Inner ear

The inner ear, or auris interna, is the deepest component of the vertebrate auditory and balance system. It is responsible for converting mechanical stimuli into neural signals that the brain interprets as sound, spatial orientation, and equilibrium. Present in all vertebrates, the inner ear shows considerable variation in form and complexity, yet its fundamental roles in hearing and balance are conserved. In mammals, it is situated within the temporal bone and consists of a complex set of bony passages known as the bony labyrinth, enclosing the delicate membranous labyrinth. Both the cochlear and vestibular systems contained within this structure are innervated by the eighth cranial nerve.

Structural organisation

The labyrinth of the inner ear can be divided into the bony labyrinth and the membranous labyrinth. The bony labyrinth comprises a network of rigid channels within the temporal bone, lined by periosteum and filled with perilymph. Its major subdivisions are the vestibule, the semicircular canals, and the cochlea. Within these spaces lies the membranous labyrinth, a flexible system of ducts and chambers filled with endolymph and suspended within the perilymph. This arrangement creates three parallel fluid-filled compartments essential for sensory transduction.
Cochlea and hearing mechanismsThe cochlea is a spiralled structure that converts mechanical vibrations into electrical impulses. Sound waves striking the tympanic membrane are transmitted through the auditory ossicles—malleus, incus, and stapes—of the middle ear. The stapes articulates with the oval window, and its movement generates pressure waves in the perilymph of the cochlea. Because the oval window is significantly smaller than the tympanic membrane, energy is concentrated to create higher pressure at the interface with the inner ear.
Pressure changes propagate along the cochlear ducts as travelling waves. These movements deflect the basilar membrane, stimulating mechanosensory hair cells in the organ of Corti. The resulting electrochemical signals pass through the spiral ganglion and auditory nerve to the brain. Mechanical tuning along the basilar membrane allows differential sensitivity to frequency: high-frequency sounds excite regions near the cochlear base, whereas low-frequency sounds induce maximal displacement near the apex.
Vestibular systemThe vestibular apparatus comprises the semicircular canals, utricle, and saccule. These structures detect angular and linear accelerations of the head, respectively. Sensory hair cells similar to those in the cochlea transduce motion and position information through deflection of stereocilia by endolymph movement or by displacement of calcium carbonate otoliths. Inputs from the vestibular system integrate with visual signals and proprioceptive feedback from muscles and joints to maintain gaze stability, posture, and balance.

Embryological development

The human inner ear begins to form during the fourth week of embryogenesis from the auditory placode, a specialised thickening of ectoderm. This placode invaginates to form the auditory vesicle, or otocyst, which differentiates into the utricular and saccular components of the membranous labyrinth. These early structures contain the precursors of the maculae, with their sensory hair cells and otoliths that detect gravity and linear acceleration.
The utricular region gives rise to the semicircular canals, endolymphatic sac, and endolymphatic duct, enabling angular acceleration detection and fluid homeostasis. During the fifth week, the cochlear duct emerges from the auditory vesicle and elongates into a coiled tube. It eventually houses the organ of Corti and the endolymph necessary for auditory transduction. Reissner’s membrane separates the cochlear duct from the scala vestibuli, while the basilar membrane separates it from the scala tympani. The stria vascularis within the lateral wall produces endolymph and maintains its high potassium concentration.
Hair cells arise from medial and lateral ridges of the cochlear duct epithelium. Along with supporting cells and the tectorial membrane, they constitute the organ of Corti, the primary site of auditory transduction.

Microanatomy

The inner ear contains highly specialised sensory and supporting cell types that ensure precise mechanical–electrical conversion.

• Hair cells: primary sensory receptors for both hearing and balance. Each hair cell bears an apical hair bundle composed of actin-rich stereocilia anchored in a cuticular plate. Damage to these bundles disrupts auditory and vestibular function.
• Pillar cells: inner and outer pillar cells form the tunnel of Corti. They contain dense arrays of microtubules and microfilaments, providing structural rigidity and mechanical coupling to hair cells.
• Boettcher cells: located on the basilar membrane beneath Claudius cells in the lower cochlear turns; supportive to outer hair cells.
• Claudius cells: situated above Boettcher cells; involved in ion transport and maintaining boundaries between endolymphatic compartments.
• Deiters (phalangeal) cells: neuroglial supporting cells underlying and stabilising outer hair cells, with one row of inner and three rows of outer phalangeal cells.
• Hensen cells: tall columnar cells adjacent to the third row of Deiters cells; associated with the tectorial membrane region.

Other notable structures include Nuel’s spaces (fluid-filled areas between outer hair cells and pillar cells), Hardesty’s membrane (a tectorial layer over outer hair cells), Reissner’s membrane (separating scala media and scala vestibuli), and Huschke’s teeth (spiral limbus projections contacting the tectorial membrane). Rosenthal’s canal houses the spiral ganglion, which contains primary auditory neurons.

Vascular supply

The bony labyrinth receives arterial supply from:

• the anterior tympanic branch of the maxillary artery,
• the petrosal branch of the middle meningeal artery, and
• the stylomastoid branch of the posterior auricular artery.

The membranous labyrinth is supplied by the labyrinthine artery. Venous drainage occurs through the labyrinthine vein, which drains into the sigmoid sinus or inferior petrosal sinus.

Functional principles

The auditory system encodes frequencies from approximately 20 to 20,000 Hz in young adults, though high-frequency sensitivity declines with age. Distinct mechanical tuning mechanisms allow separation of high and low frequencies along the basilar membrane, as demonstrated by Georg von Békésy’s studies of travelling waves.
The vestibular system detects head orientation, linear acceleration, and rotational movement. Sensory integration across vestibular, visual, and proprioceptive systems enables stable vision and posture.

Disorders

Disruption of inner ear signalling may result from inflammation, infection, or obstruction of the labyrinth. Labyrinthitis manifests with vertigo, dizziness, nausea, and disorientation. Causes include viral infection, bacterial invasion, and mechanical interference with fluid pathways. Such disorders highlight the inner ear’s central role in maintaining equilibrium and stable sensory perception.