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What is Cochlea?
Cochlea is a spiral-shaped cavity that forms a part of the inner ear. It is a hollow bone that has important roles in the process of hearing and auditory signal transduction. The name cochlea has been derived from a Greek word ‘kokhliās’ meaning snail. The cochlea has a central hollow axis called modiolus, around which it takes 2.75 turns that gives it the spiral appearance.
The modiolus contains the cochlear arteries and veins as well as fibres of cochlear nerves. The cochlea is linked to the middle ear via two holes: the oval window and the round window. The cochlea itself is composed of three fluid filled canals and two membranes.
Structure of Cochlea
The cochlea (cochleae in plural) is a hollow spiralled conical chamber of bone that propagates auditory waves from the base of the ear to the top. The spiral canal of the cochlea that surrounds the modiolus is about 30mm long. The cochlea consists of the following structures:
- It has three fluid filled canals:
- Vestibular duct: Also known as scala vestibuli, they lie on the upper side of the cochlear duct and remain attached to the oval window. They are filled with perilymph.
- Tympanic duct: Also known as scala tympani, they are located beneath the cochlear duct and terminate into the round window. They are also filled with perilymph.
- Cochlear duct: Also known as scala media, is a region of high concentration of potassium ions and is filled with endolymph.
- The vestibular duct and the tympanic duct meet at a point that is known as the helicotrema. It is found at the apex of the cochlea.
- Reissner’s membrane, also known as the vestibular membrane that separates the cochlear duct from the vestibular duct. It functions to transmit the auditory signals from the fluid of the vestibular duct to the cochlear duct.
- Osseous spiral lamina is a projection of the modiolus that separates the tympanic duct from the cochlear duct.
- Basilar membrane is a stiff structural element that separates the cochlear and tympanic ducts. It moves up and down as a response to the approaching auditory waves.
- Organ of Corti, also known as the spiral organ, is a layer of epithelial cells that allows signal transduction. It also houses the sensory hair cells which show movement on signal transduction.
- Spiral ligament is a fibrous cushion that extends above the Reissner membrane and is in contact with the vestibular canal.
Microanatomy
The walls of the cochlea are composed of bone that are lined by epithelial cells. Longitudinally, the coil is divided into two by the vestibular duct and tympanic duct. The point where the vestibular and tympanic duct meet, called the helicotrema, is the point where a reversal in the direction of the fluid happens which marks the end of vestibular duct and beginning of tympanic duct.
The cochlear duct divides a major portion of the cochlea lengthwise and occupies the central position. It is filled with endolymph that is made up of proteins and electrolytes (rich in potassium ions). The perilymph, on the other hand, is rich in sodium ions.
The cochlear duct is surrounded by three membranes, namely, the Reissner’s membrane, the basilar membrane and stria vascularis. Stria vascularis is a layer of secretory cells and capillaries that secrete endolymph.
The hair cells of the Organ of Corti are arranged in four rows, where three rows are outer hair cells (OHCs) and one row is inner hair cells (IHCs). The inner hair cells give out the neural output whereas outer hair cells receive the input from the brain.
Functions
- Hearing: As vibrations approach the inner ear from the oval window via the middle ear, the endolymph starts to vibrate. This vibration causes movement in the Organ of Corti which is sensed by the hair cells. The hair cells then convert this movement to electrical signals which are transmitted to nerve cells via neurotransmitters. The auditory neurons then convert the signals into action potentials that travel to the brain for further processing.
- Hair cell amplification: The cochlea has a unique function where it can amplify very weak sounds. The OHCs amplify faint sounds by reverse transduction. They elicit a positive feedback mechanism by converting electrical signals back to mechanical signals. A protein called prestin also couples the movement of sound waves.
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