Learning Objective: By the end of this session, the learner should be able to describe the anatomy of the inner ear, including the bony and membranous labyrinths, cochlea, vestibule, semi-circular canals, saccule and utricle, and explain their functional roles in hearing and balance, along with clinical implications such as Meniere’s disease.
The inner ear is located within the petrous part of the temporal bone. It contains the vestibulocochlear organs responsible for:
- Hearing: converting mechanical signals into electrical signals for the brain.
- Balance: detecting head position and motion.
It consists of two main components:
- Bony labyrinth: cochlea, vestibule, three semi-circular canals; filled with perilymph.
- Membranous labyrinth: cochlear duct, semi-circular ducts, utricle, saccule; filled with endolymph.
Bony Labyrinth
Vestibule
- The central part of the bony labyrinth, between the cochlea and semicircular canals.
- Contains the utricle and saccule (vestibular organs).
- Separated from the middle ear by the oval window.
Cochlea
- Spiral-shaped, contains the cochlear duct (organ of hearing).
- Surrounds a central core, the modiolus, containing cochlear nerve fibers.
- Creates two perilymph-filled chambers:
- Scala vestibuli: superior, continuous with vestibule
- Scala tympani: inferior, terminates at the round window
Semi-circular Canals
- Anterior, lateral, and posterior canals are at right angles.
- Each canal contains a semi-circular duct and ampulla, essential for balance.
Membranous Labyrinth
- Lies within the bony labyrinth; filled with endolymph.
- Includes: cochlear duct, semi-circular ducts, utricle, saccule.
Cochlear Duct
- Triangular duct within the cochlea; houses the Organ of Corti (hearing receptors).
- Roof: Reissner’s membrane
- Floor: Basilar membrane
- Lateral wall: Spiral ligament
Utricle & Saccule
- Located in the vestibule, balance organs:
- Utricle: horizontal plane movement
- Saccule: vertical plane movement
- Endolymph drains via the endolymphatic duct → sac
Semi-circular Ducts
-
Flow of endolymph detected by ampullae receptors → signals to the brain for balance.
Vasculature
- Bony labyrinth: anterior tympanic (maxillary), petrosal (middle meningeal), stylomastoid (posterior auricular)
- Membranous labyrinth: labyrinthine artery (branch of AICA or basilar artery)
- Cochlear branch → cochlea
- Vestibular branches → vestibular apparatus
- Venous drainage via labyrinthine vein → sigmoid or inferior petrosal sinus
Innervation
- Vestibulocochlear nerve (CN VIII):
- Cochlear nerve: base of modiolus → Organ of Corti → hearing
- Vestibular nerve: vestibular ganglion → utricle, saccule, semi-circular ducts → balance
- The facial nerve (CN VII) passes through but does not innervate the inner ear
Clinical Relevance: Meniere’s Disease
- Disorder of the inner ear with:
- Vertigo
- Low-pitched tinnitus
- Hearing loss
- Cause: excess endolymph → distension of the membranous labyrinth → damage to the balance and hearing membranes
