This classification of sense organs was recognized in the early 19th century by Johannes Muller.
This classification is based on the stimulating agent. According to this classification the receptors fall under following categories with respect to their sensitivity to various stimuli—mechanoreceptors, thermoreceptors, chemoreceptors and photoreceptors.
Mechanoreceptors:
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Such types of receptors are responsive to tactile stimuli, to sound, to other vibrations or to pressure changes.
The sense organs which are concerned with the pressures of the body fluid or other pressures of the body are referred to as pressoreceptors or basoreceptors.
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Thermoreceptors:
These respond to thermal or temperature changes.
Chemoreceptors:
These receptors respond to changes in concentration of specific chemical substances or classes of substances in the internal and external environment, for example the cells for taste and smell.
Photoreceptors:
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These are sense organs responded to changes in intensity or wavelength of light.
In addition to classifying the sense organs or receptors on the basis of location, function and the type of stimuli with which they are attuned, the sense organs can also be classified depending upon the structural organization and in pattern of nerve impulses produced by the sense organs.
The classification of sense organs according to structural organization:
With regard to structural organization the receptors may be simple or more complex. The simplest receptors consist of single neurons, usually multipolar, with one axon, called afferent, extending to a junction or synapse with another neuron or neurons, or in a few cases with a muscle cell, at some distance. Such receptors are called primary sense cells.
The second order of complexity finds the primary sense cells surrounded by or associated with other cells or structures constituting some sort of auxiliary apparatus.
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In some cases it is possible to infer from the nature of such auxiliary apparatus a possible function, and in some instances the function has been demonstra- the experimentally.
The lens of the eye is a familiar example which is known to function in increasing the light-gathering power of the eye and in forming a definite image on the retina, where is located ted primary sense cells.
The third order of complexity finds a number of primary sense cells and possibly auxiliary structures as well, associated to form a sense organ.
Finally as in the vertebrate eye or ear, the sense organs may have, in addition to primary sense cells and auxiliary structures, secondary sense cells—neurons with which the axons of the primary sense cells synapse.
In the retina of the vertebrate eye, for example, there are very complex synaptic connections between the primary sense cells (rods or cones) and several layers of secondary- sense cells.
The axons of the optic nerve arise in these secondary cells rather than the primary sense cells.
The classification of sense organs depending upon the pattern of nerve impulses produced by the sense organs: Such a basis for classifying the sense organs exists but to understand this classification one must know more about the action of receptors in general.
The rate and the extent of adaptation are also important characteristics of receptors and are used as a basis for classification of the receptors.
With regards to rate and extent of adaptation the receptors are of two types—(1) tonic receptors and (2) phasic receptors In tonic receptors the adaptation is slow and there is a continuing discharge as long as stimulation continues, while in phasic receptors the adaptation is rapid and ultimately complete, with no impulses discharged even though the stimulus continues to act.
Tonic receptors include the stretch receptors in the limb of vertebrates and cray fish, the pressure receptors of carotid sinus and statocyst receptors, while the phasic receptors include the Pacinian corpuscle and chemoreceptor’s.