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How does tinnitus happen? ─ Modern Tinnitus Theory

Tinnitus can originate anywhere on the auditory path. What is the auditory path? What constructs does it contain? What function does it have? To understand tinnitus, we must first understand the normal auditory anatomy and structure.

The so-called auditory path refers to the sound conduction path from the ear to the loud sound. The ear is generally divided into three parts: the outer ear, the middle ear and the inner ear (Fig. 1). The outer ear includes the auricle and the outer ear canal. The middle ear is a cavity, and important structures include the eardrum, three ossicles, and the eustachian tube. The inner ear is a lost fluid, and the most important cell for hearing is the inner ear hair cells. This is our receiving organ for sound. At present, most of the hearing impairments in the clinic, or part of the tinnitus, are caused by the destruction or death of hair cells.

The auditory nerve is formed after the nerve bundles under the hair cells are assembled. The auditory nerve enters the brainstem and connects to the cochlear nucleus. Then the nerve bundle of the cochlear nucleus goes up again and goes to the upper olive nucleus. In this position, most of the nerve bundles go to the opposite side. Similar to many other systems of sensation and movement, most of the nerves in the left ear go to the right brain, while most of the nerves in the right ear go to the left brain.

In the brainstem, the nerve bundle of the upper olive nucleus goes up to (Inferior colliculus) and MGB in thalamus. Then go up to the auditory cortex of the brain. The human brain’s auditory cortex has hundreds of millions of nerve cells, located in the temporal lobe of the brain (Fig. 2), which is also closely linked to other parts of the brain.

It is generally said that the auditory nervous system is divided into the peripheral nervous system and the central nervous system. The peripheral nervous system refers to the outer ear, middle ear, inner ear and auditory nerve. From the nucleus of the cochlea to the auditory cortex of the brain, it belongs to the central auditory system. In the early years, a scholar who studied tinnitus in order to know whether tinnitus was produced by the central or peripheral auditory system? He cut off the auditory nerves of many patients with tinnitus by surgery. As a result, half of all patients lost tinnitus and the other half remained Continued, indicating that the source of tinnitus is not simple, it is possible from the surrounding or the center.

How does tinnitus occur?

As for the transmission mechanism of sound? That is to say, how do we hear the sound of the outside world and distinguish it? The sound is a kind of vibration. For example, the voice of a person is the vibration of the vocal cords and the resonance of the throat and the mouth. .

The general sound is transmitted in the air. That is to say, the sound is the vibration of the air. Such vibrations are collected by the outer ear and shake the eardrum and the ossicles attached to the eardrum. The vibration of the small bone will be transmitted to the inner ear to cause the movement of the inner lymph fluid, and then stimulate the inner ear hair cells. When the inner ear hair cells are stimulated, the neurotransmitters are released, which in turn causes the action potential of the auditory nerve fibers. This action potential is an electrical energy that can be transmitted between nerves, so that electrical energy is routed to the auditory cerebral cortex. Whether it is simple pure tone, noise, complex music and voice, follow this path to the brain’s auditory cortex. Obviously, what kind of genre is the sound? What is the meaning? It is the brain hearing. The main function of the cortex. Aisheng hearing aid

Finally, there is a very special feature in the auditory path, called the tonotopical organization. For example, in the inner ear, the part of the bottom of the cochlea is responsible for the reception of high-frequency sound and the top ring is responsible for the reception of low-frequency signals. . This means that there is a frequency distribution map on the basement membrane of the inner ear, and different anatomical positions are responsible for the analysis of signals of different frequencies. Such a frequency map from the inner ear to the auditory nerve, to the cochlear nucleus, to the upper olive nucleus, to the IC, to the hypothalamus, until the brain auditory cortex has always existed, that is, every point on the auditory path has a different The nerve cells are responsible for receiving and analyzing the sound signals of different frequencies. After understanding this, it is helpful to understand the theory of tinnitus.

After many experts have long studied, there are now tinnitus theories that most people accept. Of all patients with tinnitus, about 80% is not worth it. Tinnitus does not have any effect on their lives. However, in patients with convulsions, tinnitus may affect their lives, or cause anxiety, irritability, and even some life barriers. It is estimated that such a serious patient accounts for about 4%. In clinical terms, there is no difference between the two patients in the measurement of tinnitus. It means that hearing tinnitus does not necessarily mean that tinnitus will cause problems. So modern tinnitus theory also separates the two, one theory explains why tinnitus is produced, and another theory explains why in a few people, tinnitus can cause serious problems.

The theory of tinnitus:

The results of animal experiments have shown that the production of tinnitus originates from damage or lesions of hair cells inside and outside the cochlea. Especially when the inner hair cells function normally and the outer hair cells have lesions, the uncoordinated function of the two is the biggest cause of tinnitus.

The hair cells in the inner ear are divided into two types. The function of the inner capillary is mainly to transmit the sound signal, or the outer hair cell is to be locally enlarged to increase the sensitivity and frequency resolution. However, the second type of auditory nerve connected by the outer hair cells can be connected upward to the dorsal cochlear nucleus of the brainstem, where it has the function of suppressing the abnormal discharge of the auditory nerve bundle. When the outer hair cell function is lost, the function of this lateral inhibition disappears. So there will be a lot of abnormal activities in the brainstem, and after receiving it by the brain, there will be a sound, that is, the feeling of tinnitus.

Such a theory can illustrate some of the clinically common phenomena. First, there are no hearing loss in patients with tinnitus with about 20~30%. In fact, these people have lost hair cells, and the inner hair cells are still normal, so they have not affected hearing. In addition, a very high proportion of patients with very severe hearing impairment do not feel tinnitus because the hair cells and outer hair cells in these patients have been lost, and there is no such problem of inconsistency. In addition, the frequency of tinnitus measured by many tinnitus patients is very close to the worst frequency of hearing, and it is also a clinical phenomenon supporting this theory.

When some inner hair cells and outer hair cells of the inner ear are lost. In the cochlear nucleus of the brainstem, the nerve cells in the frequency block lost relative to these hair cells gradually lose their function. What should be reminded here is the frequency organization diagram of the auditory path mentioned above: that is, from the inner ear to the auditory cortex of the brain, there is a frequency block diagram, and different places are responsible for receiving and parsing sounds of different frequencies. Therefore, after the hair cell function of some frequencies in the inner ear disappears, over time, the neurons responsible for those frequencies are also necrotic. The brain is plasticity, so it tries to repair or compensate for these lost functions. As a result, hyperactivity is caused by the hyperactinity of neurons near these frequency blocks, resulting in chronic tinnitus.

This is the so-called “scar theory.” In short, the lesions of the peripheral auditory system, such as the inner ear, cause the “scar” brain of the auditory cortex of the brain to try to compensate or repair the function of this lost part based on its own “plasticity.” However, excessive compensation has caused excessive activity of nearby normal neurons, and the feeling of unnecessary sound is tinnitus.

There is a similar phenomenon in other organs in the clinic. It is often compared with tinnitus that is called “phatom limb”. Phantom limb pain means that in some patients who have been amputated, they can still “feel” the presence of that part of the limb and even feel pain. This is similar to tinnitus, which is the wrong reaction and treatment made by the central center to compensate for the loss.

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