Sensory receptors in monotremes

Abstract

This is a summary of the current knowledge of sensory receptors in skin 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, Zaglossus bruijnii. The monotremes are the only group of mammals known to have evolved electroreception. The structures in the skin responsible for the electric sense have been identi¢ed as sensory mucous glands with an expanded epidermal portion that is innervated by large-diameter nerve ¢bres. A¡erent recordings have shown that in both platypuses 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 platypus, whereas there are only about 100 in the tip of the echidna snout. Recordings of electroreceptor-evoked activity from the brain of the platypus have shown that the largest area dedicated to somatosensory input from the bill, S1, shows alternating rows of mechanosensory and bimodal neurons. The bimodal neurons respond to both electrosensory and mechanical inputs. In skin of the platypus bill and echidna snout, apart from the electroreceptors, there are structures called push rods, which consist of a column of compacted cells that is able to move relatively independently of adjacent regions of skin. At the base of the column are Merkel cell complexes, known to be type I slowly adapting mechanoreceptors, and lamellated corpuscles, probably vibration receptors. It has been speculated that the platypus uses its electric sense to detect 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 been established during normal foraging behaviour, although it has been shown that it is able to detect the presence of weak electric ¢elds in water. Perhaps the electric sense is used to detect moving prey in moist soil.

Electroreception in the Guiana dolphin (Sotalia guianensis)

“…It is striking that both the electroreceptors in the monotremes as well as the vibrissal crypts in the Guiana dolphin contain intraepithelial nerve endings. This feature is not known in any F-SCderived structures other than the vibrissal crypts of the Guiana dolphin, but is believed to be the basis of electroreception in the platypus and the echidna [8]. In addition, an important component of ampullary canals related to the process of electroreception appears to be a glycoprotein-based gel in fish [36] and mucus in the platypus [37].…”

“…Passive electroreception is known in many fishes (including elasmobranches, lampreys, paddlefish and catfish [5]), in some amphibians [6,7] and in proterian mammals (i.e. platypuses and echidnas [8][9][10]). …”

Evolution of brains and behavior for optimal foraging: A tale of two predators

“…2B). Such mechanosensory organs are found on the noses of most moles (18)(19)(20) and are anatomically similar to small, domed push rods found on the snout of distantly related monotremes (21)(22)(23)(24). In star-nosed moles, each organ is about 40-60 μm in diameter and has a small (15-20 μm) central disk on the outer surface.…”

The thalamus of the monotremes: cyto- and myeloarchitecture and chemical neuroanatomy

“…The same fields (SI, deep, motor and PV) as in the echidna were described by Krubitzer et al (1995). In addition, however, the platypus SI exhibits a remarkable anteroventral expansion in which the contralateral side of the bill is represented and in which some neurons may respond to low-amplitude electrical stimulation, probably reflecting the presence of electroreceptors in the bill of the platypus Proske et al 1998). The bill representation field is distinguished by patches of enhanced cytochrome oxidase staining separated by less densely stained zones (Krubitzer et al 1995).…”