The organization of inputs to motoneurons of the locust metathoracic leg

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Abstract
1. Resistance reflexes at each joint of the leg arise from sense organs directionally sensitive to motion. Sensory impulses, acting indirectly through interneuron combinations, excite one set of motoneurons and inhibit their antagonists. 2. These resistance reflexes are not switched off during ‘voluntary’ movements elicited by touch elsewhere on the body, but instead they sum with other responses. 3. Although readily recorded in motoneurons, the resistance reflexes are mechanically so weak compared with voluntary movements that they are not a major item in the control of the leg. This applies also to posture. 4. When a voluntary movement or direct stimulation of a motoneuron causes a muscle to act against aloadthe reflexes upon motoneurons are quite different from those to motion caused by the same muscle. Therefore the reflex modification of walking patterns, in which legs are loaded by each other’s movements, and by the insect’s weight, cannot be analysed by a study of legmotions, although this has always been the method to date. 5. The positive feedback upon the motoneuron from receptors to loading of the muscle is exemplified in the jump. Between motoneurons to the femur muscles there are other neuronal interactions which accommodate the mechanics of the jump. 6. The angle of the tibial-tarsal joint depends upon sense organs which monitor the angle of the femoral-tibial joint. This inter-joint reflex sums with the local resistance reflex, centrally determined posture and voluntary movements. 7. The study of postsynaptic potentials (p.s.ps) makes possible a number of inferences about the nature of the interneuron pathways antecedent to the motoneurons. 8. Motoneurons to the same muscle usually have a high proportion of p.s.ps in common. Fast motoneurons have phasic properties because they have higher thresholds than the corresponding slow ones, as shown in situations where the p.s.ps are similar. 9. Motoneurons to antagonistic muscles can have synchronous p.s.ps of opposite sign. 10. The intraleg reflex in (6) is controlled by separate interneurons to the motoneurons that are excited together. 11. Study of the inputs upon groups of named motoneurons allows identification of the interneurons which act on them, and reveals that posture, elicited responses and spontaneous movements are determined by selection of different combinations of interneurons. 12. The same method, of recording simultaneously from several known central neurons can progressively reveal the unknown mechanisms by which interneurons integrate sensory input and generate coordinated responses.

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