Location and Structure
This structure is located within the petrous part (pars petrosa) of the temporal bone. On the posterior surface of the pyramid lies the internal acoustic opening (porus acusticus internus), which leads into the internal acoustic meatus (meatus acusticus internus). The canal runs in a medial-to-lateral direction.
From the cranial cavity perspective, the internal acoustic meatus connects the posterior cranial fossa (fossa cranii posterior) to the internal ear. The canal for the facial nerve (canalis nervi facialis) begins at the fundus of the meatus.
Contents of the Internal Acoustic Meatus
The internal acoustic meatus transmits cranial nerves and labyrinthine vessels:
- Facial nerve (nervus facialis, CN VII) — motor fibers innervating the muscles of facial expression.
- Intermediate nerve (nervus intermedius) — runs alongside the facial nerve, carrying sensory, taste, and parasympathetic secretomotor fibers.
- Vestibulocochlear nerve (nervus vestibulocochlearis, CN VIII) — macroscopically divided into vestibular and cochlear roots.
- Vestibular ganglion (ganglion vestibulare) — located within the internal acoustic meatus; contains bipolar neurons.
- Labyrinthine artery (arteria labyrinthi) — typically a branch of the basilar artery, less commonly of the anterior inferior cerebellar artery (AICA).
- Labyrinthine veins — course through the internal acoustic meatus alongside the labyrinthine artery.
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 alongside the intermediate and vestibulocochlear nerves, and enter the internal acoustic meatus. Within the meatus, the facial and intermediate nerves separate from the vestibulocochlear nerve.
Next, the facial and intermediate nerves course laterally and enter the facial canal. The labyrinthine segment of the facial canal runs perpendicular to the long axis of the petrous pyramid anteriorly toward the genu.
The cochlear nerve is formed by the central processes of neurons from the spiral ganglion (ganglion spirale). It passes through the internal acoustic meatus in close proximity to the vestibular nerve, crosses the subarachnoid space at the cerebellopontine angle, and enters the brainstem just below the inferior cerebellar peduncle.
Blood Supply to the Labyrinth
The 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, giving off branches to the spiral canal of the cochlea and the osseous spiral lamina, supplying the basilar membrane, spiral organ (organ of Corti), and the periosteum of the bony labyrinth.
- Vestibular branches (rr. vestibulares) supply the membranous vestibule, 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 alongside CN VIII, a Level 1 symptom complex develops:
- Peripheral facial nerve palsy (muscles of facial expression);
- Sensorineural hearing loss or deafness;
- Reduced vestibular function.