Ear

Auris

Also known as: vestibulocochlear organ, organ of hearing and balance

The paired organ of hearing and equilibrium, divided into the external ear (collects sound), middle ear (amplifies and transmits it via the ossicles) and inner ear (transduces sound and head motion in the fluid-filled labyrinth).

Overview

Hearing range≈20 Hz–20 kHz

Most sensitive range≈1–4 kHz

Hearing threshold (0 dB SPL)20 µPa

Dynamic range≈120 dB

The external ear — auricle and external acoustic meatus — gathers sound and funnels it to the tympanic membrane, boosting frequencies around 2–4 kHz by resonance and providing spectral cues for vertical localisation. The middle ear (tympanic cavity) is an air-filled space in the petrous temporal bone, ventilated by the auditory tube, in which the chain of malleus, incus and stapes transmits vibrations from the eardrum to the oval window, increasing pressure ≈22-fold to overcome the air-to-fluid impedance mismatch.

The inner ear lies deep in the petrous temporal bone. Its bony labyrinth (vestibule, three semicircular canals, cochlea), filled with perilymph, contains the membranous labyrinth (utricle, saccule, semicircular ducts, cochlear duct, endolymphatic duct and sac), filled with endolymph. Six sensory areas are present: the spiral organ (of Corti) for hearing, two maculae (utricle and saccule) for linear acceleration and gravity, and three cristae ampullares for angular acceleration. All use hair cells, innervated by the cochlear and vestibular divisions of the vestibulocochlear nerve (CN VIII).

Because the external and middle ear conduct sound while the inner ear and nerve sense it, hearing loss is classified as conductive or sensorineural — a distinction made at the bedside with Rinne and Weber tuning-fork tests and confirmed by audiometry.

Location

Lateral side of the head; middle and inner ear housed within the petrous and tympanic parts of the temporal bone.

Anatomy

Innervation

Special sense: vestibulocochlear nerve (VIII). General sensation: auriculotemporal (V3), great auricular (C2–C3), lesser occipital (C2), auricular branch of vagus (X), facial (VII) and tympanic branch of glossopharyngeal (IX) nerves — explaining referred otalgia from the teeth, tonsils, pharynx and larynx.

Arterial supply

External ear: posterior auricular, superficial temporal and deep auricular arteries. Middle ear: anterior tympanic (maxillary), stylomastoid (posterior auricular) and others. Inner ear: labyrinthine artery (usually from AICA) — an end artery.

Development

External ear — first pharyngeal cleft and six auricular hillocks; middle ear — first pharyngeal pouch, ossicles from first and second arch cartilages; inner ear — otic placode → otic vesicle.

Function

Hearing — detection of sound frequency (pitch), intensity (loudness) and timing (localisation)

Equilibrium — static (head position) and dynamic (linear and angular acceleration) balance

Drive reflexes that stabilise gaze and posture (vestibulo-ocular, vestibulospinal) and protect the cochlea (acoustic reflex)

Clinical significance

Hearing loss

Over 1.5 billion people have some degree of hearing loss (WHO). Conductive causes: wax, otitis externa/media, perforation, otosclerosis. Sensorineural: presbycusis, noise, ototoxic drugs (aminoglycosides, cisplatin, loop diuretics), infections (CMV, rubella, meningitis), Ménière disease, vestibular schwannoma. Unilateral sensorineural loss or tinnitus needs MRI to exclude a vestibular schwannoma.

Referred otalgia

Ear pain with a normal ear examination may be referred from the teeth/TMJ (V3), tonsils and pharynx (IX), or larynx/hypopharynx (X) — persistent unilateral otalgia in a smoker warrants examination for head and neck cancer.