Leech locomotion: swimming, crawling, and decisions

Research on the neuronal control of locomotion in leeches spans almost four decades. Recent advances reviewed here include discoveries that: (1) interactions between multiple hormones modulate initiation of swimming; (2) stretch receptors associated with longitudinal muscles interact with the central oscillator circuit via electrical junctions; (3) intersegmental interactions, according to theoretical analyses, must be relatively weak compared to oscillator interactions within ganglia; and (4) multiple interacting neurons control the expression of alternative modes of locomotion. The innovative techniques that facilitated these advances include optical recording of membrane potential changes, simultaneous intracellular injection of high and low molecular weight fluorescent dyes, and detailed modeling via an input–output systems engineering approach.