The inner ear is probably the most complicated part of the human body. It enables hearing by converting sound into electrical signals which then travel along the auditory nerve to the brain. The inner ear also plays a role in balance. The parts involved in balance (membrane maze) can detect the acceleration of the head in any direction, either in a straight line (linear acceleration) or a rotation or turn (angular acceleration). The electrical signals which are generated in response to movements of the head pass through the nerve of balance (vestibular nerve), which in its course combines with the cochlear nerve to form a single bundle (the auditory nerve), which then enters the brain.
The part of the inner ear that actually “listens” is the cochlea. This is a hollow, helical tube in very dense bone, called bone labyrinth (part of part of the lithoeidi fate of the temporal tissue). The tube is filled with fluid similar to that which circulates throughout the body (lymph) and the fluid surrounding the brain (cerebrospinal fluid). This internal fluid is called outer lymph. Inside the outer lymph is another spiral, triangular-shaped tube called a cochlear duct, which contains the very important “hair cells” (they convert sound into electrical current). These hair cells are arranged in two groups following the cochlear duct spirals upward from the bottom. There is a single row of inner hair cells near the body of the cochlia and 3-4 rows of outer hair cells. In healthy young humans there are about 3,500 inner hair cells and about 12,000 outer hair cells.