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Neuroscience > Hearing > Flashcards

Hearing Flashcards

1
Q

What is sound?

A

The oscillation of molecules or atoms in a compressible medium. the energy of the oscillations is transmitted as a longitudinal wave in which the medium is alternately compressed and rarefied, causing periodic variations in the pressure of the medium.

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2
Q

For a sine wave what is the period (T)?

A

Time taken for one complete cycle

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3
Q

What is the reciprocal of the period?

A

The frequency of the wave, the perceived pitch of the sound

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4
Q

What is the unit of frequency ?

A

The Hertz, one cycle per second

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5
Q

What is the amplitude of a sound wave?

A

The total change in pressure that occurs during a single cycle. Because of the huge range in sound wave amplitudes, P, it is expressed in a logarithmic scale as a ratio of a reference pressure P(ref). Sound pressure level = 20log^10/P(ref). Differences in sound pressure level are perceived as differences in loudness, which varies with frequency in a manner that is determined by the sensitivity of the ear.

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6
Q

What is P(ref)?

A

2 x 10^-5. A sound pressure which is at the threshold of human hearing.

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7
Q

What is the unit of SPL?

A

The decibel. Each 10-fold increase in the SPL is equivalent to 20 dB. sound pressure levels in excess of 100dB can result in damage to hearing and at 120dB auditory pain results

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8
Q

What is the frequency response of the human ear?

A

From 20 Hz to 20kHz optimally but rapidly narrows with age with most of the loss occuring at higher frequencies. By 50 years the upper limit averages 12kHz. The highest sensitivity (ability to detect quiet sounds) and auditory acuity (ability to discriminate between tones to between 1000 - 4000 Hz. Frequency range of human speech is 250 - 4000 Hz.

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9
Q

What is the function of the middle ear?

A

To convert pressure waves in the air to vibrations of perilymph in the inner ear

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10
Q

How does the middle ear fulfil its function?

A

Soundwaves pass along the external auditory meatus, striking the tympanic membrane which resonates faithfully in response. The ear drum is critically damped- stops vibrating the instant the sound ceases. Movement of the ear drum is transferred with an overall efficiency of 30 % to the fluid in the inner ear by a lever system, composed of three ear ossicles, lying in the tympanic cavity (middle ear)

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11
Q

What are the three ossicles of the middle ear?

A

The malleus (hammer) is fixed at its thin end (the handle) to the tympanic membrane. Its thick end articulates with the head of the incus (anvil) via a saddle-shaped joint. The long process of the incus makes a ball and socket joint with the head of the stapes (stirrup). The base of the stapes is attached by an annular ligament to the oval window (fenestra vestibuli).

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12
Q

What is the mechanism of the ear ossicles transferring the movement of the ear drum to the inner ear?

A

Malleus vibrates with the tympanic membrane. Inward movement locks the joint between the malleus and incus, driving the long process of the incus inward, pushing the stapes in the same direction to exert a pressure in the perilymph beyond the oval window. Outward movement reverses these motions.

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13
Q

What happens after the pressure wave is transmitted to the perilymph?

A

This pressure wave is transmitted through the perilymph to cause a compensatory bulge of the round window (fenestra cochleae).

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14
Q

What is the purpose of the auditory canal?

A

Connects the middle ear to the pharynx, which allows the air pressure to be equalised when changing altitudes.

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15
Q

Why is the pressure (force per unit area) at the oval window proportionally greater than at the tympanic membrane, why is this important?

A

The oval window is 20 times smaller than the tympanic membrane. This is important because perilymph is incompressible so must be driven to vibrate en masse, and this requires more force than it takes to transmit sound waves through the ear. It also results in an amplification of sound by about 20dB (4 fold increase in loudness by the middle ear)

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16
Q

What are the two middle ear muscles, what does their contraction cause?

A

Tensor tympani and stapedius. When they contract together the handle of the malleus and the tympanic membrane are pulled inwards and the base of the stapes is pulled away from the oval window. this reduces the sound transmission by about 20 dB, epecially for low frequencies. Reflex contraction in response to loud noises may prevent damage to the inner ear but the reaction time is 40- 60 ms, this tympanic reflex provides no protection against brief loud sounds.

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17
Q

The auditory part of the inner ear is the cochlea, what is its structure?

A

bony canal 3.5 cm long, which spirals two and three quarter turns around a central pillar, the modiolus. Within the cochlea lies a tubular extension of the membranous labyrinth, the cochlear duct, attached to the modiolus and the outer wall of the cochlea. This divides the cochlea into three compart- ments, the scala media which contains endolymph, and the scala vestibuli and the scala tympani which contain perilymph and are continuous with each other via a small gap known as the helicotrema situated at the apex of the cochlea where the cochlear duct ends blindly

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18
Q

What is the pathway of pressure waves generated at the oval window?

A

propagated through the scala ves- tibuli into the scala tympani and to the round window where the energy dissipates. During their passage the pressure waves cause oscillations of the basilar membrane, the floor of the scala media on which rests the sensory apparatus, the spiral organ of Corti

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19
Q

What is the structure of the spiral organ of corti?

A

narrow sheet of columnar epithelium running the length of the cochlear duct. The epithelium contains sensory hair cells resembling those in the vestib- ular apparatus.

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20
Q

What is the output of the single row of 3500 inner hair cells of the spiral organ?

A

form ribbon synapses with myelin- ated axons of large bipolar cells (type I) in the spiral ganglion of the cochlear nerve. Each inner hair cell is innervated by about 10 such axons, a large degree of divergence.

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21
Q

What is the output of the 3 rows of 12 000 outer hair cells of the spiral organ?

A

These are innervated by an unmyelinated axon from small bipolar cells (type II) in the spiral ganglion, each of which synapses with 10 hair cells, representing considerable convergence.

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22
Q

What is the structure of the cochlea hair cells?

A

lose their kinocilia during development and the tips of their tallest stereocilia are embedded in the overlying tectorial membrane, a matrix of mucopoly- saccharides and proteins.

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23
Q

What is the effect in the hair cells of oscillations of the basilar membrane in response to a sound stimulus?

A

They cause the basilar membrane to shear with respect to the tectorial membrane, bending the stereocilia first one way and then the other. This results in periodic depolarization and hyperpolar- ization of the hair cells, producing cyclical alterations in the tonic secretion of glutamate. The transduction mechanism for hair cells is like that of vestibular hair cells.

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24
Q

A sound stimulus causes a traveling wave to spread along the basilar membrane from base to the apex. How does this wave differ for high and low frequencies ?

A

High frequencies cause vibration at the basal end whereas low frequencies cause vibra- tion towards the apex.

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25
Q

What is the reason for the frequency sorting at the basilar membrane?

A

It is a result of the continuous variation in the width, mass, and stiffness of the basilar membrane along its length. The basilar membrane is narrow (50 μm) and stiff at the base, wider (500 μm) and less stiff at the apex. The relation- ship between frequency and length is logarithmic. At a given frequency, increasing the SPL increases the amplitude of the vibration and the length of basilar membrane responding.

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26
Q

What happens to outer hair cells when they depolarise?

A

Outer hair cells (OHCs) contract in a voltage-dependent manner. Depolarization causes them to shorten. The speed with which they change length is so fast that they are able to follow the high frequency voltage changes produced by sound stimuli

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27
Q

What is cochlear amplification?

A

When outer hair cells contract, they augment the vibrations of the basilar membrane. It probably contributes to the high sensitivity and fine tuning to frequency exhibited by the basilar membrane

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28
Q

How do aminoglycoside antibiotics affect hearing?

A

High sensitivity and fine tuning to frequency is lost due to the selective damage of outer hair cells

29
Q

What is the effect of cochlear amplification on the perilymph?

A

It causes vibrations of perilymph that are transmitted to the oval window across the middle ear in the “wrong” direction to the tympanic membrane which now acts as a loudspeaker producing inaudible otoacoustic emissions.

30
Q

What is the location of primary auditory afferents?

A

They have their cell bodies in the spiral ganglion located in the modio- lus. Their centrally directed axons project through the vestibulocochlear (VIII) nerve to synapse in the cochlear nuclei of the lower pons.

31
Q

Which cells provide the bulk of output from the cochlea?

A

about 30 000 type I afferents from inner hair cells

32
Q

Three quarters of the hair cells (the OHCs) send their output….

A

to only about 3000 type II afferents.

33
Q

How do auditory afferents fire ?

A

Tonically. In response to a tone, type I afferents show an increase in firing which adapts. When the sound stops, firing ceases for a brief period. Hence they exhibit both dynamic and static responses.

34
Q

What do responses of type I afferents plotted as tuning curves show?

A

they are sharply tuned at low sound pressure levels. The frequency to which the unit is most sensitive is the characteristic frequency (CF). At high SPLs the primary afferents respond to a much wider frequency range.

35
Q

The nervous system has two ways of encoding the frequencies of a sound. What does tonotopic mapping involve?

A

frequency maps to position along the cochlea, this occurs because afferents with successively lower CFs are found closer to its apex. This is most important for frequencies above 1–3 kHz.

36
Q

How are lower frequencies encoded?

A

coding uses the property that afferents fire with greatest probability during a particular phase of a sound wave, phase-locking. It is only necessary that an individual afferent fires during some cycles if a group of cells is involved. Moreover, if different groups phase-lock onto different parts of the cycle then a whole population of cells acting in concert can encode frequency.

37
Q

What is the range of sound levels detected by auditory afferents?

A

They have a dynamic range of about 30 dB, beyond which further increase in SPL has no additional effect. The full range of SPL (0–100 dB) is signaled by afferents with different sensitivities. Cells with the same CF may differ in threshold SPL by 70 dB. Afferent sensitivity correlates with its spontaneous rate (SR) of firing. High SR cells have the greatest sensitivity. Low SR cells are least sensitive and encode the frequencies of loud sounds.

38
Q

How are the sensitivities of auditory afferents modified

A

by efferents which have their cell bodies in the superior olivary complex (SOC) and form inhibitory synapses on OHCs. These neu- rons alter the gain of the cochlear amplifier, reducing the sensitivity of type I afferents as sound pressure levels increase.

39
Q

What is the pathway of the primary auditory afferents?

A

Primary auditory afferents bifurcate to terminate in both the ventral and dorsal cochlear nuclei. From the ventral cochlear nucleus axons run to the superior olivary nucleus (SON) on both sides and to the contralateral inferior colliculus via the trapezoid body (TB). The dorsal cochlear nucleus sends axons directly to the contralateral nucleus of the lateral lemniscus

40
Q

What is the function of the superior olivary complex in the central auditory pathways ?

A

compares input from the two ears to compute the whereabouts of a sound source. It projects to the nuclei of the lateral lemniscus.

41
Q

What is the pathway for conscious auditory perception from the lateral lemniscus?

A

The nucleus of the lateral lemniscus projects to the inferior colliculus (IC). The IC relays with the medial geniculate nucleus (MGN) of the thalamus which sends its output via the auditory radiation to the primary auditory cortex, AI (Brodmann’s areas 41 and 42) located in the superior temporal gyrus. The biggest auditory pathway is contralateral. However, reciprocal connections between the nuclei of the lateral lemniscus (via Probst’s commissure) and between the ICs, ensure extensive interactions between the input from both ears.

42
Q

Central auditory pathways process which three features of sound input in parallel?

A

tone, loud- ness, and timing. From the last two the brain calculates the location of the sound in space.

43
Q

Where does parallel processing begin?

A

The cochlear nuclei

44
Q

What do the bushy cells of the cochlear nuclei signal?

A

Reproduce the firing pattern of primary afferents faithfully, including phase-locking. Their output precisely signals the timing of sound, goes to the superior olivary nucleus

45
Q

What do stellate cells of the cochlear signal?

A

They have a much greater range than bushy cells and signal sound level

46
Q

What are the receptive fields of auditory neurons called?

A

Response maps. They are plotted in same way as the primary afferents tuning curve.

47
Q

How many classes of cells can be distinguished in the cochlear nuclei based on receptive fields?

A

5.

48
Q

What are the properties of the 5 classes of cells of the cochlear nuclei?

A

Type 1 cells have a purely excitable RF that precisely matches primary afferent tuning curves, but all other types have inhibitory responses which arise by lateral inhibition and which fine tune their frequency response. Type 4 cell axons are the main output of the dorsal cochlear nucleus.

49
Q

Where are tonotopic maps found?

A

Cochlear nuclei, superior olivary nucleus, inferior colliculus and the auditory cortex. Some structures have multiple maps

50
Q

How is the tonotopic mapp of the cochlear nuclei organised?

A

Divided into isofrequency strips, each containing cells with similar characteristic frequencies. Strips representing increasingly higher frequencies are found progressively more posteriorly.

51
Q

How is the tonotopic map of the auditory cortex organised?

A

In the primary auditory cortex, isofrequency columns running through the entire thickness of the cortex are arranged in isofrequency strips running mediolaterally, with low frequencies represented rostrally and high frequencies caudally. There are at least three other tonotopic maps in the auditory cortex. Adjacent maps are always mirror images of each other. In humans there is no great over-representation of particular frequencies. Some regions of auditory cortex, for example, secondary auditory cortex (AII), are less well tonotopically organized and contain cells that respond to a wider range of frequencies.

52
Q

Cells throughout the auditory system respond to differences in sound level and fall into two broad classes. What is the difference between these classes?

A

Monotonic cells have sigmoid plots of sound level against firing rates.
Non-monotonic cells are more finely tuned with a maximum firing rate at a characteristic
sound level. There are no maps of sound level in humans.

53
Q

What are the coordinates of a sound source in vertical and horizontal planes?

A

Elevation and azimuth respectively. Different mechanisms are involved in determining these two
coordinates.

54
Q

What acts as direction selective filters for locating sound elevation?

A

The pinna (outer ear) and ear canal. Sound waves enter the ear either directly or reflected by pinna and ear canal and hence will arrive at slightly different times at the ear drum. The delay times will depend on the elevation of the sound source and the peculiarities of the external auditory system.

55
Q

What is binaural sound localisation?

A

comparing input into both ears to localise sound.

56
Q

What are the two methods that the superior olivary nucleus uses to localise sound in the horizontal plane?

A

Interaural level differences (ILDs) and Interaural time differences (ITDs).

57
Q

How are interaural level differences produced?

A

If the head is orientated so that one ear is closer
to the sound source, then the head forms a shadow which reduces the sound level
entering the other ear. ILDs are unambiguous for frequencies > 1600 Hz for which the dimensions of the head are greater than the length of the sound waves. ILDs as low as one dB can be detected.

58
Q

How does the superior olivary nucleus detect interaural level differences?

A

Neurons of the lateral superior olivary nucleus (LSO) have a tonotopic map restricted largely to high frequency input. These cells receive inputs from both ipsilateral and contralateral cochlear nuclei. However, the contralateral route is by way of a glycinergic inhibitory neuron. Equal sound level in both ears causes overall inhibition of the LSO neuron and increasing the sound level in the contralateral only serves to augment the inhibition. However, increased sound level to the ipsilateral ear causes LSO firing. Maximum firing rate is seen when ILD is 2 dB or more. Corresponding cells in the opposite LSO will show reverse responses to the same sound.

59
Q

Where does the lateral superior olivary nucleus project to?

A

ventromedial part of the IC central nucleus. The IC connects extensively with the deep layers of the superior colliculus to form an auditory space map in register with the retinotopic map.

60
Q

What do the superior colliculus connections with the superior olivary nucleus allow it to do?

A

the superior colliculus can be implicated in the auditory reflexes organizing gaze and head rotation towards the sound source.

61
Q

How are interaural time differences produced?

A

A sound wave enters the closer ear slightly earlier
than the further one. For low frequencies (< 800 Hz) this results in a time delay less than one period which is analyzed by neurons capable of phase-locking. At higher frequencies input into the furthest ear is delayed by more than a single period, and this makes phase-locking unreliable, so ITDs cannot provide an unambiguous cue to location except by detecting delays in sound onset or offset (group delays). ITDs as short as 20 μs can be detected.

62
Q

The neural system measuring ITDs depends on cells located where?

A

cells in the medial superior olivary nucleus (MSO) acting as coincidence detectors.

63
Q

How do the inputs into the medial superior olivary nucleus detect ITDs?

A

The MSO has inputs from bushy cells in both cochlear nuclei that phase-lock in response to low frequency stimuli. If a phase difference exists between the two ears then the bushy cells corresponding to the furthest
ear fire slightly later.

64
Q

Most cells in A1 are binaural and fall into two groups of cortical columns. What are the differences of these columns?

A

Those in summation columns show bigger responses to input from both ears than from one. By contrast, cells in suppression columns have a preference for input from one ear

65
Q

How are suppression and summation columns arranged in A1?

A

arrayed alternately and at right angles to isofrequency strips.

66
Q

How is the location of a sound source thought to be encoded?

A

seems to be encoded by a broad cortical channel in each hemisphere, rather than numerous channels tuned to discrete spatial
positions.

67
Q

How can the motion of the sound be analysed?

A

seems to be encoded by a broad cortical channel in each hemisphere, rather than numerous channels tuned to discrete spatial
positions.

68
Q

What does a rising intensity activate?

A

activates a neural network responsible for space recognition, sound motion, and attention, which lies in the superior temporal lobe (planum temporale), right
temporoparietal junction, right motor and premotor cortices, the left cerebellar cortex, and part of the midbrain.

69
Q

What is the special significance of rising intensity?

A

that it results from
approaching (i.e., potentially threatening) sources in natural environments.

Neuroscience (15 decks)

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