RECEPTIVE FIELDS OF FIRST-ORDER SOMATIC SENSORY NEURONS INNERVATING RHINARIUM IN COATI AND RACCOON

When compared with the raccoon, the coati's more prominent use of its rhinarium for tactile exploration of the environment, coupled with a higher rhinarial receptor density, prompted a study to determine if there were correspondingly distinctive features in the coding of information in first-order somatic sensory neurons that innervate the rhinarium in these animals. In acute recording experiments under sodium pentobarbital anesthesia, the Vth ganglion was explored with tungsten microelectrodes. Mechanoreceptive field sizes were delinated with a series of stimulating wire probes, ranging in diameter from 250 to 25μ. For each single ganglion unit, the threshold receptive field was determined with the smallest diameter stimulus wire capable of reliably activating unit discharges (Von Frey technique). Thermal fields were activated by cool water droplets. All fields (coati = 247, raccoon = 202) were drawn on enlarged rhinarial photographs and their areas measured by polar planimeter. In contrast to our expectations, coati and raccoon median threshold receptive field sizes did not differ significantly. The importance of comparing threshold field sizes was underscored by the finding that larger diameter stimulus wires resulted in larger receptive field areas. Cutting the Vth nerve root did not affect median field size in either animal type. The median force threshold for activation of mechanoreceptive field was lower in coatis, but the proportion of modality types (touch, pressure, vibrotactile) did not differ between the two animal types. The results suggest that the coati's greater rhinarial sensibility is mainly contrived at more central levels of the somatic sensory system. © 1969.