Are muscle fibers within fish myotomes activated synchronously? Patterns of recruitment within deep myomeric musculature during swimming in largemouth bass

The myomeric axial musculature of fish has a complex three-dimensional morphology, yet within-myomere motor patterns have not been examined to determine whether all portions of each myomere are activated synchronously during locomotion. To gain insight into recruitment patterns in the deep myomeric musculature of fish, we implanted a series of fine-wire electrodes arranged in a vertical row of six electrodes and a longitudinal row of three electrodes on both the left and right sides of each of five largemouth bass (Micropterus salmoides). After recording electromyograms (EMGs) during the burst-and-glide swimming of each fish, post-mortem dissections and X-rays determined the location of electrodes with respect to (1) the longitudinal position (by counting the underlying vertebrae), (2) the position of the myomere containing the electrode, and (3) the portion within each myomere containing an electrode. Because of the convoluted overlapping shape of the myomeres, electrodes within the vertical row of sites could be located in any one of six different myomeres. Thus, we compared muscle activity for locations with a constant longitudinal position and differing myomeric position (vertical row) and among sites with both variable longitudinal and myomeric positions. We detected significant heterogeneity in EMG onset times for sites within the vertical row of electrodes; however, the durations of the EMGs from different sites were similar. EMG onset times at more posterior longitudinal positions preceded those of more anterior longitudinal positions when electrodes of the latter site were within a more posterior myomere. Thus, the timing of EMGs was consistent with the posterior propagation of muscle activity via the sequential activation of myomeres rather than the simultaneous activation of all contractile tissue within the longitudinal span of a single vertebra. In addition, extreme epaxial and hypaxial portions of myomeres showed distinct activity patterns which did not necessarily correlate with activity in the central myomeric fibers nearer the horizontal septum.