Rapid escape reflexes in aquatic oligochaetes: variations in design and function of evolutionarily conserved giant fiber systems

Abstract

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.