ArticlesAnatomyHead and neckEarInternal ear

ArticlesAnatomyHead and neckEarInternal ear

Internal ear

Author: Christina Loukopoulou, MSc • Reviewer: Michelle Vishu, MBBS, MD (DVL)
Last reviewed: October 30, 2023
Reading time: 11 minutes

The internal ear is the innermost part of the auditory system, embedded within the dense petrous part of the temporal bone. It serves two critical functions: the detection of sound and the regulation of balance.

Structurally, it consists of a bony labyrinth surrounding a fluid-filled membranous labyrinth. Functionally, it includes the cochlea for hearing, the utricle and saccule for sensing linear acceleration, and three semicircular canals that detect rotational motion.

This article provides a detailed anatomical and functional review of the internal ear, including its components, spatial relationships, blood supply and innervation.

Key facts about the internal ear
Cochlea	Converts sound vibrations into electrical nerve signals.
Cochlear duct	Contains the spiral organ (of Corti) for sound transduction.
Spiral organ (of Corti)	Houses hair cells that detect sound and send signals to the brain.
Vestibule	Central chamber connecting cochlea and semicircular canals.
Utricle & Saccule	Detect linear acceleration and head position.
Semicircular canals	Detect rotational (angular) head movements.
Ampulla	Enlarged ends of canals containing motion-sensitive receptors.
CN VIII	Vestibulocochlear nerve transmitting hearing and balance signals.

Contents

Boundaries
Cochlear component
1. Frequency and loudness detection
Vestibular component
Semicircular component
Vasculature
1. Arterial supply
2. Venous drainage
Innervation
Highlights
Sources

+ Show all

Boundaries

Located anterolateral to the posterior cranial fossa, the internal ear is bordered laterally by the promontory of the middle ear. It includes:

Bony labyrinth: a rigid framework filled with perilymph, similar to extracellular fluid.
Membranous labyrinth: suspended within the bony labyrinth and filled with endolymph, which resembles cytosol.

The membranous and bony labyrinths do not communicate directly, and the separation of their fluids is essential for proper sensory function. Bony spaces are termed canals, while membranous spaces are known as ducts.

Cochlear component

The cochlea is a spiral-shaped structure that coils around the modiolus, a central bony axis. It converts sound vibrations into neural signals, a process known as auditory transduction.

Within the cochlea:

The cochlear duct (scala media) is suspended between the scala vestibuli above and the scala tympani below, both of which contain perilymph.
These chambers connect at the helicotrema, near the apex.

The duct is bordered by:

Vestibular membrane of cochlear duct (Reissner’s membrane), separating it from the scala vestibuli.
Basilar membrane of cochlear duct, separating it from the scala tympani.

Sitting on the basilar membrane is the spiral organ (of Corti), which contains:

Inner hair cells (primary sensory receptors).
Outer hair cells (amplify sound by adjusting basilar membrane stiffness).
The tectorial membrane, which interacts with stereocilia to initiate signal transduction.

Sound-induced pressure waves move through the cochlear fluid, stimulating the basilar membrane. This bends the stereocilia of hair cells, opens ion channels, and triggers depolarization. Neurotransmitters are released, and action potentials travel via the cochlear nerve to the brain.

Frequency and loudness detection

The basilar membrane plays a critical role in pitch and loudness detection:

High-frequency sounds stimulate the stiff, narrow base of the cochlea.
Low-frequency sounds travel to the flexible apex near the helicotrema.
Amplitude of sound waves determines loudness by influencing the degree of basilar membrane displacement.

This tonotopic organization allows the brain to distinguish between different pitches and volumes in music, speech, and environmental sounds.

Vestibular component

The vestibule lies between the cochlea and the semicircular canals. It contains two membranous sacs:

Saccule, connected to the cochlear duct via the ductus reuniens (of Hensen).
Utricle, which communicates with the semicircular ducts.

Each sac contains a macula, a sensory region composed of:

Hair cells with stereocilia and a kinocilium.
A gelatinous otolithic membrane embedded with otoconia (calcium carbonate crystals).

Movement or tilt of the head causes the otolithic membrane to shift, deflecting the hair cells and generating signals related to linear acceleration and head position.

Semicircular component

Projecting from the vestibule are three semicircular canals, each oriented perpendicularly to the others:

Anterior (superior) semicircular canal projects vertically and perpendicular relative to the petrous bone.
Lateral semicircular canal projects almost horizontally and laterally, with respect to the petrous bone.
Posterior semicircular canal also extends vertically, but along the vertical axis of the petrous bone.

Each canal contains an ampulla, which houses the ampullary crest—a sensory ridge covered by a gelatinous cupula. Rotational movement of the head causes endolymph to displace the cupula, bending the embedded hair cells and stimulating the vestibular nerve.

The semicircular ducts are membranous structures located within the bony semicircular canals, each following the curvature of its respective canal. Like the canals, the anterior and posterior ducts project vertically, while the lateral duct lies nearly horizontal.

Each duct expands into an ampulla at one end, containing the ampullary crest—a ridge of sensory epithelium covered by a gelatinous cupula. Rotation of the head causes endolymph within the ducts to shift, deflecting the cupula and stimulating the hair cells embedded in the crest. These signals are transmitted via the vestibular nerve. The utricle receives input from the ampullated ends of all three ducts, while the anterior and posterior ducts converge into a common crus before entering the utricle.

This quiz will challenge to you identify the structures of the internal ear.

Vasculature

Arterial supply

The labyrinthine artery, typically a branch of the anterior inferior cerebellar artery (AICA) or sometimes directly from the basilar artery, supplies blood to the internal ear. It enters via the internal acoustic meatus and branches into:

Cochlear artery.
Anterior and posterior vestibular arteries.

A supplemental supply may come from the stylomastoid artery, a branch of the posterior auricular or occipital artery.

Venous drainage

Venous drainage of the inner ear is primarily carried out by the labyrinthine veins, which exit via the internal acoustic meatus and typically drain into the superior or inferior petrosal sinuses.

In some cases, drainage may also occur via the transverse sinus or emissary veins. More specifically, the vein of the vestibular aqueduct drains into either the sigmoid sinus or the inferior petrosal sinus, while the vein of the cochlear aqueduct drains into the inferior petrosal sinus or directly into the internal jugular vein. These venous pathways play a crucial role in maintaining proper fluid balance and pressure within the inner ear structures.

Innervation

The vestibulocochlear nerve (CN VIII) innervates the internal ear and divides into two distinct branches:

Cochlear division: carries auditory signals from the spiral ganglion (of Corti) in the modiolus to the cochlear nuclei.
Vestibular division: splits into: a superior branch, supplying the utricle and anterior/lateral semicircular ducts, and an inferior branch, supplying the saccule and posterior semicircular duct.

Notably, the facial nerve (CN VII) traverses the internal acoustic meatus along with the labyrinthine vessels and CN VIII, but does not innervate internal ear structures.

It's almost time to start revising what you've learned about the internal ear. Don't forget the importance of active recall!

Highlights

The internal ear contains structures for both hearing and balance, housed in the petrous part of the temporal bone.
The cochlea processes sound via the spiral organ (of Corti), where hair cells convert mechanical energy into nerve impulses.
The utricle and saccule detect linear acceleration; the semicircular canals detect rotational movement.
Tonotopic organization of the cochlea enables fine pitch discrimination.
The labyrinthine artery supplies blood, and the vestibulocochlear nerve (CN VIII) carries sensory input to the brain.
The auditory pathway involves multiple neural relays that contribute to sound localization, recognition, and memory linkage.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

References:

Kiernan, J. A., Barr, M. L., & Rajakumar, N. (2014). Barr's the human nervous system (10th ed.). Philadelphia, PA: Lippincott Williams & Wilkins, a Wolters Kluwer business.

Netter, F. (2014). Atlas of human anatomy (6th ed.). Philadelphia, PA: Elsevier.

Sinnatamby, C. S., & Last, R. J. (2011). Last's anatomy (12th ed.). Edinburgh: Churchill Livingstone/Elsevier.

Standring, S., Borley, N. R., & Gray, H. (2008). Gray's anatomy: The anatomical basis of clinical practice (40th ed.). Edinburgh: Churchill Livingstone/Elsevier.

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