Neural control of ventilatory movements in the aquatic insectCorydalus cornutus: the motor pattern

Abstract

1. Ventilation in the dobsonfly larvaCorydalus cornutus consists of rhythmic retraction and protraction of seven pairs of abdominal tracheal gills at variable rates up to 120/min. Movement usually begins with segment 3, and activity spreads both posteriorly, with a short (8–40 ms) delay between segments 3 and 4, and anteriorly, with a longer delay (75–150 ms) between 3 and 2. Gill retraction is produced by a single power-stroke muscle which is excited by one motoneuron during ventilation. Several muscles participate in protraction. 2. The ventilatory motor pattern consists of alternating short-duration bursts of the gill retractor motoneuron and long-duration bursts of gill protractor motoneurons. Both retractor and protractor motoneurons often fire in decelerating bursts, but interspike intervals of protractor neurons are long (40–130 ms) compared to those of the retractor neuron (8–16 ms). At high ventilatory frequencies (> 54 beats/min), decreases in cycle-time are correlated with decreases in burst lengths and interspike intervals of both retractor and protractor motoneurons. At lower ventilatory frequencies, retractor burst lengths become constant, and protractor motoneurons sometimes cease firing. 3. Both abdominal ganglia 2 and 3 can produce rhythmic bursts of ventilatory motor output when either ganglion is isolated from other ganglia and from afferent input; ganglion 3 has the highest intrinsic frequency and is thought to contain the dominant central pattern generator. 4. Two types of spiking interneurons were found that influenced the ventilatory motor pattern when stimulated intracellularly. One type, the R cell, results in excitation of the retractor motoneuron and inhibition of protractor motoneurons. Excitation of the other type, the P cell, results in inhibition of the retractor motoneuron, but has no effect on the output of protractor motoneurons.