 Dear students, in this topic we shall discuss the inner ear cochlea. Cochlea is the coiled sensory structure in the inner ear that is specialized for hearing. It takes vibrations from the middle ear bones through its oval window and transforms them into nerve impulses. We shall discuss the structure of cochlea in detail. The cochlea has three fluid filled canals or dubs along its length. These include first the upper scala vestibulari or vestibular duct. This duct is filled with perilymph. The second is the scala media which lies in the middle. It is also called cochlear duct. It is filled with endolymph. And the third lower duct is called the scala tampani or tampanic duct. It is filled with perilymph. In this way we can see that vestibular and tampanic canals are filled with perilymph and in the middle lies the cochlear duct which is filled with endolymph. The partition between the vestibular duct and the cochlear duct is called the Riesner's membrane. While the partition between cochlear and tampanic ducts is the Besseller membrane. The cochlear duct contains the sensory organ which is specialized for hearing and is called the organ of corti. This organ of corti has hair cells which transduce the auditory stimuli into sensory signals. This organ enables human ear to detect and distinguish different frequencies of sound. We shall discuss the detailed structure of organ of corti. It contains hair cells. It also contains on its lower surface a Besseller membrane. It is supplied with auditory nerve endings and on its upper side it has a tectorial membrane. There are two types of hair cells in the organ of corti. These are the outer and inner hair cells. Both types differ in their functional anatomy. The inner hair cells are about 3500 in number and they are arranged in a straight line or a wide U shape. These hair cells are the true sensory cells which send impulses through auditory nerve. The outer hair cells are more numerous in number. They range in number from 12000 to 25000 in mammals. They are arranged in 3 or 4 rows and form a characteristic W shape. The outer hair cells have both sensory and motor elements and they contribute to hearing sensitivity and frequency selectivity. Now we shall discuss the role of Besseller membrane. Besseller membrane forms the floor of cochlea duct. This Besseller membrane is the structure which is involved in the detection of sound according to frequency range. Now we shall discuss the role of tectorial membrane. Tectorial membrane is a fibrous sheet which lies on the apical surface of the organ of corti. The upper layer of the Besseller membrane is the tectorial membrane. There is a fine layer of gelatinous mucus on the tectorial membrane. The mucus is embedded in the lower stearocelium of the hair cells of the tectorial membrane. Now the tectorial membrane plays an important role when the Besseller membrane is displaced. When the Besseller membrane moves because of sound vibrations, the tectorial membrane moves on the top of the hair cells. As a result, it produces a shearing force. The bending of hair cells generates graded potential. That is transmitted to the sensory exons of auditory nerve.