Hearing relies on specialized sensory cells that detect mechanical sound vibrations and translate them into a language the brain understands. These hearing receptors are housed deep within the inner ear, a highly protected structure encased within the temporal bone of the skull. Here, the mechanical energy of vibration is converted into an electrical signal that travels along the auditory nerve to the brain.
The Inner Ear: General Location of Hearing
The ear is divided into three main regions: the outer, middle, and inner ear. Sound waves are collected by the outer ear and amplified through the middle ear via three small bones. The inner ear, also known as the labyrinth, holds the sensory apparatus for both hearing and balance.
The inner ear is positioned within the dense bone of the skull, providing a protective shell for its delicate contents. This location ensures that the fine structures responsible for translating sound are shielded from external damage. The bony labyrinth contains two main components: the semicircular canals, which manage balance, and the cochlea, which is exclusively dedicated to the sense of audition. The sensory receptors for audition reside entirely within the cochlea.
The Cochlea: The Receptor Housing Structure
The cochlea is a spiraled, hollow, conical chamber of bone that resembles a snail shell. This coiled tube in humans makes approximately 2.75 turns around a central bony axis. The cochlea is built to receive and process the vibrations sent from the middle ear’s stirrup bone.
Inside the cochlea, a thin partition divides the spiral canal into three fluid-filled compartments, or scalae. The upper and lower canals contain perilymph, a fluid similar in composition to cerebrospinal fluid. The central compartment, known as the cochlear duct or scala media, is filled with a unique fluid called endolymph.
Sound vibrations traveling through the middle ear are transferred to the perilymph fluid, creating pressure waves. These fluid waves move through the canals, causing the membranes that separate the compartments to ripple. This fluid-filled environment delivers mechanical energy to the sensory cells.
The Organ of Corti and Hair Cells
The sensory receptors for hearing are situated within a specialized strip of tissue called the Organ of Corti. This organ rests atop the basilar membrane within the central cochlear duct. The Organ of Corti contains thousands of supporting cells and the auditory sensory receptors.
These sensory cells are known as hair cells, named for the bundles of bristle-like projections, or stereocilia, that extend from their top surface. Hair cells are arranged in distinct rows along the basilar membrane. There is one row of inner hair cells, which are the primary transducers of sound, and three rows of outer hair cells, which amplify the sound signal.
The stereocilia are graded in height and are embedded in or brush against the overlying gelatinous tectorial membrane. This mechanical arrangement allows the cells to detect movement. Humans are born with about 12,000 hair cells, and their loss due to loud noise or age results in permanent hearing loss.
How the Receptors Convert Sound
The conversion of sound energy into a neural signal begins when fluid waves in the cochlea cause the basilar membrane to vibrate. This movement causes the Organ of Corti, including the hair cells, to move up and down. Since the stereocilia are in contact with the tectorial membrane, this motion shears the hair bundles, causing them to bend.
This mechanical bending of the stereocilia is the moment of transduction, where mechanical force is changed into an electrical event. The deflection of the hair bundles opens specialized, mechanically gated ion channels near the tips of the stereocilia. Positively charged ions, primarily potassium, flow into the hair cell.
The influx of ions causes an electrical change in the hair cell, called depolarization. This triggers the release of chemical neurotransmitters at the base of the cell, which activate the neighboring auditory nerve fibers. The resulting electrical impulse travels along the auditory nerve to the brain, which interprets the signal as sound.