Audition and vestibular system

Purves et al., Chapters 13 and 14 respetively


(not all of this is in the book)
Intensity dB = 20 log (pressure 1/pressure2)
standard is 0.0002 dynes/cm2
Threshold amplitude of vibration is 10-11 m (10 pm)

Fig. 13.1 p. 278
waves of compressions and rarefactions of air (must have medium) described by sine wave
Frequency Hz cycles per sec
vibration - 20 - 20,000 Hz, above which is ultrasound .
Audibility curve - Intensity [dB] vs log (freq) [Hz] very sensitive


Fig. 13.3 p. 282
Ear structure
pinna, eardrum=tympanic membrane, ossicles, cochlea, part of nerve VIII = cochlear nerve
hammer, anvil, stirrup=malleus, incus, stapes - to match impedance of air -> fluid
Eustachian tube
oval window is "inner ear drum"
20:1 "amplification" tympanic to oval
cochlea near vestibular apparatus

Fig. 13.4 p. 285
higher magnification, most importantly basilar and tectorial membrane
also inner hair cells (with afferent neurons) and outer hair cells with efferent axons
possibly outer hair cells do some motor thing to sharpen frequency discrimination

Frequency discrimination


At about 1000 Hz, you can tell the difference of a few Hz. This is explained by Helmholtz's place theory as modified by lateral inhibition as described in Bekesy's (1961) Nobel Prize winning work. You can get the audio oscillator calibrated to be slightly different from a tuning fork by listening for beats. At low frequencies, frequency discrimination is better explained by Rutherford's telephone theory. Here, frequencies to both ears can cause neural impulses that stay true to the frequency so that beats can be from neural comparison from the two ears.


Two tuning forks that are near but not identical give beats if you listen to them simultaneously. Beats arise from the constructive and destructive interference of sound waves at the physical level. There would be one beat per second if the two tuning forks differed by 1 Hz. Then, if the tuning forks differ by a few Hz, you can hear the difference in pitch if you listen to one then another.

For low frequencies:

Landmark paper
G. Oster , Auditory Beats in the Brain, Scientific American, Vol 229, October 1973, pp. 94-102
Binaural beats

Back to Lecture.

Fig. 13.5 p. 286
Vibration of basilar membrane is mapped by tonotopy
fluid vibration at oval window through helicotrema
released at round window
Frequency discrimination is mapped at high frequencies this way
Frequency discrimination very good - 2 Hz at 1000 Hz
Georg von Bekesy's data pertaining to Helmholtz's place (resonance) theory
1961 Nobel "physical mechanism of stimulation within the coclea"

Fig. 13.11 p. 293
"tuning curves" at different frequencies
for receptor is broad, while for higher order nerves, it is sharp
Lateral inhibition in ascending path sharpens tuning curve
Basilar membrane - high vs low maps to "place" in cochlear nerve
- there is a frequency mapping on the cortex
tonotopy - in A1 = Brodman # 41

Fig. 13.11 p. 293
Frequency discrimination at low frequencies
there was another theory, Rutherford's "telephone" theory
phase-locking gives volley principle up to 4 kHz

Fig. 13.15 p. 298
map of cortex tonotopy

Auditory transduction

Fig. 13.4 p 285 (again)
Fig. 13.6 p. 287
hair cells on basilar and tectorial membranes
3,500 inner hair cells
many more outer hair cells
Bend as basilar membrane vibrates relative to tectorial membrane

Recent paper
IABelyantseva et al., Myosin-XVa is required for tip localization of whirlin and differential elongation of hair-cell stereocilia, Nature Cell Biol, 7, 148-156, 2005
Myosin-XVa is product of shaker2 gene
whirlin is product of whirler gene
mutants cause deafness and improper detection of head movement.
Stereocilia need to be different lengths, shaped like a staircase, deficient in mutant
Myosin transports whirlin to tips

Fig. 13.7 p. 288
EM. Note kinocilium vs stereocilia (B) and tip links (D)

Fig. 13.8 p 288
Fig. 13.9 p. 289
kinocilium (real cilium, missing in post-natal human hair cells)
plus about 30 stereoocilia
mechanoreception assisted by tip links - depolarization if move toward kinocilium
hyperpolarize if in opposite direction
Threshold displacement is about 0.3 nm, electric potential in 10 micro seconds

Recent paper
SSidi et al., NompC TRP channel required for vertebrate sensory hair cell mechanotransduction, Science 301, 96-99, 2003.
TRP discussed n somatosensation. and smell and taste.
NompC=no mechanoreceptor potential (in Drosophila bristles) also in C. elegans.
In this paper, it is shown that this is a hair cell channel in zebrafish Danio rerio.

Fig. 13.10 p. 290
perilymph is fluid of scala vestibuli and scala tympani is like CSF - bathes baso-lateral hair cell
High K+ in endolymph of scala media (bathing hairs)
stria vascularis (endothelium lining scala media) pumps ions to produce this unusual extracellular fluid
thus when channels open, K+ comes into cell

endocochlear potential endolymph 80 mV more + than perilymph


Fig. 13.12 p. 294
Very complex- but eye does have synapses in eye (retina), while ear does not
Auditory nerve to dorsal and ventral cochlear nucleus - no crossing
Then connect in superior olivary nucleus ipsi- & contra- lateral
whose postsynaptic cells, in turn, go to inf. colliculus
Postsynaptics of inferior colliculus go to Medial Geniculate Body
Medial Geniculate to ipsilateral auditory cortex

Fig. 13.15 p. 298
various parts of auditory cortex

Auditory localization

difference in time of arrival and intensity (in big headed animals) [human 700 micro sceond difference]
(speed of sound 1087 ft (331 m) / s in air)
Localization up and down does not rely on 2 ears, may relate to pinna
small-headed animals are extraordinary

Fig. 13.13 p 295
medial superior olivary nucleus important for coincidence detection of time of arrival
phase locking important in input - barn owls good at this

Fig. 13.14 p 296
lateral superior olive (and median nucleus of the trapezoid body) calculates on the basis of intensity difference


bat echolocation biosonar
bat nocturnal, predator, insect "flickers"
moths avoid bats
medial geniculate important


Box A hearing loss p 279
conduction deafness, nerve deafness
also tinnitus - ringing in the ears

Vestibular sense
lecture is not as detailed as text.

Fig. 14.1 p. 304
utricle and sacculus linear motions
3 semicircular canals - rotations

Fig. 14.3 p. 308

Fig. 14.4 p. 308
stones (otoconia) provide mass for bending in utricle and sacculus
striola divides hair cells with differing polarities

Fig. 14.7 p. 311
Ampulla and cupula displaced as semicircular canal fluid is displaced

Fig. 14.10 p. 313
circuit for eye movements
involving Scarpa's ganglion, vestibular nucleus, abducens (VI) nucleus and oculomotor (III) nucleus

Box C pp. 314-315
neurology done by irrigating one ear with cold water

Fig. 14.11 p. 318
vestibulo spinal control from vestibular nucleus (integrates with cerebellar input) to lateral vestibulospinal tract and medial longitudinal fasciculus

Fig. 14.12 p 319
also projection to integrate with somatosensory and muscle spindle senses

Exam questions from 2005 - 2011 relating to this outline

The human audibility curve plots Y as a function of X. Answer either what is on the Y OR X axes.

ordinate is threshold plotted in intensity, plotted logarythmically, abscissa is frequency in Hz where log plotting is for convenience

What is the relevance of the value 0.0002 dynes/cm2 in hearing?

that is the denominator, the pressure standard in defining dB

What is the relevance of the value 20,000 Hz in human and animal hearing?

it is the upper limit of frequency for hearing by young people, above that is ultrasound that dogs and bats can hear

Pressure is passed from the scala vestibuli through the helicotrema (cochlear apex) to the scala tympani and released at the round window. The pressure is delivered to the cochlea by (name the third [final] bone OR the "membrane" [inner ear drum]).

stirrup=stapes, oval window

Cells in the auditory cortex are responsive to a narrow range of frequencies. By contrast, the tuning curve for a cell in cranial nerve VIII is wider. Bekesy, the Nobel Prize winner, argued that this was because of what type of processing?

lateral inhibition

A figure was shown to you with hair cells and the axons they are connected to. On the basis of how that figure was labeled, it was stated that outer hair cells do not function as receptors. What was it about the axons connected to outer hair cells that raised this issue?

those axons were called efferent

Tip links assist channels (answer either) (1) for what ion? Or (2) located on what specific subcellular component?

K+, stereocilium

Regarding the inferior colliculus, answer either (1) It's cells synapse where? Or (2) It receives input from what nucleus?

auditory cortes, cochlear nucleus and olive

How do insectivorous bats use sound to catch their prey?

echolocation (sonar)

In addition to input from vestibular apparatus, the vestibular nuclei that form the vestibulo-spinal tracts receive input from what major brain location?


The human audibility curve is the threshold for hearing as a function of frequency. The Y axis could be plotted as intensity in dynes per square centimeter (plotted logarithmically from 0.0001 to 100) or, alternatively, what more customary term for intensity (from 0 to 120)?


Answer either: (1) what the tonotopic organization of the primary auditory cortex looks like; or (2) how would you demonstrate the tonotopic organization of the primary auditory cortex?

cells that respond to low frequency at one end, high at the other, record from places and run through a range of stimulus frequencies and see which frequency each place responds to best

What causes the channels that mediate hearing to open?

mechanosensitive, would be membrane deformation, assisted by tip links

While jogging, you trip and start to fall. Name a sensory organ or a spinal pathway involved in the elicited responses of your spinal motor neurons.

vestibular apparatus, vestibulospinal tract

Two tuning forks give 5 beats per second if you listen with one ear. Under what circumstances would you hear beats if you held each near opposite ears? (Assume you cannot hear either with the ear on the other side.) Alternatively answer this: What theory does this phenomenon of binaural beats support?

low frequency, Rutherford telephone theory

Imagine that you have come up with a nearly magical technique to record and graph the responses of an auditory hair cell as a function of frequency. How would that graph compare with a similar presentation of the tuning curve of a cell in the primary auditory cortex?

much wider on the frequency axis

Why is it important that auditory receptor cells have voltage-gated calcium channels?

for release of transmitter vesicles

What is the difference between a kinocilium and stereocilia?

kino is real cilium with 9 plus 2, stereocilia are not

Where (if at all) does the auditory input (from one ear) become bilateral?

superior olive

Otoconia are used Answer either (1) in what structures? Or (2) to stimulate what kind of cells?

utricle and saccule, hair cells

Information from the vestibular system integrates with information (from what major brain structure?) for the vestibulospinal tract.


What does the striola divide?

two sides of utricle or saccule with mirror imazge hair cells

A tip link helps in the opening of a channel to what ion?


Bekesy won a Nobel Prize for showing that Helmholtz's place theory was fundamentally correct but that the localization was much more crude than Helmholtz envisioned. How did
he rationalize this discrepency to account for very narrow tuning curves and high resolution localization higher up?

lateral inhibition

What is unusual about the axons connected to the outer hair cells (in contrast with those for the inner hair cells)?

they are efferent

The medial lemniscus is part of what system?


What kind of stimulus do bats use to find moths at night?


The cupula and the ampulla are part of what system?

vestibular (semicircular canals)

How do stereocilia differ from the kinocilium structurally?

they lack 9+2 microtubules

For what aspect of hearing is the speed of sound relevant?

auditory localization

A pathway from vestibular nuclei through nuclei for cranial nerves III and VI is important for what behavior?

eye movement, vestibular ocular reflex

When the audio oscillator and tuning fork are a few Hz apart at about 1000 Hz, what explains beats that you hear with one ear?

constructive and destructive interference of sound waves

Endolymph and perilymph are both extracellular fluid compartments. Why is there an 80 mV potential where the endolymph is more positive than the perilymph?

because of the high potassium ion concentration in the endolymph

Depolarization of the inner hair cell causes entry of calcium ions. What effect do these calcium ions have?

release of synaptic transmitter vesicles

What is the difference in localization of vibrations for low vs high frequencies in the basilar membrane?

low toward helicotrema, higher toward stapes

Neurons of the olive were diagrammed in your book involved in sound localization. Why are there no such neurons in the spiral ganglion or in the cochlear nuclei?

they do not have inputs from both ears

Between the inferior colliculus and the auditory cortex is a synapse in what specific relay station?

medial geniculate of thalamus

Hair cells are located only in a bulge, not throughout the semicircular canal. What is this bulge called?

ampulla where cupula resides

Name one of the two compartments connected by the helicotrema.

scala vestibuli and scala tympani

Describe the results leading to the conclusion that there is tonotopic organization of the primary auditory cortex.

rostral part responds to low freq & caudal to high

Mutants of the whirler gene cause improper detection of head orientation. Why is it no surprise that the animals are also deaf?

affects stereocilia, and hair cells are used in hearing and balance

There is an 80 mV endocochlear potential between the endolymph and the perilymph. Why?

endolymph has high K+, perilymph low

In terms of ions or potential, what causes release of transmitter vesicles in auditory receptor cells?

influx of K+ causes depolarization causes transmitter release

What is missing in the following list of sites for auditory synapses: (Cochlear nuclei, nucleus of lateral lemniscus, inferior colliculus, medial geniculate of the thalamus, and primary auditory cortex)?

superior olive

What notable control does the vestibular apparatus exert in addition to descending influences to the ventral horn of the spinal cord and input to the sensory cortex?

eye movements

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