INHIBITION IN MOLLUSCAN HEARTS AND 

 THE ROLE OF ACETYLCHOLINE 



Ernst Florey and Harriet J. Merwin 



Department of Zoology, University of Washington, Seattle, Washington 



INTRODUCTION 



A DISCUSSION of cardiac inhibition would be incomplete if it would not 

 include references to the inhibitory phenomena observed in the hearts of 

 molluscs. Of all the invertebrates only the molluscs have a heart that has 

 morphological and physiological properties that make it comparable with 

 that of the vertebrates. The moUuscan heart is chambered; blood enters the 

 atria through large veins and leaves the ventricle through one or two aortae. 

 In most forms the heart receives a double innervation of acceleratory and 

 inhibitory nerve fibers. The great sensitivity of several mollusc hearts to acetyl- 

 chohne and the fact that acetylcholine causes cardiac inhibition constitutes 

 a further correspondence between the mollusc and the vertebrate hearts. 



It is unfortunate that the physiological and biophysical analysis of the 

 molluscan heart is still in a rather infantile stage and lags decades behind the 

 advances made in the study of vertebrate hearts. We will attempt in this 

 paper critically to evaluate the available evidence and to point out some of 

 the interesting problems. 



It is worth recalling that the phylum Mollusca includes about ten times the 

 number of species found in the subphylum Vertebrata. Their morpho- 

 logical types range from the snail to the chiton and from the clam to the 

 octopus; a diversity that certainly matches that of the vertebrates. The 

 coiTimon features of their anatomy make it possible, however, to group them 

 together and thus we are, in a way, justified in speaking of "the" mollusc 

 heart. 



Contrary to textbook doctrine that the heartbeat in molluscs is myogenic 

 there is good evidence that in a number of species there are ganglion cells 

 present in the wall of the atria as well as of the ventricle. In 1934 and 1935 

 Suzuki described ganglion cells in the auricles and the ventricle of the pearl 

 oyster Punctada marfensi, of Ostrea cinumpicta, Ostrea gigas (lamellibranchs) 

 and in the auricle and ventricle of six gSisXropods: Janthina jant/iina, Hipponyx 

 pilosus, Lementina imbricata, Cypraea tigris, Cellena nigrolineata and Cellena 

 eucosmia (Suzuki, 1934a, b, 1935). Morin and JuUien (1930) found nerve cells 



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