Impedance and the Middle Ear

What is impedance?

the apparent opposition in an electric circuit to the flow of an altering current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current

What is another definition of impedance?

the ratio of the pressure to the volume displacement at a given surface in a sound transmitting medium

what are three ways the system helps to match impedance?

1. pressure ratio

2. lever action

3. buckling of the TM

describe the pressure ratio

the same force applied on a smaller area will apply more pressure such as the manubrium against the stapes footplate/oval window

how much db is added by the pressure ratio?

25 dB (17:1)

describe the lever action of the ossicles

the length of the manubrium and head of the malleus are longer than the long process of the incus

how much db is added by the lever action?

2 db

describe the buckling action of the TM

the TM buckles in with sound energy

how much db is added by the buckling action?

6 dB (2:1)

what is the osseous outer labyrinth?

composed of a series of ducts and cavities within the petrous portion of the temporal membrane

pinna attachment

attached to the temporal lobe by extrinsic muscles and ligaments

what are the two labyrinth systems of the inner ear?

-osseous outer labyrinth

-membranous internal labyrinth

what are the two sections of the osseous labyrinth?

the vestibular system (semicircular canals, vestibule) and the cochlea

what are the three semicircular canals?

superior/anterior, posterior/inferior, lateral/horizontal

describe the semicircular canals

lateral most portion of the bony labyrinth

describe the cochlea

medial most portion of the bony labyrinth

what is the modiolus?

the middle of the cochlea whee nerves from cranial 8 run

how long is the cochlea?

35 mm long

what is the turn of the cochlea?

2 3/4 from base of apex (not fully turned=SNHL)

what is the entrance of the cochlea?

the oval window

what is the function of the oval window?

provides a point of expansion for fluid movements within the cochlea

what are the sections of the membranous labyrinth?

vestibule (utricle and saccule), membranous semicircular canals, membranous cochlear labyrinth (cochlea)

the utricle is _______ than the saccule

larger

what are similarities between the utricle and saccule?

-both contain endolymph and are surrounded by a similar fluid called perilymph

-contain a macula

what is the macula?

the end organ for balance

-responsible for angular acceleration sensation

what is the end organ for hearing?

cochlea

describe the membranous semi-circular canals

arise from the utricle and contain endolymph and are bathed in perilymph

parts of the membranous semi circular canals?

superior, lateral posterior

each membranous semi circular canal returns to the utricle through _____ which contain an end organ called _________

ampullae, crista

canals are arranged _________ to each other

perpendicular

what are the canals of the membranous cochlear labyrinth?

-scala vestibuli

-scala media

-scala tympani

scala vestibili

contains endolymph

scala media

cochlear duct, contains endolymph and the organ of corti

scala tympani

closest scala to the modiolus where cochlear implants are placed through

what is reissner's membrane?

extends from the osseous spiral lamina to the outer bony wall and joins the basilar membrane at the helicotrems at the apex of the cochlea

what does reissner's membrane divide?

scala vestibili from scala media

what does basilar membrane divide?

scala media from the scala tympani

describe the basilar membrane

formed at the osseous spiral lamina and extends out to the outer wall of the cochlea via the spiral ligament

tonotopic organization of basilar membrane

wider and more flexible at the apex (LF) than the base (HF)

where does the organ of corti sit?

on the basilar membrane

what kind of sound transduction occurs at the organ of corti?

mechanical to electrical energy

name the traveling wave characteristics

-wave always starts at base of cochlea

-amplitude changes as it moves along the length of the cochlea

-greatest amplitude depends on frequency of stimulus

the basilar membrane acts like a _________ analyzer

frequency

how is the BM a frequency analyzer?

each place along the basilar membrane has a frequency to which it responds maximally

what is the range of human hearing?

20-20,000 Hz (not equally sensitive throughout

what is the range of human speech?

250-4/5,000 Hz

what range do audiologists test?

250-8,000 Hz

what does the minimum audibility curve show?

dB SPL vs frequency (hz) and dynamic range

threshold at birth are _________ than those in older children and adults

poorer

what frequencies are more sensitive in infants?

low frequencies

which types of frequencies improve first in infants?

high frequencies

how long does it take for low frequency thresholds take to be as good as adults?

about 8-10 years