Internal Ear
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Direct answer
Within the petrous temporal bone, the densest in the body, lies the internal ear: a bony labyrinth of cochlea, vestibule and three semicircular canals, enclosing a membranous labyrinth of cochlear duct, utricle and saccule, and three semicircular ducts suspended in perilymph. The cochlea makes about two and three-quarter turns around its modiolus; the maculae of utricle and saccule detect static position and linear acceleration through otoliths; the cristae ampullares of the canals detect angular acceleration as endolymph displaces the cupula. Endolymph, uniquely high in potassium and produced by the stria vascularis, bathes the hair cells of the organ of Corti, and the whole apparatus is supplied by the labyrinthine artery — an end artery whose occlusion silences hearing and balance together.
What you must remember
- Bony labyrinth: cochlea anteriorly, vestibule in the middle, three semicircular canals (lateral, superior, posterior) posteriorly; the lateral canal is set about 30 degrees to the horizontal plane.
- The cochlea's central bony pillar is the modiolus, with the spiral lamina dividing each turn into scala vestibuli above the cochlear duct and scala tympani below, communicating at the helicotrema at the apex.
- The membranous labyrinth holds endolymph; the space between membranous and bony labyrinths holds perilymph, continuous with CSF through the cochlear aqueduct.
- Endolymph, high in potassium, is produced by the stria vascularis and absorbed at the endolymphatic sac; sodium-rich perilymph resembles CSF.
- Maculae (utricle for horizontal, saccule for vertical acceleration) contain otoconia; cristae in each ampulla detect angular acceleration, and in the lateral canal ampullopetal flow is excitatory — Ewald's second law.
- The organ of Corti on the basilar membrane carries a single row of inner hair cells (about 3,500, serving some 95 per cent of the afferent fibres) and three to five rows of outer hair cells that actively amplify the travelling wave.
- The vestibulocochlear nerve divides within the internal acoustic meatus into superior and inferior vestibular and cochlear parts; the labyrinthine artery, usually a branch of the antero-inferior cerebellar artery, supplies the whole labyrinth.
How to work through the caloric test and a labyrinthine stroke
The caloric test is the internal ear's bedside experiment. Water at 30 degrees Celsius is irrigated into the external meatus with the head positioned so the lateral canal is vertical; conduction cools the endolymph, which sinks and deflects the cupula, generating nystagmus. The direction is remembered as COWS — cold opposite, warm same — the fast phase of nystagmus being named away from the ear with cold water and towards it with warm. In bilateral caloric weakness the labyrinth is dead; in a normal caloric response with vertigo, the lesion lies central, the single most useful bedside separation of peripheral from central vertigo.
Now the vascular catastrophe. The labyrinthine artery is an end artery; its occlusion in an antero-inferior cerebellar artery territory infarct produces sudden deafness with violent vertigo and vomiting together — a combination no other single vessel can cause. Meniere's disease, by contrast, is endolymphatic hydrops: excess endolymph distends the cochlear duct and vestibular apparatus, presenting as episodic rotational vertigo with fluctuating low-frequency sensorineural loss, aural fullness and tinnitus, treated per current ENT practice with salt restriction and diuretics first. Noise-induced hearing loss targets the outer hair cells first at the 4 kHz notch, where the basilar membrane resonates maximally for that frequency.
Where students slip
Endolymph and perilymph compositions are reversed in half the answers: endolymph is the intracellular-like, potassium-rich fluid inside the membranous labyrinth, perilymph the sodium-rich fluid around it — and only endolymph bathes the hair cells' apical surface, which is the electrophysiological heart of transduction. Second, the maculae and cristae are interchanged; the maculae detect static position and linear acceleration with otoconia, the cristae angular acceleration, and the utricle's macula is horizontal while the saccule's is vertical — the pair examiners pick to test. Third, the internal acoustic meatus contains the facial nerve, nervus intermedius, vestibulocochlear divisions and the labyrinthine artery — a facial palsy with hearing loss localises to that meatus alone.
Frequently asked questions
How many turns does the cochlea make?
About two and three-quarters turns around the modiolus, forming a cone with the base directed towards the internal acoustic meatus.
What is the difference in function between maculae and cristae?
Maculae of the utricle and saccule detect static head position and linear acceleration using otolithic membranes; cristae ampullares of the semicircular ducts detect angular acceleration through endolymph displacement of the cupula.
What is the composition and source of endolymph?
A high-potassium, low-sodium fluid produced by the stria vascularis of the cochlear duct and absorbed at the endolymphatic sac.
What does the COWS mnemonic stand for?
The direction of the fast phase of caloric nystagmus — cold water produces nystagmus to the opposite side, warm water to the same side.
Which artery supplies the labyrinth, and what happens when it occludes?
The labyrinthine artery, usually from the antero-inferior cerebellar artery; occlusion causes sudden simultaneous deafness and vertigo because it is an end artery to the entire inner ear.