The cochlea is the most critical component of the inner ear. It is divided into three fluid-filled chambers, called scalae, that spiral around a bony core. The scala media, or cochlear duct, contains the organ of Corti, which perceives sound. The organ of Corti consists of supporting cells and many thousands of sensory hair cells. Each hair cell has up to 100 bristle-like hairs that translate mechanical movement into electrical sensory impulses that are transmitted directly to the brain. 

The organ of Corti is activated when vibrations caused by sound waves travel through the ear and reach the oval window, a membrane at the entrance of the inner ear. When this membrane vibrates, it creates wavelike motions in the fluid that fills the cochlea. These waves stimulate the hair cells to send messages to the brain.

The inner ear is also responsible for helping maintain balance. Sensory structures within the vestibule and semicircular canals control this.

The vestibule contains two sacs, the utricle and the saccule, and each contains a sensory patch called a macula.

  • The maculae monitor the position of the head in relation to the ground.
  • Tiny hairs projecting from sensory cells are embedded in a gelatinous mass.
  • When the head is tipped, gravity pulls the mass down and stimulates the hair cells.

Each semicircular canal contains an ampulla, or bulge, that contains a receptor structure called the crista ampullaris.

  • The crista ampullaris responds to rotational movements. Its hair cells are embedded in a cone-shaped gelatinous mass called the cupula.
  • When fluid in the semicircular canals swirls during movement, it displaces the cupula, which stimulates the hair cells.

Our sense of balance relies on the sensory structures of the inner ear as well as visual input and information received from receptors in the body, especially those around the joints.