HIS 125 Functions of the Middle Ear Mechanisms

The tympanic membrane is the receiver and collector of acoustic pressure directed against the ear by vibratory energy in the air around us.

The tympanic membrane (TM) is conical, pointing medialward so that more surface area of the membrane can be available without having a larger opening for a ear canal.

The TM is found to be at approximately a fifty-five degree angle (not perpendicular to the ear canal) and has generally an elliptical shape.

The tympanic membrane (TM)/eardrum

1. Is composed of three layers of tissue.

2. Separates the external ear from the

middle ear.

3. Is connected to the malleus and

reflects the “cone-of-light”.

The three layers of tissue of the TM are:

1. The cutaneous layer (same layer as the EAM)

2. The fibrous layer (the “compliance” layer)

3. The mucous layer (same as the inner liner of the middle ear cavity)

The TM attachment at its superior portion to the malleus creates a movement which would more resemble a swinging motion than a piston (in and out) motion.

One tenth of one decibel of sound pressure will create movement of a healthy tympanic membrane by as little as 1/100 the thickness of a hydrogen molecule—it is very sensitive to sound pressure—yes even barometric pressure.

The TM moves much more easily at its inferior portion.

Note: Due to gravity, this is the area which may become most easily compromised by ear canal debris or middle ear fluid.

When the TM becomes too flaccid (floppy) or too tense, the sound transfer to the middle ear bones (ossicles) becomes easily influenced.

The three middle ear bones (ossicles) are:

1. The malleus (attached to the TM)

2. The incus (attached to the malleus and stapes)

3. The stapes (attached to the oval window)

Two muscles are attached to the middle ear bones.

A tendon attaches the stapedius muscle to the stapes. (innervated by the VII cranial/facial nerve)

A tendon attaches the tensor tympani muscle to the malleus. (innervated by the V cranial nerve)

The movement of the three middle ear bones creates a “mechanical amplifier”.

It is also found to be an impedance transformer changing low-impedance sound waves into a high impedance match with the inner ear fluid.

As the movement of the malleus and incus bones represent the to-and-fro motion of a swing, they actually rock the stapes in and out of the oval window (do not push it in and out like a plunger or piston).

This rocking action of the stapes actually decreases once the brain senses sound pressures greater than seventy decibels due to the contraction of the middle ear muscles.

The most widely accepted theory regarding this middle ear muscle contraction is to protect the ear from too much sound pressure—mostly from low frequency energy (below 2000hz). In other words, their contraction creates a high-pass filter.

The Eustachian tube

It equalizes the air pressure between the middle ear cavity and the external auditory meatus. It usually opens and closes every time we swallow.

The Eustachian Tube

It also serves as a drainage conduit when any fluid has collected within the middle ear such as the effects of a head cold.

Let’s review the middle ear functions detailed in Northern page #12

The decibel level effects are detailed quite nicely.

Note: It is interesting the descriptions regarding any attempts to improve human middle ear function as detailed in the last paragraph of Northern chapter #1.

Let’s review it together.