Modulation of the ventilatory rhythm of the hellgrammiteCorydalus cornutus by mechanosensory input

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1. Rhythmic and non-rhythmic sensory inputs during ventilation were recorded from nerves innervating mechanoreceptors on or near the gills. Removal of sensory input by dissection did not alter the frequency of the rhythmic, metachronal retractions of the seven pairs of abdominal gills. 2. Bursts of motor impulses were recorded extracellularly from the gill retractor motor neuron at the same time that movement of water around the gills was produced by delivering a puff of air to the saline covering them. Mechanical stimulation of the gills usually advanced the next burst of impulses to the gill retractor muscle, thereby shortening one cycle of the rhythm. Stimulation advanced the next retraction of all gills as opposed to a single gill or pair of gills. 3. In some animals, a continuous stream of air delivered for 60s to the saline covering the gills increased the ventilatory frequency during that 60-s period. Removal of the stimulus was followed immediately by a return to the unstimulated frequency. 4. A single electrical pulse to the nerve innervating mechanoreceptors on abdominal segment 3 advanced the next burst of motor impulses, as did an air puff, but only if delivered during the last 70% of the expected cycle. A stimulus administered earlier in the cycle had no effect. A shortened cycle was followed by a cycle of average or slightly longer duration; thus the stimulus probably reset the oscillator that generates the rhythm. 5. Rhythmic electrical stimulation of ganglion 3 afferents at a frequency between 100% and 135% of the mean unstimulated ventilatory frequency entrained all segments to the administered frequency of stimulation. The entrainment occurred such that each stimulus was followed at a constant latency by one ventilatory burst. These results suggest that each pulse in the rhythmic series of pulses shortened one cycle in the same way that a single electrical pulse shortened a cycle. 6. Electrical stimulation of afferents in segments other than abdominal segment 3 had little effect on the rhythm. Phasic input is better able to modulate the ventilatory rhythm if it is delivered to the dominant oscillator in abdominal ganglion 3 than if it is delivered to a subordinate oscillator elsewhere.