Chapter 13 - The Auditory System Flashcards
The external ear consists of four structures. Which?
- Pinna
- Concha
- Auditory meatus
- Tympanic membrane
What is the most important function of the auditory meatus?
It selectively boosts the sound pressure 30- to 100-fold for frequencies around 3 kHz via passive resonance effects.
What is the major function of the middle ear?
The major function of the inner ear is to match relatively low-impedance airborne sounds to the higher-impedance fluid of the inner ear. The term “impedance” in this context describes a medium’s resistance to movement.
How much is the auditory signal strengthened in the middle ear?
The middle ear ensures transmission of the sound energy across the air-fluid boundary by boosting the pressure measured at the tympanic membrane almost 200-fold by the time it reaches the inner ear.
How does the middle ear achieve its pressure amplification?
Two mechanical processes:
- The first and major boost is achieved by focusing the force impinging on the relatively large-diameter tympanic membrane onto the much smaller-diameter oval window.
- A second and related process relies on the mechanical advantage gained by the lever action of the ossicles.
Mention a clinical test to determine whether hearing loss is due to conductive problems or to damage either to the hair cells of the inner ear or to the auditory nerve itself.
A Weber test uses a tuning fork placed against the scalp, which causes vibrations in the skull itself. The ossicles will conduct these vibrations onto the oval window despite tympanic membrane damage.
What is the oval window?
An area of the cochlea where the bone is absent, and in which the stapes projects the vibrations to the cochlea.
What is the round window?
An area of the cochlea where the bone is absent, and in which the sound vibrations exit the tube.
What is the cochlear partition?
A flexible structure that bisects the cochlea from its basal end almost to its apical end.
What are the names of the different fluid environments of the cochlea?
- Scala vestibuli
- Scala tympani
- Scala media
What is the scala vestibuli?
The scala vestibuli is the fluid environment in which the sound waves first enter. It becomes the scala tympani at the apical end. It contains perilymph. (Low K+ and high Na+, 0mV)
What is the scala tympani?
The scala tympani is the fluid environment in which the sound waves travel from the apex of the cochlea toward the round window. The scala tympani contains perilymph. (Low K+ and high Na+, 0mV).
What is the scala media?
The scala media is the fluid environment within the cochlear partition. It contains endolymph. (High K+, +80mV)
Frequency tuning within the inner ear is attributable to…
It is attributable in part to the geometry of the basilar membrane, which is wider and more flexible at the apical end and narrower and stiffer at the basal end.
Two membranes are embedded in the cochlear partition. Which?
- Basilar membrane
2. Tectorial membrane
The points responding to high frequencies are at the … of the basilar membrane.
base.
The points responding to low frequencies are at the … o the basilar membrane.
Apex
The cochlear hair cells are arranged in rows. How?
One row of inner hair cells and three rows of outer hair cells.
Which row of hair cells are the actual sensory receptors?
The inner hair cells are the sensory receptors, and 95% of the fibers of the auditory nerve that project to the brain arise from this subpopulation.
Current evidence tells us what about the outer hair cells?
Current evidence suggests the the outer hair cells play an important role in modulating basilar membrane motions and the they function as an important component of the cochlear amplifier.
Describe the hair cell.
The hair cell is a flask-shaped epithelial cell named for the bundle of hairlike processes that protrude from its apical end into the scala media. Each hair bundle contains anywhere from 30 to a few hundred stereo cilia, with one larger kinocilium.
The hair cell has a resting potential of..
Between -45 to -60 mV relative to the fluid that bathes the basal end of the cell.
What happens in the hair cell when the hair bundle is displaced in the direction of the tallest stereocilium?
It causes more transduction channels to open, which causes an influx of K+ from the K+-rich endolymph.
Depolarisation in the hair cell due to K+-influx causes..
It causes the opening of voltage-gated calcium channels, and calcium ions causes the release of neurotransmitter onto the auditory nerve endings. It also causes K+ channels located on the basal surface of the hair cell to open onto the perilymph. This causes K+ efflux and thus repolarisation.
What is meant by: the receptor potential of the hair cell is biphasic?
It means that the hair cell can both become more likely to fire (depolarised) and less likely to fire (hyperpolarised), depending on the direction of the force. This is because some transduction channels are open at rest.
Von Békésy’s model of cochlear mechanics was a passive one (the cochlea was taken post mortem), resting on the premise that the basilar membrane acts like a series of linked resonators, much as a concatenated set of tuning forks. More recent studies indicate that normal hearing depends on the activity of a biological amplifier within the cochlea. The rational for this is based on three observations:
- The tuning of the auditory periphery, whether measured at the basilar membrane or recorded as the electrical activity of auditory nerve fibers, is too sharp to be explained by passive mechanisms alone.
- At very low sound intensities, the basilar membrane vibrates 100-fold more than would be predicted by linear extrapolation from the motion measured at high intensities.
- The ear can generate sounds under certain conditions.
How are outer hair cells related to the cochlear amplifier?
- The high sensitivity of auditory nerve tuning curves is lost when the outer hair cells are selectively inactivated.
- Mutant mice lacking inner hair cells, although deaf, nonetheless produce otoacoustic emissions.
- Isolated outer hair cells contract and expand in response to small electrical currents, thus providing a potential source of energy to drive an active process within the cochlea.
The auditory nerve exits the ear and enters which major division of the central nervous system?
The brainstem
The auditory nerve enters the brainstem, where it branches to innervate …
The three divisions of the cochlear nucleus.
The auditory nerve enters the brainstem, where it branches to innervate the three divisions of the cochlear nucleus. The neurons in these nuclei give rise to several different pathways. These pathways bring auditory information to the following structures (5):
- Superior olive (MSO) in the pons
- Nucleus of the lateral lemniscus of the Pons-midbrain junction
- Inferior colliculus of the Caudal midbrain
- The medial geniculate complex of the thalamus
- The primary auditory cortex in the dorsal temporal lobe.
The ability to localise the horizontal position of sound sources is computed by the ..
medial superior olive (MSO) in the pons.
How is horisontal sound localisation computed for sounds below 3kHz?
A given MSO neuron responds most strongly when the two inputs from the two ears arrive simultaneously, as occurs when the contralateral and ipsilateral inputs precisely compensate (via their different lengths) for differences in the time of arrival of a sound at the two ears. This is a model, not evidence.
How is horisontal sound localisation computed for sounds above 3kHz?
The lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB) are involved in computing the differences in intensity of the sound, as sounds of these frequencies in theory should be high enough so that the human head begins to act as an acoustical obstacle.
What is the role of the nuclei of the lateral lemniscus in auditory processing?
Their role is not yet known. Some cells in the lateral lemniscus nuclei signal the onset of sound, regardless of intensity or frequency.
What is the role of the inferior colliculus in auditory processing?
- Experiments in the barn owl, animal that is extraordinarily proficient at localising sounds, indicate that neurons in the inferior colliculus create a topographical representation of auditory space.
- Many neurons in the inferior colliculus respond only to frequency-modulated sounds, while others respond only to sounds of specific durations. It might therefore be important in our ability to process sounds with complex temporal patterns.
What is the role of the medial geniculate complex (MGC), in the thalamus, in auditory processing?
- In bats it is the first station in the auditory pathway that shows a selectivity for combinations of frequencies.
Divide the auditory cortex.
The primary auditory cortex (A1) and the belt areas.
How is sound information organised in, and what are the functions of the auditory cortex?
The primary auditory cortex, which is organised tonotopically, supports basic auditory functions, such as frequency discrimination and sound localisation, and also plays an important role in processing of intraspecific communication sounds. In non-human primates.
Which part of the auditory cortex do you know anything about..?
Wernickes area is considered a part of the belt areas of the auditory cortex.
