Propioreceptors:
Propio = self.
Propioreceptors commonly called “stretch receptors” are internal sense organs that respond to mechanically induced changes caused by stretching, compression, bending or tension. These receptors give an annual information about the movement of its body parts and their position relative to each other. Proprioreceptors have been most thoroughly studied in arthropods where they are associated with appendage, joints and body extensor muscles. In these animals the sensory neurons involved in proprioreception are associated with and attached to some part of the body that is stretched. These parts may be specialized muscle cells, elastic connective tissue fibres or various membranes that span joints. As these structures change shape, sensory nerve endings of the attached nerves distort accordingly and initiate a generator potential.
Tactilereceptors and Thermoreceptors:
Tactilereceptors: Tactile = touch. Tactilereceptors are derived from modification of epithelial cells associated with sensory neurons. Most tactile receptors of animals involve projection from the body surface e.g. various bristles, spines, setae and tubercles. When an animal contracts an object in the environment these receptors are mechanically deformed. These deformations activate the receptor which in turn activates underlying sensory neurons initiating a generator potential. Most tactile receptors are also sensitive to mechanically induced vibrations propagated through water or a solid substrate. Tube dwelling polychaetes bear receptors that allow them to retract quickly into their tubes in response to movements in their surroundings. Web building spiders have tactile receptors that can sense struggling prey in webs through vibrations of the web threads.
Termoreceptors: Thermo = heat. They respond to temperature changes. Some invertebrates can directly sense differences in environmental temperatures. Protozoa paramecium collects in areas where water temperature is moderate and it avoids temperature extremes. A heat sensing mechanism draws Leeches and ticks to warm blooded hosts. Certain insects, some crustaceans and horse shoe crab (limulus) can also sense thermal variations.
Lateral line system and Electrical sensing:
Specialized organs for equilibrium and gravity detection, audition and magnetoreception have evolved from the lateral line system of fishes. The lateral line system for electrical sensing is in the head and body areas of most fishes some amphibians and platypus. It consists of sensory pores in the epidermis of the skin that connect to canals leading into electroreceptors called ampullary organs. These organs can sense electrical currents in the surrounding water. Most living organisms generate weak electrical fields. The ability to detect these fields helps a fish to find mates, capture prey or avoid predators. This is valuable sense in deep musky water, where vision is of little use. In fact some fishes actually generate electrical fields and then use their electroreceptors (electro communication) to detect how surrounding objects distort the field. This allows these fishes to navigate in musky waters.
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