Physiology of hearing

Question 1

What are the functions of hearing?

Answer 1

Communication (speech)
Alert of danger
Localising objects
Recognition

Question 2

What is sound?

What does intensity have to do with ear damage?

What is the human hearing range?

Answer 2

Travellin pressure waves that propagate through air at. a speed of 340m/s

Frequency: Hz
and intensity measured (dB-logarithmic scale)
=10 x log (sound intensity/reference intensity)
OR 10x log(sound pressure/reference pressure)
**Intensity range is over 14 orders of magnitude

> 90dB causes permanent hearing damage but only feel discomfort above 100dB and pain above 140dB

20-20000 Hz (progressively narrows with age due to hair cell death. Only 15000to begin with)

Question 3

The cochlear is fluid filled and the outside is air. How does sound travel efficiently through these modalities?

Answer 3

Middle ear is designed to amplify the sound

Impedence matching device: increases pressure by approximately 45x by the ratio of the tympanic membrane and the oral window areas

+lever action (to a lesser extent) of the middle ear oscicles (malleus, incus, stapes)

=PREVENTION OF SOUND FROM BEING REFLECTED BACK FROM THE FLUID-FILLED COCHLEAR

Question 4

Consider pathologies in the ear.

What are otitis media and glue ear?

Answer 4

Otitis media = infection/inflammation of the middle ear

• Usually self limiting
• Common in children and often from URTI

Glue ear is the secetory form with effusion

• If chronic –> conductive learning loss
• May need draining or grommets

Question 5

Consider pathologies in the ear.

What is otosclerosis?

Answer 5

Fusion of stapes with oral window

-Surgicalfix

Question 6

What is the oral window?

What is the round window?

Answer 6

Oral window- where the sound comes into the inner ear. lies between stapes and scala vestibuli

Round window- membrane at end of ear, function is to relieve the pressure of sound coming in

Question 7

Describe the structure of the cochlear and the composition of the cochlear fluids

Answer 7

Like a test tube within a larger test tube.

Superior chamber = scala vestibuli. In contact with oral window and contains perilymph

Middle chamber= scala media, sepereated from scala vestibuli by Reissner’s membrane. Contains endolymph and organ of corti

Inferior chamber= scala tympani, seperated from scala media by basilar membrane- cochlear nerve lies here. Contains perilymph

Question 8

What is the difference between the endolymph and the perilymph

Answer 8

Perilymph is like normal extracellular fluid with a raised Na+ concentration and low K+ concentration

Endolymph is unusual extracellular fluid rich in k+ and low in Na+ with electrical potential of +80mv
-Produced by stria vascularis

Question 9

What is the function of the organ of corti?

Answer 9

Detects sound-induced motions of the basilar membrane

Apical membrane of hair cells is bathed in endolymph and basolateral membrane in perilymph

Inner hair cells innervated by afferents and uter by efferent

Question 10

Outline the mechanotransduction in hair cells

Answer 10

Deflection of the hair bundle opens non selective cation channels (the MET channels) at the lower end of the tip links, between neighboring stereocilia (‘hairs’)

K+, the major cation in endolymph enters and depolarises the hair cell, driven by its electro-(chemical) gradient (the electrical gradient is 120+ to 140+mv; little/no chemical gradient)

Ca2+ also enters and causes adaptation

Voltage gated Ca2+ channels open, Ca2+ triggers vesicle release

Afferent nerve fibres are activated

Inner hair cells are sensory, outer hair cells are sensori-motor cells

(Stapes vibration -> oral window -> scala vestibuli –> perilymph –> vibrations to Reissners membrane–>scala media endolymph –> basilar membrane contracts and rebounds –> hair cells stimulated –> AP –> cochlear nerve –> CN8 –> 1 auditory cortex)

Question 11

Explain the electromotility of outer hair cells

Answer 11

They amplify basilar membrane motion

When they depolarise -shorten; hyperpolarise- lengthen

PRESTIN: A modified anion exchanger in the basolateral membrane, it is the OHC moto

Question 12

Consider the innervation of the cochlea

Describe the afferent nerve supply

Answer 12

Neurons in cochlear ganglion innervate hair cells and project axons to the brain via auditory branch of CN8

Each inner hair cell is innervated by axons from 10-20 type 1 spiral neurons that signal the reception of sound over a wide range of intensities to the brain

Outer hair cells are innervated by type 2 spiral neurons that signal the reception of painfully loud sound that cause damage to cochlear

Question 13

Consider the innervation of the cochlea

Describe the efferent nerve supply

Answer 13

From medial olive innervate the outer hair cells directly

From lateral olive synapse on the type 1 afferent fibres

Activation of the efferent system modifies the sensitivity of the cochlear

Question 14

Consider the 4 causes of sensorineural hearing loss

How can noise cause sensorineural hearing loss?

Answer 14

1. Noise- physical effects on hair bundle structure, mitochondrial damage, cytotoxic free radics, glutamate excititoxicity

Question 15

Consider the 4 causes of sensorineural hearing loss

How can ageing cause sensorineural hearing loss?

Answer 15

2. Ageing (presbyacusis)- in 30% of over 70, hair cells, stria vascularis and cochlear ganglion

Question 16

Consider the 4 causes of sensorineural hearing loss

How can ototoxic drugs cause sensorineural hearing loss?

Answer 16

3. Ototoxic drugs- cisplatin, aminoglycoside antibiotics, loop diuretics, solvents, salicylate

Question 17

Consider the 4 causes of sensorineural hearing loss

Outline the prevalence of genetic mutations that cause sensorineural hearing loss

Answer 17

4. Genetic mutations
   - High frequency : 1/2000
   - Syndromic or non-syndromic
   - Over 50 genes identifies
   - Approximately 50% of congenital deafness caused by mutations in gap junction genes

Question 18

How are cochlear implants used?

Advantages and disadvantages?

Answer 18

Surgically implanted electronic device that provides a sense of sound to proundly deaf

ADVANTAGES:
-Good results, recognise and comprehend speech

DISADVANTAGES:

• Expensive; limited to western world
• Speech sounds robotic
• Music sounds aweful
• Maximum 24 channels to substitute 15000 hair cells

Question 19

Consider the central auditory system

Outline the sequence of structures involved

Answer 19

Primary auditory cortex
Medial geniculate body
Inferior colliculus
Nuclei of the lateral leminiscus
Superior olivary complex
Cochlear nucleus

*Most auditory nuclei are tonotopically organised

Question 20

Consider the central auditory system

Describe the cochlear nucleus

Answer 20

Parallel processing starts here
Auditory nerve fibres from cochlear from cochlear ganglia innervate many types of neuron
Neurons extract information about level, onset and timing of sounds

Question 21

Consider the central auditory system

Describe the superior olivary complex

Answer 21

2 binaural cues are used to localise sounds in space

• Interaural level differences are detected in the lateral superior olive
• Interaural time differences are detected the medial superior olive

Question 22

Consider the central auditory system

Describe the inferior colliculus

Answer 22

• Obligatory synaptic station for all afferents
• Laminar organisation in ICC, iso-frequency sheets
• Combines complex frequency and amplitude analysis of DCN with information on sound localisation from SOC
• may encode complexity and localisation of sounds
• Auditory reflex centre; reflexive orientation to stimuli

Question 23

Consider the central auditory system

Describe the auditory cortex

Answer 23

On upper surface of temporal cortex
-Lesions cause defecit in: sound localisation, discrimination of tempora pattern, intelligibility of speech

-Lesions in brocas (motor aphasia) and Wernicke’s (sensory aphasia) areas also impair the production and comprehension of speech