Inner Ear

BM vibration in living subjects

Nonlinear and active processes: Tuning curves in the cochlea are sharp and the sharpness increases as the stimulus level decreases.

Stria Vascularis

-Vascularized epithelial structure -Covers the spiral ligament within the scala media -Provides the basic metabolic control of the cochlea -Believed to be the source of the endolymph and of its electrical polarization within the cochlea

Divisions of Inner Ear

-bony labyrinth -membranous labyrinth

Membranous Labyrinth

Connections between structures: • three semicircular canals connect to the utricle • cochlear duct (scala media) connects to saccule via ductus reuniens • utricle and saccule connect via endolymphatic duct

Organ of Corti

Contains: Two types of sensory cells, inner and outer hair cells Supporting cells Nerve fibers

Cochlear Micromechanics

SUMMARY Sensitivity, sharpness of tuning, and compression (e.g., dynamic range) are all qualities that reside in micromechanics of BM-‐‐OHC-‐‐TM-‐‐complex. Understanding these contributions to hearing has important implications for re/habilitation. The success of eventual treatments will depend on how well they reproduce normal micromechanics.

Cochlear Macromechanics

SUMMARY: Broadly tuned responses to high level stimuli serve as a passive, nonlinear process in the cochlea. Active component, acting in parallel, is the cochlear "motor" (aka, amplifier). Provides ≈ 50-‐‐dB boost to the input signal from the stapes. Most observable at very low stimulus levels. Saturating nonlinearity: Gain decreases with increasing stimulus levels. Sharpens the frequency response of the cochlea significantly.

Outer Hair Cell Function

When the cochlear partition moves up/down... Basilar membrane moves up/down. Tectorial membrane reacts with a shearing motion because it has no firm point of attachment to the outer (bony) wall of the scala media. Effect on outer hair cells: Tallest row of stereocilia embedded in tectorial membrane. When tectorial membrane shears, bends the tallest stereocilia. Shorter stereocilia will also bend because of tip links (cross bridges).

The Bony Cochlea

sections of the cochlea: -scala vestibuli -scala tympani -scala media- part of the membranous labyrinth

The Bony Vestibule

-"hallway" of the inner ear -contains two sensory end organs: -utricle -saccule organs of the vestibular system (balance) -defined by walls: lateral wall- defined by the oval window (tympanic wall). -medial- features elliptical recess (accommodates the utricle) and spherical recess (accommodates the saccule). includes the vestibular/endolymphatic aqueduct. posterior- entrance and exit of SCC 3 entrances and 2 exits

Blood Supply to Cochlea

-Blood supply from cochlear artery -Cochlear artery is derived primarily from labyrinthine artery (enters through the internal auditory meatus (IAM); travels with CN VII and VIII) -Labyrinthine artery End artery - no collateral circulation -if blocked circulation cannot shoot off to other arteries - blocks blood supply to inner ear -Cochlear artery - small branches supply the capillary network of stria vascularis -Unique b/c there is no direct connection between the blood supply and the OC

Differences between OHC and IHC

-Cell Nucleus Position -Cilia IHC - thicker and coarser OHC - thinner -Support OHC - not supported on side IHC - supported on sides -Shape of the cells; shape of the cilia (W vs. U) -IHCs do don't touch tectorial membrane, OHCs do - OHC are greater in number and have more cilia -Fewer Mitochondria are present in the IHC More OHC -Different anatomy = Different Functions!

Supporting Cells of the Organ of Corti

-Cells of Held -Border cells of Inner Sulcus -Deiter's Cells -Hensen's Cells -Cells of Claudius

Reissner's Membrane

-Delicate cellular membrane -Attached on the inner wall to the spiral limbus and on the outer wall to the spiral ligament -Separates the scala media from the scala vestibuli -Forms superior wall of the cochlear duct

Organ of Corti

-Divided into inner and outer portions by the pillars, or rods, of Corti - Approximately in the middle of the OC - Forms a triangular tunnel in a cross section; runs the length of the cochlear partition - Space between the rods is called the tunnel of Corti • Contains a fluid called cortilymph - Similar in chemical composition to perilymph - Inner portion of the OC is on the modiolar side (inside of the rods) - Outer portion lies toward the bony cochlear wall (stria vascularis)

Basilar Membrane

-Fibrous plate extending from the osseous spiral lamina to the spiral ligament on the outer wall of the cochlea -Separates the scala media from the tympani -Supports the OC and the Tectorial Membrane

Landmarks within the OC

-Horizontal Plane (inner to outer walls) -Spiral Ligament - periosteum-like outer wall of the scala media. Attached to the outward boarder of the BM (basilar crest), covered by the stria vascularis -Periosteum= membrane that lines the surface of most bones of the body, including the bony labyrinth.

Landmarks within the OC

-Horizontal Plane (inner to outer walls) -Osseous spiral lamina - bony ledge that projects from and winds around the modiolus for 2 ¾ turns from the base of the cochlea to the apex. -BM is attached to its free border & runs parallel to it. -Spiral Limbus - extension of the osseous spiral lamina toward the scala vestibuli. Near it's inner edge it is attached to Reissner's membrane; and on its outer edge, which also forms the internal spiral sulcus, it is attached to the tectorial membrane.

Landmarks within the OC

-Horizontal Plane (inner to outer walls) -Cells of Held - Border cells (supporting cells) -Border Cells of Inner Sulcus - tall supporting cells lying on the inward side of the IHCs (provide direct support to the IHCs) -Space between these cells and the tectorial membrane is the inner spiral sulcus -Pillars (rods) of Corti - stiff, tall, bristle-like supporting cells (produce an A-Frame effect) - provides basic strength of Organ of Corti

Landmarks within the OC

-Horizontal Plane (inner to outer walls) -Tunnel of Corti is formed by the inner and outer pillars: the space between the two pillars is the tunnel of corti which contains cortilymph -Deiter's Cells - several rows of long supporting cells, extend from the BM and support the OHCs -Long processes from these cells (phalangeal processes) form a portion of the reticular lamina - Hensen's Cells - several rows of tall supporting cells on the outward side of the Deiter's cells -Processes from these cells also form a portion of the reticular lamina -Cells of Claudius - low, undifferentiated cells lying on the outward side of Hensen's cells (supporting)

Inner Hair Cells (IHC)

-Lie in a single row close to the inside of the cochlear spiral. - ~3,500 - Cilia form a flattened 'U' shape - Cell nucleus in the center • Flask shaped (larger at the bottom, bottle neck) • Surrounded by supporting cells (Border Cells of Inner Sulcus) with the exception of the surface containing the stereocilia

Cochlear Macromechanics

-Mechanical tuning of the basilar membrane Active and passive processes -Tapered tube divided by a bony shelf, with a slit in it and a hole at one end. -Stretched across is the BM. -The taper of the BM is the exact opposite to the taper of the cochlea

Sensory Cells of the Organ of Corti

-Sensory Cells (2 kinds in the cochlea) -Inner Hair Cells (IHC) -Single row -Outer Hair Cells (OHC) -Three Rows The IHCs and OHCs slant toward each other and are held in place in 3 ways -Supporting cells -Reticular lamina -Phalangeal process of the Deiter cells

Tectorial Membrane

-Soft semigelatinous ribbonlike structure -Non-cellular (93% protein) -Attached along its inner edge to the spiral limbus and along the outer edge to the outer boarder of the OC -In intimate contact with the cilia of the OHCs

Stereocilia

-Stereocilia - (stiff cilia) • Cilium - microscopic hair-like projections on the surface of some cells - Atop each hair cell, these occur in several rows of increasing length (staircase fashion) • Side links • Tip links -Cuticular plate: a network of actin filaments (protein bundles) and root structure found in hair cells of the cochlea, believed to hold the stereocilia in place

Reticular Lamina

-Stiff membrane formed by processes of the supporting cells -Incorporated in this are the cuticular plates of the hair cells -Holds the upper ends of the hair cells and assists in maintaining their alignment -Prevents endolymph from entering the area of the hair cells and is formed by a flattened top surface of supporting cells (Deiters' cells) and the cuticular plate at the top of the hair cells. -Comes in contact with the tectorial membrane -Inner spiral sulcus: includes the space between tectorial membrane and reticular lamina

Perilymph

-clear fluid contained within the bony labyrinth -similar in composition to cerebrospinal fluid, heavier than water. -volume in inner ear is roughly equivalent to two drops from an eye dropper -secreted from epithelial cells lining the bony labyrinth -chemical composition- high in sodium (Na+) and low in potassium (K+). purpose- protects sensory organs & transmit pressure changes in fluid

Inner Ear

-cochlea (organ of hearing) -vestibule -semicircular canals (organs of balance)

The Bony Semicircular Canals

-comprise the balance system -semicircular canals lie at right angles to each other. -anterior- detects horizontal movement -posterior- detects vertical movement -lateral- detects rotational movement

Fluid Filled in Inner Ear

-perilymph -endolymph

Organ of Corti

-sensory end organ of hearing -lies on top of the Basilar Membrane (BM) -comprised of: -supporting structures -sensory cells -nerve fibers

George von Békésy

3 characteristics of the BM: Shape: Base - narrow and thick Apex - thin and wide Stiffness: Base - stiff Apex - flexible Mass: Apex is more massive Acts as a low pass filter; better transmission of low-frequency vibrations Restricts transmission of high frequencies First to report on the existence of traveling waves. Two important observations: 1.) The traveling wave always moved from base to apex. BM motion is controlled by the graduated stiffness and mass of the OC; independent of stapes position. 2.) Traveling wave decreased in velocity as it traveled from base to apex.

Application

Classes of OAEs Spontaneous Present in approx. ½ of young people with normal hearing Evoked Occur in response to stimulus Transient evoked OAEs Broadband response to click stimulus Distortion Product OAEs (2f1-f2) Frequency specific, can plot a "DP-gram"

Application

Clinical utility of OAEs Assess integrity of cochlea, specifically OHCs Screening Quick, non-invasive Can assess sleeping baby Limitations Affected by noise Can't evaluate below 1kHz Does not distinguish between degrees of HL

Helicotrema

For a very low frequency sound (e.g., ≈ <20 Hz), pressure wave length will be very long, will take time for pressure to build. Slow build-up of pressure travels up to the helicotrema through scala vestibuli and travels back through scala tympani causing round window to bulge out.

The Traveling Wave

Frequency characteristics The point of maximum displacement (maximum amplitude of vibration) varies according to frequency. High frequencies à max. displacement in base. Mid frequencies à max. displacement in medial turn. Low frequencies à max. displacement in apex. (Very low frequencies à no displacement). This allows the cochlea to function as a mechanical frequency analyzer. In other words, the cochlea is tonotopically organized.

Landmarks within the OC

Horizontal Plane (inner to outer walls) -Osseous spiral lamina -Spiral Limbus -Cells of Held -Border Cells of Inner Sulcus -Pillars/Rods of Corti -Tunnel of Corti -Deiter's Cells -Hensen's Cells - Cells of Claudius -Spiral Ligament

Inner Hair Cell Function

IHCs are the transducers within the inner ear IHC transduction process Mechanical input to IHC: bending of cilia Fluid movement over cilia causes bending Neural connections at the base of the cells -> mechanical signal must be converted into neural signal

Outer Hair Cell Function

Implications of motile action of OHCs A feedback mechanism. Alters fluid flow on top of IHC. Makes IHCs more sensitive to low-level sounds Sharpens frequency selectivity Adds gain: i.e., cochlear amplifier OHCs are source of feedback mechanism OHCs are mechanically active sensory cells Byproduct: otoacoustic emissions

IHC Transduction

Influx of K+ creates a receptor potential (an electrical charge) at the top of the hair cell which is carried down to the base of the IHC. Change in voltage in presynaptic area at the base of the cell. Causes the release of a neurotransmitter (glutamate) to the nerve endings at the base of the cell. Neurotransmitter absorbed by neuron and tells neuron to fire IHCs predominately involved in afferent transmission. IHCs are mechanically passive transducers.

Application

Several cochlear potentials and electrical events. Endocochlear potential: resting electric polarization of the endolymph of the scala media, positive relative to the perilymph in the SV and ST. Intracellular potentials: resting potentials from inside the hair cell. Cochlear microphonic: an AC potential that is a faithful reproduction of the stimulus waveform, believed to stem from the cuticular plates of the hair cells in the OC. Summating potential: positive or negative changes in the DC polarization of the cochlea in response to a sound stimulus. Otoacoustic emissions: low level acoustic energy that can be recorded in the external auditory canal, believed to reflect indirect functional integrity of the cochlear outer hair cells. Movement of the OHCs causes a "reverse" traveling wave; acoustic energy travels back through the middle ear into the external auditory canal.

Membranous Labyrinth

Six structures of membranous labyrinth: 1. three membranous semicircular canals 2. utricle 3. saccule 4. cochlear duct (aka: membranous cochlea, scala media)

Traveling Wave

The traveling wave moves from base to apex. The speed of the wave will slow as it travels. Maxima of the wave is tuned to the frequency of the stimulus. Frequency and intensity are captured. Amplitude of the wave diminishes rapidly to the right of maximum displacement.

Cochlear Micromechanics

Two cascading, parallel events in transduction: Feed‐forward loop (basilar membrane to hair cells) Feedback loop (OHCs to basilar membrane)

Membranous Labyrinth

Vestibular portion: -membranous semicircular canals, utricle, saccule -sensory organs located within each: cristae: located in semicircular canals (n=3) maculae: located in utricle and saccule (n=2) -each of the sensory end organs is bathed in endolymph

The Traveling Wave

What about VERY low frequencies? They do not cause basilar membrane displacement. Therefore, do not produce sensation of sound. Why not? Because of the helicotrema.

Herman von Helmholtz

What directs movement of the BM? Should it move up and down as a single unit in response to pressure changes from the stapes? How would frequency information be retained/coded? Movement would be restricted to areas of "resonance" if the structure of the BM changes as it moves from base to apex. Argued that a structure that varies in thickness and stiffness would resonate at different places depending on the frequency of the incoming stimulus; like the tuned strings of a piano or a harp.

Mechanical Responses of OHCs

What do OHCs contribute to hearing? Isolating intact cells Placing them in a chamber of artificial perilymph Delivering depolarizing current pulses Watching Somatic mobility Active force generation by hair cell bundles

Cochlea

coiled around a spongy core, the modiolus 2 3/4 turns basal, medial, apical regions

The Bony Labyrinth

complicated series of chambers and tunnels set in the temporal bone: -bony cochlea -bony vestibule -bony semicircular canals fluid filled cavity: perilymph

Why is the Vestibule Important?

provides communication/connection between the oval window and the cochlea

Anatomy around the OC

• Basilar Membrane (BM) • Tectorial Membrane • Reticular Lamina • Stria vascularis • Reissner's membrane

Membranous Cochlea (cochlear duct, scala media)

• Basilar membrane (floor) • Reissner's membrane (ceiling) • outer wall: lined by the stria vascularis • sensory end organ for hearing: Organ of Corti (located inside cochlear duct)

Outer Hair Cells

• Three rows that lie more toward the outer edge of the cochlea - ~ 12,000-15,000; Dominant cell group in OC - Cilia form a 'W' shape - At the base of the stereocilia there is an area with no reticular lamina and no cuticular plate known as the cutical free pore • In OHCs, this area is packed with mitochondria = high metabolic activity • Elongated, cylindric cell body (test tube shape) •Cell Nucleus - bottom 1/3 of cell •In contact with the supporting cells only at their very top and bottom (Deiter's cells at the bottom, reticular lamina at the top) •Bulk of the cell body is suspended in the fluid spaces inside the OC - cortilymph

Endolymph

• fluid that fills the membranous labyrinth • produced by endolymphatic sac • pumped into membranous labyrinth by endolymphatic aqueduct • chemical composition: high in K+, low in Na+

Membranous Cochlea

• slightly shorter than bony cochlea • creates a space in apical turn of bony cochlea • provides direct communication between scala tympani and scala vestibuli • the pathway is called the helicotrema - Narrow aperture within the apex of the cochlea that allows communication between scala vestibuli and the scala tympani