The auditory system

Question 1

What is sound?

Answer 1

Sound is 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.

Question 2

Define the amplitude of a sound wave. How may it be expressed? State the unit. Differences in amplitude may be perceived as what?

Answer 2

• The amplitude of a sound wave is the total change in pressure that occurs during a single cycle.
• It is expressed in a logarithmic scale as a ratio of a reference pressure (a sound pressure which is at the threshold of human hearing = 2 x 10-5 Pa).
• The unit of sound pressure amplitude is the decibel (dB)
• 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.

Question 3

How would you define the frequency of a sound?

What is the audible range for humans in terms of frequency?

Answer 3

The frequency of a sound wave, the perceived pitch of the sound, is the reciprocal of the period (no. of cycles per second)
The frequency response of the human ear is from 20 Hz to 20 kHz optimally.

Question 4

What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?

Answer 4

Pinna

Question 5

Describe the shape of the external auditory meatus and its importance.

Answer 5

It is conical – starts off wide and narrows to the tympanic membrane/ear drum
This focuses the noise and increases the pressure on the tympanic membrane

Question 6

Describe the functioning of the middle ear structures in sound conduction

Answer 6

• Sound waves pass along the external auditory meatus striking the tympanic membrane which resonates faithfully in response.
• The movement of the ear drum is transferred with an overall efficiency of about 30% to the fluid in the inner ear by a lever system, composed of three ear ossicles, lying in the tympanic cavity (air-filled).
• The malleus is fixed at its thin end (the handle) to the tympanic membrane. Its thick end (the head) articulates with the head of the incus via a saddle- shaped joint. The long process of the incus makes a ball and socket joint with the head of the stapes. The base of the stapes is attached by an annular ligament to the oval window of the labyrinth. The malleus vibrates with the tympanic membrane. Inward movement locks the joint between the malleus and the incus, driving the long process of the incus inward, pushing the stapes in the same direction to exert a pressure on the perilymph beyond the oval window.

Question 7

What is the point of the middle ear? Why isn’t the tympanic membrane continuous with the cochlea?

Answer 7

The cochlea contains fluid, in which you are trying to induce a pressure wave
If the tympanic membrane was continuous with the cochlea, you would go straight from air to fluid and 99% of the energy will bounce back due to impedance
Sound waves require more energy to travel through fluid than air so the increase in pressure of vibration allowed by the ossicles is crucial for this conduction

Question 8

State the 2 ways by which the ossicles increase the pressure of vibration of the tympanic membrane by the time it reaches the inner ear

Answer 8

Since the area of the oval window is 20 times smaller than the tympanic membrane, the pressure at the oval window is proportionally greater.
The mechanical advantage gained by the lever action of the three ossicles.
They amplifies the sound by about 20-30 dB

Question 9

State the two middle ear muscles

Answer 9

Tensor Tympani

Stapedius

Question 10

How do these two middle ear muscles help to prevent damage of the ossicles by excessive vibration due to loud noise?

Answer 10

• 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 sound transmission by 20 dB, especially for low frequencies.
• Reflex contraction of these muscles in response to loud noise (or self-generated vocalisation) may prevent damage to the inner ear but since the reaction time is 40–60 ms this ‘tympanic reflex’ affords no protection against brief loud sounds.

Question 11

State the purpose of the auditory canal/Eustachian tube

Answer 11

The auditory canal/Eustachian tube connects the middle ear to the pharynx which allows the air pressure to be equalised when changing altitude.

Question 12

What is hyperacusis? What can it be caused by?

Answer 12

Painful sensitivity to low intensity sounds – can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles (e.g. Bell’s Palsy)

Question 13

Describe the anatomy of the auditory part of the inner ear i.e. the cochlea

Answer 13

• The cochlea is a bony canal/tube 3.5 cm long, which spirals around a central pillar, called the modiolus.
• The cochlea is bisected from its basal end almost to its apical end by the cochlear partition (or cochlear duct) which is bounded on three sides by the basilar membrane, the stria vascularis, and Reissner’s or vestibular membrane.
• A distinct channel runs within the cochlear duct called the scala media, which contains endolymph.
• The floor of the scala media is the basilar membrane, on which rests the sensory apparatus = the spiral organ of Corti
• The cochlear duct also supports the tectorial membrane when lies immediately above the organ of Corti
• The compartments on either side of the cochlear duct, called the scala vestibuli and the scala tympani, contain perilymph and are continuous with each other via a small gap known as the helicotrema situated at the apex of the cochlea.
• Both the oval window and round window (regions where bone is absent surrounding the cochlea) are at the basal end of the tube.

Question 14

Describe the conduction/vibration transmission in the cochlea, and the importance of the round window

Answer 14

• Pressure waves generated at the oval window (due to vibration of the oval window by the footplate of the stapes) are propagated through the perilymph of the scala vestibuli into that of the scala tympani and to the round window where the energy dissipates (by causing a compensatory bulge of the round window)*.
• The pressure wave importantly causes the flexible basilar membrane (and vestibular membrane) to vibrate.

*Note that if it were a completely closed and inflexible system, then the movement of the stapes would be unable to displace the perilymph

Question 15

What is the Organ of Corti?

Answer 15

A narrow sheet of columnar epithelium running the length of the cochlear duct that contains, as well as other supporting cells, sensory hair cells.
The Organ of Corti includes the basilar and tectorial membranes.

Question 16

What are the two types of sensory receptor cells in the Organ of Corti?

Answer 16

Inner and outer hair cells

Question 17

Describe the features and overall function of inner hair cells.

Answer 17

• Single row of cells
• Approx. 3500 per human cochlea
• Stereocilia not in contact with the tectorial membrane
• They are the sensory receptors; 95% of afferent projections (sensory axons that carry signals from the cochlea towards the brain) project from inner hair cells

Question 18

Describe the features and function of outer hair cells.

Answer 18

• Three rows of cells
• Approx. 12000 per human cochlea
• Steriocilia in contact with the tectorial membrane
• Most of the efferent projections (from the brain to the cochlea) terminate on outer hair cells
• Outer hair cells contract in a voltage-dependent manner. Depolarisation causes them to shorten (=electromotility) and this is due to the reorientation of the protein prestin. They are therefore able to augment the vibrations of the basilar membrane, a process called cochlear amplification.

Question 19

Describe the innervation of the two hair cell types

Answer 19

Each inner hair cell is innervated by about 10 myelinated axons of large bipolar cells with their cell bodies in the spiral (or cochlear) ganglion of the cochlear nerve (part of CN8) found in the modiolus.

An unmyelinated axon from small bipolar cells in the spiral ganglion synapses with 10 outer hair cells.

Question 20

What are otoacoustic emissions?

Answer 20

They are (inaudible) sounds of cochlear origin that are caused by the motion of the outer hair cells as they energetically respond to auditory stimulation.

Question 21

Describe the appearance and organisation of the stereocilia

Answer 21

• The hair cell is a flask-shaped epithelial cell named for the bundle of hair-like processes that protrude from its apical end into the scala media.
• Each stereocilium tapers where it inserts into the apical membrane, forming a hinge about which it pivots.
• The stereocilia are graded in height and are arranged in a bilaterally symmetrical fashion (with the cluster shaped like an ‘arc’ in the case for outer hair cells)
• Fine filamentous structures, known as tip links, run in parallel to the plane of bilateral symmetry, connecting the tips of adjacent stereocilia

Question 22

Describe and explain the topographical mapping of frequency (tonotopy)

Answer 22

• The basilar membrane is wider and more flexible at the apical end and narrower and stiffer at the basal end.
• A membrane that varies systematically in its width and flexibility vibrates maximally at different positions as a function of the stimulus frequency.
• Points responding to high frequencies are at the base; points responding to low frequency are at the apex.
• This allows for encoding the frequencies of a sound.

Question 23

Explain how a propagating pressure wave initiates sensory transduction

Answer 23

• Because the basilar membrane and the overlying tectorial membrane are anchored at different positions, the vertical component of the traveling wave is translated into a shearing motion between these two membranes.
• This motion causes the stereocilia in contact with the tectorial membrane to bend first one way and then the other => voltage changes across the hair cell membrane

Question 24

With respect to the hair cells themselves, describe and explain the process of mechanoelectrical transduction

Answer 24

• Bending of the stereocilia in the direction of the tallest stereocilium stretches the tip links, directly opening cation-selective mechanoelectrical transduction (MET) channels located at the end of the link => depolarising the hair cell (i.e. K+ channels open leading rapid influx of K+ from the endolymph) - the opening of MET channels relaxes the tip link
• Movement in the opposite direction compresses the tip links, closing the channels => hyperpolarising the hair cell
• Depolarisation leads to transmitter/glutamate release (calcium influx mediated exocytosis) from the basal end of the hair cell.
• This triggers action potentials in cranial nerve fibres the follow the up-and-down vibration of the basilar membrane at relatively low frequencies.

Question 25

Describe the difference in K+ and Na+ concentration in the different compartments of the cochlea. What maintains these differences?

Answer 25

Scala Media = High K+ and Low Na+
Scala Tympani = High Na+ and Low K+

stria vascularis maintains this concentration

Question 26

Give the reasons for why the ear’s sensitivity arises from an active biomechanics process as well as from its passive resonant properties

Answer 26

1. Tuning of the auditory periphery (measured at the basilar membrane or recorded as auditory nerve electrical activity) is too sharp to be explained by passive mechanics alone
2. At very low sound intensities, the basilar membrane vibrates 100-fold more than would be expected
3. Spontaneous otoacoustic emissions

Question 27

Why are the outer hair cells believed to be an essential component of this ‘active’ process

Answer 27

Their ability to expand and contract in response to electrical currents may provide a potential source of energy to drive an active process of cochlear amplification.

Question 28

Describe the auditory pathway from the cochlea to the primary auditory cortex.

Answer 28

• Primary auditory afferents (bipolar spiral ganglion cells in the cochlea) 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.
• The SON projects to the nuclei of the lateral lemniscus which then projects to the inferior colliculus (IC).
• Note that the dorsal cochlear nucleus sends axons directly to the contralateral nucleus of the lateral lemniscus
• The IC relays with the medial geniculate nucleus (MGN) of the thalamus which sends its output via the auditory/acoustic radiation to the primary auditory cortex

Question 29

Up to what point is the auditory pathway from one ear ipsilateral?

Answer 29

Cochlear nuclei

Beyond this point there is bilateral representation

Question 30

The inferior colliculus receives input from both cochlea. What is the inferior colliculus responsible for?

Answer 30

Reflex associations – turning your head towards loud noise

Question 31

Describe a phenomenon that is involved in sharpening the signal coming from the cochlea.

Answer 31

Lateral inhibition

Question 32

To which parts of the CNS do collaterals from the auditory pathway go?

Answer 32

Reticular formation

Cerebellum

Question 33

Where is the primary auditory cortex located?

Answer 33

Superior temporal gyrus

Question 34

What is the name given to the group of axons that project from the medial geniculate nucleus to the primary auditory cortex?

Answer 34

Acoustic radiations (they travel via the internal capsule)

Question 35

Explain how the pinna and ear canal act as direction selective filters for locating sound elevation

Answer 35

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

Question 36

The superior olivary complex uses two methods to localise sound in the horizontal plane. What are they? What frequency ranges of sound are they used for?

Answer 36

Interaural intensity difference; frequencies higher than 2kHz
Interaural time difference; frequencies below 3kHz

Question 37

Describe the interaural intensity difference method

Answer 37

• 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.
• Neurons of the lateral superior olivary nucleus (LSO) receive inputs from both ipsilateral and contralateral cochlear nuclei. However, the contralateral route is by way of a glycinergic inhibitory neuron; this contralateral inhibitory signal is carried out via large axons with large synapses (called the large calyces of Held).
• 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.
• Corresponding cells in the opposite LSO will show reverse responses to the same sound.
• The LSO projects to the 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. Hence the superior colliculus is implicated in the auditory reflexes organizing gaze and head rotation towards the sound source.

Question 38

Define ‘phase-locking’ of auditory afferents, used for coding of sound frequency

Answer 38

For lower frequencies 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.

Question 39

Describe the interaural time difference method

Answer 39

• A sound wave enters the closer ear slightly earlier than the further one. For low frequencies this results in a time delay less than one period which is analysed by neurons capable of phase-locking.
• The medial superior olivary nucleus 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.

Question 40

What is sensorineural hearing loss and what can it be caused by?

Answer 40

Caused by damage to hair cells of the inner ear or the auditory nerve; most widespread type of hearing loss

Causes:
• Loud noises
• Many genetics mutations affect the Organ of Corti
• Aminoglycoside antibiotics are toxic for hair cells
• Congenital diseases (rubella, toxoplasmosis)
• Acoustic neuroma (tumor on the cochlear nerve)
• Demyelination (due to inflammation or viral). Most common in multiple sclerosis MS.

Question 41

What is conductive hearing loss and what can to be caused by? State how this may be partially overcome?

Answer 41

Involves damage to the external or middle ear (e.g. stiff ossicle joints) => lowers efficiency at which sound energy is transferred to the inner ear.
Causes:
• In children, fluid accumulation in the inner ear is a common cause of conductive hearing loss (cold)
• A perforated tympanic membrane
• Otitis media
• An abnormal growth of bone (otosclerosis) can obstruct the ear canal.
• Barotrauma (temporary form

Can be partially overcome by artificially boosting sound pressure levels with an external hearing aid.

Question 42

Name and describe the test that can be used in clinic to distinguish between conductive and sensorineural hearing loss

Answer 42

A Weber test uses a tuning fork placed against the scalp (since acoustical vibrations can still be transferred directly through the bones and tissue of the head to the inner ear, even without an intact tympanic membrane/ossicles)

Question 43

How do cochlear implants work?

Answer 43

Hair cells do not regenerate in mammals.
- One solution is to bypass the dead cells and stimulate the nerve fibres directly: detect sounds, break them down into their constituent frequencies and send the signal directly to the auditory nerve via antennas.