Rapid escape reflexes in aquatic oligochaetes: variations in design and function of evolutionarily conserved giant fiber systems
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This study provides neuroanatomical and electrophysiological evidence that an arrangement of three dorsal giant fibers, functioning as two distinct and dichotomous conduction pathways, has been evolutionarily conserved within the three major orders of aquatic and terrestrial oligochaetes. The medial giant fiber (MGF), activated by afferents of anterior segments, initiates anterior shortening; whereas, the two lateral giant fibers (LGFs), activated in synchrony by afferents of posterior segments, initiate a different response (usually tail withdrawal). Notwithstanding these common features, the design and function of LGF systems differ considerably in aquatic and terrestrial groups. In posterior segments of aquatic species, LGFs are disproportionately larger and conduct faster than MGFs. This contrasts with posterior segments of earthworms in which LGFs are smaller and conduct slower than MGFs. In addition, in aquatic tubificids, a single LGF spike is sufficient to evoke rapid and complete tail withdrawal, whereas a pair of closely-spaced LGF spikes are needed to elicit posterior shortening in earthworms. The graded nature of earthworm escape seems appropriate for worms that burrow in relatively hard substrates and may frequently encounter inanimate stimuli that evoke meaningless giant fiber spiking. On the other hand, the all-or-none nature of the tubificid escape appears advantageous for relatively sedentary worms that are vulnerable to intense predation but reside in aqueous sediments where triggering of giant fiber spikes by non-threatening stimuli is infrequent. Our studies suggest that anatomical and physiological modifications of giant fiber pathways in aquatic and terrestrial worms have occurred during the evolution of oligochaete nervous systems. We hypothesize that differential predation pressures, together with fundamental differences in lifestyle and habitat, have led to this divergence in the structure and function of evolutionarily conserved sets of homologous giant interneurons. 1987 Springer-Verlag.
