Cochlea

The organ that is designed for hearing and for balance in mammals is the ear. In mammals, it usually consists of three parts, the outer ear, the middle ear, and the inner ear. The outer consists of a pinna and ear canal. The middle ear consists of the tympanic cavity and three ossicles. The inner ear is present in the baby labyrinth and that consists of the semicircular canals, the utricle, the saccule, and the cochlea. The cochlea is present in the inner ear, which enables the process called hearing. The cochlea is derived from the Greek word, “kokhlias” which means “snail shell or spiral”. 

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Structure of Cochlea

The cochlea is the spiral, conical, and hollow chamber where the waves propagate from the base to the apex of the ear. The cochlea’s spiral canal is found in the bony labyrinth in the inner ear, it is approximately 30 mm long and makes about 234 turns around the modiolus. It consists of chambers and membranes, a cross section of cochlea is given below.

• It consists of three chambers or scalae: 

1. Scala Vestibuli or Vestibular Duct: These ducts contain perilymph, and are present superior to that of the cochlear duct and touch the oval window.

2. Scala Tympani or Tympanic Duct: These ducts also contain perilymph, and are found present inferior to the cochlear duct and terminate at the round window.

3. Scala Media or Cochlear Duct: These ducts contain endolymph, and it is a region where there is a high concentration of potassium ion, and the hair cells project into.

• The place or the location where the vestibular duct and tympanic duct merge are called the helicotrema. It is present at the apex of the cochlea.

• The vestibular duct and cochlear duct are separated by a membrane called the Reissner’s membrane.

• The main structural element that separates the cochlear duct and tympanic duct is the osseous spiral lamina.

• The main structural element that helps to determine the wave propagation properties of the partition of the cochlea is called the basilar membrane. It also helps to separate the cochlear duct from the tympanic duct.

• The cellular layer that is present on the basilar membrane acts as a sensory epithelium is called the organ of corti. The potential difference between the endolymph and the perilymph powers up the sensory hair cells.

• The hair-like structures called stereocilia are present on the sensory cells of the organ of Corti.

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Functioning of Cochlea

The portion of the inner ear that resembles the shape of a snail shell is called the cochlea. It is filled with a watery fluid called endolymph. Till now we have seen what is inside the cochlea. Let us now concentrate on the functioning. It receives the sounds in the form of vibrations from the middle ear through the oval window, which makes the thousands of hair cells present inside the ear sense the motion through the stereocilia and make it move. These stereocilia help to convert the vibrations into the nerve impulses that are sent to the numerous nerve cells. The electrochemical impulses called action potentials receive the signals from the neurons and these signals are traveled through the auditory nerve to the brain for further process of interpreting. There are three fluid sections among which two are canals that include the vestibular canal and tympanic canal and the third one is the organ of the Corti. This detects the impulses that travel to the brain along the auditory nerve. 

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Cochlea Ear in other Animals:

The cochlea in mammals is found unique as compared to the other animals. It is sometimes found as a blind-ended tube called a cochlear duct. This difference is found due to the difference in the frequency range of hearing that is present between the mammals and the other animals. As mammals have unique hearing frequencies it is due to the pre-amplification process of sound done by the hair cells. Mammals lack better frequency resolution, where birds cannot hear frequencies more than 4-5 kHz, some marine mammals can hear up to the frequency range of 200 kHz. Rather than having the short and straight cochlea presence of a long coiled compartment helps to increase the range of hearing.

Conclusion:

It not only receives sound waves but the structure of cochlea is designed in such a way that whenever it is necessary it amplifies the sound. Sometimes the loud noises can damage the cochlea and reduce the sensitivity of hearing. And due to any reasons if the cochlea gets damaged it can be repaired by transforming the stem cells present in the umbilical cord.