 Dear students, in this topic, we shall discuss the hair cells, which are ciliary cells found in many sensory organs of vertebrates. They are highly sensitive mechanoreceptors. They are responsible for transducing mechanical stimuli into electrical signals. We shall take few examples of systems which are based on hair cells. The first example is of lateral line system, which is found in fishes and amphibians. This system is involved in the detection of motion in surrounding water. The other examples of hair cells' sensory structures are the organ of herring, that is the ear, and the organ of equilibrium in vertebrates. These systems are also based on hair cells. Dear students, now we shall discuss the cilia of hair cells, which are the major components of hair cells involved in sensory transduction. Many cilia project from the epical end of each cell. The reason is that these cells are called hair cells, because cilia is emerging from the upper surface of the cells like hair. There are two types of cilia. One is the kinosilium and the others are stereosilium. There is a single kinosilium in each hair cell. This kinosilium has a typical motile cilia-like structure. That contains 9 plus 2 arrangement of microtubules, like this one. The hair cells also have many stereocilia. The number of stereocilia may vary from 200 to 300 in each hair cell. These stereocilia have a structure which is not like the motile cilia. They are non-motile ciliary structures. They are structurally and developmentally distinct from kinosilium. They are also not formed of microtubules. Rather, they are formed of actin filaments. These stereocilia are arranged in the order of increasing length from one side of the cell to the other. Dear students, now we shall discuss the working mechanism of hair cells. The stimulus, which is mechanical and may be pressure or force, that moves the bundles of stereocilia, which produce electrical signal. When the stimulus acts in such a way that cilia bends towards the tallest cilium, the cell depolarizes. When they bend in the opposite direction, the cilia and the cell hyperpolarizes. Dear students, hair cells do not produce action potentials. Rather, they transmit or release neurotransmitters in a graded fashion. They form chemical synapses with afferent neurons and carry information to the central nervous system.