Exam 2 Flashcards
What is an otoscope?
A handheld device w/ a light used for looking through the ear canal or nose.
What is the role of the eustachian tube?
Equalizes pressure so that the pressure behind the eardrum equals the pressure in front of it. (Auditory tube in picture)
Medial wall of the middle ear
- AKA Labyrinthine Wall
- Deep inside the ear; inner ear behind it
- Oval & round windows
- communicate w/ cochlea
- Pic: fenestra cochlea = round window; fenestra vestibuli = oval window
Lateral wall of the middle ear
- AKA tympanic wall
- Formed by the tympanic membrane
- First thing an audiologist would see when looking into the ear
Superior wall of the middle ear
- AKA tegmental wall
- Formed by a thin layer of bone, the tegmen tympani
- Roof of the cavity
- Separates middle ear cavity from cranial cavity
- Infection that spreads through superior wall causes meningitis.
Inferior wall of the middle ear
- Formed by the fundus tympani, a thin bony plate
- Lies above the jugular fossa (hole where the jugular bulb goes in) w/ the jugular bulb (holds jugular vein)
Anterior wall of the middle ear
- AKA carotid wall
- Connects to the back of the throat through the eustachian tube
- Contains the internal carotid artery (goes through carotid canal close to the anterior wall)
Posterior wall of the middle ear
Communicates with mastoid air cells through an opening, the aditus
Tympanic membrane: position & size
- Held in a groove called the tympanic sulcus
- Inclined at 55º angle
- Total area: 85 mm2
- Active area: 55 mm2
Tympanic membrane: layers
- Outer: epithelial, like our skin. Continuous w/ the ear canal skin
- Middle: fibrotic. Fibrous bridge between epithelial & mucosal. Gives the membrane flexibility, allowing it to move. Inactive portion lacks this.
- Inner mucosal: lining of nose continous with the back of the throat & eustachian tube. Secretes mucous when it’s stimulated
Tympanic membrane: Pars Tensa
Active portion of the tympanic membrane (55 mm2)
Tympanic membrane: Pars Flaccida
- AKA Sharpnell’s Membrane
- Inactive portion of the tympanic membrane (remaining 30 mm2)
Tympanic membrane: Cone of light
Reflection of light from the umbo to the lower surface of the eardrum that shows up when viewing eardrum through otoscope (inferior-anterior).
Tympanic membrane: Chorda Tympani Nerve
- Branch of facial nerve (7th)
- Sensory: supplies anterior 1/3 of tongue
- Motor: supplies muscles of facial expression
Tympanic membrane: umbo
Point of maximum retraction of the TM where the handle of the malleus attaches.
Middle ear ossicles
- Malleus, incus & stapes
- Serially connected
- Form an ossicular chain
- Suspended by 6 ligaments
- Handle of malleus attached to TM
Malleus
- AKA hammer
- Manubrium = handle
- (Blue arrow indicates umbo)
- 25 mg
Incus
- AKA anvil
- 25 mg
Stapes
- AKA stirrup
- Footplate rests on oval window of medial wall
- 2.5 mg (smallest bone in the body)
Ligaments of the malleus
- 3 ligaments:
- Superior: attaches to head
- Lateral: attaches to neck
- Anterior: attaches to anterior (lateral) process
Ligaments of the incus
- 2 ligaments:
- superior: attaches to body
- posterior: attaches to short process
Ligaments of the stapedius
Annular ligament: holds footplate onto oval window
Tensor tympani
- Attached to the handle of the malleus
- Held by semicanal for tensor tympani on medial wall
- Innervated by the trigeminal nerve (5th)
- Reflex contraction to loud sounds
- Increases tension of TM for sounds of 85-90 dB SPL or greater and < 1000 Hz
Stapedius
- Attached to the neck of the stapes
- Held by pyramid on posterior wall
- Innervated by the facial nerve (7th)
- Reflex contraction to loud sounds
- Stiffens the ossicular chain & puls the stapes outward & away from the cochlea
- Translates to 20 dB of protection (but only for sounds < 1000 Hz)
Contralateral/Ipsilateral response pathways
- In this diagram, sound is presented to the right ear
- Ipsilateral pathway = same side
- When you’re exposed to loud sounds, you also have protection on the opposite side because of the contralateral (opposite side) pathway
- bilateral protection; but, with high-frequency sounds, events happen too quickly for the stapedius to respond
- Pressure is funneled from larger to 17x smaller area
- Results in +25 dB SPL increase
- Pressure & area are indirectly related
Pressure, force & area formula
Mechanical leverage related to the ossicles of the middle ear
- Malleus & incus work together as 1 unit that pulls the stapes in & out of the oval window
- incus/stapes connection is the fulcrum
- stapes is the mass
- As you decrease the distance between the fulcrum & the mass, the output force increases
- 5 dB benefit
Divisons of the inner ear (labyrinth)
-
Bony labyrinth:
- vestibule (balance)
- semicircular canals (balance)
- cochlea (hearing)
-
Membranous labyrinth:
- utricule & saccule (balance)
- semicircular ducts (balance)
- cochlear duct/scala media (hearing)
Vestibulo-cochlear nerve
8th cranial nerve. Carries hearing (cochlear branch) & balance (vestibular branch) information to the brain.
Cochlea
- Coiled like a snail shell
- Wrapped around a central bony core “Modilus”
- Contains 8th nerve cell bodies
- 35 mm (uncoiled) in adults
- Chambers: scala vestibuli, scala tympani, & scala media
Cochlea chambers & membranes
- Scala vestibuli & scala tympani: bony labyrinth
- Scala media: membranous labyrinth
- Reissner’s membrane: separates scala vestibuli & media
- keep the fluids separate, but can rupture when exposed to loud noise
- Basilar membrane: separates scala tympani from scala media
- Tectorial membrane: proximal to organ of corti
- Helicotrema: inlet through which scala vestibuli & tympani communicate.
Basilar membrane anatomy
- AKA cochlear partition
- Stretched b/w bony spiral lamina & spiral ligament
- Primary site for Fourier analysis
- Base is tuned to high-frequency sounds; apex is tuned to low-frequency sounds
- Contains organ of corti
Basilar membrane & traveling wave theory
- We can hear 20-20,000 Hz
- Sound goes from oval window to apex & travels as a waveform
- Frequency-place principle: for a pariticular frequency, basilar membrane vibration reaches a peak amplitude at a specific point
- Tonotopic organization: for every pure tone we can hear, there is a place on the BM w/ maximal activity of the traveling wave (organization continues through brain)
- Stiffer at base – responds to higher frequencies
Organ of corti anatomy
- Located on basilar membrane
- Inner hair cells: 5000, 1 row
- Outer hair cells: 20,000, 3-4 rows
- have cilia, tallest of which is “stereocilia”
- have many supporting cells (Hensens, Dieter’s and Claudius)
- Habenula perforata: tiny opening from which all nerve fibers enter organ
- OHCs make contact with tectorial membrane; IHCs don’t
Organ of corti physiology
- OHC respond to sounds 0-40 dB SPL
- IHC respond to sounds > 40 dB SPL
- don’t have the benefit of direct contact, so you have to crank up the volume to make the TM move more
- Fewer IHCs, but they get more attention from nerve cells
- IHCs respond to higher intensities because you need many nerve fibers for faster transmission to the brain
Outer & inner hair cells innervation
-
IHC
- supplied by inner radial fibers (IRF)
- 95% of 8th nerve fibers
- one IHC supplied by 20 IRFs
-
OHC
- supplied by outer spiral fibers (OSF)
- 5% of 8th nerve fibers
- 1 OSF supplies 20 OHCs
*IHC get undivided attention of nerve cells; one nerve fiber connects w/ many OHCs
