Sensory receptor cells

The sensitivity of the antennae is astounding. The energy of a single photon, the most minute amount of light, is sufficient to stimulate the visual cell. The olfactory cells sense a single molecule of odoriferous substance. For the auditory cell to be excited, it is sufficient for the ear-drum to vi­brate with an amplitude of 0.0000000006 millimetre, which is one-tenth of the diameter of the smallest atom, that of hydrogen.

The antennae are continuously moving so as to pick up stimuli coming from the outside world. These motile antennae actively search for external stimuli.

The sensory receptor cells of different organs are not absolutely alike. There are some substantial differences. For example, the visual cells contain a special substance, known as visual purple, which can change under the impact of light. This photochemical reaction makes the perception of light possible. There is no visual purple in the receptor cells of the other sense organs. The substances by which they perceive stimuli have not yet been known.

The great similarity in the structure of the various re­ceptor cells is hard to explain. Probably, the overall design scheme proved to be so practical that this prompted nature to make use of a standard model in constructing all the diverse sense organs.

Over millions and millions of years, the animal kingdom on our planet trod the long path of evolution from primitive unicellular creatures, which were hardly able to per­ceive external stimulation, to modern humans with their numerous, perfect and highly receptive sense organs. One might think that man could have nothing in common with an infusorium, but this is not so. The receptor cells of a man and a bird, a fish and an insect, a mollusk and any other animal perceive the outside world and respond to any stimulation from the environment using motile antennae which are very similar in design. Even unicellular organisms, such as the Euglena, use motile antennae. These antennae are so well designed that they have survived for thousands of mil­lions of years. The living organisms of the Earth have preserved their motile antennae from the very origin of life up to the present day.

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