Overview
The cochlea is the auditory portion of the inner ear, shaped like a tapered spiral. It transforms mechanical energy from sound into electrical impulses that the brain interprets as hearing. The sensory epithelium responsible for this conversion is the Organ of Corti, which runs along the length of the coiled tube.
Anatomy and principal parts
Internally the cochlea is organized into three longitudinal fluid-filled compartments (scalae) separated by membranes and a central partition. Key components include:
- Scala vestibuli and scala tympani, filled with perilymph
- Scala media (cochlear duct), filled with endolymph
- Basilar membrane, which supports the Organ of Corti
- Tectorial membrane and rows of inner and outer hair cells
How it converts sound
Sound causes the stapes bone at the oval window to set the cochlear fluids into motion. Pressure waves travel along the scalae and cause the basilar membrane to vibrate at specific locations according to frequency, a phenomenon called tonotopy. Movement between the basilar and tectorial membranes bends hair cell stereocilia, opening ion channels and generating receptor potentials that modify neurotransmitter release onto auditory nerve fibers.
Origin and evolution
The term cochlea derives from the Greek kokhlias, meaning a spiral or snail shell. In most mammals the cochlea is tightly coiled; some primitive groups show a less coiled form. For example, monotremes differ in cochlear morphology from many other mammals, reflecting evolutionary variation in hearing mechanics (see monotremes).
Clinical importance and applications
Damage to hair cells or abnormal fluid balance can cause sensorineural hearing loss. Audiological tests infer cochlear function by measuring responses to sound. Cochlear implants bypass damaged sensory elements by delivering electrical stimulation directly to the auditory nerve, restoring hearing sensations for many with severe loss.
Notable distinctions
- The cochlea encodes pitch along its length, low frequencies near the apex and high frequencies near the base.
- Perilymph and endolymph differ in ionic composition, a critical factor for hair cell transduction.
- Structural variations across species reflect ecological and vocal adaptations.