Evidence of hysteresis in the neuromuscular system of 3rd instar Sarcophaga bullata
Hysteresis in neuromuscular systems is a marked deviation in force production from the predicted magnitude based on motoneuron rate. Third instar Sarcophaga bullata larvae were selected as model organisms for the investigation of hysteresis. Motoneurons exciting the longitudinal muscles of the larvae were stimulated using two different stimulus paradigms: (1) a constant stimulus frequency train, or tetanic and (2) a high frequency burst embedded within an otherwise constant frequency stimulus train, or hysteretic . The resulting isometric force production and intracellular muscle voltage were recorded. Force production following the hysteretic burst(s) produced larger peak force than that predicted by tetanic stimulus alone. The largest changes in peak force occurred at the lowest tetanic stimulus frequencies. Tetanic and hysteretic stimuli paradigms yielded statistically indistinguishable forces by 20 Hz. The increased force production was not accounted for by the occurrence of synaptic facilitation during, or after, the high frequency burst as EJPs before and after hysteretic bursts were statistically indistinguishable. Taken together, these findings provide evidence for the action of hysteresis in third instar Sarcophaga. Such findings suggest hysteresis could be a property mediated by the post-synaptic cell: the muscle itself. In an attempt to isolate the contribution of different contractile proteins to the hysteretic phenomenon, a high frequency burst was given at t = 1.5 s, during maximal contraction cycling of actin and myosin. These trials were compared to those with a high frequency burst at t = 2.5 s. The later onset time (t = 2.5 s) was chosen as a time at which myosin and actin cycling was approaching a constant cycle rate. There was no statistical different between force production from the varied onset stimulus time paradigms. Moreover, multiple burst stimuli resulted in uneven increases in force production with no muscle length change. Collectively, these results suggested hysteretic forces were not dependent upon the amount of overlap between actin and myosin.