A developmental analysis of spontaneous and reflexive reversals in the nematode Caenorhabditis elegans.

Reversals of forward locomotion in the nematode Caenorhabditis elegans are thought to be mediated by a common neural circuit, the touch withdrawal circuit. Despite substantial neuroanatomical changes over post-embryonic development, one reversal behavior, the head-touch withdrawal reflex, does not appear to change over development (Chalfie and Sulston, 1981). The experiments reported here indicate that two other reversal behaviors, spontaneous reversals and the tap reversal reflex to vibratory stimuli, show developmental changes. Young adult animals showed higher frequencies of spontaneous reversals than all other developmental stages, while larval stages differed from adults in their pattern of responses to tap. Although animals of all stages reversed in response to touch, taps elicited both reversals and accelerations of forward movement. In response to single taps, larval stages reversed on approximately half the occasions; young adult and 4-day-old adults almost always reversed. Increasing stimulus magnitudes increased the probability of accelerations at all developmental stages, but larval stages showed fewer reversals and more accelerations than adults. The behavioral changes observed coincide with known periods of neuroanatomical change in the touch withdrawal circuit. The addition of a late-developing sensory neuron, AVM, is implicated in the behavioral differences between juveniles and adults.