Non-linear summation of tendon-movement primitives in the rat tail

Adrian Brazell

Abstract

In spite of computational complexity, the central nervous system executes movement with speed and accuracy. One strategy that the CNS may use to overcome complexity arising from the large number of redundant muscles is through the use of synergies. Previous studies have explored identification of muscle synergies in a novel model system, the rat tail, which is controlled by more than 300 muscles fascicles, each giving rise to individual tendons. In a previous study aimed at understanding the role of synergies in the rat tail, a new technique was developed that involved the direct pulling of tendons in the rat. The movement resulting from the pulling of the tendons was summed linearly in an attempt to recreate the tail movement elicited during the nociceptive withdrawal response. Our study tests the assumption that individual tendons can be added linearly to recreate what would happen if they were pulled simultaneously. To test this assumption, we first pulled two tendons individually, then both tendons simultaneously and finally tested to see if the linear summation of the two individual pulls resulted in the same movement as pulling them simultaneously. We were able to show that tendons in the tail do not sum linearly, but that a mathematical transformation could be devised to more closely approximate the addition of two tendons. In conclusion, we propose that our transformations could improve the accuracy of the novel technique for identifying synergies.