Receptors

Receptors are structures specialized to receive certain environmental stimuli and generate nerve impulse. The receptors may be a cell or neuron ending or organ. According to stimuli in human, receptors can be classified  into six types.

(1) Chemoreceptors: These are sensitive to chemical substances. The sense of smell and taste are well known chemoreceptors. There are other various organs that are sensitive to hydrogen ion, concentration in blood, CO₂, oxygen, blood glucose, amino acids and fatty acids e.g. receptors in the hypothalamus.

(2) Mechanoreceptors:  These are sensitive to mechanical stimuli e.g. touch, pressure, hearing and equilibrium.

(3) Photoreceptors: These are sensitive to wavelength of the light e.g. rods and cones in the eyes.

(4) Thermoreceptors: These are stimulated by changes in temperate in the hypothalamus and surface thermo-receptors in the skin.

(5) Proprioceptors: These sense the degree of muscle contraction, the stretch tendons, and the movement of ligaments.

(6) Nociceptors: ( no-si-septers; L. noceo means hurt) These are pain receptors. They are naked dendrites that respond to chemical released by damaged tissues or to excess stimuli of heat or pressure.

Each type of sensation i.e. pain, touch, sight, sound etc is called modalities (primary methods) of sensation, but nerve fibers transmit only impulses. Each nerve tract terminates at a specific point in the central nervous system, and the type of sensation is determined by the point in the central nervous system to which the fiber leads. Each receptor organ is specialized to receive a particular type of stimulus and this is carried to the particular area of the brain. For example touch stimulus is earned by swerve impulse in the touch area of the brain and fibers from the retina terminate the “visual area” of the brain.