Inner Ear 2

Organ of Corti

IHC & OHC

Inner Ear Physiology When the oval window is moved in by the stapes, the perilymph of the scala vestibuli at the basal end of cochlea is displaced. This results in a wave-like movement that propagates to the apex of the cochlea - Traveling Wave.

Traveling Wave

Inner Ear Physiology Because the whole inner ear compartment is bony, the increased pressure caused by the sound wave needs to be displaced at the round window. Depending upon the frequency of the sound, different areas of the basilar membrane are stimulated.

Inner Ear Physiology Depending on the frequency, the vibration has a maximum effect (resonance) at different points along the basilar membrane This hierarchical arrangement of frequency information is called tonotopicity. Tonotopicity exists at all levels in the auditory pathway. Thus the basilar membrane is a spectrum analyzer, performing a kind of Fourier analysis on input complex waves, albeit with limited power of resolution.

Tonotopicity of the Basilar Membrane Animation of BM vibration for different frequencies

Tonotopicity of the Basilar Membrane

Tonotopicity of the Basilar Membrane Basilar membrane displacement patterns

Cochlear micromechanics Hair cells are small. But hair cells are themselves complex micromechanical systems whose function relies on an array of even smaller mechanical parts, such as the steriocilia

Cochlear Micromechanics Inner Hair Cell

Cochlear micromechanics The tectorial membrane is in direct contact to the stereocilia of the outer hair cells. When the traveling wave results in relative displacement of the tectorial membrane & the basilar membrane (shearing action), the stereocilia are deflected.

Shearing action of the Tectorial & Basilar Membranes When hair cell gets depolorized it means that the cells are going from negative to positive and activates the nerves.

Cochlear micromechanics Inner hair cells are the sensory receptors that signal the oscillations of the organ of Corti to the central nervous system. We know from our anatomy that it is the inner hair cells that receive the majority of the afferent innervation. However, the inner ear cell stereocilia have no direct contact with the tectorial membrane. Inner hair cells cilia are moved by a swishing motion from the outer hair cells.

Cochlear micromechanics Recent research has suggested that Inner hair cells sterocilia are moved by the force of the fluid streaming in the narrow channel between the tectorial membrane and the organ of corti.

Cochlear Transduction The hair cells are responsible for converting the mechanical movement of the sterocilia into electrical energy that can be conducted by the auditory nerve. This process is called Cochlear transduction.

Cochlear Transduction Movement of the cochlear partition produces deflection of the sterocilia. Hair cell depolarization is based upon a mechanical opening of ion channels located on top of stereocilia. Due to its high concentration in the endolymph, potassium (K+) enters the cell. This action results in depolarization of the hair cell - a positive change in the voltage across a cell's membrane. Depolorization- increase in nerve firing.

Cochlear Transduction The current produced due to the sterocilia movement is conducted by the cochlear nerve (CN VIII) to the brainstem and cortex.

Cochlear Transduction The stereocilia movements are direction sensitive. Movement towards one direction results in a positive current while in the other results in a negative current. sara\Intro to Audio - CSDI4100\anat&physio\probelft8.mov

Cochlear transduction - Depolarization and Hyperpolarization Hyperpolarization- the opposite of depolerization. Animation

Cochlear transduction - Depolarization and Hyperpolarization

Electrical Potentials of the Cochlea Electrocochleography (ECochG) is the name given to the recording of cochlear potentials. Under local anesthetic, a thin needle electrode is placed through the tympanic membrane onto the promontory, near the round window niche.

Electrical Potentials of the Cochlea Endocochlear Potential (EP) – Resting potential of the organ of Corti relative to the surrounding tissue Is a positive DC voltage of 80 - 100 mV seen in the endolymphatic space of the cochlea and is generated at the stria vascularis. Cochlear Microphonic - Alternating currents due to hair cell depolarization Closely resembles the sound stimulus, and mainly originates from the OHCs. 4 potentials you can measure: (EP) Ednocochlear Potential, Cochlear Microphonic, Summating potential, and Action Potential.

Electrical Potentials of the Cochlea Summating Potential – Change of EP in response to sound stimulation (DC current) Is mainly a reflection of IHC activity Action Potential – Is the result of synchronous activity of the auditory nerve fibers All or none response of auditory nerve fibers

Electrical Potentials of the Cochlea

ACTIVE PROCESS FOR NARROW TUNING Gold (1948) Postulated: Narrower Mechanical Tuning requires an “Additional Supply Of Energy” O2 Deprivation Degraded Sharp Tuning To Broad Tuning Evidence For Otoacoustic Emissions – Sound Production By Inner Ear (Kemp 1978) Passive BM travelling wave Active BM travelling wave von Bekesy (1961 nobel prize laureate) mechanical model von Bekesy and the place theory

Role of OHC Outer hair cells function is to sharpen the displacement pattern of the basilar membrane. OHCs also improve sensitivity to sound -- they make response thresholds lower. Motility is one means by which OHCs could accomplish these functions. Mammalian OHCs are able to change their length upon direct electrical stimulation Electrical Stimulation OHC In Vitro Generate Length Change Elongate/Contract Depending On Polarity Hyperpolarize → Free End Elongates Depolarize → Free End Shortens

OHC Electromotility

Role of OHC It is hypothesized that the OHC electro-motility is initiated by the fluctuations of the endocochlear potential as a result of the BM displacement (which in turn is a result of the incident sound) This is believed to be the force-generating mechanism for cochlear amplification This OHC electro-motility response provides positive mechanical feedback that increases movement of the cochlear partition especially for sounds close to the threshold (i.e., low level sounds).

Role of the IHC & OHC

OHCs – The source of Otoacoustic emissions Ohc source of Otoacustic emissions…this is an objective test (used a lot in infant hearing screenings) measuring sound coming out of the cochlea instead of the TM. OHC-reverse transduction transforms electical energy to machanical.

Why the inner-ear is snail-shaped? Too boost sensitivity to low frequencies! Although the spiral shape of the cochlea had little impact on the average vibrational energy traveling along the tube, as the wave progresses, this energy increasingly accumulates near the outside edge of the spiral, rather than remaining evenly spread across it. D. Manoussaki, E.K. Dimitriadis & R.S. Chadwick (2006) Low frequencies travel the furthest into the spiral, so the effect is strongest for them.

Why the inner-ear is snail-shaped? Serpentine Water Slide Straight Water Slide

Cochlear physiology – Synopsis The acoustic energy of the sound waves in the outer ear is converted to mechanical energy by the tympanic membrane. This mechanical energy is conducted by the middle ear ossicles to the oval window of the inner ear where it results in fluid movement (hydraulic energy).

Cochlear physiology This fluid movement results in deflection of the stereocilia of the inner hair cells. The deflection of the stereocilia is converted into electrical energy at the base of the hair cells & conducted by the auditory nerve fibers to higher auditory centers of the brainstem and cortex. The brain interprets it as sound! In short, the auditory Hair cells are specialized so that motion of their stereocilia changes their electrical potential, resulting in neurotransmitter release and action potentials in the nerve fibers that contact the hair cells.

Overall, how sound travels through the ear... Outer ear: Acoustic energy, in the form of sound waves, passes pinna, ear canal. Sound waves hit the ear drum, causing it to vibrate like a drum. Middle ear: It sets three ossicle bones (malleus, incus, stapes) into motion, changing acoustic energy to mechanical energy. These middle ear bones mechanically amplify sound and compensate mismatched impedance. Inner ear and Central auditory nervous system: When the stapes moves in and out of the oval window of the cochlea, it creates a fluid motion, hydrodynamic energy. It causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrochemical signal which is sent via the auditory nerve to the brain.

Cochlear physiology

Cochlear physiology Cochlear Physiology - Animation