General Structural Plan
The inner ear is organized on a nested-box principle, comprising two interlinked levels:
- Bony labyrinth (labyrinthus osseus) — the outer casing formed by dense bone tissue.
- Membranous labyrinth (labyrinthus membranaceus) — a system of closed epithelial channels and cavities. It lies strictly inside the bony labyrinth, precisely mimicking its shape but possessing a significantly smaller diameter.
Parts of the Bony Labyrinth
Anatomically, the bony framework is divided into three parts arranged anteriorly to posteriorly:
- Cochlea — the anterior part. It is a spiral bony canal making exactly 2.5 turns around a central bony axis (the modiolus).
- Vestibule (vestibulum) — the middle part. This is a central ovoid cavity communicating anteriorly with the cochlea and posteriorly with the semicircular canals. Its lateral wall features two "windows" bordering the middle ear:
- Oval window (fenestra vestibuli) — closed by the base of the stapes.
- Round window (fenestra cochleae) — 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 called the bony ampulla.
Membranous Labyrinth and Fluid Compartments
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 base and the secondary membrane). Inside the membranous labyrinth itself circulates endolymph within a closed system.
Membranous labyrinth structures are distributed across the bony regions:
| Bony Labyrinth Region | Membranous Labyrinth Structure |
|---|---|
| Cochlea | Cochlear duct (scala media), occupying about 1/3 of the canal lumen |
| Vestibule | Two sacs: the anterior saccule (sacculus) and the posterior utricle (utriculus) |
| Semicircular canals | Membranous semicircular ducts with their own ampullary expansions |
Localization of Receptors and the Mechanism of Hearing
All sensory (receptor) cells are located exclusively within the structures of the membranous labyrinth. The organ of hearing is called the spiral organ (or organ of Corti) and is located inside the cochlear duct.
Brief mechanism of sound perception:
- Sound vibrations travel from the tympanic membrane through the auditory ossicles to the oval window.
- Vibration is transmitted first to the perilymph and then to the endolymph.
- Fluid movement causes the sensory cells of the organ of Corti to interact with the overlying tectorial membrane.
- This mechanical stimulation triggers the generation of a nerve impulse.
Embryology of the Auditory Organ
Development of the inner ear begins from the head ectoderm at the level of the hindbrain. The process goes through several stages:
- Formation of the otic placodes (ectodermal thickenings).
- Invagination of the placodes to form the otic pits.
- Detachment of the otic vesicles (otocysts) lined by stratified epithelium.
Next, the vesicle is divided by a constriction into two parts. The anterior division forms the saccule and the cochlear duct, while the posterior division forms the utricle and semicircular canals. The epithelial cells undergo cytodifferentiation, transforming into the organ of Corti, maculae, and cristae.
Concurrently, innervation is established. The spiral ganglion sends dendrites to the organ of Corti, while the vestibular ganglion sends fibers to the receptors of the maculae and cristae. The axons of these ganglia merge into a single trunk — the vestibulocochlear nerve (cranial nerve VIII, n. vestibulocochlearis) — through which signals travel to the brain.