CARDIAC INHIBITION IN 

 DECAPOD CRUSTACEA* 



Donald M. Maynard 

 Department of Zoology, University of Michigan, Ann Arbor 



DECAPOD Crustacea possess a neurogenic heart. Each heartbeat is normally 

 initiated by nervous activity originating in a cardiac ganghon located on 

 the inner dorsal wall of the heart. The ganglion is considered spontaneously 

 active for it continues to discharge with httle change in pattern when com- 

 pletely isolated from the myocardium. The spontaneous discharge is modula - 

 ed however, in crabs and lobster by activity in three regulator fibers originat- 

 ing in the central nervous system and terminating in the heart on the ganglion 

 neurons and possibly on the myocardium. One of the regulator fibers ,s 

 inhibitory, the other two excitatory. This paper considers the effect of 

 inhibitor fiber activity on the ganglion neurons. Where necessary, detai s of 

 anatomy, interaction among the ganglion neurons, and action of the accelera- 

 tor fibers will be presented. Two major aspects of inhibition will be dis- 

 cussed" (T) What are the characteristics of the processes which lead to 

 inhibition of spontaneous or postsynaptic activity in the individual cardiac 

 neurons^ (2) What are the effects of depressed excitability in the indrv.dual 

 neurons upon the pattern of integrated activity in the total ganglion burst . 



ANATOMY 



The cardiac ganglion in lobsters, and ^arine decapods generally, norrnally 

 contains but nine intrinsic neurons (Alexandrowtcz 1932. '" W"'™'"^ 

 a,nencanus these are arranged along a ■'Y-i-Ped" gang .on trunk^ m 

 PanuUms argus and Panulirus intenupius. along a linear trunk (Figs. 1 and 2). 

 « anterior neurons are larger than the four posterior neurons and are 

 lo or units directly responsible for the he-tbeat.t The posterior unit 

 Tpparently serve as pacemakers in normal cardiac aeon, but they are not 



. Porlions of this investigation were supported by the Grass Trust for Researeh in 



pertorLd a" the Mental Health Research Instttnte, Untverstl, of Mtchtgan. 



