Anatomy quiz 8

• pinna
• EAM (tympanic membrane)

• the prominence we refer to as the ear
• serves as a sound collector and aids in localizing sound

• external auditory meatus
• ear canal

ear drum

• cartilaginous meatus, osseus meatus, isthmus, tympanic membrane (eardrum)
• lateral third is cartilage
• inner two-thirds is (temporal) bone

approx. 7 mm in diameter

• S-shaped (not straight)
• first headed inward, forward and up, then turns down

• earwax
• lines the cartilaginous meatus along with hairs to trap foreign substances

• 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

where the tympanic membrane attaches to malleus bone

• ossicles:
• malleus
• incus
• stapes

• hammer
• head (articulates with body of incus)
• manubrium (handle)- attached to TM

• anvil
• body, short process, long process (tipped by lenticular process which articulates with head of stapes)

• stirrup
• 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)

• stapedius
• tensor tympani
• embedded in bone
• only tendons found in middle ear space
• both muscles stiffen middle ear system
• protection against loud sounds

• inserts in upper manubrium of malleus
• places more tension on TM
• reduces movement of TM

• inserts in neck of stapes (near head)
• pulls stapes away from oval window

• medial wall
• anterior wall
• posterior 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)

• entrance to Eustachian tube
• brings oxygen to middle ear
• helps maintain equilibrium between middle ear and atmospheric pressure (ear "pop")

prominence of stapedial pyramid (tendon of stapedius arises here)

• scala vestibuli- oval window
• scala tympani- round window
• scala media (basilar membrane; organ of corti)- middle chamber)

the basilar membrane

consists of inner and outer hair cells and converts movement to electrochemical energy which is carried by cohclear/acoustic nerve (cranial nerve VII)

• 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)

entryway to cochlea and vestibular system

• bony casing of the cochlea
• lined by the membranous labyrinth

osseous cochlear labyrinth

• scala vestibuli
• scala tympani
• scala media (cochlear duct)

divides scala vestibuli and media

• VII vestibulocochlear nerve
• both afferent and efferent connections (cochlea can receive signals from CNS)
• inner HC
• outer HC

type I nerve fibers innervate IHC, many fibers to one HC innervations

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

• serves as a "funnel" for sound and has resonant frequencies
• it enhances the intensity of sounds in the 1500-8000 Hz range

• impedence- resistance to energy flow
• the decibel scale is logarithmic, which means that a change of +40 dB is 10^4 more intense

can identify duration of signal, can identify timing of signals to each other

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

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

• "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

send sensory information up the auditory pathway

• when only OHCs are damaged, intensity thresholds go up and frequency discrimination is impaired
• when IHCs are damaged, complete hearing loss results

• 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

• many IHC activate a single VIII nerve fiber
• individual fibers respond best to specific frequencies (characteristic frequency)

shows frequency of respoinse at levels of variable

• 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

• 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

an auditory evoked potential extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp