Command fiber control of crayfish abdominal movement

Abstract

1. The control of abdominal extension and flexion movements in the crayfishOrconectes rusticus, O. virilis andProcambarus clarkii was examined by monitoring the afferent and efferent unit activities in the dorsal nerve of the second abdominal segment during unrestrained abdominal extensions and flexions which were evoked by stimulation of command fibers (CFs) in the circumesophageal connective (CC). 2. CFs which when stimulated generate coordinated extensions and flexions are present in the CC ofP. clarkii. 3. Two classes of extension CFs are present in the CC ofP. clarkii: 1. those which excite the motor neuron (#4) which innervates the tonic receptor muscle (RM1) of the muscle receptor organ; 2. those which excite one or more of the excitatory motorneurons which do not innervate RM1 (#2, #3 and #6). 4. The extensor motorneurons are activated in reciprocal sets by CF stimulation. The inhibitory motor neuron (#5) is silent during CF evoked extensions (inP. clarkii.) 5. During flexion CF stimulation the inhibitory extensor motorneuron is excited while activity in the excitatory extensor motorneurons is suppressed. 6. The strong reciprocity observed in the superficial extensor motor neuron (SEMN) activities evoked by CF stimulation contrasts with the weak reciprocity which is characteristic of extensor motorneuron activities generated by the intact animal which moves its abdomen in response to a strong sensory stimulus (Sokolove, 1973). This difference in reciprocity supports the hypothesis that an intact animal initiates a postural movement of its abdomen by the conjoint excitation of several CFs. 7. Few if any extension CFs are present in the CC ofO. rusticus orO. virilis.