SENSORY RECEPTORS IN MONOTREMES

This is a summary of the current knowledge of sensory receptors in ski n of the bill of the platypus, Ornithorhynchus anatinus, and the snout of the echidna, Tachyglossus aculeatus. Brief mention is also made of the third living member of the monotremes, the long-nosed echidna, Za glossus bruijnii. The monotremes are the only group of mammals known t o have evolved electroreception. The structures in the skin responsibl e for the electric sense have been identified as sensory mucous glands with an expanded epidermal portion that is innervated by large-diamet er nerve fibres. Afferent recordings have shown that in both platypuse s and echidnas the receptors are excited by cathodal (negative) pulses and inhibited by anodal (positive) pulses. Estimates give a total of 40 000 mucous sensory glands in the upper and lower bill of the platyp us, whereas there are only about 100 in the tip of the echidna snout. Recordings of electroreceptor-evoked activity from the brain of the pl atypus have shown that the largest area dedicated to somatosensory inp ut from the bill, S1, shows alternating rows of mechanosensory and bim odal neurons. The bimodal neurons respond to both electrosensory and m echanical inputs. In skin of the platypus bill and echidna snout, apar t from the electroreceptors, there are structures called push rods, wh ich consist of a column of compacted cells that is able to move relati vely independently of adjacent regions of skin. At the base of the col umn are Merkel cell complexes, known to be type I slowly adapting mech anoreceptors, and lamellated corpuscles, probably vibration receptors. It has been speculated that the platypus uses its electric sense to d etect the electromyographic activity from moving prey in the water and for obstacle avoidance. Mechanoreceptors signal contact with the prey For the echidna, a role for the electrosensory system has not yet bee n established during normal foraging behaviour, although it has been s hown that it is able to detect the presence of weak electric fields in water. Perhaps the electric sense is used to detect moving prey in mo ist soil.