The cochlea is the main structure of the human auditory system. It is not actually an organ itself, but a bony structure in the inner ear that contains the auditory organ. Sound waves travel into the outer ear canal, vibrating the structures of the air-filled middle air, which transmit the waves to the fluid of the inner ear by the stapes bone hitting a membrane called the oval window. The sound waves are carried by the fluid into the scalae of the cochlea to move the stereocilia of hair cells connected to a cranial nerve, allowing the sound waves to be heard. The cochlea has a number of structures and functions that make this physiology possible.
Structure of the cochlea
Branching off the vestibule of the inner ear, the cochlea consists of a bony labyrinth spiraling around a central bony core. Within this bony spiral is a membranous labyrinth. The bony labyrinth comprises two chambers, called scalae. The scala vestibuli receives sound waves from the oval window and transmits them to the membranous labyrinth, the scala media, via the vestibular membrane. The outer bony labyrinth, the scala tympani, ends at the round window – a membrane that acts as a pressure valve for sound waves in the cochlea. The scala tympani is partly separated from the scala vestibuli by the osseus spiral lamina but can communicate via an opening called the helicotrema. They both contain a fluid called perilymph.
The scala media is also known as the cochlear duct. This chamber of the cochlea is separated from the scala vestibuli by the thin Reissner’s membrane, and from the scala tympani by the basilar membrane. The structure is supported by the spiral ligament. Attached to the length of the basilar membrane and projecting into the scala media is the organ of Corti, the auditory organ.
Organ of Corti and hair cells
The organ of Corti, also known as the basilar papilla, is a membranous tissue on top of nerve endings from the spiral ganglion in the center of the cochlea. This ganglion serves the auditory, or vestibulocochlear, nerve – cranial nerve VIII. The outer membrane of the organ is the tectorial membrane. Protruding from this surface are thousands of stereocilia from specialized hair cells. The hair cells are entrenched in a gelatinous substance supported by the spiral lamina.
The hair cells are supported by pillar cells and other connective cells and tissues that maintain the structure. Two types of hair cells are found in the human cochlea – inner hair cells and outer hair cells. When the basilar membrane is moved by sound waves, the hair cells also move. Via action potentials, the inner hair cells signal the nervous system to the presence of the sound.
Fluids of the cochlea
The sound waves are able to reach the hair cells of the cochlea because of the fluids present in the inner ear. Perilymph is thought to flow through the cochlear aqueduct, which contains the perilymphatic ducts and meets the end of the scala tympani. The duct connects the inner ear to the subdural space of the cranium. Perilymph is similar to cerebrospinal fluid and high in sodium ions.
Endolymph is high in potassium ions and secreted by the cochlea, specifically the stria vascularis, the epithelial lining of the scala media. The fluid is thought to flow out of the cochlea and through the saccule of the inner ear to the endolymphatic sac. This fluid is also found in the other membranous labyrinths of the inner ear, the semicircular canals, where balance is maintained.
The anatomy of the inner ear is complex, and the cochlea is no exception. Intricate communication between bone, membrane and fluid guides sound waves to affect the hair cells of the auditory organ. The brain then deciphers the nerve signals sent by these specialized receptors, achieving the sense of hearing.