Abstract
The opener muscle in the walking legs of the crayfish (Procambarus clarkii) has three distinct phenotypic regions although innervated by only one excitatory motor neuron. These regions (distal, central, and proximal) have varied biochemistry and physiology, including synaptic structure, troponin-T levels, fiber diameter, input resistance, sarcomere length, and force generation. The force generated by the central fibers when the excitatory neuron was stimulated at 40 Hz was more than the force generated by the other regions. This increase in force was correlated with the central fibers having longer sarcomeres when measured in a relaxed claw. These data support the idea that the central fibers are tonic-like and that the proximal fibers are phasic-like. The addition of serotonin directly on the fibers was hypothesized to increase the force generated by the central fibers more than in the other regions, but this did not occur at 40-Hz stimulation. We hypothesized that the central distal fibers would generate the most force due to the arrangement on the apodeme. This study demonstrates how malleable the motor unit is with modulation and frequency of stimulation.
Highlights
Arrangement of muscle fibers within a muscle serve particular functional capabilities to optimize the outcome of movements and force development
The arrangement of tension receptors in series and in parallel with muscle fibers occurs in the legs of a crab, Macmillan and Dando [5], and Cooper and Hartman [6] showed that tension receptors are situated on the apodeme in legs of crabs among muscle fiber insertions
It is interesting that could show that these fibers contribute a small amount the total interesting that the distal the distal fibers appear to contribute the least to the total force generated because the tension receptor fibers appear to contribute the least to the total force generated because the tension receptor neurons neurons that sense force development of theare muscle areinlocated in the distal region of the apodeme
Summary
Arrangement of muscle fibers within a muscle serve particular functional capabilities to optimize the outcome of movements and force development. The mechanical arrangement of the Golgi tendon organ in cats has been shown to occur both in series and in parallel with adjacent muscle fibers. In such configuration, the in-series receptor displays a brisk discharge when a muscle is in a stretched position under isometric conditions and a single motor unit is stimulated [3]. The in-series receptor displays a brisk discharge when a muscle is in a stretched position under isometric conditions and a single motor unit is stimulated [3] This was shown to occur during passive stretch of muscle [4]. The pinnate attachment of the muscle fibers is directly in series with some tension receptors and in parallel with others
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