Lecture 11: The Auditory System

scala vestibuli (red), scala media (blue), scala tympani (green)

What three fluid-filled channels are located within the cochlea (shown uncoiled in the figure)?

outer (efferents; Notice the characteristic V-shape of outer cochlear hair cells arranged in rows of three.)

What type of cochlear hair cells are depicted in the figure?

A (Hearing aids artificially boost sound pressure levels. This is helpful for conductive hearing loss, in which damage to the external or middle ear causes a decrease in sound amplification. However, it will not aid in sensorineural hearing loss because no amount of mechanical amplification can compensate for the inability to generate or convey a neural impulse from the cochlea.)

What type of hearing loss can be partially overcome through the use of hearing aids? A.) Conductive B.) Sensorineural C.) Both D.) Neither

Wernicke's area

What region important in comprehending speech is just posterior to the primary auditory cortex?

lateral superior olive (LSO)

What region of the brain contains the neural circuity to compute interaural intensity differences?

medial superior olive (MSO)

What region of the brain contains the neural circuity to compute interaural time differences?

cochlea (Latin for "snail"; 10 mm wide, 35 mm long uncoiled)

What structure of the inner ear (depicted in figure) is responsible for transforming the energy from sonically generated pressure waves into neural impulses?

sound

In physical terms, ______ refers to pressure waves generated by vibrating air molecules.

parallel

As in the visual system, the ascending auditory system is organized in ______. This arrangement becomes evident as soon as the auditory nerve enters the brainstem, where it branches to innervate the three divisions of the cochlear nucleus.

medial nucleus of trapezoid body (MNTB)

Excitatory axons project directly from the ipsilateral anteroventral cochelar nucleus to the LSO. Where does the LSO receive inhibitory input from the contralateral ear?

tuning curves

Because a single auditory nerve fiber innervates only a single inner hair cell (although several or more auditory nerve fibers synapse on a single hair cell), each auditory nerve fiber transmits information about only a small part of the audible frequency spectrum. The properties of specific fibers can be seen in electrophysiological recordings of responses to sound, creating threshold functions called ______ ______ (superimposed in figure from multiple auditory nerves).

phase locking

Because hair cells release transmitter only when depolarized, auditory nerve fibers fire only during the positive phases of low-frequency (< 3 kHz) sounds. What term is used to describe this response?

biphasic, receptor potential, 3

Because some transduction channels are open at rest, the receptor potential is ______: movement toward the tallest stereocilia depolarizes the cell, whereas movement in the opposite direction leads to hyperpolarization. This situation allows the hair cell to generate a sinusoidal ______ ______ in response to a sinusoidal stimulus, thus preserving the temporal information present in the original signal, up to frequencies of around ______ (#) kHz.

oval window, round window

Both the ______ ______ (at the base of the stapes) and the ______ ______ (another region where the bone is absent surrounding the cochlea) are located at the basal end of the cochlea.

VIIIth nerve, tectorial membrane

Current evidence suggests that the outer hair cells play a role in: 1.) helping sharpen ______ ______ tuning curves 2.) contracting and expanding in response to small electrical currents thus stiffening the ______ ______.

electrode array, auditory nerve (which must be intact for CI to function)

Each cochlear implant has a(n) ______ ______ (1), typically a series of tiny metal rings that electrically stimulate ______ ______ (2) endings to create sound sensations. Thus CIs can only be used for those with sensorineural hearing loss that involves hair cell damage.

loudness, pitch

For a human listener, the amplitude and frequency of a sound pressure change at the ear roughly correspond to that listener's experience of ______ and ______, respectively.

acoustical trauma, infection, ototoxic, presbycusis

Hearing loss can result from any of the following reasons: • ______ ______, such as loud sounds (especially around 3 kHz, which is amplified best by the auditory meatus). • ______ in the inner ear (e.g. scarlet fever, meningitis, measles). • ______ drugs, such as hair cell damaging antibiotics like gentamycin. • ______, or old age.

axons that project from the anteroventral cochlear nucleus vary systematically in length

How do coincidence detectors compensate for sounds arriving at slightly different times at the two ears so that the resultant neural impulses arrive at a particular MSO neuron simultaneously?

kinocilium (In mammalian cochlear hair cells the kinocilium disappears shortly after birth.)

How does the vestibular hair bundle shown in figure A differ from that of cochlear hair cells found in mammals?

1

How many row(s) of inner cochlear hair cells exist in humans?

3

How many row(s) of outer cochlear hair cells exist in humans?

malleus, incus, stapes

List the ossicles of the middle ear.

K+, Ca2+

Mechanoelectrical transduction of sound waves is mediated by hair cells. When the hair bundle is deflected toward the kinocilium, cation-selective channels open near the tips of the stereocilia, allowing ______ to flow into the hair cell. The resulting depolarization opens voltage-gated ______ channels in the cell soma, allowing the release of neurotransmitter onto the the nerve endings of the auditory nerve.

fundamental frequency (determines pitch), harmonics (contribute to timbre)

Most natural sounds are complex, meaning they differ from the pure tones or clicks that are frequently used in neurophysiological studies of the auditory system. The figure depicts the communication call of a mustached bat. Each syllable has its own ______ ______ (fa0, fb0) and multiple _______.

coincidence detectors

Neurons of the medial superior olive (MSO), more specifically the anteroventral cochlear nuclei, work as ______ ______ that respond best when both excitatory signals arrive at the same time.

time

Phase differences have corresponding ______ differences. This concept is important in appreciating how the auditory system locates sounds in space.

interaural time differences (ITDs)

Phase locking provides temporal information from the two ears to neural centers that compare ______ ______ ______ for frequencies below 3 kHz. This comparison provides a critical cue for sound localization and the perception of auditory "space."

lower, increased

Sounds arising closer to the listener's midline will elicit (LOWER / HIGHER) firing rates in the ipsilateral LSO because of (DECREASED / INCREASED) inhibition arising from the contralateral MNTB.

higher, decreased

Sounds arising directly lateral to the listener will elicit (LOWER / HIGHER) firing rates in the ipsilateral LSO because of (DECREASED / INCREASED) inhibition arising from the contralateral MNTB.

stria vascularis

The K⁺-rich, Na⁺-poor endolymph of the scala media is produced by dedicated ion-pumping cells in the ______ ______.

belt areas (also accept secondary auditory cortex)

The ______ ______ (red) of the auditory cortex are less precise in their tonotopic organization compared to the primary auditory cortex (from which they receive input) as well as more diffuse input from areas of the medial geniculate nucleus.

primary auditory cortex (A1)

The ______ ______ ______ (blue) has a topographical map of the cochlea, just as V1 and S1 have topographical maps of their respective sensory epithelia.

pivot point, stereocilia

The ______ ______ of the basilar membrane is offset from that of the tectorial membrane so that when the basilar membrane is displaced by a sine wave, the tectorial membrane moves across the tops of the hair cells, bending the ______.

stereocilia, negative (-45 mV), efflux, influx, Ca2+-dependent K+, efflux

The ______ of the hair cells protrude into the endolymph (80 mV, high K⁺). When tip-links are stretched, K⁺ rushes into the hair cell because its resting potential is (POSITIVE / NEGATIVE). Depolarization of the hair cell soma opens K⁺ (INFLUX / EFFLUX) and Ca²⁺ (INFLUX / EFFLUX) channels. Finally, _______ channels open and K⁺ (INFLUX / EFFLUX) into the perilymph leads to fast repolarization. NOTE: Blanks may contain more than one word.

low, wider, more

The apex of the basilar membrane (near helicotrema) is "tuned" for (LOW / HIGH) frequencies. This region of the membrane is (WIDER / NARROWER) and (LESS / MORE) flexible.

high, narrower, less

The base of the basilar membrane (near oval window) is "tuned" for (LOW / HIGH) frequencies. This region of the membrane is (WIDER / NARROWER) and (LESS / MORE) flexible.

helicotrema, perilymph, round window

The cochlear partition does not extend all the way to the apical end of the cochlea; instead, an opening, known as the ______, joins the scala vestibuli to the scala tympani, allowing their fluid, known as ______, to mix. One consequence of this structural arrangement is that inward movement of the oval window displaces the fluid of the inner ear, causing the ______ ______ to bulge out slightly and deforming the cochlear partition.

endocochlear potential

The compartment containing endolymph is about 80 mV more positive than the perilymph compartment, a difference that is referred to as the ______ ______.

auditory meatus, 3 kHz

The configuration of the _______ _______ allows it to selectively amplify up to 30-100X for frequencies around ______, which is the natural resonance and energy of human speech.

pinna, concha, auditory meatus, tympanic membrane

The external ear, which consists of the ______ (1), _______ (2), and ______ ______ (3), gathers sound energy and focuses it on the eardrum, or ______ ______ (4).

interaural intensity differences (IIDs)

The head produces an acoustic shadow (a sound coming from a source located to one side of the head will be louder at the ear nearest the sound source), creating ______ ______ ______, that are critical for sound localization.

ipsilateral, contralateral

The lateral dendrites of the MSO receive input from the _______ anteroventral cochlear nucleus, and the medial dendrites receive input from the ______ anteroventral cochlear nucleus. Both inputs are excitatory.

bipolar dendrites, anteroventral cochlear nuclei

The medial superior olive (MSO) contains cells with ______ ______ that extend both medially and laterally and receive excitatory inputs from the ______ ______ ______.

200

The middle ear ensures transmission of sound energy across the air-fluid boundary by boosting the pressure measured at the tympanic membrane almost ______(#)-fold by the time it reaches the inner ear.

basilar, tectorial

The organ of Corti (black) sits on a thin, flexible membrane called the ______ membrane (red) and under a relatively immobile membrane called the ______ membrane (blue).

Organ of Corti

The scala media houses the actual hearing apparatus, called the ______ ______ ______, composed of hair cell (auditory) receptors and support cells.

assymetrical

There is a(n) ______ representation of sound detection in the brain. For example, speech is mostly localized to the left side of the human brain, whereas music is on the right.

False (The tip links provide the means for rapidly translating hair bundle movement into a receptor potential. Displacement of the hair bundle PARALLEL to the plane of bilateral symmetry in the direction of the tallest stereocilia stretches the tip links, directly opening cation-selective channels located at the end of the link and depolarizing the hair cell. Orthogonal (90°) displacement will not stretch the tip links and thus does not result in a receptor potential.)

True or False: Displacement of hair bundles orthogonal to the plane of bilateral symmetry in the direction of the tallest stereocilia will result in a receptor potential.

True (This is because most transduction channels are closed while "at rest.")

True or False: Equivalent displacements of hair bundles generate larger depolarizing responses than hyperpolarizing responses.

False (Hair cells can still signal at frequencies above 3 kHz, although without preserving the exact temporal structure of the stimulus. The tonotopically organized basilar membrane provides an alternative to temporal coding referred to as a "labeled-line" coding mechanism.)

True or False: Hair cells cannot signal at frequencies above 3 kHz.

True (This occurs even at the level of the brainstem. Thus, damage to central auditory structures is almost NEVER manifested as a monaural hearing loss, or damage to sound arriving at one ear.)

True or False: Information from each ear reaches both sides of the auditory system.

False (Outer hair cells are efferent; therefore they travel away from the brain, specifically the superior olivary complex. This statement is true for the case of INNER hair cells: 95% of the fibers of the auditory nerve project to the brain.)

True or False: Outer hair cells comprise of 95% of the fibers of the auditory nerve that project to the brain.

False (The inside of the hair cell is about 45 mV more negative than the perilymph and about 125 mV more negative than the endolymph.)

True or False: The inside of the hair cell is more negative than the endolymph (+80 mV) but more positive than the perilymph (0 mV).

tympanic membrane, oval window, ossicles

Two mechanical processes occur within the middle ear to boost the pressure by 200X. 1.) Focusing the force impinging on the relatively large-diameter of the ______ ______ onto the much smaller-diameter ______ ______, the site where the bones of the middle ear contact the inner ear. 2.) Lever action of three small, interconnected middle ear bones, or ______.

shearing, depolarization, hyperpolarization

Vertical movement of the basilar membrane is translated into a(n) ______ force that bends the stereocilia of the hair cells. Bending towards the kinocilia (upward phase) causes ______, while the opposite (downward phase) causes ______.

interaural time differences (ITDs), interaural intensity differences (IIDs)

What are the two most important localization cues that result from comparing signals from both ears?

tubulin (characteristic "9+2" structure), actin

What cytoskeletal protein makes up the kinocilium? the stereocilia? HINT: The cross section of the vestibular hair bundle reveals an important structural element that is characteristic of a certain protein filament.

tip links

What fine filamentous structures connect adjacent stereocilia and are believed to be mechanical linkages that open and close transduction channels?

perilymph, scala vestibuli, scala tympani (basal end of the hair cell body also exposed to this fluid)

What fluid of the inner ear is K⁺-poor and Na⁺-rich (as in most extracellular fluids)? Which cochlear canal(s) does it bathe?

endolymph, scala media (apical end of the hair cell, including the stereocilia, protrudes into this canal)

What fluid of the inner ear is K⁺-rich and Na⁺-poor? Which cochlear canal(s) does it bathe?

waveform, phase, amplitude (dB), frequency (Hz)

What four quantities define a sound wave?

K+ (This enables the hair cell's K+ gradient to be largely maintained by passive ion movement alone.)

What ion serves to both depolarize and repolarize hair cells?

700 ms

What is the maximum interaural time difference (ITD) detected between the two ears?

10 ms, 1 degree

What is the minimum interaural time difference (ITD) detected between the two ears? What degree difference in the angling of one ear towards the sound causes this ITD?

match low-impedance air sounds to high-impedance fluid of the inner ear (NOTE in this context, impedance describes a medium's resistance to movement)

What is the role of the bones in the middle ear?

20 Hz to 20 kHz

What is the standard range of audible frequencies for humans?

Barn owl

What organism has very sensitive coincidence detectors (phase-locking up to 9 kHz) important for locating and capturing prey?

temporal lobe

Where is the primary auditory cortex located?

inner

Which cochlear hair cells act as the actual sensory receptors (afferents)?

outer

Which cochlear hair cells form efferent axons from the superior olivary complex?

scala media

Which tube(s) in the cochlea is/are filled with endolymph?

scala vestibuli, scala tympani

Which tube(s) in the cochlea is/are filled with perilymph?

Conductive

Which type of hearing loss involves damage to the external or middle ear?

Sensorineural

Which type of hearing loss involves damage to the hair cells of the inner ear or to the auditory nerve itself?

Georg von Bekesy, tonotopy

Who showed that a membrane that varies systematically in its width and flexibility vibrates maximally at different positions as a function of the stimulus frequency? What term is used to describe this organization of the human cochlea by which the place of max amplitude is frequency dependent?

respond to environmental conditions (e.g. in response to loud noises, an an autonomic reflex causes contraction of the middle ear to decrease amplification; another example is stretching of outer hair cells as they move with stimulation.)

Why does the ear possess efferent signals (outer hair cells) when it primarily acts as a detector?

apical (Auditory nerve fibers related to the apical end of the cochlea respond to low frequencies, and fibers that are related to the basal end respond to high frequencies.)

Would you expect the upper-most graph to represent a tuning curve from auditory nerve fibers innervating a single inner hair cell in the apical or basal cochlea?

Tones

______ correspond statistically to peaks of power in normalized human speech.

Sensorineural, cochlear implant

_______ hearing loss is best treated with a medical device known as a(n) _______ _______, which replaces the function of the damaged inner ear to provide sound signals to the brain.

passive

von Bekesy performed his studies on human cochleas taken from cadavers. What type of model did he employ?