General structure
The inner ear is organized on a " matryoshka doll" principle and includes two nested levels:
- Bony labyrinth — the outer case formed by dense bone tissue.
- Membranous labyrinth — a system of closed epithelial canals and cavities. It lies strictly inside the bony labyrinth, exactly mirrors its shape, but has a significantly smaller diameter.
Sections of the bony labyrinth
Anomically, the bony framework is divided into three sections arranged from anterior to posterior:
- Cochlea (cochlea) — the anterior part. It is a spiral bony canal making exactly 2.5 turns around a central bony axis.
- Vestibule (vestibulum) — the middle part. It is a central oval cavity communicating anteriorly with the cochlea and posteriorly with the semicircular canals. On its lateral wall are two "windows" bordering the middle ear:
- Oval window (vestibular window) — closed by the base of the stapes (stapes).
- Round window (cochlear window) — closed by the secondary tympanic membrane.
- Three semicircular canals — the posterior part. The canals lie in three mutually perpendicular planes. Each opens into the vestibule via an expanded portion — the bony ampulla.
Membranous labyrinth and fluid media
Unlike the middle ear, which contains air, the inner ear is filled with fluids. The space between the bony walls and the membranous labyrinth contains perilymph (isolated from the tympanic cavity by the stapes and the secondary membrane). Inside the membranous labyrinth itself circulates endolymph — forming a closed system.
The structures of the membranous labyrinth are distributed across the bony sections:
| Section of the bony labyrinth | Structure of the membranous labyrinth |
|---|---|
| Cochlea | Membranous cochlea (cochlear duct), occupying about 1/3 of the canal lumen |
| Vestibule | Two sacs: the anterior spherical (sacculus) and the posterior elliptical, or utricle (utriculus) |
| Semicircular canals | Membranous semicircular canals with their own ampullary expansions |
Receptor localization and mechanism of hearing
All sensitive (receptor) cells are located exclusively within the structures of the membranous labyrinth. The organ of hearing is called the spiral or organ of Corti, and is located inside the membranous cochlea.
Brief mechanism of sound perception:
- Sound vibrations travel from the tympanic membrane through the chain of auditory ossicles to the oval window.
- Vibration is transmitted first to the perilymph and then to the endolymph.
- The movement of fluids leads to the interaction of the sensory cells of the organ of Corti with the tectorial membrane hanging over them.
- This mechanical stimulation triggers the generation of a nerve impulse.
Embryogenesis of the auditory organ
Development of the inner ear begins from the head ectoderm at the level of the medulla oblongata. The process goes through several stages:
- Formation of otic placodes (ectodermal thickenings).
- Invagination of placodes and formation of otic pits.
- Pinning off of otic vesicles lined with stratified epithelium.
Next, the vesicle is divided in two by a constriction. The anterior section forms the saccule and membranous cochlea, while the posterior forms the utricle and semicircular canals. The stratified epithelium cells undergo cytodifferentiation, turning into the organ of Corti, maculae, and cristae.
In parallel, innervation is established. The spiral ganglion sends dendrites to the organ of Corti, and the vestibular ganglion to the receptors of the sacs and canals. The axons of these ganglia merge into a single trunk — the VIII cranial nerve (n. vestibulocochlearis), via which the signal travels to the brain.
Briefly: answers to topical questions
What specific cell types (sensory and supporting) make up the organ of Corti?
The spiral (Corti) organ includes the following source-verified cell elements:
- Inner hair cells — receptor cells, arranged in a single row.
- Outer hair cells — receptor cells, arranged in multiple rows.
- Phalangeal cells (Deiters cells) — supporting cells; inner cells form 1 row, outer cells form 3–5 rows. They form a recess for the base of the sensory cell and hold it with a thin finger-like process ("phalanx").
- Border cells — inner cells line the spiral sulcus of the limbus; outer cells, or Hensen cells, cover the lateral part of the basilar lamina and transition into the epithelium of the stria vascularis.
- Pillar cells — mentioned in the source as a landmark: phalangeal cells are located on the sides of the pillar cells.
What specific receptor zones are located in the spherical and elliptical sacs?
Specific receptor areas — maculae (maculae) — are located in the spherical and elliptical sacs.
- Macula of the elliptical sac (macula utriculi) — located at the base of the sac, parallel to the base of the skull.
- Macula of the spherical sac (macula sacculi) — located vertically on the medial wall of the sac.
These structures consist of sensory hair cells and supporting epitheliocytes. The cells are covered on top by a gelatinous otolithic membrane containing calcium carbonate crystals — otoliths (statoconia).
What receptor apparatuses are located in the ampullae of the membranous semicircular canals?
Receptor apparatuses — ampullary cristae (cristae ampullares) — are located in the ampullae of the membranous semicircular canals. These structures are covered with specialized receptor epithelium comprising:
- Sensory hair cells — form the upper row of cells, being pear-shaped and cylindrical.
- Supporting epitheliocytes — form the lower row of cells.
- Gelatinous cupula (cupula) — a transparent gel-like structure covering the receptor epithelium of the crista and shifting under the influence of endolymph.
Which structure of the membranous labyrinth is responsible for the secretion and production of endolymph?
Endolymph production in the inner ear is carried out by:
- Stria vascularis of the cochlea.
- Secretory areas of the epithelium of the saccule, utricle, and semicircular ducts.
Endolymph fills the space inside the membranous labyrinth. The secretory areas of the epithelium of the saccule, utricle, and semicircular ducts are also involved in the metabolism of supporting structures—the cupula and the otolithic membrane.
To easily remember the arrangement of the vestibular sacs from anterior to posterior, use the mnemonic «SUP»: S — Spherical saccule (anterior, near the cochlea), U — Utriculus (posterior), P — Semicircular canals (most posterior).
Class and exam questions
Where exactly is the spiral organ located?
The spiral (cochlear) organ is located strictly within the membranous labyrinth, specifically inside the membranous cochlea (cochlear duct).
What fills the space between the bony and membranous labyrinths?
This space is filled with perilymph. It is reliably isolated from the air environment of the tympanic cavity by the base of the stapes in the oval window and the secondary tympanic membrane in the round window.
From which germ layer does the inner ear develop?
The inner ear is of ectodermal origin. It develops from the head ectoderm at the level of the medulla oblongata through the stages of otic placodes, pits, and vesicles.
What else to study on this topic
- Histological structure of the spiral (cochlear) organ
- Cellular composition of the maculae of the sacs and ampullary crests
- Mechanism of transformation of mechanical vibrations of endolymph into a nerve impulse
- Structure and functions of the spiral and vestibular nerve ganglia
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