The Organ of Corti.
The Organ of Corti is a gelatinous mass about 4 cm long and is composed of some 7500 interrelated parts.The Organ of Corti is enclosed in the cochlea which is deeply imbedded in the temporal bone (the hardest in the body) is one of the best protected parts of the body. It is related to a series of tiny sensing bumps in fishes that are located along the body in rows just under the skin. These tiny bumps are used by fish to sense slight movements of water. The Organ of Corti operates in a similar way. It is filled with fluid, surrounded by other fluid and responds to movements in these fluids - those movements induced by sound waves.

The fluids filling and surrounding it act as shock absorbers, and so do the springy membranes which support it. It is even isolated from the normal body supply lines, for the faint pulsing of blood through capillary vessels would be detected as background noise.
The capillaries nearest to the organ of Corti end at the wall of the cochlea; nutrients on their way out are carried to and from the capillaries by the endolymph fluid that bathes the organ.

The organ of Corti is shaped like the jam in a jam roll. It spirals around within the cochlea. The basilar membrane supports the organ which contains a mass of cells almost touching the branch endings of the auditory nerve. From these cells sprout fine hairs, (23,500 of them) rising in orderly rows like the bristles of a very soft brush. The hairs stick through the dome of the organ, their ends embedded in a thick overhanging sheet, the tectorial membrane.
These hairs are transducers. As the basilar membrane bellies in and out, it pushes and pulls the complex of tissues above it.
The hairs' cells of the organ of Corti ride with the basilar membrane. The hairs have their tops embedded in the tectorial membrane and their roots fixed in the hair cells, so the motion of the basilar membrane bends and twists and pulls and pushes the hairs. Under these physical stresses the hairs generate electrical signals which stimulate the auditory nerve (also known as the acoustic nerve and the eighth cranial nerve) - a bundle of about 30,000 individual fibres.

Eventually, in a way still not fully understood, the electrical signals running through the auditory nerve stimulate the hearing centres of the brain. In the cells of the auditory cortex lies the mystery of the sensation of hearing.

The organ of Corti serves two vital functions:

1 It Converts mechanical energy into electrical energy.
2 It Dispatches to the brain a coded version of the original sound - information not only about fundamental frequency but about intensity and timbre as well.

As the organ of Corti, which is attached to the basilar membrane, bends to outside pressure, it moves laterally to the left whilst the tectorial membrane moves to the right.
This shearing action within the cochlear duct activates the hair cells of the organ of Corti to send their electrochemical signals into the central nervous system.