Inner Ear Processing
Introduction
The
inner ear consists of a thin diaphragm (oval window bordering the middle ear) and a maze of tubes and passages (labyrinth) that contain the vestibular (organ of balance) and the
cochlea, a transducer that converts the mechanical sound energy to electrical impulses. Humans have a coiled (snail shaped) form of cochlea as do other mammals. The outer hair cells (OHC) of a
mammalian cochlea provide enhanced sensitivity and frequency resolution; in contrast to other animals.
The function of the inner ear is to transduce mechanical vibrations received from the middle ear into nerve impulses (electrical signals) that can be perceived by the human brain. It is within the course of this transduction process that humans perceive
pitch (the human interpretation of frequency) and
loudness (the human interpretation of sound intensity).
The
oval window (fenestra ovalis) is a connective tissue membrane between the middle ear and the inner ear. The oval window connects the tiny bones of the middle ear to the scala vestibule (upper part of the cochlea).
The middle ear transmits motion of the eardrum to the inner ear, thereby increasing the pressure on the connective tissue of the oval window. This pressure is transmitted through the stapes (bone in the middle ear) which presses against the oval window to the cochlea of the inner ear.
The cochlea within the inner ear, shaped like a snail shell, has two and a half turns and incorporates the
membranous labyrinth, a collection of fluid filled tubes and chambers containing receptors for the human senses of equilibrium (balance) and hearing. The
endolymph is the fluid within the membranous labyrinth.
The membranous labyrinth is found within the bony labyrinth in the inner ear and is of the same general composition. However, it is substantially smaller and partly separated from bony walls by the
perilymph, a fluid that is essentially incompressible. The membranous labyrinth is separated into three perilymph-filled sections, by a membranous sac of triangular cross-section which run the length of the cochlea. The two outer sections are the scala vestibule, connected to the oval window, and the scala tympani connected to the round window. The three sections connect at the apex by a small opening (helicotrema) which serves as a pressure equalizing mechanism at frequencies well below the audible range.
The volume of the cochlea is approximately 0.2 mL, and within this volume up to 30,000 hair cells are present which transduce vibration into nerve impulses. Nerve fibers transmit signals both to the brain and from the brain.
The cochlea is continuous with the
vestibular labyrinth, the organ of balance that serves as an accelerometer (both linear and angular) to enable the human brain to ascertain head position with respect to the surroundings, and have a relational sense of gravity and other forces.
Vibration of the foot plate of the stapes vibrates the perilymph in the bony cochlea. A counter opening is present in the labyrinth to enable fluid expansion when the stapes foot plate moves inwards. The counter opening is provided by the round window membrane beneath the oval window (in the inner wall of the middle ear). It is covered by a fibrous membrane which moves in synch (but opposite in phase) with the foot plate in the oval window.
The mammalian cochlea contains receptors that allow for transduction of mechanical waves into electrical signals. The
basilar membrane is the essential mechanical element within the length of cochlea that serves as a mechanical analyzer, as it curls toward the center. The vibration patterns of the basilar membrane separate incoming sound into component frequencies that activate different cochlear regions. The basilar membrane is non-linear exhibiting less accurate behavior with lower frequencies.
Mass and stiffness properties of the basilar membrane vary in accordance with its length; it is thicker, narrower and taut where the cochlea is largest; and thinner, broader, and less taut near the apex of the whorl (where the cochlea is smallest).