NERVE-TERMINALS OF MUCOUS GLAND ELECTRORECEPTORS IN THE PLATYPUS (ORNITHORHYNCHUS-ANATINUS)

Platypus mucous gland electroreceptors differ from electroreceptors de scribed for fish in that they lack an associated specialized sensory c ell. Thus a bare nerve terminal is used to detect electrical stimuli, and also to generate local and action potentials. Previous studies hav e identified these terminals (an average of 16 per mucous gland), but had not shown whether the terminals have direct contact with the duct of the mucous gland. This poses the problem of how the electrical stim ulus reaches the nerve terminals. This study demonstrates the portions of the nerve terminals responsible for electroreception, and shows ho w these portions use the surrounding epidermal tissue to overcome the combined problems of lacking a sensory cell and making physical contac t with the conducting medium in the duct of the gland. A terminal axon al filament is described which accommodates for these problems. the te rminal filament provides a low-resistance pathway for the electrical s timuli, and is embedded with its proximal and distal portions in high and low resistance epidermis, respectively. Lateral interactions occur between adjacent terminal filaments via a plexus that is directed cir cumferentially around the duct from the proximal portion of the termin al filament. These circumferential arbors form an interconnecting ring between all 16 terminal filaments, and may be used to lower the signa l-to-noise ratio of the electroreceptor and thus enhance overall sensi tivity.