The Role of Push Rods in Platypus and Echidna - Some Speculations

摘要

This is a review of the structure and innervation of the mechanosensory organ, the push-rod, in skin of the platypus bill and echidna snout. Four receptor types can be identified in association with push rods in platypus and echidna: (ⅰ) central vesicle chain receptors, (ⅱ) peripheral vesicle chain receptors, (ⅲ) Merkel endings and (ⅳ) paciniform corpuscles. Function of the vesicle chain receptors remains unknown. Merkel endings are known to be slowly adapting with irregular discharge (SAI) while paciniform corpuscles are rapidly adapting vibration-sensitive (RA). Recordings made from echidna nose skin have identified both SAIs and RAs. In addition, responses typical of SAII endings (regular discharge) and rapidly adapting, but vibration insensitive, responses were observed. It was concluded that the push rod in monotremes is not associated with mechanoreceptors unique to the group. Skin of the platypus bill and the echidna nose contains erectile tissue. It is conjectured that blood engorgement inflates the skin to facilitate contact between push rods and the external environment. In addition platypus push-rods have a ring of contractile tissue around their tips which, on contracting, restricts mobility of the rod, perhaps when the platypus leaves the water. Possible cooperative roles between electroreceptors and mechanoreceptors are discussed.