Auditory TransductionThe Inner Ear

11.6.12

Ossicles as levers

Levers increase force

To oval window

Stapes footprint

Ear drum

Oval window

22 times amplification of sound pressure due to difference in surface area of ear drum and oval window

Importance of Middle EarImportance 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

• The impedance Z is the product of the mass density ρ and speed of sound

• It determines 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

Importance of Middle EarImportance of Middle Ear

• 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 oval window, 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

Basic parts of Human Ear

I. Ear anatomy

II. Outer ear

III. Middle ear

IV.Inner ear

Semicircular canals

Cochlea (Latin for snail.)

Inner Ear

Cochlea (Transducer/ Microphone)

Semicircular Canals (Balance)

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

Uncoiled 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.

• Each hair cell has up to 80 tiny hairs projecting out of it into the endolymph

Organ of Corti

Generation of Receptor Potentials by Hair Cells

• 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 membraneHair cells

Basilar membrane

Sheared hairs

• Endolymph is rich in K+ ions• The bending of hairs depolarizes the hair cells

producing receptor potentials across the hair cell membrane.

• K+ from endolymph enters into hair cells Neurotransmitters are released. Nerve endings are at the base of hair cells. These impulses travel to the auditory areas of the brain for processing

Generation of Receptor Potentials by Hair Cells

Action Potential by Sensory Neurons

• Sensory receptors like hair cells do not directly generate action potentials

• Instead sensory receptors generate receptor potentials which vary in intensity with stimulus

• These changes in membrane potential are passed to adjacent sensory neurons which may generate an action potential, if the incoming stimuli are sufficient for the neurons to reach threshold

• Receptor potential is a graded potential for sensory neurons

Pitch Perception and Resonance of Basilar Membrane (Bekesey theory of pitch perception)

• Pitch can be distinguished through differences in sound wave frequencies

• Different areas of the basilar membrane are sensitive to different pitches due to different levels of flexibility along the membrane

• 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

Sensitivity of Ear to Pressure Variations

• The sensation of hearing is produced by the response of the nerves in the ear to pressure variations in the sound wave

• The nerves in the ear are not the only ones that respond to pressure, as most of the skin contains nerves that are pressure sensitive.

• However, the ear is much more sensitive to pressure variations than any other part of the body