Lecture Notes for 10/05/99

Lecture Notes for 03/29/00

I. _______ labyrinth

A. A series of canals and cavities housed in the dense petrous

portion of the temporal bone

B. Houses organs for hearing and balance

C. _________-filled

D. Divided into 3 parts

a. Has ______ turns in humans

1) Basal turn (bottom)

2) Medial turn

3) Apical turn (partial turn; top)

b. Coiled around central bony core called ___________

1) ____________________ –projection of central bony

core out of modiolus into cochlea

a) Divides turn into 2 spaces

i) Scala tympani (bottom)

ii) Scala vestibuli (top)

b) Has tiny perforations called ________________

which nerve fibers pass through

a) _______________ – bunch of nerve fibers in

periphery found inside modiolus

b) _______________ – channel that opens at

base

i) 8^th Nerve fibers – both aud & vest portions

ii) 7^th (facial) Nerve fiber

3) ________________ – fluid that fills bony labyrinth

a) Continuously circulating

b) Total fills 2 drops

c) Secreted by membrane that lines scala vestibuli

d) Clear, denser than water, similar in composition to

extracellular fluids like CSF

e) Potassium (K+) is ____ & sodium (Na +) is _____

f) Functions – involved in transmitting sound

pressure variations in cochlea

2. Bony ____________

a. Cavity w/ 4 walls, floor, and ceiling

b. Lateral wall – oval window

c. Posterior wall – entrances & exits to 3 semicircular

canals

d. Medial wall

e. Anterior wall

1) Opening to basal turn of cochlea

2) Above opening is circular recess which houses

another organ of membranous labyrinth (saccule)

f. Nothing important to note about superior or inferior walls

3. Bony semicircular canals

a. All at right angles to each other

b. 3- superior, posterior, lateral

II. _______________ labyrinth

A. Location

1. Inside bony labyrinth

2. Follows same shape, just skinner and shorter

B. Fluids

1. Space between bony labyrinth and membranous labyrinth

is filled with perilymph

2. Membranous labyrinth is filled with ______________

a. _______ in K⁺ and ______ in Na⁺

b. Opposite of perilymph composition

1. Formed by two membranes and attached to bony cochlea

a. Floor =

1) Flexible, dense membrane that varies in width and

stiffness - becomes wider and less stiff towards the

apex (top)

b. Ceiling =

c. Side wall =

2. _______________ = opening at apical end that allows

communication between the scala vestibuli and scala

tympani because the membranous cochlea is shorter than

the bony cochlea

3. The membranous cochlea houses the ______________

which is the sensory end organ for hearing

III.

A. General

1. Runs entire length of cochlear duct

2. Complex cellular structure that sits on top of the Basilar

membrane

3. Basic architecture

a. Framework of supporting cells

b. Hair cells with fine hairs (cilia) that protrude are embedded

in supporting cells

B. Hair cells

1. Outer hair cells (OHCs)

b. ________ rows (some evidence exists there may be _ rows

at the apex)

c. Anchored at the base by support cells and at the top by

the reticular lamina (see definition below) and

completely unsupported on the sides

d. Lower ____ of cell body is cell nucleus with packets of

mitochondria (higher metabolic activity) found on top

and lining sides

e. Subsurface layer in OHCs are similar to muscles and

contain proteins associated with muscle contractions

f. The hair cells have a thickening called the ___________

which covers the top of the cell except for a small section

called the _____________

g. Out of the top of the cuticular plate protrude ___________

1) _________ out of each OHC

2) Form a _____ shape

3) Vary in length

a) innermost are shortest, outermost are longest

b) longest hairs are embedded in the Tectorial

membrane (see below)

2. Inner hair cells (IHCs)

a. Irregular flask-shaped and larger in volume than the OHCs

b. _______ row

c. Completely surrounded by support cells

d. Cell nucleus in center of cell body with mitochondria

found in area of cuticle-free pore but not up the sides

e. Minimal subsurface layers

f. Also find cuticular plate/ cuticle-free pore

g. Stereocilia protrude through cuticular plate

1) ________ out of each IHC

2) Form a ____ shape

3) Also vary in length but do not make direct contact with

________________

3. Similarities between OHCs and IHCs

4. Differences between OHCs and IHCs à suggest differences

in function

C. Reticular lamina

1. Formed from the cuticular plates of the hair cells and parts of

the support cells

2. Creates a stiff plate covering for the hair cells which provides

a source of support for the upper surfaces of the hair cells

3. Also effectively isolates the structures and spaces of the

Organ of Corti from the endolymph filled portions of the

cochlear duct

D. Tectorial membrane

1. Mass of gelatin-like substance mostly composed of protein

Cochlear Physiology

I. Transduction process of energy

A. Input= vibration of footplate of stapes into oval window

which moves perilymph

1. Pressure is increased in perilymph

2. Increased pressure is in region of base

3. Pressure needs to be dissipated (move pressure in Scala

Vestibuli [ST] then Scala Tympani [ST])

4. Difference in pressure is relieved by downward motion

of displacement of Scala Media [SM]

5. Now ST has increased pressure

6. Round window bulges out and relieves pressure

B. Diagram (done in class)

C. When footplate moves out, sequence is reversed

1. - pressure in SV

2. + pressure in ST

3. Upward bending of SM

4. - pressure in ST

5. + pressure in SV

6. Round window is pulled inward

D. Up and down displacement of SM

1. High frequency stimulus

a. Rapid pressure increase in perilymph

b. Rapid pressure differential at base

2. Low frequency stimulus

a. Slow increase in pressure will travel all the way to the

helicotrema

II. Traveling wave

A. Up and down displacement of the SM because of the

transfer in pressure in the perilymph

B. Always seems to travel from base to apex of cochlea

1. Base is first because it is closest to the oval window

2. Figure (in class)

C. Amplitude is not uniform throughout traveling wave

D. For a given stimulus frequency, the maximum amplitude

will vary

1. High frequency (4-8 KHz) - max displacement

2. Low frequency (125-250 Hz) - max displacement

3. Mid frequency (500 - 2000 Hz) - max

E. From origin in base, there is a gradual increase until you

reach maximum point of displacement then it dies down

quickly

F. These waveforms show that place of max displacement is in

a very specialized place which is dependent upon stimulus

frequency

G. Max amount of time it takes for wave to travel from base to

apex is

H. von Bekesy determined that cochlea is mechanical frequency

analyzer (cochlea’s max point of displacement varies with

stimulus frequency)

III. Basilar membrane mechanics

A. There is a ____ change in width from base to apex

1. Base is _________________

2. Apex is ___________________

B. There is a _____ change in stiffness from base to apex

1. Base is very

2. Apex is

C. This and the fluid properties discussed earlier allow the

cochlea to act as a mechanical frequency analyzer

IV. Action of hair cells

A. IHCs

1. IHCs are true _________ centers

2. They convert acoustic energy into electrical signals

B. OHCs

1. OHCs are not ________ cells - they do not convert acoustic

energy into electrical signals

2. OHCs are ___________amplifiers (_________) which actively

put out energy

a. Can make themselves long & thin or short & fat in response

b. Respond together in phase which greatly _______ tuning of

cochlea

c. Reinforce _____________ movement

V. Otoacoustic Emissions (OAE)

A. In normal-hearing subjects, if you present a brief burst of sound,

then _______ ms later, a continuous emission of sound may be

present in outer ear

1. OAE s are ______ in intensity

2. People w/sensorineural hearing loss have elevated

thresholds of OAE s

a. OAEs may be absent at certain frequencies

B. Source of OAEs

1. Evidence that OHCs contract (active process)

2. Done by ________________

a. w/signal - cilia bend - produce electrical response - lateral

walls of OHC are altered (motility)

b. OHCs contract due to change in chemical characteristic in

cells - ions & H₂0 are pushed in & out (a fast hydraulic

pressure system)

3. Movement of OHC initiates a __________________

a. OHC movement - changes stiffness of rect. lamina & cilia

- alters coupling w/ tect. mem. & sound is returned back

out of cochlea to middle ear & ear canal

4. Prominent researchers in OAEs (a few of many)

a. Lonsbury-Martin - major proponent of electro-osmosis

theory

b. Kemp - discovered OAEs

c. Glattke

C. Clinical measurement of OAEs

1. Noninvasive - present low level sound to ear canal, then a

microphone picks up and records emissions

2. Three types - spontaneous (SOAEs), click-evoked (TEOAEs

or CEOAEs), and distortion-product (DPOAEs)

3. Can get frequency-specific info esp in __________ Hz range

4. Can tell us status of OHCs

5. Used primarily in newborn screening but also useful in adults

(malingering) and identifying auditory neuropathy