Inner ear (labyrinth)

Auris interna; labyrinthus

Also known as: labyrinth, bony labyrinth, osseous labyrinth, membranous labyrinth, otic capsule, endolymphatic sac, internal acoustic meatus

A system of fluid-filled channels in the petrous temporal bone: a bony labyrinth (vestibule, semicircular canals, cochlea) containing perilymph, enclosing a membranous labyrinth containing endolymph and the sensory epithelia for hearing and balance.

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Overview

The bony (osseous) labyrinth is a series of cavities in the dense otic capsule of the petrous temporal bone — the hardest bone in the body. It has three parts: the central vestibule, the three semicircular canals posterosuperiorly and the cochlea anteromedially. It is filled with perilymph (similar to extracellular fluid; high Na⁺) and communicates with the subarachnoid space through the cochlear aqueduct.

Suspended within it is the membranous labyrinth, a closed system of interconnected sacs and ducts filled with endolymph (high K⁺): the utricle and saccule (in the vestibule), the three semicircular ducts (in the canals), the cochlear duct (in the cochlea), and the endolymphatic duct ending in the endolymphatic sac in the posterior cranial fossa (under the dura), where endolymph is absorbed. The saccule communicates with the cochlear duct via the ductus reuniens, and with the utricle via the utriculosaccular duct.

Six sensory epithelia lie in the membranous labyrinth: the spiral organ (cochlear duct), the maculae of utricle and saccule, and three cristae ampullares. Their hair cells are innervated by bipolar neurons whose cell bodies lie in the spiral (cochlear) and vestibular (Scarpa) ganglia; the central processes form the vestibulocochlear nerve, which passes with the facial nerve through the internal acoustic meatus to the cerebellopontine angle.

Location: Petrous part of the temporal bone, medial to the middle ear; its axis lies roughly parallel to the long axis of the petrous ridge.

Parts & subdivisions

Bony labyrinth
Vestibule, semicircular canals and cochlea; perilymph-filled.
Membranous labyrinth
Utricle, saccule, semicircular ducts, cochlear duct, endolymphatic duct and sac; endolymph-filled.
Vestibule
Central chamber containing utricle and saccule; oval window in its lateral wall.
Semicircular canals
Anterior, posterior and lateral; detect angular acceleration.
Cochlea
Snail-shaped spiral of 2½–2¾ turns; organ of hearing.
Endolymphatic duct and sac
Runs through the vestibular aqueduct to a dural sac on the posterior petrous surface; absorbs endolymph and has immune functions. Enlarged vestibular aqueduct is a common inner-ear malformation causing childhood hearing loss.
Internal acoustic meatus
Canal on the posterior petrous surface transmitting VII, VIII (cochlear and vestibular divisions), nervus intermedius and the labyrinthine artery. At its fundus: facial nerve anterosuperior, cochlear anteroinferior, superior and inferior vestibular posteriorly ("7-Up, Coke down").

Anatomy

Innervation: Vestibulocochlear nerve (VIII): cochlear division (spiral ganglion) and vestibular division (superior and inferior vestibular nerves, vestibular ganglion). Efferent olivocochlear and vestibular efferent fibres modulate hair-cell sensitivity.
Arterial supply: Labyrinthine (internal auditory) artery — usually a branch of the anterior inferior cerebellar artery (AICA), sometimes the basilar — an end artery dividing into the common cochlear and anterior vestibular arteries. Minor supply from the stylomastoid artery.
Venous drainage: Labyrinthine veins and veins of the cochlear and vestibular aqueducts → inferior petrosal and sigmoid sinuses.
Development: Otic placode → otic pit → otic vesicle (otocyst) (week 4), which divides into a dorsal utriculosaccular (vestibular) part (utricle, semicircular ducts, endolymphatic duct) and a ventral saccular–cochlear part (saccule, cochlear duct). The cochlear duct coils to 2½ turns by week 10. Surrounding mesenchyme chondrifies to form the otic capsule, which ossifies from 14 centres and is adult-sized at birth.

Function

Transduce sound (cochlea)
Detect head position and linear acceleration (utricle, saccule)
Detect angular acceleration (semicircular canals)

Clinical significance

Labyrinthine infarction (AICA stroke)

Because the labyrinthine artery is an end artery, its occlusion causes sudden unilateral deafness and vertigo — sometimes the first sign of an AICA territory stroke (with facial weakness, ipsilateral ataxia, Horner and facial sensory loss).

Ototoxicity

Aminoglycosides (gentamicin — mainly vestibulotoxic; amikacin, neomycin — mainly cochleotoxic) concentrate in endolymph and kill hair cells irreversibly (risk increased by mitochondrial m.1555A>G variant); cisplatin damages outer hair cells (high frequencies first); loop diuretics affect the stria vascularis (often reversible); high-dose salicylates cause reversible tinnitus and hearing loss.

Vestibular schwannoma (acoustic neuroma)

Benign Schwann-cell tumour of the vestibular nerve in the internal acoustic meatus/cerebellopontine angle: progressive unilateral sensorineural hearing loss and tinnitus, later facial numbness (V) and ataxia; bilateral tumours define neurofibromatosis type 2. Diagnosed with MRI.

Mnemonics

Internal acoustic meatus

"Seven-Up, Coke down": facial nerve (VII) anterosuperior, cochlear nerve anteroinferior.

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.