripheral facial nerve palsy;
• hearing impairment or deafness;
• decreased excitability of the vestibular apparatus.

Class and exam questions

What else to study on this topic

• Facial canal (Canalis nervi facialis)
• Posterior cranial fossa (Fossa cranii posterior)
• Vestibulocochlear nerve (Nervus vestibulocochlearis)
• Inner ear (Auris interna)

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.