BODY FLUIDS AND CIRCULATION 



115 



tension or application of NaCl, but the resulting contractions are slow, 

 local or peristaltic, of quite a different kind from the neurogenic heart 

 beat (1, 3, 4, 75). 



Electrocardiograms of the neurogenic hearts of arthropods show fast 

 and slow components like those of myogenic hearts, but in addition a 

 complex oscillatory pattern is present (Astacus, Libinia, Limulus) (Fig. 

 3.18). The larger slow waves in the arthropod ECG correspond to 

 processes of muscular contraction and repolarization, while the fast 

 oscillations represent nervous discharge. Electrical recording from the 

 isolated pacemaker ganglion of Limulus shows bursts of impulses corres- 

 ponding to each heart beat. Each neurone discharges several times during 



(a) 



Seg. 6-8 



Se£.8-9 wmm 



U 



Fig. 3.18. Records of Slow Potentials from the 

 Cardiac Ganglion of Limulus 



(a). Records from three preparations showing axon spikes of single units superimposed 

 on slow spikes. 1: Spikes from two units; 2: One unit discharging twice during each 

 slow wave; 3: Spikes of one large unit. (6). Records from different segments of one 

 ganglion. Position of leads on segments noted. Time signal, 01 sec. (From Prosser (106).) 



a burst lasting about 0-5 sec (range, 2-15 discharges per heart beat). The 

 rate of discharge of each neurone is initially high and gradually falls off. 

 Records secured from the middle segments where the large unipolar neu- 

 rones are located reveal slow waves lasting more than 0- 1 sec, with super- 

 imposed axon spikes. The excitatory wave originates in these large 

 unipolar cells, the pacemakers, and their slow potentials electrically 

 synchronize the discharge of the smaller multipolar neurones. In fatigue, 

 or after treatment with abnormal concentrations of K + and Ca ++ , the 

 individual ganglion cells begin to discharge at random and asynchrony 

 appears (106, 107, 108). 



The normal heart beat in higher crustaceans likewise originates in a 

 burst of nervous activity in the cardiac ganglion. The ganglionic cells fire 

 off a series of impulses at regular intervals : the frequency of bursts deter- 

 mines the frequency of heart beats, and the number of motor impulses in 



