physiology of hearing Flashcards
threshold
plot quietest sound that can be heard at each frequency
Sound pressure defined as 20 x log10 P/Pref
This is called dB SPL
Human ear is more sensitive to sounds at certain frequencies
So we get curves which are difficult to interpret
mammalian ear
Outer Ear
-Pinna
-Ear canal
Middle Ear
-Tympanic membrane
-Ossicles
Inner Ear
-Cochlea
-Vestibule
importance of two ears
Localisation of sound in horizontal plane
Primarily inter-aural time difference
Also difference in loudness
outer ear- pinna
Pinna amplifies & filters incoming sounds
Directional-dependent filtering at certain frequencies
outer ear- canal
For a tube closed at one end, the wavelength of the resonant frequency (f0) is related to 4 × tube length (L)
“f” _“0” “= “ “c” /”4 × L” “ = “ “344 m/s” /”4 × 2.5 cm” “≅ 3.4 kHz”
This resonance results in a ~10 dB (3×) increase in level for speech frequencies
The pinna & canal combined increase sound pressure level by up to 20 dB
Frequency filtering useful for sound localisation in vertical and front-back planes
middle ear overcoming impedance mismatch
Collect sound energy over a large area
tympanic membrane
Convert vibration of air (not v dense) into vibration of bone (dense)
Concentrate all the energy onto small area (oval window)
Use the bone as a piston to transfer energy into the fluid
importance of tympanic membrane and stapes
Impedance matching
99.9% of sound bounces off an air-fluid interface
transmission of sound from air to fluid-filled inner ear inefficient
area ratio TM : oval window amplifies sound by concentrating energy
middle ear bones and muscles
Three wee bones
Malleus - Incus - Stapes
Protective muscles
Stapedius reflex
Tensor tympani
Eustachian tube
Connects the nasopharynx to the middle ear
Allows air to enter and leave the middle ear
Closed at rest, opens during swallowing and Valsalva
Keeps the air pressure in the middle ear space the same as the ambient atmospheric pressure
This is important to enable the tympanic membrane to vibrate
Dysfunction leads to blocked feeling and poor hearing (e.g. aeroplane flight)
hearing loss
Conductive hearing loss caused by:
Ear canal – wax, foreign body, congenital atresia
Tympanic membrane – perforation
Ossicles – congenital fusion, damage from infection
Middle ear space – fluid instead of air
Inner ear works fine if you can get the sound to it by another route
Vibrate the skull – bone conduction
the inner ear
Two sensory structures in one organ
Vestibular apparatus
contains sensory structures for balance and head movements
Cochlea
Contains sensory epithelium for hearing: the organ of Corti.
how does the vibration get into the cochlea
- Oval window faces into vestibule
- Vestibule contains sensory epithelia for saccule & utricle
- Vestibule leads into scala vestibuli (upper cochlear duct)
- Pressure waves travel along scala vestibuli then back through scala tympani (lower cochlear duct)
- Waves terminate at the round window
scala media
Organ of Corti
Sensory epithelium containing auditory hair cells.
Stria vascularis
Regulates ionic and metabolic functions of scala media.
Scala media full of endolymph
Rich in potassium, low sodium
Scala vestibuli/tympani full of perilymph
high sodium, low potassium
more typical extracellular fluid
hair cell structure
Hair cells are of epithelial origin, resembling cells that line the stomach
Stereocilia form bundle at apical pole of the hair cell
Stereocilia arranged from shortest to tallest
depolarisation and spike generation
Stereocilia pushed towards the tallest > depolarisation
K+ channels opened, K+ flows into cells from endolymph
Stereocilia pushed towards the shortest > hyperpolarisation
K+ channels closed
