 Dear students, in this topic we shall have an introduction of three components of a sensory system that is the sensory stimuli, sensory organs and sensory receptor cells. Dear students, the nervous system detects and responds to many external and internal factors. Such factors which provoke the response of nervous system are called sensory stimuli. All stimuli are actually some form of energy. The energy may be mechanical, for example, in the stimuli of sound, vibration, gravitation and pressure. Energy may be chemical, for example, in odorants, that is, smell-producing substances, testants and allergens. Energy may be light energy which is in the form of photons. Dear students, a stimulus is detected by the receptor cells when the energy in the stimulus is at its lowest detectable level that is called threshold. A stimulus which has not enough energy is called sub-threshold stimulus that is not detectable by the sensory receptor cells. Dear students, the structures which are specialized to collect the information contained in a particular type of stimulus are called sensory organs. In sensory organs, sensory receptor cells are arranged in a particular way. These organs can collect or gather sensory information more accurately than scattered, separate or individual receptor cells. Sensory organs are located at different parts of the body, both on the surface and in the deeper parts of the body. However, the most specialized sensory organs are concentrated at the anterior end of the vertebrate animals. The major sensory organs of vertebrates, including the human, are eyes, ear, nose, tongue and skin. Dear students, the cells specialized to receive and respond to a particular kind of stimulus are called the receptor cells. Receptor cells generate sensory signals in response to the stimulus and transmit this signal to the nervous system. The receptor cells for some stimuli are concentrated in specialized sensory organs. For example, olfactory, visual, gastritory and sound receptors are concentrated in the sense organs, nose, eyes, tongue and ear. The receptor cells for many stimuli are found scattered in the skin and also in the deeper parts of the body. For example, receptors for heat, cold, pressure and pain receptors.