Middle ear anatomy and physiology

consists of

-inner skin of TM

-closed middle ear space lined with mucous layer and surrounded by porous mastoid bone

-Eustachian tube close otherwise forced open

-middle ear ossicles: malleus, incus, stapes, ligaments

-two muscles: stapedius VII facial nerve, tensor tympani V trigeminal nerve

closed middle ear space is about

2cm3

Soft mastoid portion- houses outer ear canal, mastoid bone (air pockets)

Hard petrous portion- houses most of middle ear space and cochlea

mucous membrane

lines entire space (including inside of TM and Eustachian tube)

jugular vein

lies just below floor of middle ear space

epitympanic recess

middle ear space just above TM and malleus

aditus ad antrum

area just above incus and stapes (more medial than epitympanic recess) mastoiditis begins here

carotid artery

sits behind anterior wall (not in overhead)

chorda tympani nerve

branch of VII facial nerve that passes through middle ear space

-can get in way of middle ear surgery

-carries taste info 2/3 one side of tongue

oval window

filled by footplate of stapes

round window

covered by tough membrane

promontory

landmark bump between oval and round windows

Eustachian tube

connects middle ear to throat

cilia mucous lining move particles down

-closed unless swallowing, yawning, blowing nose

-opens once/min while awake and 1/5min when asleep

Air pressure must be

EVEN ON BOTH SIDES of TM for middle ear to work optimally middle ear mucous lining absorbs o2

-alleviates negative pressure

malleus

manubrium embedded in TM extends from umbo to top of TM

incus

long process and short process are the 2 main landmarks

stapes

head attached to incus

crura attached to footplate

footplate in oval window

ligaments

extend from walls of middle ear space to hold ossicles in place

stapedius muscle

7mm long, goes from posterior wall of middle ear space to neck of stapes

-innervated by V11 facial nerve- STRONGEST 2 MUSCLES

tensor tympani muscle

25mm long inserts into manubrium of malleus

innervated by weaker mostly sensory cranial nerve V or trigeminal

Physiology #1

air pressure has to be equal both sides of TM for middle ear to do job

-TRANSDUCES or changes vibration energy of sound into mechanical energy

ROCKING MOTION of footplate

Physiology #2

OVERCOMES IMPEDANCE MISMATCH

-as sound goes from AIR TO FLUID of cochlea

-EX swimming pool head under water CANT HEAR

AIR BORNE SOUND has to activate fluid filled cochlea

-or else loss of energy 99.9% (30-35dB)

Rinne tuning fork test

compares clients own HL by AC and BC

-what middle ear adds

normal ear should hear tone louder when fork is outer ear

1. area of TM (-) larger than footplate of stapes

17 times

17:1

2. leverage action

of ossicles increase force from TM to footplate by ratio

1.3:1

3. buckling action of TM

2:1

combine of increase SPL at oval window

is 44:1

17 x 1.3 x 2 = 44.1