Internal acoustic canal
The internal acoustic canal (IAC), also known as the internal auditory canal or meatus (IAM), is a bony canal within the petrous portion of the temporal bone that transmits nerves and vessels from within the posterior cranial fossa to the auditory and vestibular apparatus.
The opening of the IAM, the porus acusticus internus, is located within the cranial cavity, near the posterior surface of the temporal bone. The margins of the opening are smooth and rounded, and the canal is short (1 cm), running laterally to the bone. The canal narrows laterally, and the lateral boundary is the fundus, where the canal splits into three distinct openings, one of which is the facial nerve canal.
Contents of the IAM include:
- facial nerve
- vestibulocochlear nerve
- vestibular ganglion
- labyrinthine artery (usually a branch of the AICA or basilar artery)
There are five nerves that run through the IAM:
- nervus intermedius (sensory component of CN VII)
- facial motor root (motor component of CN VII)
- cochlear nerve (component of CN VIII)
- inferior vestibular nerve (component of CN VIII)
- superior vestibular nerve (component of CN VIII)
Their position is most constant in the lateral portion of the meatus which is anatomically divided by the falciform crescent. This horizontal ridge divides the canal into superior and inferior portions:
- superior: facial nerve and superior vestibular nerve (SVN); the facial nerve is anterior to the SVN and is separated from it laterally by Bill's bar, a vertical ridge of bone
- inferior: cochlear nerve and inferior vestibular nerve (IVN); the cochlear nerve is situated anteriorly
See mnemonic for the position of the nerves in the IAC.
In addition to the three nerves which enter it, it also contains the vestibular ganglion (ganglion of Scarpa). From here three bundles emerge: superior and inferior division of the vestibular nerve and the nerve from the posterior semicircular canal (see artice: vestibulocochlear nerve (CN VIII) for further details).
- narrow duplicated internal acoustic canal