Physiology of hearing Flashcards
What are the functions of hearing?
Communication (speech)
Alert of danger
Localising objects
Recognition
What is sound?
What does intensity have to do with ear damage?
What is the human hearing range?
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)
The cochlear is fluid filled and the outside is air. How does sound travel efficiently through these modalities?
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
Consider pathologies in the ear.
What are otitis media and glue ear?
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
Consider pathologies in the ear.
What is otosclerosis?
Fusion of stapes with oral window
-Surgicalfix
What is the oral window?
What is the round window?
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
Describe the structure of the cochlear and the composition of the cochlear fluids
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
What is the difference between the endolymph and the perilymph
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
What is the function of the organ of corti?
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
Outline the mechanotransduction in hair cells
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)
Explain the electromotility of outer hair cells
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
Consider the innervation of the cochlea
Describe the afferent nerve supply
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
Consider the innervation of the cochlea
Describe the efferent nerve supply
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
Consider the 4 causes of sensorineural hearing loss
How can noise cause sensorineural hearing loss?
- Noise- physical effects on hair bundle structure, mitochondrial damage, cytotoxic free radics, glutamate excititoxicity
Consider the 4 causes of sensorineural hearing loss
How can ageing cause sensorineural hearing loss?
- Ageing (presbyacusis)- in 30% of over 70, hair cells, stria vascularis and cochlear ganglion
Consider the 4 causes of sensorineural hearing loss
How can ototoxic drugs cause sensorineural hearing loss?
- Ototoxic drugs- cisplatin, aminoglycoside antibiotics, loop diuretics, solvents, salicylate
Consider the 4 causes of sensorineural hearing loss
Outline the prevalence of genetic mutations that cause sensorineural hearing loss
- 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
How are cochlear implants used?
Advantages and disadvantages?
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
Consider the central auditory system
Outline the sequence of structures involved
Primary auditory cortex Medial geniculate body Inferior colliculus Nuclei of the lateral leminiscus Superior olivary complex Cochlear nucleus
*Most auditory nuclei are tonotopically organised
Consider the central auditory system
Describe the cochlear nucleus
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
Consider the central auditory system
Describe the superior olivary complex
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
Consider the central auditory system
Describe the inferior colliculus
- 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
Consider the central auditory system
Describe the auditory cortex
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
