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