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69 Cards in this Set

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Outer ear structures
Pinna- primarily made of cartilage
Concha- deep center portion
External auditory meatus- opening to the external auditory canal
External auditory canal- approximately 2-3 cm in length. First 1/3 cartilaginous with glands and hair follicles. Final 2/3 bony.
Outer ear function
Pinna collects auditory stimuli and funnels it into the ear canal.
Sound localization using two ears. Based on interaural time and level differences.
Wax, hair and orientation of canal protects TM.
Tympanic membrane
Most lateral portion of the middle ear. Cone shaped, pearl-white color.
Three landmarks:
1) Pars flaccida- most vulnerable portion for damage.
2) Pars tensa
3) Umbo- point of greatest retraction.
Middle ear ossicles
Malleus- manubrium of the malleus is embedded in the TM.
Incus- short process and a long (inferior) process.
Stapes- smallest bone in the body. Footplate of the stapes in the most medial portion of the middle ear.
Middle ear muscles
Tensor tympani- inserts into the manubrium of the malleus. Stiffens manubrium --> stiffened TM.
Stapedius- inserts into the posterior portion of the neck of the stapes. Smallest muscle in the body.
These muscles are the acoustic reflex.
Eustachian Tube
Most inferior portion of the middle ear cavity. Equalizes the pressure between the middle ear cavity and the external auditory canal.
Function of the middle ear
1) Converts acoustic energy into mechanical energy by using the lever action of the ossicular chain.
2) The TM and the ossicular chain increase pressure in order to displace the fluids of the inner ear (because of the higher impedance of the inner ear fluid). Area ratio and lever action work together to overcome impedance mismatch.
3) Limit distortions during transmission through the ossicular chain.
Middle ear impedance
1) Resistance- cochlear fluid on the footplate of the stapes.
2) Mass reactance- a result of the mass of the middle ear structures.
3) Compliant reactance- spring-like characteristics of the ligaments and muscles in the middle ear.
Utricle
Larger of the two sacs in the vestibular labyrinth.
Vestibule
Also an end organ of equilibrium but gives access to the cochlea.
Cochlea
Small, snail-shaped organ encased in the temporal bone. Base, apex and modiolus. Coils approximately 2.5 times and is about 35 mm in length, 1 cm wide, and 5 mm high.
Scala vestibuli
Region of the cochlea that connects the inner ear to the middle ear via the oval window/ stapes footplate. Contains perilymph.
Scala tympani
Lower region of the cochlea that terminates at the round window. Contains perilymph.
Scala media
Aka cochlear duct. Portion of the cochlea between the scala vestibuli and scala tympani. Bounded by Reissner's membrane, the basilar membrane, and the stria vascularis. Contains endolymph. Endolymph has a resting potential of +80-100 mV.
Stria vascularis
Produces endolymph and supplies oxygen and other nutrients (predominately blood) to the cochlea. Very important for survival of the structure.
Basilar membrane
Supports the organ of Corti which is the end organ of hearing. Inferior support for the organ of Corti.
Tectoral membrane
Superior support for the organ of Corti.
Tunnel or Corti
Triangular space between the OHCs and IHCs
Stereocilla
Tiny hairs that are rooted in the cuticular plate at the tip of each OHC and IHC. The tips of the tallest stereocilia are in contact with the tectorial membrane.
For OHCs, there are approximately 150 stereocillia arranged in three or more rows on each OHC in the shape of a V or W.
For IHCs, there are approximately 40 stereocilla arranged in two or more parallel rows that form a shallow U.
Function of the cochlea
To translate the mechanical vibrations of the stapes and the inner ear fluids into neural responses in the auditory branch of the 8th cranial nerve. Also to convert complex stimuli (multifrequency, such as speech) into individual frequency components.
Depolarization
The vibration of the fluid causes the basilar membrane to be displaced up, which causes the stereocilia of the hair cells to bend. The bending of the stereocillia causes the nerve at the base of the hair cell to initiate a neural potential which is sent along the auditory nerve.
Single hair cell depolarization
When the stereocilia are deflected small ion channels at the tip of the hair cell are opened.
Potassium (K+) ions enter the hair cell depolarizing the cell. Enters at base and tiplinks.
Depolarization (when IHC has about -20 mV) leads to a release of a transmitter (Glutamate and Aspertate) at the base of the hair cells.
Resting potentials for IHC and OHC
IHC about -45 mV
OHC about -70 mV
Place theory
Each point along the basilar membrane that is set in motion vibrates at the same frequency as the stimulus. The amplitude of the membrane vibration, however, is different at different locations, depending on the frequency and level of the input stimulus.
Traveling wave
The basilar membrane becomes wider as it gets to the helicotrema. Because of the variation in stiffness, different frequencies will cause maximum vibration amplitude at different points along the basilar membrane.
Tonotopic organization
For low frequencies, the maximum displacement of the basilar membrane is near the apical end. For high frequencies, the maximum displacement is near the basal end.
Low frequencies displacement
Low frequencies stimulate not only the apical end where the point of maximum displacement occurs but also at the basal end. The amount of displacement past the maximum displacement is reduced gradually.
High frequencies displacement
Higher frequencies stimulate only the basal end of the cochlea. The amount of displacement past the point of maximum displacement is reduced more rapidly.
Basilar membrane
For a given frequency, the basilar membrane acts as a low-pass filter with a sharp high-frequency roll-off. Furthermore, the basilar membrane also acts as a high-pass filter with a more gradual low-frequency roll-off. As a result, the specific frequency on the basilar membrane resembles a band pass filter of vibrating motion.
Vestibular function
Motion detection and sense of position in space. Vestibular, visual, and somatosensory information put together to produce fine control of visual gaze, posture, autonomotic reflexes, and spacial orientation.
Vestibular dysfunction
Dizziness, vertigo, and motion sickness are associated with dysfunction of the vestibular system. Most likely see an audiologist first for the above problems. Mineers: leak in the vestibular system.
Types of motion
Rotational (angular acceleration) - head turns.
Linear (linear acceleration) - walking, falls, vehicle travel, elevators, and head tilts from gravity.
Bony labryinth
Embedded within the petrous portion of the temporal bone and protects the sensory structures.
Membranous labyrinth
Contains 5 sensory structures.
Three semicircular canals: horizontal, anterior, and posterior.
Two otolith organs: utricle and saccule.
Peripheral Vestibular Labyrinth
Horizontal semicircular canal and utricle - lie in a 30 degree horizontal plane.
Anterior and posterior semicircular canals and saccule - lie in a vertical plane.
Structures on ipsilateral side work in opposition of structures on the contralateral side.
Vestibular labyrinth fluid
Perilymph - fluid surrounding the membranous labyrinth. High in sodium (Na+) and low in potassium (K+).
Endolymph - fluid within the membranous labyrinth. 7-10 mV resting potential. High in potassium (K+) and low in sodium (Na+). From cerebral spinal fluid.
Ampulla
Membranous portion of the semicircular ducts connect to the utricle with an enlarged sac called the ampulla.
50-100 stereocilia and a single, longer kinocilium. Type I - surrounded by afferent nerve fiber. Type II - single synaptic connection at the base of the hair cell. Afferent nerve fibers are the vestibular portion of the vestibulocochlear nerve (VII cranial nerve). Hair cells are embedded in supporting cells of neuroepithelium - crista. Fluid-tight (endolymph) structure covers the crista - cupula.
Rotational head movement
Endolymph in the membranous semicircular duct is displaced. Pushes the cupula in the opposite direction of the head turn. Stereocilia and kinocilium deflect either causing excitation or inhibition.
Linear acceleration
Hair cells in the otolith organs are in neuroepithelium called the macula. Stereocilia and kinocilium are located in the otolith membrane. Above this membrane, there are calcium carbonate crystals called otoconia.
Otoconia
More dense than endolymph and hair cells are displaced by inertia not endolymph.
Tip links
Staggered height of the stereocilia permits the presence of tip links. Fine strands (possibly elastin) that connect the top of the shorter stereocilium with the lateral wall of its taller neighbor. When the stereocilia are deflected toward the tallest one, opening the "trap door" transduction channels.
Scala Media resting potential
+ 80-100 mV
Also known as the endocochlear or endolymphatic potential. Generation of the EP is in the stria vascularis.
IHCs resting potential
-45 mV
When K+ enters the cell and the voltage increases to -20 mV, the cell depolarizes and releases glutamate and aspartate (excitatory neurotransmitters).
OHCs resting potential
-70 mV
Summating potentials
A component potential of electrocochleography (ECochG) measured using rarefaction and condensation clicks. Stimulus-related direct current (DC) electrical responses recorded from both the outer and inner hair cells. Some components are thought to be a reflection of the nonlinear distortion in the transduction process. Representative of the stimulus duration.
Cochlear microphonic
Another component of ECochG. An alternating current (AC) potential that only occurs during the presentation of an acoustic stimulus. This potential appears to mirror the waveform of the acoustic stimulus. The output stems primarily from the OHCs.
Action potentials
Neural potential: the sum of the potentials of many individual nerve fibers firing synchronously. Similar to the CM, the AP appears to be an AC (alternating current) potential. Reflects the neural output of the cochlea, specifically the AP is associated with wave I of the auditory brainstem response (ABR).
Otoacoustic emissions
Pre-neural potentials that are acoustic signals detected in the ear canal. These emissions are thought to be echoes that are a result of OHCs contracting to acoustic stimuli. Must do otoscopy and tympanometry!
Spontaneous OAEs (SOAEs)
OAEs that are measured without any acoustic simulation. We don't know what the absence of OAEs means.
Transient Evoked OAEs (TEOAEs)
Evoked by click stimuli.
Distortion Product OAEs (DPOAEs)
Evoked by two pure-tone frequencies.
Structure of the auditory nerve
Afferent fibers - sensory fibers that carry information from the organ of Corti to the brainstem and brain.
Efferent fibers - bring information from higher neural centers to the auditory periphery.
Type I Afferent Nerve Fibers
Radial or Type I fibers innervate the IHCs only.
95% of afferent fibers innervate IHCs.
Type I fibers tend to be thicker than Type II fibers.
Convergence: 1 afferent nerve fiber to 1 inner hair cell.
Type II Afferent Nerve Fibers
Outer spiral or Type II fibers innervate the OHCs.
5% of afferent fibers innervate OHCs.
Divergent: 1 afferent nerve fiber to many hair cells.
Efferent nerve fibers
Fibers from the lateral superior olive project predominantly to the IHCs. These fibers synapse on the afferent fibers leaving the IHCs. Fibers from the medial superior olive project to the OHCs and these fibers synapse directly on the OHCs.
Summary of hair cell function
IHCs serve as the main neurologic transducer of acoustic information whereas OHCs serve to maintain and/or alter the biomechanics of the inner ear.
Nerve bundle organization
Nerve fibers exit the cochlea through the habenula perforata and are organized in a bundle within the modiolus. Fibers from the apex run down the middle and those from the base are on the outside.
Single neuron threshold
The minimum stimulus level that will cause an increase in the discharge rate above the spontaneous activity.
Rate-level functions
Increase the level of the acoustic stimulus and measure the change in discharge of a single neuron above the spontaneous rate. All auditory nerve fiber discharge rates increase over a range of 20 to 50 dB (neuronal dynamic range). After the discharge rate will remain the same or decrease slightly when the stimulus level is increased.
Response areas
Plots each frequency the nerve fiber responds to for a fixed stimulus level. Characteristic frequency - the frequency at which the neuron fires the most.
Tuning curve
Plot the level of tone required for the neuron to discharge at a threshold amount of firing as a function of the stimulus frequency. Characteristic frequency - the frequency of the neuron's lowest threshold.
What do Response Areas and Tuning Curves tell us?
Demonstrate that auditory nerve fibers are very selective to frequency. Each fiber fires best to a limited range of frequencies. Maintain the frequency selectivity found along the basilar membrane (tonotopic organization).
Histograms
Graphic displays of a single neuron's response to repeated presentations of a given stimulus.
Phase-locking
Occurs when nerve fibers discharge about once per sycle (i.e., 1000 times for a 1000 Hz stimulus). Usually occurs for CFs below 4000 or 5000 Hz.
Two-tone suppression
Present one tone to auditory nerve fibers that will cause it to fire above its spontaneous rate. Adding a second tone with a certain frequency and level may cause a decrease of suppression in the discharge rate of the fiber to the first tone.
The efferent system
Research suggests that this system is inhibitory. These inhibitory effects may decrease the neural activity in intense noise situations, protecting the neural system against noise-induced damage. Also may decrease the effect of background noise so you can detect certain desired acoustic signals.
Frequency encoding
Basilar membrane is tonotopically organized, the afferent auditory nerve innervates the basilar membrane in a systematic way, and as a result the auditory nerve is tonotopically organized. Also called place theory.
Intensity encoding
Intensity is thought to be encoded by an increase in the discharge rate within the auditory system. Combining low-, medium-, and high- threshold fibers and fibers with different CFs may serve to code for level.
Time encoding
Aka phase locking
When nerve fibers discharge in synchrony to the phase of the acoustic stimulus, that is what codes the dynamic timing of that stimulus. 5000 Hz is the upper limit where timing codes are effective.