The Myogenic Automatism of the Contraction of the Heart of Insects

Abstract

It was observed by Carlson (1904) that when the median, cardiac nerve cord (ganglionic) of the “horseshoe crab,” Limulus is isolated up to the third segment the amplitude of the contractions of the anterior segments is about halved. He also noted, in a now classical but simple series of experiments (1905) that lesion of the median nerve cord and the two lateral nerves in any segment of the heart destroyed the coordination of the two ends of the heart on either side of the lesion; and conversely, a cross-section of the heart in any segment, leaving the longitudinal nerves intact, did not interfere with the coordination. “The abolition of coordination by sectioning the longitudinal nerves is immediate and permanent. Both ends of the heart continue to beat but with independent rhythm. Therefore, conduction of the heart in this animal is in the nervous and not in the muscular tissue.” This work was repeated on the isopod, Ligia Oceania , by Alexandrowicz (1932a) with the same results. Carlson further found that total extirpation of the median nerve and the lateral nerves caused the heart to cease beating immediately. It was concluded that the heart beat as well as the transmission of that beat throughout the heart is of purely nervous origin and the result of rhythmic nervous impulses sent out from the median cardiac nerve which contains neural cell bodies. Carlson and Meek (1908) later found that the embryonic heart of Limulus begins to beat before any anlage of the dorsal median cardiac nerve cord, the dorsal nerve plexus, or of the lateral cardiac nerves appears. They then concluded that there takes place a transfer of automatism from the myocardium to the nervous tissue at some stage. This observation is sufficient to show that myocardial transmission is at least feasible.