What are the two sections of the outer ear?
- EAM (tympanic membrane)
What is the pinna?
- the prominence we refer to as the ear
- serves as a sound collector and aids in localizing sound
What is the EAM?
- external auditory meatus
- ear canal
What is the tympanic membrane?
What is the EAM made up of?
- cartilaginous meatus, osseus meatus, isthmus, tympanic membrane (eardrum)
- lateral third is cartilage
- inner two-thirds is (temporal) bone
How big is the EAM?
approx. 7 mm in diameter
What is the shape of the EAM?
- S-shaped (not straight)
- first headed inward, forward and up, then turns down
What is cerumen?
- lines the cartilaginous meatus along with hairs to trap foreign substances
What are characteristics of the tympanic membran?
- boundary between outer and middle ear
- 3-layered disk of tissue
- a fibrous layer is "sandwiched" between continuations of the epithelial linings of the outer and middle ear
What is the umbo?
where the tympanic membrane attaches to malleus bone
What are the bones of the middle ear?
What is the malleus?
- head (articulates with body of incus)
- manubrium (handle)- attached to TM
What is the incus?
- body, short process, long process (tipped by lenticular process which articulates with head of stapes)
What is the stapes?
- head (articulates with the lenticular process of the incus)
- crura (the neck of stapes bifurcates to become the crura)
- footplate (meets oval window of tempral bone, transfers energy to cochlea)
What are the muscles of the middle ear?
- tensor tympani
- embedded in bone
- only tendons found in middle ear space
- both muscles stiffen middle ear system
- protection against loud sounds
What is the tensor tympani?
- inserts in upper manubrium of malleus
- places more tension on TM
- reduces movement of TM
What is the stapedius?
- inserts in neck of stapes (near head)
- pulls stapes away from oval window
What are the middle ear cavity landmarks?
- medial wall
- anterior wall
- posterior wall
What is the medial wall?
- oval window is where the stapes transfers energy to the cochlea
- round window is below oval window (secondary tympanic membrane)
- promontory (bulge between windows; created by nasal curvature of cochlea)
- above oval window (prominence of lateral semicircular canal; prominence of facial nerve)
What is the anterior wall?
- entrance to Eustachian tube
- brings oxygen to middle ear
- helps maintain equilibrium between middle ear and atmospheric pressure (ear "pop")
What is the posterior wall?
prominence of stapedial pyramid (tendon of stapedius arises here)
What are the three sections of the cochlea?
- scala vestibuli- oval window
- scala tympani- round window
- scala media (basilar membrane; organ of corti)- middle chamber)
What divides the scala tympani from scala media?
the basilar membrane
What is the organ of Corti?
consists of inner and outer hair cells and converts movement to electrochemical energy which is carried by cohclear/acoustic nerve (cranial nerve VII)
What is the vestibular system?
- the vestibular system makes up the sensory organs for balance
- the parts of this system are the saccule and ultricle, which are fluid-filled bulges that are located centrally and contain otoliths (crystals)
What is the vestibule?
entryway to cochlea and vestibular system
What is the osseous labyrinth?
- bony casing of the cochlea
- lined by the membranous labyrinth
What is shaped like a snail shell?
osseous cochlear labyrinth
What are the chambers within the cochlea?
- scala vestibuli
- scala tympani
- scala media (cochlear duct)
What is Reissner's membrane?
divides scala vestibuli and media
Innervation of hair cells
- VII vestibulocochlear nerve
- both afferent and efferent connections (cochlea can receive signals from CNS)
- inner HC
- outer HC
What innervates the inner hair cells?
type I nerve fibers innervate IHC, many fibers to one HC innervations
What innervates outer hair cells?
type II nerve fibers innervate OHC, one fiber branches to many HC innervations
- sound energy travels as a wave through the perilymph of the scala vestibuli
- the basilar membrane is then deflected toward the scala tympani and hair cells are excited
- different places on basilar membrane respond to different frequencies of sound
- higher frequencies near base
- lower frequencies near apex
Physiology of outer ear
- serves as a "funnel" for sound and has resonant frequencies
- it enhances the intensity of sounds in the 1500-8000 Hz range
Physiology of middle ear
- impedence- resistance to energy flow
- the decibel scale is logarithmic, which means that a change of +40 dB is 10^4 more intense
What is temporal analysis?
can identify duration of signal, can identify timing of signals to each other
What is spectral analysis?
refers to the process of extracting or defining the various frequency components of a given signal, most sounds are made up of many frequencies (except pure tones)
- when the stapes compresses the perilymph in the middle ear, the basilar membrane is displaced and a traveling wave is initiated
- the direction of the traveling wave is basal to apical
- when the wave reaches the greatest point on the basilar membrane, it is then quickly dampened
When moving from the base to apex, what happens to the basilar membrane?
It becomes less stiff, wider, and more massive
- the stereocilia are embedded in the tectorial membrane
- the movement of the basilar membrane bends the stereocilia in a shearing action
- the OHC are then depolarized
- the inner hair cells are then excited and the stereocilia are deflected by fluid motion
What are outer hair cells?
- "cochlear amplifiers"
- they contract when excited
- when the frequency of this mechanical action matches the natural frequency of that part of the basilar membrane, vibration is enhanced
What are inner hair cells?
send sensory information up the auditory pathway
What happens when OHCs and IHCs are damaged?
- when only OHCs are damaged, intensity thresholds go up and frequency discrimination is impaired
- when IHCs are damaged, complete hearing loss results
What happens at resting potential?
- differences in electrical charge in cochlea at rest
- scala media is more positive than scala vestibuli and scala tympani
- hair cells are negative relative to endolymph
- generates electrical impulses
- outer hair cells
- closely follows pattern of input signal
What is whole-nerve (compound) potential?
- many IHC activate a single VIII nerve fiber
- individual fibers respond best to specific frequencies (characteristic frequency)
What is a histogram?
shows frequency of respoinse at levels of variable
Post-stimulus time histograms
- how many times does a given nerve fiber fire following stimulus onset?
- responds a lot when tone of characteristic frequency begins
- drop down to random level
nerve fiber will respond at much lower dB level for sounds at characteristic frequency
Interspike interval + period histograms
- fiber fires in phase with tone at its characteristic frequency
- interval between firing matches period of vibration (time of 1 cycle of vibration)
- fiber can only fire every 1 ms
- frequencies higher than 1000 Hz, fire with every x cycles of vibration
What is the auditory brainstem response?
an auditory evoked potential extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp