About 2200 years ago, Aristotle identified five human senses — sight, hearing, smell, taste and touch. Zoologists have identified many more senses in animals, some of which are given below:
In invertebrates, there are:
(a) tactile receptors that sense touch;
(b) georeceptors sense the pull of gravity;
(c) hygroreceptors detect the water content of air;
(d) proprioceptors respond to changes caused mechanically by stretching, compression,bending, and tension;
(e) phonoreceptors that are sensitive to sound;
(f) baroreceptors, that respond to pressure;
(g) chemoreceptors that respond to chemicals;
(h) photoreceptors that sense light; and
(i) thermoreceptors that respond to temperature.
Most vertebrates have a sense of equilibrium (balance) and a sense of body movement. They are also sensitive to fine touch, touch pressure, heat, taste, vision, olfaction, audition, cold, pain etc. Receptors in blood vessels register changes in blood pressure, blood levels of carbon dioxide and hydrogen ions, and perception of hunger and thirst.
Q. 19. What are sensory receptors? What are some basic features of receptors?
Ans. Sensory receptors are cells or neuron endings that can convert environmental information (stimulus) into nerve impulses. Sensory receptors have the following basic features:
- They contain sensitive receptor cells or finely branched peripheral endings of sensory (afferent) neurons that respond to a stimulus by creating a generator potential.
- Their structure is designed to receive a specific stimulus.
- Their receptor cells synapse with afferent nerve fibres that travel to the central nervous system along specific neural pathways.
- In the central nervous system, the nerve impulse is translated into a recognizable sensation, such as sound.
Classification of receptors:
Receptors are traditionally classified on the basis of their location into exteroceptors, interoceptors and proprioceptors. The exteroceptors are near the external surface and keep the animal informed about the external environment. The interoceptors provide information about internal parts of the body. Proprioceptors, which are sensitive to changes in tension of muscles and give the animal a sense of body position are present in muscles, tendons, and joints.
Sometimes receptors air:classified by the form of energy to which the receptors respond, such as chemical, mechanical, light, thermal, gravity etc.