mechanicalcochlea


The Cochlea

What is the Cochlea?

The cochlea is an integral part of hearing. The human ear is separated into three main parts: The outer ear, the middle ear, and the inner ear. The outer ear contains the visible part of the ear and moves along the channel collecting sound waves and passing them to the middle ear. At the middle ear the sound waves hit the tympanic membrane (ear drum) which vibrates the three tiny ear bones. These bones react and transfer the sound into a wave vibration that is transferred to the cochlea. The cochlea is part of the inner ear, and is described as a bony structure filled with fluid and thousands of hair cells that react to the vibrations from the middle ear. The hair cells are each linked to a certain frequency and when that frequency is made, the hair cell sends a nerve signal to the brain. Figure 1 shows a representation of the parts of the ear. The cochlea is located on the right past the three ear bones.

Figure 1:

As seen in figure 1, the cochea is a snail like structure, with an unrolled length of 3cm. Inside, the cochlea contains two curved fluid-filled ducts. These ducts are called the scala vestibuli and the scala tympani. The fluid is useful because it responds to the sound vibrations from the middle ear and moves through the ducts. The vibration moves the thousand of hair cells that are contained in a third canal called the scala media, shown in figure 2.

 
Figure 2: Fluid filled ducts of the Cochlea

 The hair cells in the scala media lie on the organ of corti. This organ is the key to the cochlea. It takes the pressure changes from the surrounding fluid ducts, and causes the movement of the hair cells, or the acoustic sensor cells. The hair cells product electrical signals, which are converted to electrical potentials that propagate to the brain for processing. In order for the cochlea to pick up different frequencies, the hair cells located in Figure 3 are tuned to by way of their location in the cochlea.

Figure 3: Hair Cells within the Cochlea

A further detailed look at the anatomy in Figure 2 will show that the walls of the hollow cochlea are comprised of bone, with a thin epithelial tissue lining. The mean length is between 33-36 mm. The spiral shape of the cochlea expands when sound enters it, and this movement acts as a sound dampener. In human beings, the audible range of frequencies is usually said to be 20 Hz to 20,000 Hz, which is picked up through the cochlea. Lastly, Figure 4 shows where the different frequencies are picked up within the cochlea, as well as how the level of frequency vibrates through the cochlea channel.

Figure 4: Cochlea Frequencies (Britannica.com)

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