Anatomy and Cell Biology 3309 Lecture Notes - Lecture 23: Ear Canal, Eardrum, Oval Window
15 May 2018
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Lecture 33 – Ear
Anatomy of the ear
- Pinna: ear – mostly cartilage tissue
o Functions like a funnel and funnels sound waves into the ear canal (external
auditory canal)
o Sound is funneled by pinna into outer ear canal
- Outer ear canal contains glands that secrete ear wax
o Keeps the ear canal open and clean
o Hairs in the external auditory gland prevent insects from entering canal and expel
dirt from the external canal
- External auditory canal ends with the ear drum (tympanic membrane)
o When sound waves enter the canal, they hit the tympanic membrane and ear drum
moves according to the sound waves
- Ear drum: border between external and middle ear
- Middle ear: air filled cavity behind the ear drum
o Ends with the oval window
o Border are the ear drum and oval window
o Contains ossicles (three bones)
▪ Malleus, incus and stapes
▪ Smallest bones in the human body
o Stapes: ends towards the oval window in the back of the middle ear
o Malleus: picks up the movement of the ear drum and conveys movement to incus
o Incus is connected to stapus
o Ossicles pick up the movement of the ear drum and amplify it → convert it to the
movement of the stapes against the oval window
o Ossicles move when sound hits ear drum causing them to move
o Three ossicles are connected to each other by two joints and there are some muscles
that insert at the ossicles that can dampen or enhance the transmission into
movement
o AIR FILLED SPACE → air can expand depending on pressure or temperature
o = Air filled space of middle ear must be connected to the exterior or else you get
high or low pressure in the middle ear
o E.g. go to high altitude – middle ear hurts and feels uncomfortable because the
pressure in the middle ear is too high compared to the exterior word
▪ = pressure on ear drum and it impairs the hearing because ear drum
becomes less sensitive to sound
o To have high or low pressure adapted to the exterior air pressure, the middle ear is
connected to the nasal pharynx (mouth) with the Eustachian tube
o Tube is normally not open and collapses → when you crack your ear the tube
opens and allows you to release air pressure in middle ear
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o Eustachian tube only opens at the age of 5 months → small babies cannot crack ears
= if they are taken on a flight and air pressure drops a lot, their ear drums will
rupture = painful! → it grows back so it is okay
o Middle ear infections when ear drum ruptures and inflammation can escape
through a hole in the ear drum
o If ear drum ruptures too often, it can become scarred and impact hearing
o Someone with many middle ear infections → put tubes into ear drum so it stays
continuously open and does not rupture all the time (impacts hearing later on)
- MIDDLE EAR: air filled cavity connected to the nasal pharynx by the eustachian tube
(normally closed unless chewing gum or crack ear)
- Inner ear: vestibular system + cochlea
o Cochlea: organ used for hearing – looks like a snail
o Cochlea is innervated by auditory and vestibular nerve
o Nerve is a joint nerve
o 8th nerve innervates the vestibular and auditory system
- Organ of Corti: sensory organ we use to hear – transmits sounds to electrical signal
Organization of the auditory system
- Sound hits ear drum, makes ossicles move and the stapes moves against the oval window
- Oval window: membrane that separates the cochlea from the middle ear
- From the oval window onward is the inner ear (contains coiled cochlea)
- There is a space filled with liquid called perilymph
o Perilymph: extracellular fluid
o Oval window moves against the liquid
o Liquids cannot be compressed
- At the other end of the structure, there is a round window (membranous window)
- As the stapes presses against the oval window, the entire liquid moves up the coil to apex
and back down through the perilymph space against the round window
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- Round window bulges in and out depending on how the stapes presses against the oval
window
- When sound enters ear, the stapes presses against the perilymph space and the liquid starts
to move all the way up the coil to the apex and down the other side of the coil through
perilymph filled space
- Perilymph embeds another space filled with endolymph (high potassium solution)
o Endolymph: similar to intracellular fluid
o Within endolymph filled space is the Organ of Corti (sensory organ)
Organization of the auditory system
- Stapes moves against perilymph space all the way up to the center of the coil and then down
to the round window
- Endolymph filled space is embedded and contains the basilar membrane
- Basilar membrane has a specific structural organization
o It is stiff at the beginning and as it extends towards the helicotrema (tip of the coil),
it becomes more floppy
- Depending on which frequency our stapes pounds against the oval window, basilar
membrane resonates and swings
o Resonance depends on frequency movement of the perilymph in the inner ear
- High frequency (20 kHz) makes the bottom of the basilar membrane vibrate
- Low frequency sounds (0.2 Hz) makes the basilar membrane vibrate at the tip
- Different frequencies make the basilar membrane swing at different areas
o They are distributed along the basilar membrane according to where it swings
optimally in response to a certain sound frequency
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Pinna: ear mostly cartilage tissue auditory canal: functions like a funnel and funnels sound waves into the ear canal (external, sound is funneled by pinna into outer ear canal. Outer ear canal contains glands that secrete ear wax: keeps the ear canal open and clean, hairs in the external auditory gland prevent insects from entering canal and expel dirt from the external canal. External auditory canal ends with the ear drum (tympanic membrane: when sound waves enter the canal, they hit the tympanic membrane and ear drum moves according to the sound waves. Ear drum: border between external and middle ear. = if they are taken on a flight and air pressure drops a lot, their ear drums will rupture = painful! It grows back so it is okay: middle ear infections when ear drum ruptures and inflammation can escape through a hole in the ear drum.
