Audition and vestibular system

Purves et al., Chapters 13 and 14 respetively
Note that the Washington University Medical School's Neuroscience Tutorial has good coverage on this topic:
Auditory and Vestibular sense

Sound

(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
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

Ear

Fig. 13.3
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
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

Background.

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.

Demonstration.

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

Back to Lecture.

Fig. 13.5
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
"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
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
map of cortex tonotopy

Auditory transduction

Fig. 13.4
Fig. 13.6
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
EM. Note kinocilium vs stereocilia (B) and tip links (D)

Fig. 13.8
and
Fig. 13.9
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
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

Projection

Fig. 13.12
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
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
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
lateral superior olive (and median nucleus of the trapezoid body) calculates on the basis of intensity difference

Ultrasound

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

Disorders

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

Vestibular sense
lecture is not as detailed as text.

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

Fig. 14.3
stones

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

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

Fig. 14.10
circuit for eye movements
involving Scarpa's ganglion, vestibular nucleus, abducens (VI) nucleus and oculomotor (III) nucleus
Box C - neurology done by irrigating one ear with cold water

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

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

Exam questions from 2005 - 2007 relating to this outline

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?

K+

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?

auditory

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

ultrasound

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

Return to Syllabus

Return to Stark home page

This page was last updated 3/18/08