The Organ of Corti is a specialized sensory structure responsible for the sense of hearing. Located deep within the inner ear, this intricate strip of epithelial cells acts as the body’s primary sound receptor. Its purpose is to perform auditory transduction, converting the mechanical energy of sound waves into electrical nerve signals. These neural impulses are then transmitted to the brain, which interprets them as recognizable sounds.
Anatomical Placement in the Inner Ear
The Organ of Corti is situated inside the cochlea, a snail-shaped, fluid-filled component of the inner ear. This organ rests upon the flexible Basilar Membrane, which separates two of the cochlea’s main fluid-filled compartments. Specifically, the Organ of Corti resides within the central cochlear duct, also known as the scala media.
The scala media is filled with endolymph, which has a high concentration of potassium ions (K+). Compartments above and below the scala media—the scala vestibuli and scala tympani—contain perilymph. This separation of fluids, with their distinct ion compositions, establishes the electrical potential necessary to power the transduction process. The entire structure is housed within the temporal bone of the skull.
Key Structural Components
The complex architecture of the Organ of Corti relies on a precise arrangement of cells and membranes to function correctly. The organ’s foundation is the Basilar Membrane, a fibrous sheet that vibrates in response to fluid movement within the cochlea. Overlying the hair cells is the Tectorial Membrane, a gelatinous shelf anchored at one end that interacts with the sensory receptors below.
The sensory receptors are the hair cells, divided into two populations. A single row of Inner Hair Cells (IHCs) is primarily responsible for sending auditory information to the brain. These flask-shaped cells are the main transducers, relaying approximately 90% of the neural signals to the auditory nerve.
Separated from the IHCs are three to five rows of Outer Hair Cells (OHCs). While they possess stereocilia, OHCs function more as biological motors or amplifiers rather than primary sensors. Supporting cells, such as pillar cells and Deiters cells, provide the mechanical scaffold to maintain the organ’s precise structure.
Bundles of tiny, stiff, hair-like projections called stereocilia project from the top of both hair cell types. These stereocilia are arranged in rows of ascending height, with adjacent tips connected by fine filaments known as tip links. The movement of these stereocilia, often in contact with the Tectorial Membrane, initiates the entire conversion of sound into an electrical impulse.
The Mechanism of Hearing Transduction
Hearing begins when sound waves, amplified by the middle ear bones, cause the oval window to push against the fluid inside the cochlea. This pressure creates a traveling wave that propagates along the Basilar Membrane, causing it to ripple. Since the Basilar Membrane is flexible and the Tectorial Membrane is rigid, this ripple creates a mechanical shearing force between the two structures.
This shearing action physically bends the stereocilia bundles. When the stereocilia deflect toward the tallest row, the connecting tip links pull open mechanically gated ion channels. The sudden opening of these channels allows positively charged potassium ions (K+) to flow rapidly into the hair cell from the surrounding endolymph.
The influx of positive potassium ions changes the electrical charge inside the hair cell, a process known as depolarization. This change in voltage triggers the opening of voltage-gated calcium channels near the base of the hair cell. The resulting calcium influx causes the hair cell to release the neurotransmitter glutamate into the synaptic cleft.
Glutamate binds to receptors on the afferent nerve fibers, generating an action potential that travels to the brain. The Outer Hair Cells actively refine this process through electromotility. These cells rapidly change length in response to membrane voltage, which physically increases the movement of the Basilar Membrane, amplifying weak sound signals and enhancing IHC sensitivity.