Flashcards in Audition: Peripheral and Central Deck (22):
Understand the physical nature of sound
Sound comes in the form of pressure waves. The waves are created by increases and decreases in air pressure, which correspond to compression and rarefraction of the sound wave. Sound has two distinct qualities that contribute to audition: intensity (loudness) and frequency (pitch).
What is auditory threshold, how is it measured in the audiogram?
Auditory threshold is the lowest intensity of sound that can be heard by the human ear at a given frequency. An audiogram is a frequency (x-axis) vs. intensity (y-axis) plot of auditory threshold.
The audiogram is constructed by playing a series of tones for a patient of progressively lower intensity at each frequency. The patient signals when they hear the tone and the lowest intensity tone (and corresponding frequency) are plotted to construct the audiogram.
Understand the concept of acoustic impedance mismatch and the role of the middle ear in overcoming impedance
mismatch between the air-filled middle ear and fluid-filled inner ear.
Acoustic impedance mismatch refers to the differences in impedance between the middle ear and the inner ear. Impedance can be thought of most simply as resistance to flow. Fluids are much more dense than air and as result they have a higher impedance. Because of these differences in impedance, when a sound wave moves from air to fluid, a significant portion of the wave is deflected at the air/water interface. In going from the middle ear to the inner ear, appx. 30 dB of intensity would be lost due to impedance. But fortunately the ear has a mechanical way to compensate for this---the ossicles!
Understand the difference between sensorineural and conductive hearing loss.
Conductive hearing loss results from damage to any of elements involved in mechanical transmission of the sound waves through the ear (i.e. the ear canal, the tympanic membrane, the ossicles)
Common causes of conductive hearing loss:
•Otitis media—fluid disrupts conduction
•Otosclerosis—ossicles can’t move as well
•Atresia—the ear canal is misshapen
•Perforation/rupture of the tympanic membrane
•Increased pressure in the middle ear—due to altitude changes
Sensorineural hearing loss results from damage to the hair cells or the nerve fibers of the auditory system.
Common causes of sensorineural hearing loss:
•Excessive loud noises
How does sound elicit movement of the BM?
The oval window of the middle ear opens into a coiled inner ear structure called the cochlea. A collection of inner hair cells called the organ of Corti sits on top of the basilar membrane within the scala media of the cochlea. The organ of Corti houses the inner hair cells.
When the oval window is compressed by the compression portion of the sound wave (high pressure), the fluid within the scala vestibuli is compressed. Because the outer walls of the cochlea are incompressible, the compression causes the downward movement of the basilar membrane. This compresses the fluid of the scala tympani and bulging of the round window (another opening into the middle ear). Some of the pressure from the scala vestibuli is transmitted directly to the scala tymapani through the helicotrema. During rarefraction, pressure drops and the sequence of events is reversed. Together, compression and rarefraction produce oscillation of the basilar membrane. This does not occur symmetrically along the entire length of the basilar membrane.
What is the tonotopic map?
The basilar membrane is not uniform in structure along its length. The end of the basilar membrane nearest the oval and round windows is called the base. This portion of the membrane is small widthwise and rigid. The opposite end, or apical end, is larger widthwise and “floppy.” The intermediate basilar membrane has properties somewhere between those of the apical end and the base. As a result of these differences in structure, waves traveling down the basilar membrane will have a particular point at which their amplitude is greatest, depending on the frequency of the wave. The base of the basilar membrane vibrates best (has the greatest amplitude) with high frequencies. The apical end vibrates best with low frequencies. This variation in response to different frequencies down the length of the basilar membrane creates a tonotopic map of sound within the cochlea.
Why do hair cells located along the length of the BM respond maximally to different frequencies?
The IHC move in response to movement of the basilar membrane. Because portions of the basilar membrane are vibrated maximally at certain frequencies, hair cells also respond to different frequencies based on their lengthwise position in the basilar membrane.
How does the IHC respond to bending of the stereocilia?
1. A sound wave selectively vibrates a portion of the basilar membrane according to differences in the properties of the membrane (the tonotopic map).
2. This wave moves the stereocilia situated on that portion of the membrane.
3. If the waves move the stereocilia in the direction of the longest stereocilia, the tip links will pull open the NSC channels.
4. K+ will rush into the NSC channels and the cell depolarizes.
5. The opposite happens if the wave moves the stereocilia in the other direction: NSC channels close and the cell hyperpolarizes
What are the properties of the transduction channels located at the tips of the stereocilia?
•Bathed in endolymph (K+ rich)
•Channels connected to shank of neighboring, longer stereocilia by “tip links”
What is the role of the endocochlear potential in auditory transduction?
The endocochlear potential is around +80 mV. It is the potential difference between the fluid of the endolymph and perilymph. The endolymph is the fluid of the scala media and is rich in K+. Perilymph is the fluid of the scala vestibuli and scala tympani and it is low in K+. Combined with the potential difference across the hair cell membrane, the endocochlear potential contributes to the driving force for K+ movement into the hair cell and subsequent depolarization. Without this potential, the driving force would be greatly diminished, K+ would not rush into the hair cell when the NSC channels were opened, there would be no depolarization and sensorineural deafness would result. This is the basis for most cases of congenital deafness.
Understand the role of OHCs in the cochlear amplifier. Know that OHCs are of clinical significance because of their susceptibility to damage by ototoxic antibiotics and prolonged exposure to loud sounds.
Cochlear amplifier: the mechanical amplification of the displacement of the BM by OHCs. The OHC enhances the BM movement in a frequency-dependent manner, resulting in a larger and sharper response of the BM to pure tone sounds (like do – ray – me).
•Efferent innervation from the central auditory system act upon OHCs to amplify BM movements.
•OHCs = electromotile ==> respond to changes in voltage w/ change in length.
•Prestin: motor protein that causes the change in hair cell length due to its voltage sensitive nature during the response to sound.
•Change in length of OHC pulls the BM toward OR away from the tectorial membrane, and thus changes the mechanical frequency selectivity of the BM.
Understand the response of spiral ganglion cells to sounds of different frequencies.
Describe the anatomical pathway for the auditory system in the brainstem, diencephalons and cerebral cortex. Where
do axons decussate in this pathway?
1) ANFs with cell bodies in the spiral ganglion relay signals from the cochlear hair cells to the brain. Axons of these ANFs form the auditory portion of the VIIIth nerve
2) Enter brainstem and immediately bifurcate – both found on dorsal and lateral aspects of inf cerebellar peduncle. One branch innervates the ventral portion of the cochlear nucleus (VCN). The other is the dorsal cochlear nucleus (DCN).
3) Some axons from cells in cochlear nucleus cross the midline to opposite side of brain in dorsal acoustic stria (from DCN) and trapezoid body (from VCN).
4) Tracts regroup as lateral lemniscus and ascend to inferior colliculus of midbrain.
5) Many axons terminate in nuclear complexes in the pons: Superior olivary complex and Nuclei of the lateral lemniscus
6) The Medial olivocochlear neurons (MOC) of the superior olivary complex feed back to the OHCs.
7) Projections from these stations join the lateral lemniscus en-route to the inferior colliculus.
8) Other axons from cells in cochlear nucleus join lateral lemniscus ipsilaterally and terminate in the inf. colliculus.
What is the clinical significance for decussation of axons in the brainstem in the auditory pathway?
Inferior colliculus receives both projections from the cochlear nuclei and multisynaptic input from the pontine nuclei (of sup. olivary complex). This is an obligatory relay and integration center for ascending auditory info
• Fibers then project to the ipsilateral medial geniculate in the thalamus or to the contralateral inferior colliculus and medial geniculate.
• Medial geniculate sends projections to primary auditory cortex (A1) part of the superior temporal gyrus via auditory radiations.
• On the same side, specific regions of auditory cortex are linked by association fibers.
• For regions on opposite sides of the brain, these are linked by the antierior commisure.
• The cortical surface actually contains the cochleo-topic (tonotopic) map.
Understand the two main mechanisms involved in sound localization. When is a difference in intensity used by the
auditory system to localize sound? When is time of arrival used?
Interaural time differences: (ITDs) when is time of arrival is used, from the ears physically separating the head in space, the direction-dependent differences in path lengths that sound must travel to reach each ear from the source will generate different times of arrival of the sound at the two ears (ITDs).
Intraural level differences: the head results in the ILD cue to sound location. For sounds of high frequency above 1.5kHz. Resulting sound arrives at ear farthest from source that is effective attenuated creating direction-depedent differences in amplitudes of sounds reaching the two ears. ILDs are for high frequency sounds!
What is the role for the auditory cortex? Understand that auditory cortex is arranged in a tonotopic map.
What is sound intensity?
Intensity corresponds to the pressure compressing the air of the sound wave. It is measured in dB SPL. Normal hearing spans 7-8 orders of magnitude. A given intensity of sound is converted to units of dB SPL using a standard reference pressure (P2) of 20x10-6 Newton/m2 and the following equation. The standard reference pressure is based on normal auditory threshold at 1000 Hz.
dB SPL = 20log(P1/P2)
What is sound Frequency?
Frequency is the number of times per second a sound wave reaches its peak compression (maximum intensity) or rarefraction (minimum intensity). Frequency is measured in Hz (s-1). The normal range of frequencies for human hearing is 20 Hz to 14 kHz
Understand the role of the middle ear in overcoming impedance
mismatch between the air-filled middle ear and fluid-filled inner ear.
The ossicle bones form a mechanical connection between the tympanic membrane of the middle ear and the oval window. The oval window is the connection between the inner and middle ear. The bones amplify the intensity of the sound wave by appx. 28 dB in order to compensate for the intensity loss to impedance mismatch.
This amplification is the result of two factors:
1.The pressure sound wave from the tympanic membrane is concentrated onto the much smaller surface of the stapes footplate. Pressure = F/A, so as area goes down pressure (which correlates with intensity) increases.
2.The middle bones are oriented like a lever, which increases the force transmitted to the inner ear.
What are the three ossicles?
The three ossicles (from medial to lateral) are the stapes, incus and malleus.
Parts of the cochlea
The cochlea is divided into three fluid-filled compartments: the scala media, scala tympani and scala vestibuli. The scala media and tympani are divided by the basilar membrane.
A collection of inner hair cells called the organ of Corti sits on top of the basilar membrane within the scala media of the cochlea. The organ of Corti houses the inner hair cells.