Analysis Of The Scaphognathite Ventilatory Pump in the Shore Crab Carcinus Maenas III. Neuromuscular Mechanisms

Abstract

The motor output pattern to one of the ventilatory muscles of the scaphognathite (SC) in the shore crab, Carcinus maenas, was analysed from extracellular nerve recordings. During ‘forward mode’ bursting, an increase in the burst rate of L2b motor neurones is accompanied by an increase in the average intraburst firing frequency. The number of action potentials per burst, although variable, does not change consistently as a function of burst rate. The influence of individual aspects of the motor pattern on isotonic contractions of muscle L2b was examined. Increasing the intraburst frequency leads to greater contraction and work output, and allows the muscle to lift heavier loads. This effect is correlated with an increase in the level of postsynaptic depolarization, due, at least in part, to greater summation of EPSPs. Increasing the burst rate alone also enhances muscle contraction and work, and results in greater depolarization. This latter effect appears to involve an accumulation of short-term facilitation, which becomes more acute as the time interval between the bursts is progressively shortened. In addition, contraction and depolarization are augmented by increasing the number of impulses per burst or the number of axons recruited. These observations indicate some aspects of the motor output pattern which are appropriately modified to accommodate the changes in force and work demands which accompany a change in gill ventilation rate.