SLHS 2203

Where is antitragus located?

Exam 3 Study Guide

The Ear 

The Inner Ear

A. Pinna (aka Auricle)

a. The visible portion that is commonly referred to as "the ear" b. Composed entirely of cartilage and skin except for the ​lobule​, which contains no cartilage

c. Pinna Parts

i. Helix​ is the outer frame of the auricle, it is the rolled up edge. ii. Tragus​ - Small projection just in front of the ear canal

What are some examples of outer ear malformation?

iii. Concha​ - The hollow bowl like portion of the outer ear next to the canal

iv. Lobe (lobule)​ - fleshy lower portion of the ear

v. Scapha​ - scooped out section medial to the helix. This is the outer depression near the ear edge

vi. Anti Helix (antihelix)​ - folded "Y" shaped part of the ear.

Elevated ridge of cartilage between concha & scapha. The upper parts of this "Y" are the superior crux and the inferior crux.

vii. Triangular​ ​Fossa​: Triangular depression between the superior and inferior crusa.

viii. Antitragus​ - Lower cartilaginous edge of the conchal bowl just above the fleshy lobule of the ear.

How does a silicone ear function?

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ix. External Auditory Meatus​: Entrance to outer ear canal

B. Development of External Ear

a. Embryonic External Ear

i. Images of the lateral view of the human embryonic head from week 5 (stage 14) through to week 8 (stage 23) showing

development of the auricular hillocks that will form the external ear. The adult ear is also shown indicating the part of the ear We also discuss several other topics like What are the stages of the alternation of generations life cycle of a flowering plant?

that each hillock contributes.

b. Notice how developmentally, the ear is located beneath the mandible C. Variety of Outer Ear Malformations

a. (Otia/Otic = ear)

b. Microtia​: Born with deformed pinna

c. Anotia​: Born with completely underdeveloped pinna

d. Atresia​: Born without ear canal

e. Synotia​: A congenital malformation characterized by the union of the ears in front of the neck, often accompanied by the absence or defective development of the lower jaw

D. Microtia

a. Born with deformed or absent ears

i. Genetic and environmental factors

ii. Usually unilateral

iii. Sometimes detectable via 3D ultrasound

b. Gradients of Microtia

c. Microtia Grading System

i. Grade 1

1. Small but almost normal We also discuss several other topics like What does it mean to human according to shinto beliefs?

ii. Grade 2

1. Small but recognizable anatomy

iii. Grade 3

1. Small rudiment of soft tissue and no ear canal

iv. Grade 4

1. No external ear and no ear canal

d. Cosmetic Approaches to Microtia

i. Surgical Reconstruction, multiple different techniques, e.g., use cartilage from another part of the body (ribs) to build an ear. 1. Rib cage framework

ii. Implantation of a silicone ear

1. Rib cage adhered with bone anchored post + magnet or

medical adhesive

e. 3D printing ears

E. Atresia

a. Born without ear canal

b. Transverse CT scan

i. Notice lack of ear canal (atresia) on the left side but normal ear canal on the right.

F. Function of the Outer Ear?

a. Just cosmetic?

i. Piercings

b. Collect Sound

i. Sound collector, intercepting sound energy and deflecting it to the auditory canal

c. Localization

d. Resonator (Amplify Sound)

e. Protection

f. Sensitive (Earlobe)

g. Having two, bilaterally located ears helps to detecting the location of a sound in the environment.

h. But you also need a brain to interpret the information

G. The Ear Affects the Sound We Hear

a. Each individual's pinna creates a distinctive imprint on the acoustic wave traveling into the auditory canal, that helps in sound localization. H. Outer Ear Resonance

a. Influence of Pinna (p)

b. Influence of Ear Canal (m)

c. Combined Influence (t)

d. At 3000 Hz, the final amplification (t) is 20dB

e. The specific amount of amplification is unique for each person f. This is also called

g. “Head-Related Transfer Function” (HRTF).

h. The net effect of the head, pinna, and ear canal is that sounds in the 2,000 to 4,000 Hz region are amplified by 10 to 15 dB.

i. Sensitivity to sounds greatest in this frequency region

ii. Noises in this range are the most hazardous to hearing I. Pinna is Important for Detecting Sound Elevation

a. The human pinna filters the acoustic spectrum in a

direction-dependent way.

i. Normal pinna transfer functions of the right ear of one subject. b. The linear acoustic transfer functions are shown as a function of frequency (ordinate) and sound direction (abscissa) in the midsagittal plane (azimuth zero degrees; elevation from -40 to +50 degrees). c. Color encodes the amplitude (in dB) of the transfer function. A value of zero dB indicates that the presence of the head and pinna does not change the sound pressure amplitude of a tone at that particular frequency and elevation. Light colors correspond to sound

amplification; dark areas refer to sound attenuation.

d. Altering the shape of the pinna can cause temporary disruptions to localization ability but then eventually the brain learns to deal with the

altered input. However, the brain doesn’t forget the original

confirmation.

J. External Auditory Meatus: External Ear Canal

a. External ear canal

b. Extends from the pinna to the tympanic membrane

i. About 26 millimeters (mm) in length and 7 mm in diameter in adult ear.

ii. Size and shape vary among individuals.

c. Protects the eardrum

d. The superior wall is about 5mm shorter than its anterior inferior wall, thus accounting for the oblique positions of the tympanic membrane e. Outer half is cartilaginous and the inner half is bony

f. The skin lining the cartilaginous canal is thick and contains fine hairs (cilia), sebaceous glands that produce oils, and special glands that produce cerumen (wax).

g. Cerum

i. Earwax, also known by the medical term cerumen, is a yellowish waxy substance secreted in the ear canal of humans and other mammals.

1. Wax will darken with age

ii. It protects the skin of the human ear canal, assists in cleaning and lubrication, and also provides some protection from

bacteria, fungi, insects and water.

1. If this local defense system fails, infections of the external auditory canal may arise

iii. Excess or impacted cerumen can press against the eardrum and/or occlude (block) the external auditory canal or hearing aids, potentially hindering hearing.

K. Head-Related Transfer Function

a. Our bodies alter the sounds that we hear.

b. Sound is shaped/filtered by the body, leading to some sounds being amplified (gain added) and other being attenuated.

c. A ​head related transfer function​ is a mathematical representation of the ways in which the positions of the head, trunk, and ears filter sounds.

L. The External Auditory Meatus Terminates at the Tympanic Membrane (TM)

a. The TM is the boundary between the ear canal and the Middle Ear (ME) M. Normal Tympanic Membrane (TM)

a. The tympanic membrane is an oval, thin, semi-transparent membrane that separates the external and middle ear.

b. It functions to transmit sound from the air into the small bones in the middle ear

c. If the TM is ruptured or perforated this can lead to a type of hearing loss called: conductive hearing loss.

d. The bones of the middle ear, ossicles, are visible in many cases e. A healthy TM is pulled inward by the ME bones, toward the middle ear, creating a concave shape.

f. The part of the TM that is most depressed is the ​umbo​.

N. 2 Main Parts of TM

a. The Tympanic Membrane is divided into 2 parts:

1. The ​Pars Flaccida

a. (Upper region, delicate)

2. The ​Pars Tensa

a. (Larger lower region, more robust)

b. When illuminated during an otoscopic exam, a concentration of light is observed in the lower quadrant.

O. The Pars Tensa

a. Three Layers

1. Skin (outside)

2. Fibrous tissue, encloses the handle of the malleus (the most superficial of the ME bones)

3. Mucosa (inside)

P. Normal Tympanic Membrane

a. The color will vary naturally as a function of skin pigment. b. Notice the different shades of color yet the eardrum still remains an opaque translucent appearance in all the pictures.

c. Notice also the cone of light

Q. Examples of Atypical Ear Canals and/or TMS

a. Acute Infection with bulging of the tympanic membrane due to pressure from purulence (pus) behind it in the middle ear cavity. R. Ear Tubes

a. Aka: tympanostomy tubes, ventilating tubes, pressure equalizing tubes (PE tubes). They are tiny hollow tubes made of a soft material. b. A surgically implanted vent to relieve middle ear fluid build-up associated with chronic otitis media (middle ear infections).

c. The small tubes that are used do not cause hearing loss or long-term damage to the eardrum. The fall out naturally over time.

S. External Otitis Media

a. Aka Swimmers Ear: an infection of the ear canal itself. Notice the swelling of the ear canal.

b. Can often occur if water is trapped in the ear, leading to spread of bacteria or fungal organisms.

T. Ear Drum Perforations/Rupture​: ​Causes of TM Perforations a. Middle Ear Infection (Otitis Media)​: Pressure from the infection can lead to rupture.

b. Barotrauma​: Extreme imbalance between pressure in the ME and environmental pressure. Generally the result of air travel but also can arise from scuba diving or a blow to the ear (e.g., from airbag). c. Loud Sounds or Blasts (Acoustic Trauma)

d. Foreign Objects in your Ear.​ Small objects, such as a cotton swab or hairpin, can puncture or tear the eardrum.

e. Severe Head Trauma​. Skull fracture can dislocate the TM.

U. Ear Wax

a. Ear wax can either partially or fully block the ear canal.

b. Totally occluded with wax.

i. This can be very harmful in a young child that is just learning to speak due negative affects hearing loss can have on speech

development.

V. Normal Ear Drum

a. Tympanic Membrane​: A window to the middle ear

W. Otoscopic Exam

a. Visual exam of the ear canal using an otoscope (a magnifying glass + light).

The Middle Ear

A. The Middle Ear

a. The middle ear begins with the ​tympanic membrane ​at the end of the ear canal.

b. An ​air-filled​ space (unless infected).

i. The Eustachian tube and mastoid cells help to keep the space fluid-free.

c. Eustachian tubes drains fluid from the ME to the pharynx.

d. Contains three tiny bones, called the ​ossicles​.

i. Functionally coupled the eardrum to the​ fluid-filled inner ear​. B. The Three Ossicles

a. The ossicles are the smallest bones in the body and are at their adult size at birth--they don’t grow.

b. Instead of being attached to other bones the ossicles are suspended within the cavity by a series of ligaments and tendons.

c. Malleus (hammer)

d. Incus (anvil)

e. Stapes (stirrup)

C. The Malleus, “The Hammer”, 8-9 mm long

a. Connected to TM and the Incus

b. 4 Parts

i. Manubrium

ii. Lateral Process

iii. Neck

iv. Head

c. Manubrium​ (handle)-- Firmly embedded between the fibrous and mucous membrane layers of the TM. This is the most lateral

attachment of the ossicular chain.

d. The neck is a narrowing between its manubrium and head. e. The lateral process is what produces the bulge on the

Lateral View​ eardrum when we look otoscopically.

f. The head of the malleus connects with the body of the next middle ear ossicle--the incus.

D. The Incus

a. Aka Anvil

b. Short process

c. Long process

d. Lenticular process

e. Incudostapedial joint

E. The Stapes, Smallest Bone in the Body

a. Head

b. Neck

c. Anterior crus

d. Posterior crus

e. Footplate, makes contact with the oval window of the inner ear F. Middle Ear

a. The middle ear is bound by a roof, floor, and 4 walls, essentially, it can be conceptualized as a cube, although (it isn’t actually cube-like in shape)

G. The Ossicular Chain Suspended by ​Ligaments​ & Tendons a. Superior malleal ligament

b. Superior incudal ligament

c. Anterior malleal ligament

d. Posterior incudal ligament

H. Ligaments

a. The ossicles are suspended in the middle ear cavity by four ligaments. b. The Superior Malleal Ligament​: Runs from the superior face of the cavity (tegmen tympani) to the head of the malleus

c. The Anterior Malleal Ligament​: Runs from the anterior face of the cavity to the anterior process of the malleus

d. The Superior Incudal Ligament​: Runs from incus to the tegmen tympani

e. The Posterior Incudal Ligament​: Runs from the mastoid (posterior wall) to the short process of the incus

I. The Ossicular Chain Suspended by Ligaments & ​Tendons a. Tensor Tympani Tendon

b. Stapedial Tendon

J. Otosclerosis

a. Derived from the Greek words for "hard" (scler-o) and "ear" (oto). b. Abnormal bone growth that causes the ossicles (ME bones) to become an immovable mass. This leads to a conductive hearing loss.

c. Treatments

i. Hearing Aids

1. Only for conductive hearing loss (early stage otosclerosis) ii. Fluoride

1. Strengthens inner ear bone

iii. Stapedectomy

1. Remove the stapes bone and replacing it with a new

prosthetic bone

K. The Anterior Face (Missing Wall)

a. Also called the carotid wall because the internal carotid artery runs behind this wall. The carotid supplies oxygen to the brain.

b. Houses Eustachian tube

L. The Eustachian Tube

a. For the middle ear to work effectively (to transfer sound waves into the inner ear), the air pressure within the middle ear cavity must equal the air pressure in the outer ear.

b. The Eustachian tube helps to accomplish this pressure equalization. c. Connects with nasopharynx.

i. From from brain, leaks into ME, which drains in the

nasopharynx where it can be effused into the nose.

d. The Eustachian tube helps maintain equal air pressure on both sides of the eardrum by allowing outside air to enter the middle ear. If the Eustachian tube is blocked, air cannot reach the middle ear, so the pressure there decreases. When air pressure is lower in the middle ear than in the ear canal, the eardrum bulges inward. The pressure difference can cause pain and can bruise or rupture the eardrum. e. Child’s Ear

i. Eustachian tube is oriented more horizontally in children, leading to less efficient draining.

ii. Making children are more susceptible to middle ear infections. M. Dehiscence of the Tegmen Tympani

a. Tegmen tympani, very thin bony plate that separate the ME and the cranium.

b. Dehiscence = split, tear

c. Dehiscence can lead to a herniation of the meninges or brain (basically, the brain starts to fall into the middle ear).

d. Possible symptoms,

i. Cerebral spinal fluid will leak into the middle ear,

ii. Otorrhoea: Ear discharge

iii. Rhinorrhoea: Nose discharge

iv. Conductive Loss

e. How is it possible for Rhinorrhoea of brain fluid to occur?

N. The Ossicular Chain Suspended by Ligaments & Tendons a. Superior Malleal Ligament

b. Superior Incudal Ligament

c. Anterior Malleal Ligament

d. Posterior Incudal Ligament

e. Tensor Tympani Tendon

f. Stapedial Tendon

O. Ligaments

a. The ossicles are suspended in the middle ear cavity by four ligaments. b. The Superior Malleal Ligament​: Runs from the superior face of the cavity (tegmen tympani) to the head of the malleus

c. The Anterior Malleal Ligament​: Runs from the anterior face of the cavity to the anterior process of the malleus

d. The Superior Incudal Ligament​: Runs from incus to the tegmen tympani

e. The Posterior Incudal Ligament​: Runs from the mastoid (posterior wall) to the

f. short process of the incus

P. Middle Ear Muscles

a. Contract Reflexively When Encountering Loud Sounds.

b. Dampen the movement of the ossicles to prevent noise-induced damage to the auditory system.

c. “The Auditory Reflex”

Q. Schematic View of the Middle Ear

a. Many authors view the middle ear cavity as a box--to get a perspective regarding where the middle ear structures are found.

b. Fig 1-2 shows the names of the six faces of the box.

i. Superior face (ceiling)

ii. Inferior face (floor)

iii. Posterior face (left side wall)

iv. Anterior face (right side wall)

v. Lateral face (front wall)

vi. Medial face (back wall)

R. Function of the Middle Ear

a. Sound from the outsides is being propagated through air. To be converted to an electrical signal (in the inner ear) that energy needs to be passed into a fluid environment.

S. Sound Propagation

a. What happens when sound hits a boundary?

b. A Boundary ​= A difference in impedance

T. Transmission

a. Transmission from one medium to the next depends on the impedances of the medium

b. Impedance (Z): opposition to the flow of sound within a medium i. Characteristic property of each medium

c. More efficient transmission when the impedances are the same d. If they’re not the same, not all of the sound energy will pass through. e. In other words there’s a loss of energy transference when there is an impedance mismatch.

f. There’s a high mismatch between air and water.

g. Characteristic Acoustic Impedance (Z)

h. Reflected

i. Incidence Wave vs. Reflective Wave

ii. Most of the energy will get reflected when a surface is

encountered, only a small amount will pass through the

boundary.

i. Proportion of Transmission (H)

i. H = 4 ZbZa/(Zb+Za)2

1. Where Zb and Za are the characteristic impedances of the

two media in question.

ii. Air to water?

1. H = 4 (412 x 1.52*106)/ (412+ 1.52*106)

2. H = ~0.001

iii. So only 0.1% of the sound is transmitted across the air-water boundary

U. Middle Ear is Air-Filled

a. he impedance of cochlear fluids is approximately equal to that of sea water

i. (i.e. 1.5 x 106 N.sec/m3 ).

b. Because of this impedance mismatch, only 0.1 % of incident energy would be transmitted between the middle-ear to cochlear boundary. V. The ME Overcomes This Mismatch in TWO Ways:

1. The area of the tympanic membrane is larger than that of the stapes footplate in the cochlea. Pressure concentrated over a smaller area.

a. The area of the tympanic membrane is larger than that of the stapes footplate in the cochlea. The forces collected over the ear drum are concentrated over a smaller area, thus increasing the pressure over oval window. The pressure is increased by the

ratio of these two areas i.e. 18.75 times.

2. Lever action of the middle ear bones.

a. The second process is the lever action of the middle ear bones. b. The arm of the incus is shorter than that of the malleus, and this produces a lever action that increases the force and decreases

the velocity at the stapes.

c. Since the malleus is 2.1 times longer than the incus, the lever action multiplies the force by 2.1 times.

d. Remember: Pressure = force/area

Inner Ear

A. Bony Labyrinth: Bony Casing of the Inner Ear

a. Encases the membranous labyrinth

b. You see there are 3 semicircular canals that make up part of the vestibular, balance, system

c. Vestibule - refers to the middle area of the bony labyrinth and includes the oval and round window as well as the promontory that separates the 2 windows

d. Bony cochlea - the section of the bony labyrinth that covers the hearing portion of the inner ear

e. Bony Structure Contains Sacs of Fluid

i. The boney shell is filled with an interior membranous labyrinth, filled with ​Endolymph​.

ii. Perilymph​ fills the space between the membranous labyrinth, and the boney casing.

iii. Inner membrane filled with endolymph

iv. Outer membrane filled with perilymph

B. Composition of the Inner Ear Fluids Are: Cerebral Spinal Fluid (CSF) a. Equal amounts of Chloride and BiCarbonate

b. Endolymph has higher concentration of Potassium (K+) than the other fluid

c. This K+ is essential for the firing of hair cells.

C. Cochlea

a. Named for its resemblance to a snail shell

b. Surrounded by a very hard bony structure.

c. Supports the membranous labyrinth inside but also provides a certain amount of protection

d. Coiled: ~2.5 turns in humans

e. When uncoiled: 35 mm in length from the base, or basal end, to the apex (or apical end).

D. Osseous Spiral Lamina

a. Looks like a beehive

b. Modiolus

i. The core of the spiral lamina, the ​modiolus​, is hollow.

ii. The core houses the auditory nerves fibers in the cochlea, which then radiate across the cochlea.

iii. Spiral ganglion, dense mass of cell bodies occurring in the modiolus giving off radiating axons that comprise the cochlear nerve

iv. The nerve fibers on the last slide were part of the cochlear branch of the 8th nerve, aka Vestibulocochlear Nerve

E. 8th Cranial Nerve (AKA Vestibulocochlear Nerve)

a. Transmission of ​afferent information​ ​from​ ​inner ear​ ​to​ central nervous system

b. 2 subdivisions

1. Vestibular (sense of balance/equilibrium)

2. Cochlear (sense of hearing)

a. Aka “auditory nerve” or AN

F. Cranial Nerves

a. 12 pairs

b. Sensory, motor and mixed (sensory & motor function)

i. Sensory: Send sensory information to brain (i.e., pressure) ii. Motor: Send impulses from brain to body

c. Roman numerals

i. I = most anterior, VII = most posterior.

G. The Central Auditory System Spans the Brainstem, Diencephalon, and Cerebral Hemisphere

a. Information from the cochlear nerve is relayed to the brainstem. H. Organ of Corti: Organ of Hearing

a. Located within the cochlea

b. Receptors = hair cells on the basilar membrane

c. Gel-like tectorial membrane is capable of bending hair cells d. Cochlear nerve attached to hair cells transmits nerve impulses to the central auditory system (brainstem).

e. Sensory Reception: 2 Types of Hair Cells

i. Inner​: (IHC) ​one row​; convert movement of basilar membrane into neural signal

ii. Outer​: (OHC) ​3 rows​; participate in movement & amplification of basilar membrane

iii. On the top of each hair cell is a set of mechanoreceptors called stereocilia​, collectively called the ​hair bundle

iv. The stereocilia are arranged from shortest to tallest (laterally to medially).

v. In humans, roughly 15,000 hair cells (3/4 of which are OHCs). vi. The OHC Amplifier

1. OHC are motile: they expand and contract.. spring

action…

2. Motor protein: “Prestin”

3. Adds energy back into the basilar membrane.

f. Cochlear nerves (branches of the 8th nerve) at the base of the hair cell transmit nerve impulses to (​afferent​) and from (​efferent​) the central auditory system (i.e., the brain)

g. Efferent nerve fibers: Feedback from brain that modifies the movement of the OHCs.

h. Movements of the basilar membrane are detected by hair cells situated along its length; their electrical responses trigger firing in

nerve fibers of the 8th nerve that communicate information to the brain.

i. Each nerve fiber has a characteristic frequency, meaning that it has a preferred frequency that it will respond to.

j. In this way, the auditory nerve fibers are also tonotopically arranged. k. Schematic diagram of the basilar membrane and hair cell tuning. A 4 kHz sound results in a peak in the travelling wave at position B. The hair cell at this position is stimulated by the bending of the stereocilia. The depolarization results in transmitter release and the generation of an action potential in the auditory nerve fiber.

I. The Basilar Membrane

a. The ​basilar​ ​membrane​ is a stiff structure that separates the scala media and scala tympani.

b. For the next few slides, we are going to uncoil the basilar membrane to talk about its vibrational characteristics.

c. BM Runs from the base to the apex of the cochlea.

d. It varies in width and stiffness:

i. Base is narrowest and most stiff at the base and then it

becomes progressively narrower and less stiff at the apex.

e. This physical gradient (stiffness, width) impart is resonance feature (i.e., it’s ability to move in response to sound energy).

f. Each position along the basilar membrane is finely tuned to a particular frequency.

g. Each position will respond maximally to a specific frequency h. This mapping between place and frequency is called ​tonotopicity​. i. Tonotopy​: Different regions of the BM tuned to different frequencies i. At low intensities

1. Base responds BEST to high frequency, apex responds

BEST to low frequency

2. Each frequency activates a very particular region of the

basilar membrane

j. Non-Linearly Spaced Frequencies

i. Covers human range of hearing: 20-20000 Hz

ii. <20 & >20000HZ

1. No receptors for this, we don’t hear them

k. Basilar Membrane (BM): Moves Like a Wave

i. Pressure waves entering the cochlea, lead to a traveling wave along the basilar membrane.

ii. To visualize the motion of a travelling wave, think of a wave that travels along a piece of ribbon if you hold one end in your hand and give it a flick.

l. Air-Conduction Hearing

i. Activation of the Basilar sound displacing air molecules that then activate the tympanic membrane → middle ear → ear

m. Bone Conduction Hearing

i. Basilar membrane can be activated by vibration of the skull bones

ii. Remember that the cochlear is encased in bone.

J. Bone Conduction

a. In cases of outer or middle ear infection, there may only be weak transmission of sound (via air) due to a conductive hearing loss. b. Using bone-conduction audiometry (vibrating the skull bones) can be used to test the function of the middle. This technique allows you to bypass the ME and activate the inner ear directly.

c. Bone-conduction hearing aids (BAHA)

i. The surgically implanted sound processor converts sounds into vibrations, which are then sent through your skull bone and

directly on to your inner ear.

ii. A bone anchored hearing system consists of two parts:

1. A small titanium implant placed in the bone behind the ear 2. A sound processor that attaches to the implant

iii. Titanium post allows for the external device to be snapped on and off.

iv. Consumer Grade Bone Conduction Headsets

d. Von Békésy

i. Awarded Nobel Prize in 1961.

ii. We know a lot about the basilar membrane thanks to Von Bekesy, the only person to have be awarded a Nobel prize for hearing-related topics.

iii. Developed method for dissecting the inner ear of human cadavers while leaving the cochlea partly intact.

iv. He found that the basilar membrane when stimulated by sound will vibrate, with the vibration pattern depending on the

frequency (i.e., he discovered the tonotopic organization).

v. Critically this happens even in a dead person, telling us that this vibrational pattern is in response to the mechanical (not

biological properties of the basilar membrane).

K. Stereocilia

a. On the top of each hair cell is a set of mechanoreceptors called stereocilia​, collectively called the hair bundle

b. The stereocilia are arranged from shortest to tallest (laterally to medially).

c. Stereocilia Damage

i. Scanning electron micrograph showing the normal organization of the organ of Corti. View is of the apical membrane of the

single row of IHCs (top) and 3 rows of OHC (bottom). Notice the orderly arrangement of stereocilia.

ii. Disruption of IHC stereocilia and loss of OHC in the basal turn of the cochlea following noise exposure (90 dBA noise for 8 hours) 6 months earlier. This damage produced a profound hearing loss.

1. NOISE EXPOSURE

iii. Disruption of OHC stereocilia following administration of the aminoglycoside antibiotic kanamycin​. Depending upon dose, drug administration can cause either a temporary or permanent threshold shift.

1. OTOTOXICITY

iv. The most common causes of stereocilia damage are noise exposure and administration of ototoxic drugs

d. Tip Links

i. Stereocilia​ arranged in three rows of graded lengths.

ii. The stereocilia within a row are attached by transverse lateral links

iii. The stereocilia between rows are connected by tip links, which are important for converting pressure waves into an electrical signal

iv. The basilar membrane moves when sound energy enters the cochlea.

v. When the basilar membrane moves, the hair cells “sheer” mechanically against the tectorial membrane, initiating a

cascade of molecular events that ultimately leads to the sense of hearing.

vi. Tip links are bathing in a liquid of endolymph that is rich in K+ (potassium).

vii. Mechanotransduction

1. A mechanical movement of the hair cells is translated

(transduced) into an electrical impulse that is carried to

the brain.

2. When stereocilia are bent, K+ enters the channel and

depolarizes the cell (becomes less negative), leading to an

influx of Ca+ resulting in the release of neurotransmitters

and the excitation of the afferent nerve fiber at the base

of the hair cell.

3. The closure of channels occurs prior to a return of

stereocilia to their initial position.

4. This mechanism reduces the time constant of channel

opening, thus allowing cycles of mechano-transduction to

occur in rapid succession i.e. at high frequencies.

viii. Tip links are susceptible to acoustic trauma. Zhao et al. found that tip links can regenerate, albeit imperfectly, over several

hours.

ix. The time course of tip-link regeneration suggests that this process may underlie recovery from ​temporary threshold

shifts induced by noise exposure.

1. Temporary threshold shift, not so temporary

L. BM: Passive vs. Active

a. Passive response (cadaver response) vs Active Response

b. Tonotopy is observed in a cadaver basilar membrane but activation pattern is different from that seen in a “live” basilar membrane where the activation is more focal (more frequency specific), and is also amplified. This focal activation, i.e., active amplification, theorized to be the result of ​prestin​.

i. Prestin discovered by one of former professors; Peter Dallos c. The motor action of the OHC create sounds that can be heard in the ear canal with a sensitive microphone:

d. These otoacoustic emissions are a byproduct/epiphenomenon of the active cochlear amplifier

M. Otoacoustic Emissions or OAEs

a. Low-level sounds produced by the cochlea that travel out through the middle ear and are recordable in the external ear canal.

b. Generally not audible to the naked ear.

c. Measure of cochlear health.

d. Different types of OAEs:

i. Stimulus evoked

ii. Spontaneous

e. OAE History

i. First described by Kemp (1977 & 1978)

ii. But predicted earlier by Gold (1948!), but no one believed him because he also theorized that the origins of life on earth were from a pile of waste products accidently dumped by

extraterrestrials long ago.

iii. Clinical use

1. Screening for hearing loss, especially in newborns

2. Objective measure of hearing: If the OHCs are

functioning, it’s a pretty good indication that the rest of

the system is intact, too.

3. Only tell you about the inner ear not the brain and

whether or not it can interpret the signal.

iv. Rare cases:

1. Intense Enough to Be Heard OUTSIDE the ear

2. A Child With an Unusually High-Level Spontaneous

Otoacoustic Emission

v. “Knockout” Mouse Model Reveals that Prestin is a Generator of OAEs

1. A knock-out animal model is an animal whose DNA has

been genetically engineered so that it does not express

particular proteins.

2. Side note: In humans, prestin can be found in the blood.

It is unique to the inner ear. Potential BioMarker of

Hearing

3. Prestin knockout mice​, in which the motor protein

prestin is absent from the lateral walls of outer hair cells.

OAEs are absent (i.e., in the noise floor) in the knockout

mice.

4. Tecto knockout mice​, in which the tectorial membrane, is vestigial and completely detached from the organ of

Corti. Still getting OAEs in these mice.

N. 3D Digital Model of the Cochlea, Showing just the Nerve Infrastructures a. More than 30 thousand nerve fibers extend through the cochlea b. Spiral Ganglion​: A dense mass of cell bodies occurring in the

modiolus (center core of the osseous spiral lamina) giving off radiating axons that comprise the cochlear nerve.

c. Ganglion​: A group of nerve cell bodies outside of the central nervous system. These cell bodies can seen by staining them.

O. Nerve Fibers

a. Cochlear nerves (branches of the 8th nerve) at the base of the hair cell transmit nerve impulses to (​afferent​) and from (​efferent​) the central auditory system (i.e., the brain)

b. Efferent Nerve Fibers​: Feedback from brain that modifies the movement of the OHCs.

Two Types of ​Afferent​ Cochlear Neurons

● OHCs

● IHCs (IHC = Type I)

○ Type I

■ Make up 90-95% of the neurons & innervate the inner

hair cells.

■ MANY to ONE Relationship​: Each type I axon innervates only a single hair cell, but each hair cell directs its output

to an average of 8 nerve fibers in humans

■ Myelinated & large diameter

○ Myelin

■ A sheath of proteins and lipids surrounding a neuron

■ Serves to insulate electrical current from the axon,

protect against leakage between neurons

■ Speeds up transmission time

○ Type II

■ Make up the remaining 5-10% of the neurons & innervate the outer hair cells.

■ Small diameter & unmyelinated.

■ ONE –to- MANY Relationship​: Each type II axon

innervates many OHC

■ Their function is less well understood but recent evidence suggest that they only respond to very high intensity

sounds (Work by Paul Fuch’s group)

■ (They sound the alarm that stuff is getting dangerous in

the inner ear!)

P. How do the Hairs Cells and the Nerve Fibers Communicate with Each Other?

a. A process called ​Mechanotransduction​. The mechanical (physical) movement of the basilar membrane results in movement of the hair

bundle which ultimately is converted (transduced) into an electrical signal.

Q. Noise-Induced Damaged to the Inner Ear

a. Damage to the tip-links

b. Synaptopathy: Damage to the synapse

c. Hair cell loss

1. Damage to the Tip-links

a. Tip links are susceptible to acoustic trauma. Zhao et al. found that tip links can regenerate, albeit imperfectly, over several

hours.

b. The time course of tip-link regeneration suggests that this process may underlie recovery from ​temporary threshold

shifts induced by noise exposure​.

2. Synaptopathy: Damage to the Synapse

a. The nerve fibers that are most susceptible to damage are those that fire to high intensity sounds (but are silent to low intensity sounds). For this reason, this type of hearing loss may be ​hidden or invisible on the standard test of hearing thresholds.

b. Hidden hearing loss

c. Nerve ganglion (dense packing of nerve cell bodies)

i. There are so many cell bodies packed in, that you can’t

make out the individual ones

ii. Primary neuronal degeneration was seen in mice that

were exposed and allowed to survive for many months.

iii. Acceleration of age-related hearing loss by early noise

exposure: evidence of a misspent youth.

3. Hair Cell Loss

a. Damage to the hair cells results in sensorineural hearing loss. b. When HCs are damaged, sensory information is not being adequately transferred to the nervous system.

c. In cases where the HC are damaged, the auditory nerve fibers can be stimulated artificially using a cochlear implant device that electrically stimulates the nerve fibers.

d. Cochlear implants are an ever evolving technology.

Experimental devices are using lasers to stimulate the cochlea! R. Hair Cell Regeneration

a. In non-mammalian species, hair cells naturally regenerate (e.g., birds, fish)

b. In mammals, hair cells don’t naturally regenerate but other cells in our bodies do.

c. Hair cell regeneration in humans was until VERY recently considered to be an impossibility.

d. Convert supporting cells into hair cells

e. Pharmacologically trigger this process and deliver to the inner ear S. What is a Cochlear Implant?

a. A prosthetic electronic device that creates hearing sensation by stimulating the auditory nerve.

b. Cochlear implants are an ever evolving technology.

c. Experimental devices are using lasers to stimulate the cochlea! d. Loss of frequency sensitivity in cochlear implants

e. Dozens of electrodes doing the work of thousands of hair cells. f. Note that the electrode array does not reach all the way to the apex, producing reduced frequency sensitivity.

T. The Central Auditory System

a. Primary auditory cortex neighbors the language centers in the temporal lobe (Wernicke’s area)

b. Preservation of tonotopy across the auditory system, all the way up to the auditory cortex

c. In CI listeners, the tonotopy in the cortex is distorted; it reflect the reduced frequency resolution at lower regions of the cortex.

d. Listening in on the central auditory system!

i. At low levels of the central auditory system the sensory

information is encoded more “literally” and then as you ascend the way the sound is encoded by the brain becomes more

abstract.

ii. The way in which the brain represents sound is determined by genetic and environmental factors.

iii. Measuring the central auditory system, using scalp electrodes U. Newest Way to Measure the Auditory System

a. Electrocorticography​: Electrodes placed right on the surface of the auditory cortex in epileptic patients

V. Vestibular System

a. Returning to the peripheral system…

i. The inner ear is involved in hearing & balance. The cochlea is involved in hearing and the vestibular system is involved in

balance.

b. Vestibular System, 2 Branches

i. Two major subsystems:

1. Otolith system (utricle and saccule).

a. Contain sensory receptors in otolith system called

maculae, that detect linear acceleration (speed of

movement).

2. Semicircular canals

a. Contain sensory receptors, called cristae that

detect angular acceleration (yaw, pitch, roll)

ii. This three dimensional movement is sensed by the semicircular canals

W. 3 Things the Auditory and Vestibular Systems Have in Common a. (and why some disease processes can affect both hearing & balance) 1. Innervation.

a. Hearing and balance are innervated by separate branches (cochlear & vestibular) of the same cranial nerve (8th nerve)

b. These nerve branches form a common trunk when exiting the internal auditory meatus.

c. The vestibular nerve conveys the impression of equilibrium and orientation in three dimensional space.

d. Cochlear branch also known as the auditory branch

2. Common Fluid System.

a. The membranous labyrinth is one continuous fluid system serving both hearing and balance. If there’s something wrong with this fluid system, both hearing and balance will be affected. Example: Meniere’s Disease.

b. Meniere's disease: episodes of vertigo (spinning) and fluctuating hearing loss, often associated with fullness/pressure in ear.

Chronic condition, usually emerging around age 20-50. Result of increased hydraulic pressure within the inner ear

endolymphatic system.

3. Hair Cell Motion Detectors.

a. Hearing and balance both involve the detection of motion: slowly varying (i.e., low frequency) in the case of balance, higher frequency vibratory motion in the case of hearing. In both cases, the motion detectors are hair cells that operate on nearly

identical principles. Meniere’s Disease affects the HCs in both the cochlea and the vestibular system – hearing and balance are both affected.

X. 1 Thing the Auditory and Vestibular Systems Do Not Have in Common

a. Crystals (Otolithic System)

b. Otolithic (Crystal) System in Saccule & Utrical

c. Otoliths made from calcium carbonate: they are gravity detectors d. From time to time, these crystals get out of alignment, causing dizziness, physical therapy can be used to restore them to their rightful place.

e. Dix Hallpike Maneuver