The cochlea is an organ within the auditory system that is responsible for the sensation of sound. It is located within the last part of the auditory canal called the inner ear. This organ gets its name from the Latin word for snail since its hollowed tube structure coils in on itself like a snail’s shell. The cochlea is a long tube that is open to the middle ear at one end and closed and tapered at the opposite end. The tube is composed of three fluid-filled sections separated by thin membranes. This particular organ has one main function: the transformation of physical waves from sound into electrical signals that can be sent to the brain. The cochlea is also attached to three separate half circle—shaped tubes that help maintain balance as a part of the vestibular system.
Anatomy and Physiology
The cochlea is the final organ of the auditory canal and is located in the inner ear. The inner ear is found within the bony structure just behind the outer ear called the temporal bone. There are two noticeable landmarks on the external surface of the cochlea, the oval and round windows. These structures are adjacent to each other, the oval window on top and the round window on the bottom. An extremely thin membrane covers each of these windows in order to contain the fluid within the cochlea. The oval window is attached to one of the ossicles located within the middle ear called the stapes. The round window is left open to the environment external to the cochlea and helps to reduce pressure within the cochlea.
The coiled snail shell—like structure is best pictured as an uncoiled stretched-out tube. Within this tube there are three compartments: (1) the scala vestibuli can be found at the top of the tube and has the oval window at one end, (2) the scala tympani is found at the bottom of the tube and contains the round window on the same end as the oval window, and (3) the scala media lies between the scala vestibuli and the scala tympani and contains the organ of Corti. The scala media is separated from the top compartment by Reissner’s membrane and from the bottom compartment by the basilar membrane. The organ of Corti rests on top of the basilar membrane and contains millions of hair cells, which respond to physical movement and produce a neural signal.
The transduction of sound into a neural signal occurs when a sound wave travels down the auditory canal and reaches the stapes. As the stapes vibrates from the sound wave, it acts like a piston on the oval window and produces a wave within the fluid of the topmost chamber. The membrane between the top and bottom chambers is thin enough that the wave can continue past it. The wave travels down the length of the top chamber and returns down the bottom chamber until it reaches the round window, which releases the pressure wave. As the sound wave disrupts the fluid within the top and bottom chambers of the cochlea, the fluid within the middle chamber moves as well, which in turn moves the basilar membrane. The basilar membrane is thin and stiff at one end and the other end is wide and floppy. This property allows high-frequency noises and low-frequency noises to be converted into a neural signal at different places along the membrane. High-frequency noises are perceived at the thin stiff region of the basilar membrane and low-frequency noises are perceived at the wide floppy region.
Hair cells have tiny hair-like projections on their top surface called cilia that are embedded into a nonmoving shelf-like membrane called the tectorial membrane. Once a sound wave causes the basilar membrane to move, the hair cells move as well. Since the cilia are stuck in place, the base of the cilia attached to the cell will bend back and forth as the sound wave hits it. It is this bending motion that produces an action potential within the hair cells. The action potentials produced in the hair cells are then transmitted down small individual nerves to join the auditory or cochlear nerve.
A normal functioning human cochlea is able to perceive sounds ranging from 20 to 20,000 hertz. The most common disease of the cochlea is deafness or loss of the ability to hear. Hearing loss specific to the cochlea and neural pathways is referred to as sensorineural hearing loss. Hearing acuity within the cochlea generally decreases with age, but it also can be due to prolonged exposure to loud noises. High-frequency sounds are typically lost first. Other causes of sensorineural hearing loss are illness, genetic disorders, head trauma, or tumors.
See also: Action Potential; Auditory System; Cochlear Implants; Deafness; Vestibulocochlear Nerve
Bear, Mark F., Barry W. Connors, & Michael A. Paradiso. (2007). Neuroscience exploring the brain (3rd ed.). Baltimore, MD: Lippincott Williams & Wilkins.