General characteristics of the inner ear
The inner ear is a component of the auditory and vestibular system. It consists of two main components: the osseous labyrinth filled with perilymph, and the membranous labyrinth located inside it, filled with endolymph.
The inner ear comprises:
- organ of hearing — represented by the cochlea;
- organ of balance — represented by three mutually perpendicular semicircular canals, as well as the saccule and utricle.
Structure of the cochlea
The cochlear canal makes 2.5 turns around the bony axis (modiolus). At the apex of the cochlea, the scalae connect through an opening — the helicotrema.
The cochlea contains two scalae filled with perilymph:
- scala vestibuli — begins at the oval window;
- scala tympani — ends at the round window.
The round window is necessary to dampen sound vibrations: if there were a rigid structure at the end of the scala tympani, the transmission of vibrations would be impossible.
The cochlear duct is separated from the scalae by two membranes:
- vestibular membrane (Reissner's membrane) — formed by a single-layered squamous epithelium on the inside and endothelium on the outside; ensures the transmission of vibrations from the perilymph to the endolymph;
- basilar membrane — contains a large number of bundles ("strings") of collagen fibers extending from the osseous spiral lamina to the spiral ligament.
On the lateral side, the cochlear duct is bounded by the spiral ligament and the stria vascularis, which is rich in blood vessels and ensures the synthesis of endolymph.
Spiral organ (organ of Corti)
The spiral organ is located on the basilar membrane. Above, it is covered by the tectorial membrane — a gelatinous amorphous substance with collagen fibers, in which the stereocilia of sensory cells are embedded.
The spiral organ includes two groups of cells:
- sensory epithelial (hair) cells — convert sound vibrations into a nerve impulse;
- supporting cells — provide a supporting function.
The inner and outer pillar cells form the tunnel of the organ of Corti, which divides the structure into medial (inner) and lateral (outer) parts relative to the bony axis.
Hair cells
Inner hair cells are pear-shaped with an expanded base and a narrowed apical part. The apical pole bears 30–60 stereocilia. The total number is about 3.5–4 thousand, which is three times less than the outer ones. They lie in the depressions of inner phalangeal cells, and afferent nerve fibers approach them.
Outer hair cells bear stereocilia arranged in a V or W shape. The stereocilia contain actin and myosin, which ensure the return of stereocilia to their original position after deflection. The amount of enzymes in the cells increases during short-term sound exposures and is depleted during prolonged sound loads. The total number is about 12 thousand.
Outer hair cells rest on outer phalangeal cells, and afferent nerve fibers also approach them.
Supporting cells and reticular membrane
The supporting cells of the spiral organ include:
- inner and outer pillar cells — form the tunnel;
- inner and outer phalangeal cells (Deiters' cells) — support the hair cells;
- border cells of Hensen — located lateral to the outer phalangeal cells;
- supporting cells of Claudius — lie closer to the stria vascularis;
- cells of Böttcher — at the border with the basilar membrane, provide secretion and reabsorption of endolymph.
The apical parts of the supporting cells, together with the phalangeal processes, form the reticular membrane, thanks to which the stereocilia of all hair cells are located at the same level and embedded in the tectorial membrane.
Mechanism of sound perception
Sound perception occurs in several stages:
- The outer ear captures sound vibrations and directs them to the tympanic membrane.
- Through the system of auditory ossicles (malleus, incus, stapes), vibrations are transmitted to the oval window.
- Vibrations propagate through the perilymph of the scala vestibuli, are transmitted via Reissner's membrane to the endolymph of the cochlear duct, and also through the basilar membrane from the scala tympani.
- Vibrations of the basilar membrane and tectorial membrane cause deflection of the stereocilia of hair cells.
- In the sensory epithelial cells, the mechanical stimulus is converted into a nerve impulse, which reaches the cortical area via the conducting pathway of the analyzer.
An important feature is tonotopic organization: cells at the base of the cochlea perceive high-frequency vibrations, and cells at the apex of the cochlea perceive low-frequency ones.
Key points
- The inner ear includes the osseous labyrinth with perilymph and the membranous labyrinth with endolymph.
- The cochlear duct is separated by Reissner's membrane from the scala vestibuli and by the basilar membrane from the scala tympani.
- The stria vascularis synthesizes endolymph, and the cells of Böttcher participate in its secretion and reabsorption.
- The spiral organ contains inner (≈3.5–4 thousand) and outer (≈12 thousand) hair cells.
- The stereocilia of outer hair cells have a V- or W-shaped arrangement and contain actin and myosin.
- Pillar cells form the tunnel, phalangeal cells support the hair cells, and the reticular membrane aligns the stereocilia.
- High-frequency sounds are perceived at the base of the cochlea, and low-frequency sounds at the apex (helicotrema).
- The conversion of sound vibrations into a nerve impulse occurs upon the deflection of stereocilia embedded in the tectorial membrane.