50 - Electroreceptors and Magnetoreceptors

Abstract

This chapter discusses electroreceptors and magnetoreceptors. The chapter considers the morphology and physiology of both ampullary and tuberous electroreceptors, including several reviews. It is mentioned that ampullary receptors are tuned and respond to low-frequency stimuli, with best frequencies generally between 0.1 and 20 Hz. Electroreception is a primitive vertebrate character, found in the common ancestor of jawless and jawed vertebrates. Phylogenetic analysis of character traits indicates that the electrosenses of all of these animals are homologous, reflecting their common phylogenetic origin. All electroreceptors, whether primitive or derived, can be broadly classified to belong to one of two categories—ampullary or tuberous. Electrosensory receptors and primary afferents exhibit common mode noise, which has important implications for central processing of electrosensory information. Tuberous electroreceptors are tuned to higher frequencies, with best frequencies in the 0.1-1.0 kHz range. Similar to derived ampullary electroreceptors, tuberous electroreceptors appear in separate and distantly related families of teleost fishes, indicating that they have evolved independently in each lineage. Gymnotid tuberous receptors exhibit a fairly uniform morphology despite the existence of several different functional subtypes of receptor. Intracellular recordings from individual tuberous electroreceptors have yet to be successfully conducted, and a considerable amount remains to be learned about the physiology of transduction in these receptors. Despite the fact that the tuberous electroreceptors in the Mormyroidea (Mormyridae and Gymnarchidae) are not homologous to those of the gymnotids, the receptors show striking similarities in both morphology and physiological responses, presenting a striking case of parallel homoplasy. As in gymnotids, the tuberous electroreceptors of mormyrids can be differentiated into rapid–timing units and amplitude-coding units.