The auditory system 09.02.23 Flashcards
What is sound?
The displacement of air particles following a sinusoidal pattern of compression and rarefacation
What does a bigger amplitude mean for sound?
The bigger the amplitude of the sound wave, the louder the sound
What is the range of human hearing?
20Hz - 20KHz
- This range will change during our life
How is sound transferred or moved in the outer, middle and inner ear?
Outer - air
Middle - air
Inner - fluid
What is the pinna?
It is the cartilaginous structure that we can see on the outside of the ear
How is the pinna formed?
- From pharyngeal arches 1 and 2
- It has 6 Hillocks of His
- Formed between the 10th and 18th week in utero
- The outer ear forms independently to the inner ear
What is the role of the pinna?
- It directs sound waves towards the ear canal
- It has folds in the ear that selectively filters sounds
What is the ear drum also called?
The tympanic membrane
What shape is the eardrum?
It is not flat but curved towards us
What are the top 1/3 and bottom 1/3 of the eardrum called?
Top: Pars Flaccida
Bottom: Pars Tensa
How big is the eardrum?
8 x 10mm diameter
14mg
84mm2 - 55mm2
(About the size of your pinky fingernail)
What are the three bones in the middle ear called?
- Malleus
- Incus
- Stapes
What are the two muscles in the middle ear called?
- Tensor Tympani
- Stapedius
What is the tube in the middle ear called?
Eustachian tube
What is the role of the middle ear?
Acoustic impedance match between air and fluid-filled inner ear
(converting air to fluid)
How much energy is lost transferring air to fluid?
97%
How does the middle ear create a sound vibration?
- The eardrum is concentrating sound onto a small area (area of stapes) (14:1 SA difference)
- This amplification makes it louder
How does the ossicles create a movement?
- It acts as a lever
- Creating a moment force which in turn creates a small force onto the stapes
How much longer is the handle of malleus compared to the incus?
1.3 times longer
What is the role of muscles in the middle ear?
- They protect the inner ear from acoustic trauma
- Stiffens the ossciular chain
What does the stapedius muscle do?
- It is stimulated acoustically
- It has a reflex arc of 3 or 4 neurons
- 25ms reaction time
What does the tensor tympani do?
It tenses the tympanic membrane
- Helps us be able to hear other sounds while chewing
What is the eustachian tube?
- tube that causes crackling
- We pop this tube on an aeroplane
What is the role of the eustachian tube?
- ventilation of the middle ear space
- Drains secretions
- Often dysfunctional in children
What is the vestibulocochlear apparatus in the inner ear?
- A set of fluid filled sacs, encased in bone
What is the cochlear responsible for?
Hearing
What is the labyrinth responsible for?
Balance
What nerve innervates the vestibulocochlear apparatus?
The Vestibulocochlear nerve
What is the structure of the cochlea?
2.5 turns of fluid filled bony tube
What are the two openings of the cochlea?
- Round window
- Oval window
What are the three compartments of the cochlea?
- Scala Tympani
- Scala Media
- Scala Vestibuli
What fluids are in the cochlea?
2 ionic fluids:
- Endolymph (High K+) - in the Scala media
- Perilymph (Na + rich) - in the Scala tympani and vestibuli
How is a gradient maintained with ionic fluids in the cochlea?
- Na, K-ATPase channel
- NKCC1 and CIC-K chlorine channels
How is a pressure wave created in the cochlea?
The oval window moves and creates a pressure wave
Where does the pressure wave travel in the cochlea?
- Through the oval window
- Into the Scala vestibuli
- Through the helicotrema
- Through the Scala tympani
- Then through the round window
What does the movement through the round window do?
Moves the cochlear as it passes through the basilar membrane and creates a movement up
Where does the organ of Corti sit?
On the basilar membrane
What does the organ of Corti contain?
Has hair cells in it that attach to the tectorial membrane
What is the structure of the basilar membrane?
- Narrow, stiff at base so high frequencies are detected here (at oval window)
- Wide, floppy at the apex so low frequencies detected here (helliocotremor)
What is tonotopy?
The spatial arrangement of where sound is perceived, transmitted, or received on the basilar membrane
How do the hairs interact with the membranes and their movements?
- The basilar membrane moves up due to pressure
- The Organ of Corti and the hairs move with it taking the pressure so that the…
- … Tectorial membrane does not move
What does the inner and outer hair cells cause a change in energy of?
Inner hair cell: Changes energy from mechanical (fluid) to electrical
Outer hair cell: Fine tuning
What occurs at the stereocillia?
- These are anchored to the tectorial membrane.
- They have shearing forces so brush against it
- They convert physical force from sound, head movement or gravity into an electrical signal, a process that is called mechanoelectrical transduction
What does movement of the stereocillia cause?
- A rapid response is required
- K+ channels open causing depolarisation
- Results in calcium channels open
- Influx of calcium
- Releases neurotransmitter
- Repolarisation
How does outer hair cells help us respond to specific frequencies?
Outer hair cells alter the stiffness of the basilar membrane to ensure maximal stimulation at one site and a dampened response at another
- Hair cells not activated droop and pull tectorial membrane down so it is only up at one activation point
How is frequency/ pitch encoded for neural transmission?
In nerves by location along the basilar membrane
How is intensity/ loudness encoded for neural transmission?
In nerves by numbers responding and by firing rate
How is sound transduction encoded for neural transmission?
By inner hair cells
How is amplification encoded for neural transmission?
Outer hair cells
How is sound transmitted from neuron to the brain?
Auditroy fibre to spiral ganglion to cochlear nerve (8th) to the central auditory pathway to medial geniculate body to auditory cortex
What does the brainstem do?
Sound localisation
What is sound localisation?
Brainstem has a coincidence detector because sound may reach one ear quicker than the other and it needs to localise that sound into one sound
What is the structures of the central auditory pathway?
ECOLI
E - Eight nerve
C - Cochlear nucleus
O - Olive (superior olivary nucleus)
L - Lateral leminiscus (contralateral cross)
I - Inferior colliculus
Then to the thalamus and then primary auditory cortex
