Auditory System

Question 1

What are the 3 sections of the ear?

Where is the ear located?

Answer 1

Outer, middle, inner

Petrous portion of the temporal bone (hardest bone)

Question 2

What is the outer ear formed by?

What are the functions of the outer ear?

Answer 2

Penna and external auditory canal

To capture sound and to focus it onto the tympanic membrane
To amplify some frequencies by resonance in the canal
To protect the ear from external threats (with hairs - cilia, and wax - mechanical and chemical protection)

Question 3

What is the middle ear comprised of?

What are the functions of the middle ear?

Answer 3

Tympanic area - tympanic membrane, tenor tympani muscle, ossicles (malleus, incus, stapes), stapedus muscle, auditory tube, oval window, cochlea

Amplify sounds by:
Focusing vibrations from the tympanic membrane (large surface) onto the ossicles (small surface) = change in surface area increases pressure
Using leverage from the incus-stapes joint to increase the force on the oval window

Question 4

What is the inner ear comprised of?

What is the function of the inner ear?

Answer 4

Cochlea

Transduce the vibrations from the middle ear into nervous impulses - this produces a frequency (pitch) and intensity (loudness) analysis of the sound

Question 5

What are the 3 compartments of the cochlea?

Answer 5

Scala vestibuli and scala tympani are joined with the scala media on the inside

Scala vestibuli and tympani share the perilymph (fluid - high is sodium) outside the membranous structure of the scala media

Scala media = membranous structure containing endolymph (fluid - high in potassium) where the Organ of Corti is located

Question 6

What is the Organ of Corti?

Where is it found?

Answer 6

The organ of hearing

Within the scala media, in the basilar membrane

Question 7

How does the basical membrane detect frequency?

Answer 7

Basilar membrane is arranged tonotopically (like a xylophone)

So it sensitive to different frequencies at different points along its length

Higher frequencies on the base, lower frequencies on the apex

Question 8

What does the organ of corti contain?

Answer 8

Hair cells:
Inner hair cells (IHC) - round shaped cells, arranged on one column
Outer hair cells (OHC) - long thin shaped cells, arranged on three columns

Question 9

What makes up the roof of the organ of Corti?

Which hair cells are in direct contact with the roof?

Answer 9

The tectorial membrane - located above the hair cells
So when the hair cells are deflected, it causes depolarisation

Only the OHC are in constant contact with the tectorial membrane, these assist contact with the inner hair cells

Question 10

The IHC carry which type of nerves (a/efferent) and what is its function?

The OHC carry which type of nerves (a/efferent) and what is its function?

Answer 10

IHC = 95% afferent info of the auditory nerve. Function = transduction of sound into nerve impulses

OHC = 95% efferent of the auditory nerve. Function = modulation of sensitivity of response (act as an active amplifier)

Question 11

Why can the OHC contract?

How does this help its function with modulation of sensitivity of response?

Answer 11

Protein present in their membrane

Contraction enables the hair to change length, taking the tectorial membrane closer or further away from the IHC = modulation of sensitivity of response

So if the tectorial membrane is pulled away from the IHC, lesser transduction of sound by the IHC and vice versa

Question 12

How does transduction occur by the IHC?

How do louder sounds lead to greater transduction?

Answer 12

Stereocilia = hairs of the hair cells
Deflection of stereocilia towards longest cilium = opens K+ channels
Causes depolarisation of the cell, NT is released

Higher amplitudes (louder) = greater vibration = greater stereocilia deflection = greater K+ openings

Question 13

How do the OHC act as active amplifiers?

Why is this important?

Answer 13

Pull the tectorial membrane closer to the IHC when sounds are quiet - to help transduction
Move tectorial membrane further away from IHC when sounds are loud - reduces transduction

Without active amplification, amplitude range that can be heard = very narrow

Question 14

Which hair cells are more commonly damaged during hearing loss?

Answer 14

OHC

Question 15

Where does the sound go after travelling through the cochlea?

(HINT: what is the auditory pathway)

Answer 15

The info travels ipsilaterally down the cochlear nerve, past the cochlea nuclei, and to the superior olive

At the superior olive, most of the information crosses over and so it travels bilaterally

It travels via the inferior culliculus and medial geniculate body to the auditory cortex - connection to language centres = understand auditory info

Question 16

Why are the auditory pathways tonotopically arranged?

Answer 16

To allow for distribution of frequency i.e. phonics organised tonotopically allows for language understanding

Question 17

What is frequency, what is its unit?

What is amplitude?

Answer 17

Frequency = pitch. Cycles per second, perceived tone, measured in Hz

Amplitude = loudness. Sound pressure, subjective attribute correlated with physical strength, measured in dB

Question 18

What range of frequencies and amplitudes can humans hear?

Answer 18

Frequency = 20-20,000 Hz
Amplitude = 0-120 dB

Question 19

Why is the decibel (dB) range useful?

Answer 19

It is a log scale, allows to compress such a large range of sensitivity on a graph

Question 20

During hearing loss, which frequencies are affected?

Answer 20

Usually higher frequencies

Question 21

What is the purpose of a hearing assessment?

What are the different ways by which hearing can be assessed?

Answer 21

If there is hearing loss, of what degree and type

Tuning fork
Pure Tone Audiometry (PTA) 
Central processing assessment
Tymponametry 
Otoacoustic Emissions (OAEs) 
Auditory evoked potentials 
Auditory Brainstem response 
Cortical potentials

Question 22

How are each of these hearing tests carried out?

Tuning fork
Pure Tone Audiometry (PTA) 
Central processing assessment
Tymponametry 
Otoacoustic Emissions (OAEs) 
Auditory evoked potentials 
Auditory Brainstem response 
Cortical potentials

Answer 22

Tuning fork - not used much anymore, tells you whether hearing is symmetric / asymmetric

Pure Tone Audiometry (PTA) - most common, person wears headphones and needs to press a button when they hear a sound (sounds are played at different freq and amp)

Central processing assessment - verbal and non-verbal tests e.g. sound localisation, filtered speech, speech in noise

Tymponametry - tests condition of middle ear, and measures mobility of the eardrum by creating variation of air pressure in the ear canal

Otoacoustic Emissions (OAEs) - normal cochleas produce low intensity sounds (OAEs), so if this cannot be recorded, then suggestive of hearing loss

Auditory evoked potentials - measure electrical changes in the nerves to see if they are picking up and transmitting the sounds

Auditory Brainstem response - electrical responses from the auditory pathway are measured

Cortical potentials - used to look at neurological or processing problems

Question 23

What are the 3 types of hearing loss and what are they determined by?

Answer 23

Type of hearing loss depends on where the damage is:

Conductive hearing loss = problem located in outer or middle ear (dissociation between what you can hear from the cells Vs the sound that travels through the air)

Sensorineural = problem with the inner ear cells or the auditory nerve (lack of transduction)

Mixed hearing loss = conduction and transduction issues due to more than one structure affected

Question 24

What are some causes of hearing loss?

Answer 24

Conductive hearing loss:
Outer ear = wax, foreign body
Middle ear - otitis, otosclerosis

Sensoneural hearing loss:
Inner ear = presbycusis (loss of hair cells, common with age), ototoxicity
Nerve = CN VIII tumour

Question 25

What are the 4 treatment options for hearing loss?

Answer 25

Treatment of underlying cause e.g. remove wax

Hearing aids - e.g. in presbycusis the hair cells are lost, so the hearing aids amplify the sound

Cochlear implants - replaces the function of the hair cells by receiving the sound and transforms it into electrical impulses (when the cells are damaged), requires functional auditory nerve to function

Brainstem implant - electrical impulses sent to a set of electrodes placed directly into the brainstem, v. risky procedure, only advised for patients with bilateral auditory nerve damage