Mechanisms of Subjective Tinnitus

The inner ear contains thousands of minute hairs which vibrate in response to sound waves. Receptor cells (hair cells) in turn send signals to the brain, which are interpreted as sound. Although receptor cells can be regenerated from the adjacent supporting Deiters cells after injury in birds, reptiles, and amphibians, in mammals it is believed that they can be produced only during embryogenesis. Although mammalian Deiters cells reproduce and position themselves appropriately for regeneration, they have not been observed to transdifferentiate into receptor cells except in tissue culture experiments. Therefore, if these hairs become damaged, through prolonged exposure to excessive decibel levels, for instance, then deafness to certain frequencies occurs. In tinnitus, they may falsely relay information at a certain frequency that an externally audible sound is present, when it is not.
The mechanisms of subjective tinnitus are often obscure. While it is not surprising that direct trauma to the inner ear can cause tinnitus, other apparent causes (e.g., TMJ and dental disorders) are difficult to explain. Recent research has proposed that there are two distinct categories of subjective tinnitus: otic tinnitus, caused by disorders of the inner ear or the acoustic nerve; and somatic tinnitus, caused by disorders outside the ear and nerve, but still within the head or neck. It is further hypothesized that somatic tinnitus may be due to "central crosstalk" within the brain, as certain head and neck nerves enter the brain near regions known to be involved in hearing.
source- Wikipedia