The Nervous Control of Ventilation in Dragonfly Larvae

Abstract

1. Ventilatory movements of the sterna are effected by the paired segmental respiratory dorso-ventral muscles in abdominal segments 4-9 (expiration) and by the natural elasticity of the cuticle aided by two transverse abdominal muscles, the diaphragm and the sub-intestinal transverse muscle (inspiration). The former are innervated by the second segmental nerves, the latter by median nerves. 2. Rhythmic motor discharges were recorded in the nerves to both expiratory and inspiratory muscles. 3. Expiratory bursts consist of a single unit, the firing frequency of which increases during the burst and then normally decreases slightly at the end of the burst. Inspiratory bursts show a rather irregular discharge of the unit(s) within them, the overall frequency of which increases slightly during the first half of the burst. 4. Each spike in the expiratory burst is accompanied by a muscle action potential in the respiratory dorso-ventral muscle it innervates. With increase in frequency of the motor unit facilitation of the muscle action potentials occurs. No such facilitation has been observed in the inspiratory muscles. 5. The mean firing frequency of the expiratory unit increases steadily with increase in burst duration over the normal range of respiratory frequency and a similar minimum interval is attained irrespective of the duration of the burst. 6. Expiratory bursts in second roots of the same ganglion are synchronous and may show a 1:1 relationship. 7. Expiratory bursts start first in posterior segments and the eighth ganglion presumably contains a pacemaker. The duration and number of impulses per burst decrease from behind forwards. However, the expiratory unit reaches its maximum frequency at about the same time in all segments. 8. Stimulation of a first segmental nerve root in the posterior region of the abdomen in an inter-expiratory period causes expiratory bursts to appear in the second roots and consequently re-sets the respiratory rhythm. 9. The inspiratory neurones start to fire very shortly after cessation of the expiratory burst and are closely coupled with the latter. 10. Rhythmic bursts containing many units were recorded in the isolated ventral nerve cord, usually after application of carbon dioxide to the preparation.