Meeting Abstract

P2.147  Monday, Jan. 5  Hysteresis in Force Production of Bodywall Muscle of Larval Diptera KRANS, JL*; PATERSON, BA; Mount Holyoke College; Mount Holyoke College jkrans@mtholyoke.edu

We describe neuromuscular hysteresis – a strong dependence of muscle force production on recent motoneuron activity – in the bodywall muscles of larval Diptera, including Sarcophaga and Drosophila. In semi-isolated nerve-muscle preparations, we show that force produced by a train of nerve impulses at a singular rate is significantly different than that produced by the same train of stimuli when preceded by a brief (< 200 ms) high frequency burst of impulses. The increased force could not be explained by a change in electrical junction potentials (EJPs) between the two impulse trains; EJPs were not statistically different before or immediately after the high frequency burst. Both single muscle and semi-intact preparations exhibited hysteresis similarly, suggesting that the mechanics of non-contractile tissue is not central to the mechanism of hysteresis. Hysteresis was most dramatic at low impulse rates – yielding ~100% increase in force predictions based on constant-rate impulse trains – and decreased in a consistent manner as impulse rate increased. The frequencies at which hysteresis was greatest correspond well to the range observed under normal physiological conditions. In an attempt to more closely model physiological activation, we modulated nerve frequency in a sinusoidal fashion across rates close to those during normal, rhythmic behavior. Each successive cycle of these modulations yielded a slightly decreased, but still notably positive hysteretic force loop. These force loops illustrate the difference between force produced while frequency was increasing and that produced while it decreased. Our hope is that this initial characterization provides a foundation upon which the underlying mechanism of this non-linearity can be investigated.