4c. The Peripheral Auditory System Flashcards by tabitha Stafford

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

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Length of Outer Ear Canal Equation

F = c/4L

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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

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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

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Impendance Matching

- Effect

30dB increase in sound pressure from airborne sound to oval window

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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

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Middle Ear Muscles

- Delay

Contract 100ms after sharp loud sounds, so cannot protect from them

Contract before self-made vocalisations

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Middle Ear Muscles

- Function

Attenuate low frequency more than high frequency, acting as a high pass filter
Protect from loud sounds and self-vocalisations

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Middle Ear Muscles

- 2

Tensor tympani

- Stapedius

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Middle Ear Muscle Reflex

- Activation

Sounds 80-90dB above hearing threshold

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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

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Hearing Loss

- 2 Types

Conductive

- Sensorineural

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Sensorineural Hearing Loss

- Definition

Cochlea or auditory pathway is damaged

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Conductive Hearing Loss

- Definition

Pathology that prevents conduction of sound to the cochlea

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Conductive Hearing Loss

- Causes

Otitis
Wax build up
Tumours
Tympanic rupture
Nerve defects

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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