Auditory Transduction The Inner Ear

Auditory TransductionThe Inner Ear 5.3.13

Outer Ear • Pinna collects the sound and directs it to ear canal • Because of the length of the ear canal, it is capable of amplifying sounds with frequencies in range 2000-3000 Hz. Ear canal acts as a resonator for this fundamental frequency

Basic parts of human ear a. Outer ear b. Middle ear • The Eardrum (tympanic membrane) • Auditory Ossicles • The Tympanic Cavity • The Eustachian Tube c. Inner ear

Middle Ear Ossicles Ear Drum Eustachian Tube

Ossicles

Auditory Ossicles • The function of the auditory ossicles is to transmit sound from the air striking the eardrum to a fluid-filled labyrinth inside the inner ear (Cochlea). • The bones are connected by small ligaments and transmit the vibratory motions of the eardrum to the inner ear.

Transmission of sound wave by ossicles to inner ear • Being connected to the hammer, the movements of the eardrum will set the hammer, anvil, and stirrup into motion at the same frequency of the sound wave. • The stirrup is connected to the inner ear; and thus the vibrations of the stirrup are transmitted to the fluid of the inner ear and create a compression wave within the fluid

Importance of Middle Ear • One may wonder why the incident sound wave collected by outer ear is not incident directly on the fluid of inner ear • The primary reason is that of a very poor matching of the impedance of the air and the cochlear fluid • Middle ear acts as an impedance matching device

Importance of Middle Ear • Acoustic impedance is a measure of the resistance of a medium to being disturbed by a change in the external pressure • When a sound wave is traveling in one medium and is incident upon an interface with a second medium, a certain fraction of sound energy will be reflected and a certain fraction will be transmitted

If the impedances of two materials are very different, sound will not easily pass from one to the other • If two stones are tapped together in air and the ear is in air, the sound made is clearly audible. Sound conducts well through air. • If two stones are tapped together underwater and the ear is underwater, the sound made is, again, clearly audible. Sound conducts well through water.

On the other hand, if two stones are tapped together in air and the ear is underwater (or the other way round), the sound made is almost imperceptible. • Sound does not conduct well from air to water or from water to air. This is because the impedances of water and air do not match, and most of the sound is reflected off the interface between the two media, remaining in the medium in which it was generated

The impedance of the fluid in the cochlea is about 30 times greater than that of air, and if the sound were applied directly to the cochlear fluid, most of it (~97%) would be reflected, leaving only 3% transmission. • It is necessary to somehow compensate for this difference, to match the characteristics of one material to that of the other • Ossicles chain works as impedance matching device

Sound amplification by middle ear Middle ear amplifies sound by a combination of three mechanisms • The area ratio advantage of the ear drum to the oval window • The lever action of ossicles

Sound amplification by middle ear • The largest contribution comes from area advantage • The force that is exerted over the large area of the tympanic membrane is transmitted to the smaller area of oval window • The area of the eardrum is about 22 times larger than the oval window. Therefore, the pressure on the oval window is increased by the same factor • This feature enhances our ability of hear the faintest of sounds

Sound amplification by middle ear • Ossicles amplify the sound reaching eardrum by lever action With a long enough lever, you can lift a big rock with a small applied force on the other end of the lever. The amplification of force can be changed by shifting the pivot point

Sound amplification by middle ear • The three tiny bones of the middle ear act as levers to amplify the vibrations (pressure) of the sound wave. • The pivot point or fulcrum is located farther from the tympanic membrane than from the stapes. • The force at the oval window is amplified. The mechanical advantage is 2 • The resulting vibrations would be much smaller without the levering action provided by the bones

Amplification of sound with frequency in range 2000-3000Hz • In the frequency range around 3000Hz, there is an increase in the pressure at the eardrum due to the resonance of the ear canal. This amplifies the sound pressure by a factor of 2 • Lever action amplifies by another factor of 2 • Smaller area of oval window amplifies the sound by a factor of 22 • amplification = 2 x 2 x 22 =88 • This accounts for the high sensitivity of ear to this frequency range

The Tympanic Cavity and the Eustachian Tube • The tympanic cavity is an air chamber surrounding the ossicles within the middle ear • The Eustachian tube is a membrane lined tube (approximately 35 mm long) that connects the middle ear space to the back of the nose (the Pharynx) • The Eustachian tube does not directly relate to the mechanical process of hearing

Functions of the Eustachian tube • Pressure equalization: Air seeps in through this tube to maintain the middle ear at atmospheric pressure A rapid change in the external air pressure such as may occur during an airplane flight causes a pressure imbalance on the two sides of the eardrum. The resulting force on the eardrum produces a painful sensation that lasts until the pressure in the middle ear is adjusted to the external pressure

Volume control by muscles of middle ear • The ossicles are connected to the walls of the middle ear by muscles that also act as a volume control • If the sound is excessively loud, these muscles as well as the muscles around eardrum stiffen and reduce the transmission of sound to the inner ear

Basic parts of Human Ear I. Ear anatomy II. Outer ear III. Middle ear • Inner ear Semicircular canals Cochlea (Latin for snail.)

Inner Ear Semicircular Canals (Balance) Cochlea (Transducer/ Microphone)

The Inner Ear • The inner ear can be thought of as two organs: the semicircular canals which serve as the body's balance organ and the cochlea which serves as the body's microphone, converting sound pressure impulses from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve

The Inner Ear • The cochlea is a snail-like structure divided into three fluid-filled compartments/ducts • The scala vestibuli and scala tympani are filled with fluid called perilymph while scala media is filled with endolymph

The Cochlea

Transmission of sound into organ of corti • The small bone called the stirrup, one of the ossicles, exerts force on the thin membrane called the oval window by piston action, transmitting sound pressure information into the perilymph of the scala vestibuli • Then through Reissner's membrane and the basilar membrane to the scala tympani. In the scala tympani, the vibrations pass again through perilymph to the round window at the base of the cochlea. The displacement in the cochlea caused by movement of the stapes is almost all across the basilar membrane. The energy dissipation at the round window is necessary to prevent pressure-wave reflections within the cochlea

Organ of Corti: The body’s Microphone • On the basilar membrane sits the sensory organ of the ear, the organ of Corti which acts as a transducer (converting sound energy into electrical energy) • It is composed of a complex of supporting cells and sensory or hair cells atop the thin basilar membrane • There are some 16,000 -20,000 of the hair cells distributed along the basilar membrane which follows the spiral of the cochlea. There are 3500 inner hair cells and 12,000 outer hair cells in each ear • Each hair cell has up to 80 tiny hairs projecting out of it into the endolymph

Organ of Corti

Function of hair cells • Research of the past decades has shown that outer hair cells do not send neural signals to the brain, but that they mechanically amplify low-level sound that enters the cochlea. • The inner hair cells transform the sound vibrations in the fluids of the cochlea into electrical signals that are then relayed via the auditory nerve to the auditory brainstem and to the auditory cortex

Generation of Receptor Potentials by Inner Hair Cells (Sensory receptors) • The upper ends of the hair cells are held rigid by the reticular lamina and the hairs are embedded in the tactorial membrane • Due to the movement of the stapes both the membranes move in the same direction and they are hinged on different axes so there is a shearing motion which bends the hairs in one direction

Hair cell shearing Tectoral membrane Hair cells Basilar membrane Sheared hairs

Generation of Receptor Potentials by Inner Hair Cells • Endolymph is rich in K+ ions while perilymph in Na+ ions • The deflection of the hair-cell stereocilia opens mechanically gated ion channels that allow K+ ions to enter and depolarize the cell. • The influx of K+ from endolymph in Scala media depolarizes the hair cells producing receptor potentials across the hair cell membrane.

Resonance Place Theory of Pitch Perception by Helmholtz • Pitch can be distinguished through differences in sound wave frequencies • Different areas of the basilar membrane resonate/ respond to different pitches due to different levels of flexibility along the membrane

Resonance Place Theory of Pitch Perception by Helmholtz • Higher frequencies stimulate the membrane closest to the oval window, lower frequencies stimulate areas further along (apex) • These regions then stimulate neurons to send signals to specific areas of the brain and thus leads to certain perception of pitch

Loudness of sound and frequency of action potentials • The louder the sound is, the greater height or amplitude of the vibrations in the sound waves, the more movement of hairs/stereocilia of hair cells and thus more action potentials • Greater the frequency of action potentials, louder the sound is • If you could hear someone talking, that means the voice is loud enough to generate action potentials in the sensory neurons of your ear.

Loudness of sound and frequency of action potentials • If they raise their voice, that causes an increase in the APs to your brain. If they lower their voice into a whisper, the frequency decreases. • If they lower their voice to the point where you can’t hear them, then that means you’re not even generating ONE action potential. So if you can’t hear a sound, it doesn’t mean there’s no sound in the room, it means the sound is too soft for you to hear.

Why do our own voices sound different to us when we hear them on a recording vs. when we hear them as we speak • This is because there are two different ways in which we hear sounds. One is through air conduction, and the other is bone conduction. • Everyday sounds we hear are primarily hear through air conduction, which is basically sound waves traveling through our ear canal and impacting our eardrum, and eventually to the cochlea of the inner ear. • When we speak, however, we hear our voice through both air conduction and bone conduction.

Bone conduction is the conduction of sound to the inner ear through the bones of the skull. The vibrating of our bones and body tissue transmits sounds directly to the cochlea. • The skull conducts lower frequencies better than air, people perceive their own voices to be lower and fuller (heavier) than others do. • When we hear our voice on a recording, that's how it sounds to everyone else, as we are then hearing it through air conduction only • You can note the difference in your voice by talking with the ears plugged

Auditory Transduction The Inner Ear