The chain of bones in the middle ear leads into the convoluted structures of the inner ear, or labyrinth, which contains organs of both hearing and balance. The three main structures of the inner ear are the cochlea, the vestibule, and the three semicircular canals.
The cochlea is a coiled tube that bears a close resemblance to the shell of a snail, which is what the word means in Greek. Along its length the cochlea is divided into three fluid-filled canals: the vestibular canal, the cochlear canal, and the tympanic canal. The partition between the cochlear canal and the tympanic canal is called the basilar membrane. Embedded in the basilar membrane is the spiral-shaped organ of Corti. The sensory cells in the organ of Corti have thousands of hairlike projections that receive sound vibrations from the middle ear and send them on to the brain via the auditory nerve. In the brain they are recognized and interpreted as specific sounds.
The vestibule, the second main structure of the inner ear, helps the body maintain balance and orientation by monitoring the sensations of movement and position. Without a sense of balance, even simple functions like walking would pose impossible challenges. With no sense of orientation, people would not know if they were in a normal position, upside down, or lying on their sides. Both balance and orientation depend on nerve impulses to reach the brain when the body is unbalanced or disoriented. The brain, in turn, sends messages to appropriate muscles, causing them to correct the imbalance or reposition the body.
The vestibule is made up of two sacs, the utriculus and the sacculus. Special sensory areas in the walls of the utriculus send impulses to the brain indicating the position of the head. These sensory areas consist of hairlike projections embedded in gelatin. Covering the surface of the gelatin are small mineral particles. Depending on the position of the head, the gelatin and mineral particles exert varying pressures on the sensory cells. The cells, in turn, send particular patterns of stimulation to the brain, where the patterns are interpreted.
For example, when the head is upright, the gelatin and mineral particles press down on all the hairlike cells equally. When the head is tilted straight forward by dropping the chin, the gelatin and mineral particles pull on all the hairlike cells equally. If the head is tilted to one side or the other, the cells receive unequal stimulation, varying with the direction and amount of tilt. If the utriculus of both ears is destroyed by injury or disease, the head will hang down limply unless its position can be judged with the eyes. The utriculus is also used to detect the body’s starting or stopping. If a person stops suddenly, the gelatin and mineral particles continue to move, exerting a forward pull on the hairlike cells. The cells then send a specific pattern of nerve impulses to the brain.
The structure of the sacculus is similar to that of the utriculus, but its function is not well understood. The sacculus may aid in determining body orientation, but it may also have a function in hearing.
Arising from the utriculus is the third main structure of the inner ear, the three semicircular canals. These canals direct body balance when the body moves in a straight line or rotates in any direction. Each canal also contains sensory areas with sensory hair cells that project into a cone-shaped cap of gelatin. Two of the semicircular canals are in a vertical position and are used to detect vertical movement, such as jumping or falling. The third canal is horizontal and detects horizontal movement, such as turning or spinning.
The action of the canals depends on the inertia of the fluid inside. When the motion of the body changes, the fluid lags behind, causing the hair cells in the canal to bend. The bending of the hair cells sends nerve impulses to the brain, which in turn informs the body of changes in the direction of movement.