Step Response Analysis of Thermotaxis in Caenorhabditis

Introduction Caenorhabditis elegans orients to both chemical (chemotaxis) and thermal (thermotaxis) gradients (Ward, 1973; Hedgecock and Russell, 1975), making it a promising experimental system for investigating the neuronal basis of spatial orientation. Previous studies have established a plausible behavioral mechanism for chemotaxis in C. elegans (Dusenbery, 1980; Pierce-Shimomura et al., 1999). Locomotion consists of periods of relatively straightforward movement punctuated approximately twice per minute by bouts of turning (Rutherford and Croll, 1979). Two main kinds of turns are recognized in C. elegans: "reversals," in which the animal moves backward for several seconds and then goes forward again in a new direction, and "omega turns," in which the animal's head bends around to touch the tail during forward locomotion, momentarily forming a shape like the Greek letter (Croll, 1975b). Statistical analysis reveals that reversals and omega turns occur in bursts that have been ter