The Ear

Resonance

A system oscillates at a greater amplitude than its input at some frequencies. The resonance in the ear canal results from the standing waves (input wave + reflected wave). EX: ear canal

Middle Ear

Matches the impedance difference between air and the fluids/tissues of the inner ear; direct stimulation of the inner ear; protection

Traveling Wave

The Basilar Membrane moves up and down according to the sound pressure waveform, but it moves more at one point along the Basilar Membrane than at other points, depending on the frequency of the sound pressure waveform. High frequency= movement at base, low frequency= movement at apex

Physiology

the study of the function of the anatomical parts. (i.e., what is it for?)

Anatomy

the study of the structure of the body and their interconnections. (i.e., what is it?)

Mechanical Response of Cochlea

(1) Reliability The vibration pattern of the BM at any point faithfully follows the pressure changes over time in the sound pressure waveform (2) Traveling time It takes time for the traveling wave to move from Base to Apex. As a result, the BM does not move in sync or in phase at all points. (3) Place code (4) Compressive nonlinearity

Impedance Mismatch

(Problem 1 of air transmission) Sound that is traveling in air eventually has to cause the fluids and tissues in the Inner Ear to vibrate in order for the process of hearing to take place. The Inner Ear fluids and tissues offer an impedance to sound transmission to the Inner Ear, and have a much larger Characteristic Impedance than air.

Round and Oval Windows both Stimulated in the Same Direction

(Problem 2 of air transmission)

Transmission is Ineffective if Middle Ear Cavity is Enclosed

(Problem 3 of air transmission) Eustachian Tube solves this

Stereocilia

(the "hairy" top), bathed in Endolymph The body of a hair cell is in Perilymph

How the Middle Ear protects the Inner Ear

1) The Middle Ear provides a sort of buffer against "debris" reaching the Inner Ear 2) Due to a reflex action of the Middle Ear Muscles, loud sounds are somewhat attenuated by the Middle Ear, protecting the Inner Ear from overstimulation and perhaps undue nonlinearities.

Tasks of the peripheral auditory system

1. Allows the auditory system to be as sensitive as possible. 2. Provides a neural code for the sound. The central auditory nervous system (CANS) analyzes that code. 3. Provides a compressed neural representation of the large dynamic range of sound level.

main functions of the middle ear

1. To effectively deliver sound vibration to the inner ear fluids; 2. To help prevent the inner ear from being overdriven by excessively strong vibration 3. The primary function of the middle ear is to compensate for the impedance difference, so that the auditory system can be maximally sensitive

Middle Ear (Tympanic cavity)

3 ossicles Malleus, incus, stapes 2 chambers Epitympanum i.e., the attic Tympanum "Middle-Ear Cavity" i.e., the atrium 1 tube Eustachian Tube tympanic cavity oval window round window pharyngotympanic tube tensor tympani stapedius

Cochlear Wave

As the cochlear wave travels from the Base to the Apex, it causes the basilar membrane to move in an up down kind of motion. The vibration is strongest at the base of cochlea when high frequency sound is played and the vibration is strongest at the apex when low frequency sound is played, High Frequencies cause the Base to move more than the Apex, with the Apex often not moving; while low frequencies cause a motion from Base to Apex, but more at the Apex

Tympanic Membrane

Boundary between the external and middle ear Sound waves make the tympanic membrane vibrate, which transfers the sound to the ossicles, the thinnest tissue in the body.. this helps insure that the TM vibrates well at high frequencies, does not alter the vibration of the incoming sound wave very much

Perilymph

CSF like fluid between the bony labyrinth and membranous labyrinth

Vestibule

Central egg shaped cavity in the bony labyrinth contains saccule and utricle

Bony Labyrinth

Channels running through the temporal bone

Scala Media

Cochlear duct, contains the organ of corti, filled with endolymph (low concentration of sodium Na+; high concentration of potassium K+).

Middle Ear Muscles

Completely encased in bony canals and only their tendons enter the middle ear cavity. This arrangement reduces (1) muscular vibration and (2) effective mass of the ossicular chain. Contract with loud sound. Contraction of stapedius muscle alters the rotational axis of the stapes.

Spiral Organ of Corti

Contains the nerve cells that convert and transmit the sound, the sensory organ of the auditory system, It contains receptor cells for hearing The hair cells are arranged in four rows along the entire length of the cochlear coil. Three rows consist of outer hair cells and one row consists of inner hair cells. The inner hair cells provide the main neural output of the cochlea.

Membranous Labyrinth

Continuous series of membranous sacs within the bony labyrinth

Equilibrium

Control of coordination and balance Receptors in vestibular apparatus include: semicircular ducts contain crista ampullari saccule and utricle contain macula

Vestibulocochlear Nerve

Cranial Nerve VIII, branches from the semicircular canals, and runs through the cochlea

Middle Ear Transfer Function

Difference between Tympanic Membrane and Fluids of Inner Ear, Largely Explains Thresholds of Hearing, along with that for the Outer Ear provides the gain necessary to overcome the Impedance Mismatch over a significant range of audible frequencies. So the Auditory System can operate at peak efficiency

Outer (External) Ear

Directs sound to the middle ear, amplifies or attenuates sound in frequency specific manner depending on relative location of the sound source. (pinna, concha, external auditory meatus, external auditory canal, tympanic membrane (ear drum))

Scala Tympani

Duct inferior to the cochlear duct, terminates at the round window, filled with perilymph (high concentration of sodium Na+; low concentration of potassium K+).

Scala Vestibuli

Duct superior to the cochlear duct, filled with perilymph (high concentration of sodium Na+; low concentration of potassium K+).

Maculae

Equilibrium receptors in the saccule and utricle that respond to the pull of gravity and report on changes of head position

Endolymph

Fluid within the membranous labyrinth

Connection of the middle ear to the Inner Ear

Footplate of the stapes fits into the Oval Window of the Inner Ear Within Inner Ear there are three chambers - Oval Window is at the top chamber; Round Window is at the bottom chamber

Round Window

Inferior opening in the middle ear Acts as a pressure release from the inner ear

Auditory Transduction

Inner Hair Cells are the Biological Transducers for hearing, changing mechanical vibrations into neural responses (action potentials) that are relayed to the brain stem and brain by the Auditory Nerve. BM mechanical wave (Mechanical)--> Cilia motion of hair cells (Mechanical)--> Voltage potential changes inside a hair cell (Electrical)

Eustachian Tube

Maintains Equal Pressure Across Tympanic Membrane

Ossicles of the Tympanic cavity

Malleus Incus Stapes

isointensity measure

Measuring the amount of displacement at one place for stimulation sinusoidal sounds of a fixed level but different frequencies, The most sensitive frequency is called the characteristic frequency (CF) of a given partition of BM.

isosenstivity measure

Measuring the sound level necessary to displace the BM a fixed amount at one place as a function of the stimulation sinusoidal frequency, The most sensitive frequency is called the characteristic frequency (CF) of a given partition of BM.

Tensor Tympani

Muscle connected to the malleus that tenses the eardrum during loud sounds (see in later slide) pulls malleus away from eardrum muscle of middle ear

Stapedius

Muscle that pulls the stapes from the oval window during loud sounds muscle of middle ear

Hair Cell Damage and AN Tuning

Normal AN-fiber tuning curves have excellent sensitivity and sharp tuning Outer-hair-cell damage/loss results in loss of sensitivity and broadened tuning Inner-hair-cell damage results in loss of sensitivity without broadened tuning

Malleus (hammer)

Ossicle connected to the tympanic membrane

Incus (anvil)

Ossicle that connects the malleus to the stapes

Stapes (stirrup)

Ossicle that transfers the mechanical energy to the oval window Smallest/Lightest bone in the body (by far)

Peripheral Auditory System

Outer (External) Ear Middle Ear Inner Ear Auditory Nerve

Semicircular Canals

Oval shaped ducts that respond to the angular rotation of the head Anterior/Posterior/Lateral filled with endolymph responsible for detection of angular movement, therefore assists angular movement Movement in three separate planes (i.e. forward/backward, or a rotation form left to right)

Saccule

Part of the membranous labyrinth that leads into the cochlea Contain macula

Utricle

Part of the membranous labyrinth that leads into the semicircular canals Contain macula

Quarter wavelength rule

Resonant frequency = Speed of sound /(4*length of tube) = 3500 Hz

Auricle/Pinna

Shell shaped structure on the side of the head Made from elastic cartilage and covered in skin Captures the sound waves *Function: Provides Cues for Sound Location

External Auditory Canal

Short tube that leads from the auricle to the Tympanic Membrane

Tympanic Cavity

Small air filled cavity in the temporal bone composed of ossicles, tensor tympani, and stapedius

Bone Conduction

Sound can reach the inner ear and cause inner ear vibrations: 1. Via the normal Middle Ear-Ossicular Chain processes. (only one that results in good hearing) 2. Via the sound pressure waveform directly traveling in the air of the Middle Ear . 3. Via the bones of the head that vibrate with the sound pressure wave, and in turn vibrate the structures of the Inner Ear. But only intense vibrations and only low-frequency vibrations can cause sufficient bone vibration.

Problem one of air transmission

Strategy 1: Area Difference (Area of manubrium is 17 times larger than that of the stapes footplate) Strategy 2: Lever Action (The ossicular chain operates like a lever in that a small force at the malleus is translated to a larger force at the stapes) Strategy 3: Buckling Effect (The tympanic membrane moves more in the middle part which is connected to the manubrium. This effect increases the force at the manubrium over that in the Outer Ear canal by a factor of about 2.0)

Oval Window

Superior opening in the middle ear Transfers the sound to the inner ear

Outer Hair Cell Motility

The fast motility in response to all frequencies of stimulation is due to a special protein (actins) contained in the OHCs, which enables muscle-like contraction. Slow motility is also available via the efferent fibers that come from the brainstem and innervate the OHCs.

Inner Ear Functions

The inner ear is situated inside the skull, protected by petrous portion of temporal bone. The inner ear is divided into two cavity systems: one houses the organ of balance and the other houses the organ of hearing.

Head related transfer function

The spectrum of the sound arriving at the tympanic membrane depends on the relationship between the spatial position of the sound source relative to the pinna. Thus, the spectral information in the HRTF reaching the brain might be useful for locating the position of sound sources, localization of sound

When Sound Comes In

The stapes causes the oval window to vibrate which in turn vibrates the fluids and structures of the Inner Ear, primarily the Organ of Corti in Scala Media. The stereocilla of the Inner Hair Cells are bended to trigger action potentials in the Inner Hair Cells and then action potentials in the fibers of the auditory nerve that synapse with the Inner Hair Cells

Ear Canal Transfer Function

The system transfers the input to the output, so the transfer function is: Transfer Function = Output - Input or Output = Input + System's Transfer Function, at various frequencies

Inner Ear

Transduction of sound vibration into the neural code

Auditory Nerve

Transmits neural code for sound to the brainstem

Cochlea

Turns mechanical energy to neural impulses a bony canal, wrapped around in a spiral shape (35 mm, 2.5 turns). The inner structure needs to be revealed using cross-section illustrations.

Middle Ear Reflex

When a loud sound is present, the two middle ear muscles,contract and pull the three ossicular bones tighter together, preventing them from moving as much. In so doing, the stimulation of the inner ear is limited and the possibility for overstimulation of the inner ear is reduced. triggered by sounds with levels greater than about 80 dB SPL. it takes time (10 to 150 ms) for the reflex to occur after a loud sound is turned on. doesn't protect again sudden noises, does protect against drawn out noises

Outer hair cells

adjust cochlear responses to different frequencies increase precision

Parts of the Outer Ear (External Ear)

auricle/pinna concha external auditory canal (meatus) external auditory canal tympanic membrane (eardrum)

Inner Ear (internal ear)

bony labryrinth membraneous labyrinth vestibule semicircular canal cochlea vestibulocochlear verver

Tectorial Membrane

gelatinous substance that rests on top of the stereocilia Doesn't move

Macula

hair cells with stereocilia and one kinocilium buried in a gelatinous otolithic membrane used for balance otoliths add to the density and inertia and enhance the sense of gravity and motion

Dynamic equilibrium

in car, linear acceleration detected as otoliths lag behind perceived in two locations perception of motion or acceleration linear acceleration perceived by macula of vestibule angular acceleration perceived by crista ampullari of semicircular canals

Impedance Bridge or Tympanometer

is used to measure Middle Ear Impedance as a way to determine what aspects of the Middle Ear may or may not be responsible for a conductive hearing loss

Inner hair cells

responsible for hearing

Membranous labyrinth

scala media lies in the middle

Osseous labyrinth

scala vestibuli lies opposite the vestibule, connects to the oval window and stapes scala tympani (label 3) connects to the tympanic cavity via round window

Deiters cells

supporting cells, The inner and outer phalangeal cells, have a shape of a chair (for the Hair Cells to sit on), Each phalangeal cell has two parts, a body at the base attached firmly to the basilar membrane and a phalangeal process extending upwards to TM

The Rods of Corti

supporting cells, form a triangular structure and provide rigidity to the Organ of Corti

Basilar Membrane

thick substance that separates the cochlear duct from the scala tympani below, The BM is a rubbery band and tapers in the opposite direction of the cochlea (wider at apex), BM is more flaccid and under no tension at the apex, but stiffer and under tension at the base

Static equilibrium

when head is tilted, weight of membrane bends the stereo cilia perceived by macula perception of head orientation