Auditory System Flashcards
What is sound
Displacement of air particles following a sinusoidal pattern of compression and rarefraction
Amplitude
Loudness of sound
Frequency of wavelength
Pitch
Range of human hearing
20Hz—> 20KHz
Auditory system components
Outer ear
Middle ear
Inner ear
Central auditory pathways
Which parts of the ear contain air
Outer and middle ear
Which parts of the ear contain fluid
Inner ear
Epithelial lining of Eustachian tube
Respiratory epithelium
Pinna
Unique to every individual
Cartilaginous structure
Function of folds of Pinna
Directs sound waves into the ear canal
High pitch>low pitch
Ossicles
Malleus
Incus
Stapes
When are the ossicles fully formed
At birth
Middle ear contains
Bones- ossicles
Muscles- tensor tympani and stapedius
Eustachian tube
Which nerve passes through the internal auditory meatus
Facial nerve (CN VII)
Which Foramen does the facial nerve leave the ear cavity
Stylomastoid foramen
Role of the middle ear
Acoustic impedance match
Between air and fluid-filled ear
What percentage of energy is lost when transferred from air to fluid
97%
How does the middle ear amplify sound
Amplifying the movement:
Ratio area - tympanic membrane: stapes 14:1
Lever action of ossicles
Malleus>incus
How much louder does the middle ear make sound
20-35 Db
Role of middle ear- increase in pressure
200 fold increase boost in pressure from tympanic membrane to inner ear
Role of muscles in middle ear
Protection of the inner ear from acoustic trauma
Stiffens the ossciular chain
How is the Stapedius stimulated
Acoustically
Reflex arc: 3 or 4 neurones
Length of the Stapedius reflex
25 ms (thunderclap not shotgun)
When is the tensor tympani stimulated
Voluntary and involuntary control
Stiffens when chewing
Role of the Eustachian tube
Ventilation if the middle ear space
Drainage of secretions
Inner ear
A set of fluid filled sacs encased in bone
Role of cochlear
Hearing
Role of labyrinth
Balance
Innervation of inner ear
Vestibulocochlear nerve
Cochlea
2.5 turns fluid filled bony tube
2 openings of cochlea
Round window
Oval windoe
3 compartments of cochlea
Scala tympani
Scala media
Scala vestibuli
2 ionic fluids of cochlea
Endolymph
Perilymph
Endolymph
Scala media
High K+
Low Na+
Perilymph
Scala vestibuli/tympani
Like ECF and CSF
Na+ rich
Low K+
How is ion gradient maintained between cochlear fluids
Na/K ATPase
NKCC1 CIC-K chloride channels
Ion channel abnormalities
Deafness
Helicotrema
Small hole at the end of the cochlea
Allows movement of pressure wave of fluid in cochlea
Basilar membrane
Separates Scala media from Scala tympani
Moves when pressure waves hit it- transfers mechanical energy into chemical energy
Properties of basilar membrane
Narrow at base
Wide at apex
Stiff at base
Floppy at apex
High frequencies detected at base
Low frequencies at apex
What frequencies are detected at base of basilar membrane
High
What frequencies are detected at apex of basilar membrane
Low
Tonotopy
Differential movements of the basilar membrane at different areas of the cochlea due to different frequencies
Each nerve responds maximally at a specific frequency
Outer hair cells can alter the stiffness of the basilar membrane to ensure maximal stimulation at one site and a dampened response at another
Increased resolution
Components of Scala media
Organ of corti
Inner and outer Hair cells
Tectorial membrane
Tectorial membrane
Fixed
Organ of corti
Contains inner hair cells anchored into tectorial membrane by stereocilla and outer hair cells
Cochlea nerve fibre attached to inner hair cells
How are sound waves transferred to electrical energy in organ of corti
Displacement of the basilar membrane causes movement of specialised mechanical transducing cells- inner hair cells
Movement of inner hair cells causes activation of the auditory nerve—> activated the central auditory pathway
Role of inner hair cells
Mechanical transduction
Role of outer hair cells
Fine tuning of sound by stiffening the basilar membrane in areas where the cochlea is not maximally stimulated
From waves to sparks
Movement of stereocillia
Rapid response required
Mechanically gated K+ channels open causing depolarisation (K+ Endolymph)
Depolarisation results in opening of voltage gated calcium channels
Release of neurotransmitter (mainly glutamate)
Repolarisation through K+ efflux (into K+ poor Perilymph)
Central auditory pathway
Cochlea —> cochlear nerve —> medial geniculate body —> auditory cortex
How do we localise sound
Interaural time difference
Interaural intensity difference
Interaural time difference
MSO neurones are coincident detectors- respond only when excitatory signals arrive simultaneously
anatomical differences in connectivity allow each MSO neuron to be sensitive to sound source from particular location
Ear canal
1/3 cartilage
2/3 bone
Formation of Pinna and ear canal
Forms between 10th and 18th week in utero
Formed from pharyngeal arches 1 and 2 (6x Hillocks of His)
How is frequency (pitch) encoded
In nerves by location along the basilar membrane
How is intensity (loudness) encoded
In nerves by numbers responding and by firing rate
Sound transduction
By inner hair cells (and outer hair cells)
How is sound amplified
Outer hair cells
From neuron to brain
Auditory fibre- spiral ganglion
Spiral ganglion to cochlear nerve (CN VIII)
Central auditory pathway
Conductive hearing loss
Defective outer or middle ear
Sensorineural hearing loss
Defective inner ear
Treatment of conductive hearing loss
Depends on the cause = improve conduction
eg drain middle ear fluid
Remove wax
Bypass the conductive mechanism
Constitution of Perilymph
High sodium and low potassium content
Afferent arm of pupillary light reflex
Optic nerve —> pretectal nucleus —> Edinger-Westphal nucleus
Organ of corti
specialised structure which rests on the basilar membrane
Semi-circular canals
Pouring ice cold water in the external auditory meatus can cause convection currents in the semicircular canals and nystagmus
Utricle and Saccule
Signal the position of the head at rest (i.e. when no movement is occurring).
Sensation to middle ear
Glossopharyngeal nerve
Type of joints between chain of 3 ossicles
Synovial
During aging
High tone deafness occurs first (presbycusis)
Function of semi-circular canals
Detect rotational acceleration and deceleration
Otolithic organs (utricle and saccule) function
Detect gravity and liner acceleration
Arrangement of semi-circular canals
3 different planes- roughly orthogonal
The semi-circular canals affect movement in
Both eyes simultaneously
What fills the utricle and saccule
Endolymph
After a morning of lectures you walk outside to see luis a medical student dancing non stop on the benches. Seeing a crowd gather he then proceeded to perform 5 perfect pirouettes in consistent succession(spinning round in circles) which resulted in him falling over due to dizziness.
Which organ of the vestibular system detects angular acceleration(head spinning round)?
Semicircular canals
Saccule function
verticle movements (imagine adding a sack to someone before you throw them over a bridge)
Utricle function
horizontal movements
Plate of stapes
in conduction system of sound via the bones malleus incus stapes(MIS- in order)
