4c. The Peripheral Auditory System Flashcards
External Auditory Meatus
- Effect on Sound
Increases amplitude (dB) of sounds with 2-5kHz frequency, which is the range of frequencies that coincides with human speech
Length of Outer Ear Canal Equation
F = c/4L
Middle Ear
- 3 Functions
- Impendance matching between the air and cochlear fluids
- Protection from loud sounds, especially self-vocalisations
- Anti-masking of high frequency sounds by low frequency sounds
Impendance Matching
-2 Methods
- Relative areas of tympanic and oval membrane, where the tympanic is much larger
- Ossicle lever ratio, where the malleus is longer than the incus
Impendance Matching
- Effect
30dB increase in sound pressure from airborne sound to oval window
Anti-Masking
Antimasking of high frequency sounds by low frequency sounds, particularly at high sound levels, acting as a high-pass filter
Achieved by middle ear muscles
Middle Ear Muscles
- Delay
Contract 100ms after sharp loud sounds, so cannot protect from them
Contract before self-made vocalisations
Middle Ear Muscles
- Function
- Attenuate low frequency more than high frequency, acting as a high pass filter
- Protect from loud sounds and self-vocalisations
Middle Ear Muscles
- 2
- Tensor tympani
- Stapedius
Middle Ear Muscle Reflex
- Activation
Sounds 80-90dB above hearing threshold
Middle Ear Muscle Reflex
- Pathway
- Cochlea
CNVIII
- Ventral cochlear nucleus
Trapezoid body
- Both superior olivary complexes (brainstem)
- Facial nerve nuclei
- Ipsilateral projects to both
- Contralateral only to its own
CNVII
- Stapedius muscle contraction
Hearing Loss
- 2 Types
- Conductive
- Sensorineural
Sensorineural Hearing Loss
- Definition
Cochlea or auditory pathway is damaged
Conductive Hearing Loss
- Definition
Pathology that prevents conduction of sound to the cochlea
Conductive Hearing Loss
- Causes
- Otitis
- Wax build up
- Tumours
- Tympanic rupture
- Nerve defects
Rinne Test
Differentiates between conductive and sensorineural hearing loss
Placing a vibrating tuning form on the mastoid process and the entrance of the external auditory meatus.
Sensorineural = air conduction is better than bone conduction
Conductive = bone conduction is better than air conduction
Basilar Membrane
- Function
Decompose sounds into their component frequencies
Auditory Transduction
- Speed
Direct mechanical system with no 2nd messenger cascade = fast
Basilar Membrane
- Functional Properties
Creates a tonotopic map along its length
Thick, wide, stiff base vibrates at high frequencies
Thin, narrow, less stiff base vibrates at low frequencies
Auditory Transduction
- Stereocilia Movement
Sheering towards the tallest stereo cilia (kinocilium) opens mechanically gated ion channels
- Depolarisation
Sheering away from the tallest stereo cilia (kinocilium) closes mechanically gated ion channels
= Hyperpolarisation
Auditory Transduction
- Transducer Current
K+ influx
Driving force is:
- Electrical, due to gradient of 130mV
- Concentration, as K+ is abundant in endolymph
Endolymph Composition
Low Na+
Low Ca2+
High K+ maintained by stria vascularis
Endocochlear Potential
+80mV
Perilymph Composition
High Na+
Low K+
Similar to CSF
Auditory Transduction
- Outer Hair Cells
Depolarisation decreases OHC length, feeding back on the basilar membrane augmenting its motion
Outer Hair Cells
- 3 Roles
- Increase sensitivity
- Frequency selectivity through lateral inhibition
- Oto-acoustic emissions
Inner Hair Cells
- 2 Roles
- Decline in phase locking
- Adaptation at the synapse
Frequency Resolution
The ability to detect 1 frequency in a multifrequency complex stimulus, when all components are presented simultaneously.
Determined at the basilar membrane.
Efferent Innervation of the Cochlea
- Lateral pathway
- Medial Pathway
Efferent Innervation of the Cochlea
- Lateral Pathway
Neurones with their cell body in the lateral olivocochlear nuclei synapse with dendrites of afferent type I ganglion cells that innervate inner hair cells
Efferent Innervation of the Cochlea
- Lateral Pathway Role
Very little is known about the role of these neurones
Efferent Innervation of the Cochlea
- Medial Pathway
Neurones with their cell body medial to the lateral olivocochlear nuclei, in the peri-olivary region of the superior olive synapse directly with outer hair cells
Efferent Innervation of the Cochlea
- Medial Pathway Roles
- Protection from loud sounds
- Anti-masking
Otoacoustic Emissions
Sounds generated by the movement of the sensory hair cells in the cochlea
Strongest piece of evidence suggesting that the OHCs are mediating an active process.
Otoacoustic Emissions
- Use
Measurement is used to diagnose neonatal deafness, in whom it is difficult to establish the absence of hearing.
Otoacoustic Emission
- Loss
Lost in patients with significant sensorineural hearing loss
Prebycusis
- Description
Threshold for hearing increases with age.
Hearing at high frequencies also decreases
Prebycusis
- Cause
Lateral wall degeneration causing loss of endocochlear potential
Prebycusis
- Test
Selectively lowering the endocochlear potential in 1 ear by application fo furosemide to the round window, causing a loss in threshold which is greater at high frequencies than low frequencies
