Auditory System Flashcards
What is sound?
The displacement of air particles following a sinusoidal pattern of compression and rarefacation
What can humans hear?
20Hz-20KhZ
Ranges change during life
Optimal area of hearing is 40-50 decibels for humans normally
Components of the auditory system
Outer ear- Pinna to eardrum
-air
Middle ear- eardrum to medial wall of the middle ear cavity -air
Inner ear- cochlear, vestibular system and vestibulocochlear nerve
-fluid
These are Central auditory pathways
Outer pinna and ear canal
Pinna- cartillagenous structure
Formed from pharyngeal arches 1 & 2 ( 6x Hillocks of His)
Comes up in assessment
Forms between 10th and 18th week in utero
Directs soundwaves towards ear canal
Role of the pinna is to direct sound towards the inner subsystems
Folds of ear selectively filter sound out
High pitch sounds are detected more easily than Low pitch sounds due to evolution
Ear Canal made of 1/3 cartilage & 2/3 bone
Outer ear- tympanic membrane
Top 1/3rd of eardrum is pars flaccida
2 layers- lacks a mesoderm
More likely to retract and become damaged
Bottom 2/3rds of eardrum is pars-tensa
This has 3 layers- mesoderm, endoderm, ectoderm
The middle ear contains
Bones:
Malleus, Incus & Stapes
Malleus is attached to tympanic membrane
The head of malleus is attached to incus
Incus long process is attached to head of stapes
Muscles:
Tensor Tympani & Stapedius
Tensor tympani tenses the tympanic membrane
Stapedius attaches to the stapes
Tubes
Eustachian Tube
Middle ears function
Slide 15
Is to house the the ossicular chain
What is the role of the middle ear?
Acoustic impedance match between air and fluid- filled inner ear
When sound travels from air to fluid (into cochlea fluid) 97% of loss of energy due to impedance but the amplification then cancels this loss to cause acoustic impedance match
Middle ear is the sound amplifier
The large eardrum concentrates its size down onto the foot plate of stapes which is very small
Amplifying the movement makes the airborne sound vibration louder
Because of the ratio of the tympanic membrane to stapes footplate( 14:1)
This means any force applied to the ear drum, which has air hitting it, is going to be concentrated down to a much smaller area, increasing the air pressure as force is same but area decreases and P=F/A
There’s a lever action of the ossicles- handle of maelleus is 1.3 times longer than process of incus
So movement of the tympanic membrane anchored to the malleus causes a moment force onto the incus, which then causes a little bit of force onto the stapes foot plate, which takes advantage of the fact that theres a big ear drum and small footplate, leads to gain (making things loud)
Total gain is 18.3:1 or 20-35 dB
Transferring energy from outer ear to inner ear- oscillar chain
200 fold increase boost In pressure from tympanic membrane to inner ear
What is the role of muscles in the middle ear?
Protection of the inner ear from acoustic trauma
Stiffens the ossciular chain
Stapedius stimulated acoustically (quicker of the reflexes)
Reflex arc: 3 or 4 neurones
25 ms in man
Tensor Tympani-voluntary and involuntary control
Stiffens to allow other sounds to be heard when chewing as otherwise nothin but chewing heard
What is the role of the Eustachian tube?
Ventillation of the middle ear space
Drainage of secretions
Equalises air pressure
Often dysfunctional in children – causing hearing loss and middle ear infection
The inner ear: vestibulocochlear apparatus
A set of fluid filled sacs, encased in bone
Cochlear- responsible for hearing
Labyrinth- responsible for balance
Innervation: Vestibulocochlear nerve
The cochlea
2.5 turns fluid filled bony tube
2 openings- round window & oval window
3 compartments ( Scala Tympani, Scala Media & Scala Vestibuli)
2 Ionic fluids
Scala vestibular
Label
Cochlear fluids
Scala media contains endolymph
This contains high K+
Scala vestibuli/tympani contain perilymph
These are like Extra cellular fluid or CSF
Na+ rich
Gradients are maintained by Na+/K+ ATPase channels and NKCC1 CIC-K chlorine channels
Ion channel abnormalities lead to deafness- inner ear problems cause complete deafness
The cochlea
If a fluid is compressed a wave can be generated as long as there’s somewhere for the wave to leave from
When the stapes moves near the oval window, the oval window moves in and out slightly which then sends a pressure wave coursing through the Scala vestibuli
The reason that this pressure wave moves at all is because the round window is elastic
When the pressure gets to the Scala tympani theres a tiny bit of movement upwards of the basilar membrane
The basilar membrane has the organ of corti sitting on it
Organ of corti has hair cells- inner and outer
The hair cell is attached to the tectorial membrane
Basilar membrane
At base side the basilar membrane is narrow and stiff
High frequency sounds picked up here
At the apex the basilar membrane is wide and floppy
Low pitch sounds picked up here
This shows the basilar membranes tonotopy: different parts of the basilar membrane optimally responding to different frequencies
Organ of corti
Movement in the basilar membrane causes movement in the organ of corti and specifically causing movement of the hair cells
This is where there is movement from waves to sound waves to fluid waves which then changed to electrical energy
Hair cells attached to auditory nerve fibres
Displacement of the basilar membrane causes movement of specialized mechanical transducing cells
Hair cells
-Inner Hair Cells- Mechanical transduction
-Outer Hair Cells- fine tuning
Base of hair attached to basilar membrane
Stereocillia anchored to tectorial membrane
Shearing forces at the stereocillia
From waves to electrical signals
The stereocillia of the inner hair cells move as a result of the shearing forces- this causes creation of electrical spark
Rapid response/depolarisation required
Mechanically gated (as the hair cells physically move to open up a k+ channel) K+ channels opened causing depolarization ( K+ rich endolymph)
Depolarization results in opening of voltage gated Calcium channels
Release of neurotransmitter- Glutamate (plus others)- afferent neurotransmitter in the auditory system, is thought to be the afferent transmitter between the cochlear inner hair cells and afferent neurone
Repolarization through K+ efflux ( into K+ poor perilymph)
Tonotopy and role of outer hair cells
Each nerve responds maximally at a specific frequency.
Can respond to others but responds optionally to a specific frequency
But our ability to discriminate different frequencies is not fully explained by this theory
Outer Hair Cells can alter the stiffness of the basilar membrane to ensure maximal stimulation at one site and dampened response at another
Increased resolution
How is sound information coded?
Sound analysed to encode in formation for neural transmission
FREQUENCY (PITCH) Encoded in nerves by location along the basilar membrane
INTENSITY (LOUDNESS) Encoded in nerves by numbers responding and by firing rate
SOUND TRANSDUCTION Inner Hair Cells (and OHCs)
AMPLIFICATION Outer Hair Cells
Whole process
E. C. O. L. I
Auditory fibre – spiral ganglion.
Spiral Ganglion to vestibuloCochlear nerve ( VIII)
Then hits Central auditory pathway
Central auditory pathway is
8th cranial nerve
Cochlear nucleus- in the cochlear complex which is in junction between the pons and medulla
Olive- MSO
Lateral leminiscus
Inferior colliculus
What does the Brian stem do
Localise sound
Interaural Time differences: MSO- Medial Superior Olive
MSO neurones are coincident detectors: response only when exciters signals arrive simultaneously
Anatomical differences in connectivity allow each MSO neurone to be sensitive to sound source from particular location
Slide 45
