The effects of postembryonic receptor cell addition on the response properties of electroreceptive afferents

Abstract

The weakly electric fish Sternopygus detects electric fields with a receptor organ called a tuberous electroreceptor. Previous studies have shown that an electroreceptive afferent fiber innervates a single organ in small fish that as fish grow some of these organs divide, giving rise to daughter organs; these divide in turn to produce a cluster of organs. All of the organs in a cluster are innervated by the original afferent, which sprouts new terminals to accommodate them. Other organs, however, seldom divide. Thus, the distribution of the number of tuberous organs per afferent becomes increasingly bimodal with fish body length (Zakon, 1984a). In order to investigate the effect of organ addition on neural coding within the afferent fiber, activity was recorded from single units within the anterior branch of the anterior lateral line nerve in fish of a range of sizes. It was found that, as fish increase in body length, the best frequency, sharpness of tuning, sensitivity increase in a subset of afferents, while others remain essentially unchanged. This results in an increasingly bimodal distribution of these physiological measures with increasing fish body length. These results suggest that the afferents that innervate multiple receptor organs are more sensitive possess higher best frequencies than those that innervate 1 or 2 organs. This was confirmed by dye injections of electroreceptive fibers with Lucifer yellow. Stimulation of afferents with stimuli of the intensity produced during the normal discharge of the electric organ showed that the sensitive afferents that innervate multiple organs are T receptors (phase-coders) and those that innervate a single organ are P receptors (probability coders) (Bullock and Chichibu, 1965). Our results demonstrate for the first time a morphological difference between these 2 unit types.