Hearing

 

Alexander Liss

 

11/10/2004

 

corrected 11/12/2008

 

 

     The mechanism of hearing is well studied.

     Sound processing involves Middle and Inner Ears.

     The Middle Ear is an air filled cavity separated from external environment with elastic tympanic membrane (eardrum).

The Inner Ear is a liquid filled cavity, which has two openings into the Middle Ear both covered with an elastic membrane: oval window and round window.

There is a complex mechanism of small bones suspended with ligaments and muscles in the Middle Ear:

Working of two muscles of the Middle Ear is understood only partially. The function of stapedius as a guardian against an overload of the Internal Ear, especially from internal sounds (yelling), is well studied. Some forms of vibration of stapedius, possibly caused by its "spasm", cause tinnitus (ringing in the ear).

     The cavity of the Inner Ear has complex configuration, which is often described as a "vestibule" adjacent to the oval window, which "leads" to two distinctive areas: one consists of three semicircular channels and the other is cochlea housed in a spiral dead-end cavity. 

     Semicircular channels contain small bony particles suspended in the liquid and hairs-receptors. The entire system reacts to heads and tails of pressure waves in the liquid of the Inner Ear, because these fronts of waves move particles, which in turn bend hairs-receptors.

Such waves could be caused by the body acceleration (tree semicircles allow determination of the direction of such acceleration).

The cochlea has a special membrane of varying thickness along its spiral cavity with hairs-receptors embedded in it. 

Sound in the ear is generally described as a sound wave propagating through a chain of transmitters:

1.  tympanic membrane, which passively reflect a wave of external environment; it is silently assumed that variations of movement of different points of this membrane are unimportant;

2.  ossicles chain with its muscles, which collects energy of the sound on a large area of the tympanic membrane and passes it through a small area of the oval window; muscles are assumed to be static;

3.  oval window - cochlea - round window.

It is silently assumed that after the wave exits the Inner Ear, its energy dissipates.

In sum, these assumptions are highly unrealistic.

Let us introduce a different set of assumptions and arrive to a different more realistic model.

First, we revisit role of muscles.

Maintaining static level of muscles tension is an expensive task for a body. Muscle fibers contract and relax randomly and coordinating their actions that an average tension stays in a narrow range is difficult. It is much less expensive to organize periodic contractions and relaxations of a muscle. Hence, most likely, tensor timpani and stapedius are in the process of periodic contraction and relaxation. Periods and strength of contraction of these two muscles should be independent, because they are innervated from different nerve pathways.

When stapedius contracts, it tilts stepes, reduces area of contact between bony stepes and the membrane of the oval window. Consequently, this reduces level of energy of sound in the Inner Ear (bones transmit sound better than air). In addition, it changes the form of sound wave entering the Inner Ear and distribution of its energy in the Inner Ear.

Hence, in addition to protecting the ear from energy overload, stapedius actively participates in sound analysis.

When tendon tympani contracts, it increases tension in the tympanic membrane and this changes its sound qualities. Hence, the tendon tympani also participates actively in sound analysis.

If this assumption is true, then weakening of muscles with age should lead to worsening of hearing, which is a known fact. Exercising of these weakened muscles should improve hearing. There is anecdotal evidence that it is true also, and there is a cottage industry dedicated to such exercises, but there is no scientific evidence in this area yet.

Second, we revisit role of the tympanic membrane.

The tympanic membrane is under tension; hence, it has to have particular forms of vibration, where it resonates. The set of these forms depends on degree of its tension exerted by the tendon tympani. When the membrane’s tension changes, the set of these forms changes.

Hence, the reaction of tympanic membrane depends on the form of the sound wave in external environment. Obviously, the form of a sound wave is modified by the external ear. This is not important for a sound analyzer, it is important that different forms of sound waves are still different after transformation by the external ear.

That one ear can discriminate between different forms of waves and even could help to determine location of the sound this way without help of another ear is an easily verifiable fact.

When the tympanic membrane has high tension, it resonates at higher frequencies, hence when tensor timpani changes its tension it changes sound in the ear.

Third, we assert that different characteristics of sound are analyzed in different places of the Internal Ear.

It is known that different ears have different capacity to differentiate tones and clicks and often one ear is used to analyze tones and the other to analyze clicks. This means that there are separate mechanisms in the ear to do these two forms of analysis.

Natural sounds have a head (an interval of growing strength) and a tail (an interval of diminishing strength).

Most likely, heads and tails of sounds are analyzed in semicircular channels, which are attuned to detection of displacements caused by heads and tails of sounds.

The body of the sound with relatively stable intensity of the sound most likely is analyzed by cochlea.

Fourth, we dispel the notion of the running sound wave in the ear and replace it with an assumption of the standing wave.

There is no dampening device in the ear; hence, one cannot assume that energy of the sound wave dissipates at returning into the Middle Ear; hence, a realistic model has to deal with sound environment in the entire assembly of the Middle and Inner Ears under changing external conditions.

Functioning of hairs-receptors in cochlea could be logically explained, when there is a standing wave in it.

This standing wave is defined by all elements of the ear, not only by characteristics of the cochlea. For example, the Middle Ear creates two pathways of sound, one through bones to the oval window and the other through air to the round window with differential of speed of sound over these pathways. The stapedius controls the ratio of sound energy coming via these pathways; hence it controls the standing wave.

Now we summarize an alternative model of sound analysis by the ear, based on above assumptions:

1)  Muscles of the Middle Ear (tensor tympani and stapedius) are in the process of periodic contraction and relaxation and their actions are independent. The brain controls parameters of these actions and receives back information about current state of muscles (degree of their contraction).

2)  Sound environment in the Middle Ear and in the cochlea is a slowly changing standing wave; this standing wave is changing even for constant external sound, because of actions of muscles.

3)  The tensor tympani affects the standing wave trough changing properties of tympanic membrane.

4)  The stapedius affects the standing wave trough changing the level of sound energy passing through the oval window (back and forth).

5)  The tympanic membrane affects the standing wave for two reasons:

a)  there are particular forms of vibration, where it resonates,

b)  it is affected by the external sound environment of a particular form of wave.

6)  Receptors in the cochlea detect properties of this standing wave.

7)  Rapidly changing external sound environment (heads and tails of natural sounds) is detected and analyzed in semicircular channels of the Internal Ear.

8)  Brain receives information from muscles, cochlea and semicircular channels and extracts patterns, which it interprets as characteristics of external sound environment.

This model could be used as a basis for classification of hearing disorders.

Muscles related disorders could be related to one of the muscles of the Middle Ear or to both of them.

Disruption of brain control over muscles activity leads to diminished ability to differentiate tones of sounds.

Unusually strong contractions of tensor tympani present to the brain unheard before high pitch sounds of external and internal environment; this could be confusing.

Inability of tensor tympani to make strong contractions (weakening of muscles) leads to removal from analysis of standing waves, which correspond to the highly tense state of the tympanic membrane, when it resonates on high frequency component of external sound. This leads to loss of ability to detect a high frequency component of sound and inability to hear sounds, which are completely high frequency.

Too short period or too strong contraction in the cycles of muscles contraction and relaxation could lead to generation of sound by the mechanism of the Middle Ear (ringing, clicks in the ear, tinnitus). This mechanism has a range of normal operations and this range narrows with age because of loss of original flexibility. Hence, with age, it could produce sounds on its own more often.

Disorders related to the tympanic membrane could be caused by its damage or diminished elasticity.

Damaged membrane messes up entire picture of correspondence of external sound environment and internal standing waves and this leads to misinterpretation of sound by the brain.

Membrane of diminished elasticity could resonate on high frequencies at some level of its tension, but there is no level of tension, which could make it resonate on low frequencies. This leads to loss of ability to detect a low frequency component of sound and inability to hear sounds, which are completely low frequency.

Presence of control circuitry in the system (contractions of muscles controlled by the brain) introduces possibility of a special kind of disorders, where the system flips in an undesirable pattern of operation (pattern of cycles of contraction and relaxation, similar to muscles twitching). This could manifest itself in distorted sound perception, in noises produced by the mechanism of the Middle Ear, and even in pain.

Problems with ear usually cause anxiety and stress and this actually could reinforce such bad state of the body.

Note that hearing aide does not help with any of these disorders. However, there are a few things, which could be helpful.

One group of measures includes exercises of muscles of the Middle Ear. They should become a regular routine of aging people, because muscles weaken with age.

The other group of measures includes techniques of flipping the body out of bad patterns of cycles of muscles contraction and relaxation. In some cases, it is possible to flip body back into an acceptable state with change of activity or with a special exercise. For example, sometimes, it is sufficient to change sound environment and engage in physical activity to get rid of an incident of tinnitus. The very understanding that a healthy body could accidentally fall in a bad state of operations and the only thing, which is needed in this case is to kick it back into normal state, could help solving this kind of problems.