Fig. 1. Circular muscle contraction induced by OA. (A) Diagram of a semi-intact preparation. In the middle part of the dissected body wall, a small hook is connected to a force transducer via a pulley. Electrical activities of motor neurons were simultaneously recorded from the cut end of the first segmental nerve in the same segment using a glass suction electrode. (B) Strong phasic contractions could be observed on application of 10^–4 mol l^–1 OA (upper trace). In the absence of the ventral nerve cord, no contractions were observed under the same experimental conditions (lower trace). Downward deflections of the force response indicate a circular muscle contraction. The broken lines indicate when the concentration of OA was changed. Open triangles, resting level contraction; solid triangles, strong contraction associated with crawling behavior. (C) The results of simultaneous recording of muscle contraction and electrical activity from motor neurons. In control preparations, electrical activity was only seen at the onset of a contraction. At 10^–4 mol l^–1 OA, electrical activity occurred throughout much of the period of contraction.