Tinnitus can originate from any part of the auditory pathway. What is the auditory pathway? What structures does it contain? What functions does it have? To understand tinnitus, we must first know the normal auditory anatomy and structure.

The so-called auditory pathway refers to the 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 external ear includes the pinna and the external auditory canal. The middle ear is a cavity. The important structures include eardrum, three ossicles and eustachian tube. Inner ear is a fluid filled labyrinth, and the most important cell for hearing is inner ear hair cell. This is our receiving organ for sound. At present, the vast majority of hearing impairment or part of tinnitus is caused by the destruction or death of hair cells.

The nerve bundles under the hair cells form the auditory nerve. The auditory nerve enters the brainstem and connects to the cochlear nucleus. Next, the nerve bundle of cochlear nucleus goes up to the superior olive nucleus. At this position, most of the nerve bundles are on the opposite side. Like many other sensory and motor systems, most of the nerves in the left ear move to the right brain, while most of the nerves in the right ear move to the left brain.

In the brainstem, the nerve bundle of the superior olivary nucleus goes all the way up to (inferior colliculus) and MGB in thalamus. And up to the auditory cortex of the brain. There are hundreds of millions of nerve cells in the auditory cortex of the human brain, which are located in the temporal lobe of the brain (Fig. 2). They are also closely connected with other parts of the brain.

Generally speaking, the auditory nervous system is divided into peripheral nervous system and central nervous system. Peripheral nervous system refers to the outer ear, middle ear, inner ear and auditory nerve. From the cochlear nucleus to the auditory cortex, it belongs to the central auditory system. In order to know whether tinnitus is produced by the central or peripheral auditory system, an early tinnitus researcher cut off the auditory nerves of many tinnitus patients by operation. As a result, half of the tinnitus disappeared and the other half was still special, indicating that the source of tinnitus is not simple, and it is possible to cause tinnitus from the surrounding or central area.

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How about the transmission mechanism of sound? That is to say, how do we hear the external sound and distinguish it? Sound is a kind of vibration, for example, the voice of human speech is the vibration of vocal cord plus the resonance of throat and mouth.

And the general sound is transmitted in the air. That is to say, sound is the vibration of air. This kind of vibration will vibrate the eardrum and the ossicles attached to the eardrum after being collected by the outer ear. The vibration of the auditory ossicles can be transmitted to the inner ear to cause the movement of the inner lymph and then stimulate the hair cells of the inner ear. When the inner ear hair cells are stimulated, they will release neurotransmitters, which will further cause the action potential of the auditory nerve fibers. This action potential is a kind of electric energy, which can be transmitted between nerves, so the electric energy is transmitted to the auditory cortex through brain stem. No matter simple pure tone, noise, complex music and voice, they all follow this path to reach the auditory cortex of the brain. Obviously, it is the main function of the auditory cortex to understand what kind of sound belongs to and what the meaning is. Aisheng hearing aid

Finally, there is a very special feature in the auditory pathway, which is called the frequency organization. For example, in the inner ear, the part of the cochlear bottom circle is responsible for receiving the high-frequency sound while the top circle is responsible for receiving the low-frequency signal. It 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 different frequency signals. Such a frequency map exists from the inner ear to the auditory nerve, to the cochlear nucleus, to the superior olive nucleus, to the IC, to the inferior colliculus, and to the auditory cortex of the brain. That is to say, at every point of the auditory pathway, different nerve cells are responsible for receiving and analyzing sound signals of different frequencies. After understanding this point, it is helpful to understand the theory of tinnitus production.

After a long time of research by many experts, the theory of tinnitus has been accepted by most people. About 80% of the patients with tinnitus are not satisfied. Tinnitus has no effect on their lives. But the rest of the patients, tinnitus may affect their lives, or cause anxiety, irritability, and even some life obstacles. It is estimated that about 4% of patients need to seek medical treatment. In clinic, there is no difference in the measurement of tinnitus between the two kinds of patients. Hearing tinnitus does not necessarily mean that tinnitus will cause problems. So modern tinnitus theory also separates the two. One theory explains why tinnitus occurs, and the other explains why tinnitus causes serious problems in a few people.

Theory of tinnitus production:

At present, animal experiments have proved that tinnitus is caused by damage or pathological changes of hair cells inside and outside the cochlea. Especially when the inner hair cells function normally and the outer hair cells have pathological changes, the discordant function of the two is the biggest cause of tinnitus.

There are two kinds of hair cells in the inner ear. The function of the inner capillaries is mainly to transmit the signal of sound, or the outer hair cells are locally amplified to increase sensitivity and frequency resolution. However, the second type of auditory nerve connected by outer hair cells can be connected to the dorsal cochlear nucleus of brain stem, which has the function of inhibiting the abnormal discharge of auditory nerve bundle. When the function of outer hair cells is lost, the function of this inhibition disappears. So there will be a lot of abnormal activities in the brain stem, and after being received by the brain, there will be a sound, that is, the feeling of tinnitus.

Such a theory can explain some common clinical phenomena. First, about 20-30% of tinnitus patients have no hearing loss. In fact, these people have lost the outer hair cells, while the inner hair cells are still normal, so they do not affect hearing. In addition, a high proportion of patients with very severe hearing impairment do not feel tinnitus, because these patients have lost both inner and outer hair cells, so there is no so-called discordant problem. In addition, the tinnitus frequency measured by many tinnitus patients is very close to the frequency of the worst hearing, which is also a clinical phenomenon supporting this theory.

When some inner hair cells and outer hair cells of inner ear are lost. In the cochlear nucleus of brain stem, the nerve cells in the frequency block relative to the loss of these hair cells will gradually lose function. What should be reminded here is the frequency organization chart of the auditory pathway mentioned earlier: from the inner ear to the auditory cortex of the brain, there is a frequency block chart, and different places are responsible for the reception and analysis of different frequency sounds. So after the hair cells of some frequencies in the inner ear disappeared, over time, the neurons responsible for those frequencies in the brain also died. The brain is plastic, so it tries to repair or compensate for these lost functions. As a result, the hyperactivity of neurons near these frequency blocks results in chronic tinnitus.

This is the so-called scar theory. In short, it is the lesions of the inner ear and other peripheral auditory systems that cause the “Scar” brain in the auditory cortex of the brain. Based on its own “plasticity”, it attempts to compensate or repair the lost part of the function. But the excessive compensation caused the excessive activity of the nearby normal neurons, and the unnecessary sound feeling was tinnitus.

In clinic, there is a similar phenomenon in other organs, which is often compared with tinnitus to simulate phantom limb. Phantom limb pain means that some amputees can still “feel” the existence of that part of the body, and even feel pain. This is similar to tinnitus, which is the wrong response and treatment made by the center to compensate for the loss of function.

Link:How does tinnitus happen? - modern tinnitus theory

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