Auditory and vestibular system Flashcards
what are the 3 sections of the ear
The ear is divided in 3 sections: outer , middle and inner
where is the ear found
The ear is embedded in the petrous portion of the temporal bone
What parts make up teh outer ear and what is the function of the outer ear
Capture sound and focus it in the tympanic membrane
Amplify some frequencies by resonance in the canal
Protect ear from external threats
Has hairs to protect inner ear
Formed by pina and external auditory canal
what makes up the inner ear and what is the function
ossicles , oval and round window , tympanic membrane and muscles
Function is amplification:
Focussing vibrations from large SA (tympanic membrane) to smaller SA (oval window) → change in SA increases pressure
Leverage from incus-stapes joint to increase force on oval window
what makes up the inner ear and what is its function
Cochlea is the hearing part of the ear; transduces vibrations into nervous impulses (while doing so, also produces a frequency and intensity analysis of the sound - pitch and volume)
what is the 3 compartments
scala vestibuli
scala tympany
scala media
what is the fluid in the scala vestibuli and scala tympani
perilymph ( high in Na)
what is the fluid in the scala media
endolymph high in K
what other structure is found in the scala media
Organ of corti in the media basilar membrane
how is the scala media basilar membrane arranged and why
Basilar membrane arranged tonotopically ( aka thicker at one end + tinner at the other) meaning it is sensitive to different frequencies at different points along length
what are the two types of hair cells found in the Organ of corti and what is thier function
Inner hair cells –> afferent cells carry out transduction
Outer hair cells –> can contract / elongate to act as an active amplifier by moving the tectorial membrane towards or away from IHC
what is the tectorial membrane
located above the hair cells and will allow the hair deflection which in turn will depolarise the cell
OHC hair are in constant contact with the tectorial membrane, and these assist the contact with the IHC
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Describe the process of transaction
1) Sound come in causing the tectorial membrane to move ( depends on OHC) causing vibration of hairs on the basilar membrane
2) Deflection of the stereocilia towards the longest cilium will open potassium ion channels within the inner hair cell, as the tectorial membrane is brought into contact.
3) Ionic interchange of K+ within the endolymph and inner hair cell leads to cell depolarisation.
4) Depolarisation induces the opening of voltage-gated calcium channels which release excitatory neurotransmitters (Glutamate) onto the afferent nerve.
5) Higher amplitudes ( louder) sounds will cause greater deflection of the stereocilia and K+ channel
6) If a sound it too loud the cell will hyperpolarise and close K+ channels
how does the OHC effect transduction
act as an active amplifier . OHC can contract to move the tectorial membrane away / towards the IHC.
1) For softer sounds :
OCH contract to move tectorial membrane towards IHC then more sound is transduced so more APs and visa versa
This normally happens if a sound is too soft to increase APs
2) for louder sounds;
If a sound is too loud , OHC elongate and push tectorial membrane away from IHC to prevent APs and reducing noise
Describe the auditory pathway
Spiral ganglions from each cochlea project via the auditory vestibular nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurons).
Auditory information crosses the superior olive level, beyond this point all connections are bilateral. ( aka travels up both the same and opposite side)
Ultimately information is relayed to the auditory cortices located within the temporal lobe.
Wernicke’s area: comprehension and understanding of language.
What are the areas of the brain that the auditory sensory system is connected to
Primary
Primary: exclusively carries messages from the cochlea
Short pathway: 3 to 4 relay
Each relay nucleus does a specific work of decoding and integration
First relay in cochlear nuclei of brainstem: receive type 1 spiral ganglion axons (auditory nerve)
Important decoding of brain signal here: duration, intensity, frequency
Second major relay in superior olivary complex: majority of auditory fibres synapse here having crossed the midline
Third relay in mesencephalon: localisation of sound along with final neuron
Final relay in median geniculate body of thalamus: preparation of motor (vocal) response
Fast: large myelinated fibers
Final neuron links thalamus to primary auditory cortex
PAC in temporal area within lateral sulcus
Message already largely decoded by previous neurons but at PAC it is recognised, memorised and integrated into voluntary response
Reticular sensory
non-primary/reticular sensory: carries all types of sensory messages
From the cochlear nuclei, small fibers connect with the reticular formation: this is where the auditory message joins all other sensory messages
Next relay is in the non-specific thalamus nuclei before the pathway ends in the polysensory (associative) cortex
Main function of pathways: select the type of sensory message to be treated first
Pathways connected to wake and motivation centers as well as to vegetative and hormonal systems
Central auditory cortex
What is freq? , Amplitude?
Frequency/pitch (Hz): Cycles per second, perceived tone
Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength.
what sounds are heard by humans and what happens to these sounds with age
Frequency: 20-20,000Hz (8000Hz is the discrimination maximum)
Loudness: 0 dB to 120 dB sound pressure level (SPL). ( can hear >120 but will cause damage to ear)
Hearing acuity decreases with age, particularly higher frequencies. Medium and low frequencies could be affected with the progression of a hearing loss.
How is sound measured
The decibel scale is a logarithmic scale since the range of sensitivity is very large.
A logarithmic scale enables us to graphically compress the scale. Outer hair cells support sound amplification beyond 60dB
what tests are used for hearing assessment
tuning for pure tone audiometry verbal and non verbal tests Tympanometry Otoacoustic emissions Auditory evoked potentials cortical potentials
what are the 2 tests for tuning forks and what happens in a tuning fork test
Rinne test directly stimulates vibration of hair cells ie only inner ear
Weber test tests normal hearing ie outer / middle inner
Used to establish probably presence or absence of hearing loss with a significant conductive component
Used to provide early and general information, when audiometry is not available or possible.
( NOT really used anymore main function is to see if hearing loss is symmetric or asymmetric)
what happens in a PTA
Science of measuring hearing acuity for variations in sound intensity and frequency
Audiometer = device used to produce sound of varying intensity and frequency
Machine plates different sounds and freq and person tells you if they can hear it or not
Test produces an audiogram = hearing threshold are plotted to define if there is hearing loss of not
Normal = 0 - 20dB
what happens in a verbal and non verbal test
can a person who hears sounds understand the sound ie :
Sound localisation
Filtered speech
Speech in noise
What is tympanometry and what is it used for
Used to test condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones ( ossicles) by creating variations of air pressure in the ear canal
( see google docs for results)
