Neuromuscular Organization for "Wing" Control in a Mollusc (Clione limacina) and a Bird (Columba livia): Parallels in Design

Flapping as a means of locomotion is shared by divergent groupsranging from pteropod molluscs to birds. The pteropod, Clionelimacina, exhibits two modes of locomotion, slow and fast swimming.The motor units which control swimming consist of small motoneuronsand relatively nonfatigable muscle fibers, while those involvedin fast swimming consistof large motoneurons and relativelyfatigable fibers (Satterlie et al., 1990).The pectoralis muscleof the pigeon, Columba livia, consists of two populations ofmuscle fibers distinguished by histochemistry and size. Cinematographicand electromyographic experiments suggest that the large fibersare used for takeoff and landing and the small fibers for levelflight (Dial et al., 1988). We have employed a suite of experimentaltechniques similar to those used for studies of the neuromuscularsystem of Clione to analyze a limited sample of motor unitsfrom the pectoralis of Columba. The peak tetanic tension andcontractile fatigue resistance during electrical stimulationof single alpha axons, functionally isolated from nerve filaments,was studied in 30 motor units. All units but one generated peaktetanic tensions which were less than 0.22% of whole muscletension. A high proportion (75%) of units demonstrated fatigueresistance, reflective of the demands of sustained flappingflight. These preliminary data suggest that the peripheral neuromuscularsystems of Clione and Columba share some common components forthe execution of at least two distinct modes of flapping locomotion.