Properties of Receptors
The receptors collect information concerning the change in the environment such as temperature, illumination etc. In lower animals, sensory perception is mediated by undifferentiated nerve endings. These are their sense organs. Though, the receptors or sense organs become more specialized and elaborate. Such type of elaborate sense organs are present even in Cnidaria, and they become much more complicated in higher animals. You have already seen that sensory receptors exchange one form of energy into another, electrical. So all receptors are transducers. Even one molecule of odour substance is frequently able of evoking a small electrical disturbance in a chemoreceptor. This local current is generally the result of amplification too, and is propagated along the axon of the sensory receptor. So the receptor is extremely sensitive to change in the environment. You have to also note that each sensory receptor responds to just only one kind of stimulus. For example, chemo-receptors will respond to only chemical stimuli, but not to mechanical or light stimuli. Another significant fact you have to bear in mind is that the frequency of nerve impulse increases with strength of the stimulus. In other words, information that is concerning intensity of the stimulus is transmitted by frequency code. Thus the code of information travels as impulse in axons of all receptors, be it mechanoreceptor, photoreceptor or chemoreceptor. But because each kind of receptor is sensitive to one type of stimulus only and because these axons carry impulses to specific regions of the central nervous system, brain correctly interprets the information reaching it. Depending on the type of stimulus, receptors may be divided into mechanoreceptors, chemo receptors and photoreceptors.