auditory systems

Question 1

what is the anatomy of the ear?

Answer 1

The ear is divided in three sections: outer, middle and inner ear.

Each part has a specific function during the hearing process.

The organ is embedded in the petrous portion of the temporal bone, the hardest bone in the body.
this protects it. it is a very delicate structure

Question 2

what is the outer ear and its functions?

Answer 2

Pinna and external auditory canal/meatus

functions:
To capture sound an to focus it the tympanic membrane

To amplify some frequencies by resonance in the canal.

To protect the ear from external threats. - this is done with hairs and wax. the wax capture microbes and trap or kill them (pH). the hairs expel the wax

Question 3

what is the middle ear and its functions?

Answer 3

tympanic membrane (ear drum), ossicles (malleus, incus, stapes), muscles (tensor tympani, stapedius), oval window

The main function of the middle ear is AMPLIFICATION by:

Focusing vibrations from large surface area (tympanic membrane) to smaller surface area (oval window). The change in surface area means the pressure is increased

Using leverage from the incus-stapes joint to increase the force on the oval window

Question 4

what is the inner ear and its functions?

Answer 4

cochlea (hearing), semicircular canals

hearing functions:
Its function is to transduce vibration into nervous impulses

While doing so, it also produces a frequency (or pitch) and intensity (or loudness) analysis of the sound

Question 5

what is the cochlea?

Answer 5

The cochlea contains 3 compartments:

1. Scala vestibuli and 2. scala tympani:
   Bone structures, contain perilymph (high in sodium)
   (where the hair cells are)
2. Scala media: Membranous structure, contains endolymph (high in potassium). Here is where the hearing organ or Organ of Corti is located.
   (where the stereocilia of the hair cells are)

Question 6

what is the basilar membrane of the cochlea?

Answer 6

The structure where the organ of Corti lies in is the basilar membrane

Basilar membrane is arranged tonotopically, using the same principle as a xylophone

base: narrow and tight, high frequency, high pitched sounds
apex: wide and loose, low frequency, low pitched sounds

This means it is sensitive to different frequencies at difference point along its length

Question 7

what is the organ of corti/hearing organ?

Answer 7

The organ of Corti contains thousands of hair cells: inner hair cells (IHC) and outer hair cells (OHC)
(cillia)

IHC are arranged on one column and OHC on three

The tectorial membrane is located above the hair cells and will allow the hair deflection, which in turn will depolarise the cell. Only OHC hair are in constant contact with the tectorial membrane, and these assist the contact with the IHC

Question 8

what are the hair cells of the organ of corti?

Answer 8

IHC carry 95% of the afferent information of the auditory nerve. Their function is the transduction of the sound into nerve impulses

OHC carry 95% of efferents of the auditory nerve. Their function is modulation of the sensitivity of the response. (they act as an active amplifier, they can contract, to make the tectorial membrane closer or farther from the IHC, changing the afferents) (closer means more transduction)

(there are 3 columns of outer, and one column of inner)

Question 9

how does trandsuction work?

Answer 9

The hairs of the hair cells are called stereocilia.

The deflection of the stereocilia towards the longest cilium will open K+ channels

The ionic interchange depolarises the cell, which leads to calcium influx, and the neurotransmitter (glutamate, or in its family) is liberated.

Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening
which leads to calcium influx into the cells, and release o

endolymph is high in potassium, so acts as a store

for quiet sounds, the basement membrane will vibrate less, so the tectorial membrane will be in contact with fewer hair cells, so sound is transduced less

Question 10

what happens with the tectorial membrane during depolarisation and re-polarisation?

Answer 10

Depolarisation:
as the basilar membrane moves up (upwards phase), this pushes the hair cells up so they come into contact with the tectorial membrane, which deflects the hairs in the direction of the longest hair
causes potassium influx and depolarisation

re-polarisation:
basilar membrane moves down (downward phase).
hairs still in contact with tectorial membrane are pushed the wrong way, and potassium channels close

Question 11

how do the outer hair cells help in transduction?

Answer 11

active amplifying

if the amplitude of a sound doesnt move the basement membrane sufficiently upwards for the IHCs to be displaced by the tectorial membrane
the OHCs can contract to bring it closer

if the sound is too loud, the OHCs can elongate to push the tectorial membrane away from the cilia of the IHCs

OHCs are the ones that get damaged more commonly so these abilities are lost

Question 12

what are the auditory pathways?

Answer 12

Spiral ganglions from each cochlea project via vestibulocochlear nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurons)

Auditory information crosses at the superior olive level (in brainstem)

After this point all connections are bilateral
(both ears impulses travel on both sides)

CNVIII -> cochlear nucleus -> superior olive (more decussate, some stay) -> inferior colliculus -> medial genticulate body -> auditory cortex in temporal lobe

(first 4 are brainstem, 5 is thalamus, 6 is cerebral cortex)

Question 13

how is hearing organised?

Answer 13

Hearing is tonotopically organised

by frequency, only basal cells will transmit high frequencies, only apical will transmit low frequencies (this is very necessary for language)

in primary and secondary auditory cortices in temporal lobe

Question 14

how does sound work?

Answer 14

longitudinal wave

Frequency/pitch (Hz): Cycles per second, perceived tone.

Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength

Human range of hearing:
Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB (can hear wider but its uncomfortable)sound pressure level (SPL)

(although we do start to lose the top end progressively) (150-4000Hz is the main range used in speech)

Question 15

what is sound amplitude?

Answer 15

The decibel scale (a log scale) is useful because the range of sensitivity is very large.

This allows us to compress the scale on a graph, and reflect the fact that many physiological processes are non-linear (i.e. they can respond to both very low and very high values)

(conversational speech is about 60 dB, loo flushing is 70, stadium crowd is about 130, shotgun is 170)

Question 16

what happens to hearing acuity with age?

Answer 16

Hearing acuity decreases with age

particularly higher frequencies.

Medium and low frequencies could be affected with the progression of a hearing loss.

Question 17

what is a hearing assessment, and what happens in it?

Answer 17

Aims of the assessment?

To answer these questions:

• Is there a hearing loss? If yes…
• Of what degree?
• Of what type?

Procedures:

• Tunning fork
• Audiometry
• Central processing assessment
• Tympanometry
• Otoacustic Emission
• Electrocochleography
• Evoked potentials

Question 18

what is the tuning fork method of hearing testing?

Answer 18

not really used now days

Used to establish the probable presence or absence of a hearing loss with a significant conductive component.

They are used to provide early and general information, when audiometry is not available or possible.

weber test, rinne test

weber: symmetric or asymmetric
weber: tuning fork on skull bone. vibes go directly to hairs of cochlea, stimulates endolymph internally

Question 19

what is the pure tone audiometry method of hearing testing?

Answer 19

Science of measuring hearing acuity for variations in sound intensity and frequency

An audiometer is the device used to produce sound of varying intensity and frequency

patient raises hand when they hear sounds of different frequency and amplitudes

Question 20

what is an audiogram?

Answer 20

The audiogram is where the hearing thresholds are plotted to define if there is a hearing loss or not. A normal hearing threshold is located between 0 – 20dB

in normal hearing, the line will be fairly level
if hearing loss, the line will be downhill (high freq go first

Question 21

what is a central processing assessment?

Answer 21

Assessment of hearing abilities other than detection.

Verbal and non verbal tests

Examples:
Sound localization
Filtered speech
Speech in noise

if abnormal, there is a problem with sound processing

Question 22

what is tympanometry?

Answer 22

Examination used to test the condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal.

anything below normal means there is some sort of resistance

Most common results:
A – normal
C – negative  middle ear  pressure
B 
Middle ear effusion
Perforation of tympanic membrane
Eustachian (pharyngotympanic) tube dysfunction
Occluded ear canal

Question 23

what are otoacoustic emissions (OAEs)?

Answer 23

The normal cochlea produces low-intensity sounds called OAEs

These sounds are produced specifically by the outer hair cells as they expand and contract

This test is often part of the newborn hearing screening and hearing loss monitoring.

test will be negative if their hair cells arent working, so theyll have a hearing deficit

Question 24

what are auditory evoked potentials?

Answer 24

Electrocochleography
– 0.2-4.0 ms, electrical activity from the cochlea and eighth nerve. Evoked by clicks or tone burst.

Auditory Brainstem Response (ABR)
– 1.5-10.0 ms, electrical activity from the eighth nerve and brainstem nuclei and tracts. Evoked by clicks.

Late Responses (N1-P2, P300, MMN, and more)
– 80-500+ ms, electrical activity from the primary auditory and association cortex. Evoked by tone burst and oddball paradigm.

Question 25

what is the auditory brainstem response?

Answer 25

Electrical responses from the auditory pathway

ABR is more commonly used in clinic.

Does not require attention from the patient.

Alterations in the latency of waves can point to the location of the deficit.

Objective measurement commonly used in babies and children.

Question 26

what are cortical potentials?

Answer 26

not commonly used in clinics

Cortical potentials could be affected on neurological conditions or processing problems.

Question 27

what are types of hearing loss?

Answer 27

Conductive hearing loss: Problem is located in outer or middle ear. (sound isnt getting to the inner ear)

Sensorineural hearing loss: Problem is located in the inner ear or the auditory nerve. (hair cells or nerve isnt working)

Mixed hearing loss: Conduction and transduction of sound are affected. Problem affects more than one part of the ear.

Question 28

what does audiometry look like in different types of hearing loss?

Answer 28

conductive:
will be two distinct lines depending on where the sound is coming from (inner ear hair cells, vs through air)

sensorineural:
lines are on top of eachother, so problem is not in conductance of sound.
the info isnt being transduced probably

mixed:
looks more random, two different lines

Question 29

how are degrees of hearing loss graded?

Answer 29

Depending on where the hearing thresholds are located, hearing loss can be classified as mild, moderate, severe or profound.

on audiometry

done by decibels/volume

Question 30

what are causes of conductive hearing loss?

Answer 30

conductive:
cerumen impaction (tympanic membrane dysruption)

outer ear -
wax
foreign body

middle ear -
otitis
otosclerosis

Question 31

what are causes of sensorineural hearing loss?

Answer 31

inner ear -
presbycusis (Progressive hearing loss due to
age)
ototoxisity (eg. drugs)

nerve-
VIII nerve tumour

Question 32

how is hearing loss treated?

Answer 32

underlying cause (eg. wax removal)

hearing aids

cochlear implants

brainstem implants

Question 33

what are hearing aids?

Answer 33

Depending on the type, degree and characteristics of the patient there are many options.

Hearing aids amplify the sound, does not replace any structure.

Question 34

what are cochlear implants?

Answer 34

The cochlear implant replaces the function of the hair cells by receiving sound, analysing it, transform it into electrical signals and sending an electric impulse directly to the auditory nerve
it inserts into the cochlea

It needs a functional auditory nerve to function.

helps coz lots of hearing loss is caused by hair cell dysfunction

Question 35

what is a brainstem implant?

Answer 35

When the auditory nerves are the affected structures, the electrical signals can be send to a set of electrodes placed directly into the brainstem.

It is very risky, then it is advised for people with bilateral important auditory nerve damage.
and doesnt necessarily give great results