148 DONALD M. MAYNARD 



of the third and fourth large cells; in Homarus, slightly anterior to the first 

 two large cells. The inhibitor fiber is generally slightly larger than the two 

 accelerators. Within the ganglion, it branches, sending processes throughout 

 the trunk. Small collaterals enter neuropile tangles or ramify over the large 

 ganglion cell bodies and along the basal portions of their axons (Fig. 3 and 

 Alexandrowicz, 1932). Some larger inhibitor collaterals leave the trunk to 

 end among the "dendritic arborizations" of the large and small cells. Other 

 branches travel into the myocardium in nerves presumably carrying motor 

 fibers from the large cells. There are no fiber networks over the small cell 

 somata, but the inhibitor connects with the small cell dendrites in neuropiles 

 and dendritic arborizations. The inhibitor fiber may end. therefore, on at 

 least three and possibly four regions of a ganglion neuron: (I) on dendritic 

 arborizations; (2) on the cell body (large cells only); (3) on dendrites or 

 collaterals in neuropiles; and possibly, (4) near the endings of motor axons in 

 the myocardium (Fig. 2). 



ACTIVITY IN THE CARDIAC GANGLION 

 Wire electrodes placed on the cardiac ganglion and on the dorsal nerve 

 record two kinds of electrical activity (Fig. 4). First there is the repeating, 

 intermittent burst discharge which originates in ganglion neurons and is 

 limited to the cardiac ganglion. Second there are extrinsic impulses having 

 no direct relation to burst activity or heartbeat which occur both in the 

 dorsal nerve and the cardiac ganglion. They originate in the ventral ganglia 

 and pass through the dorsal nerve to the heart. 



A 



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i 

 DN 



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B 



CG 



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DN 



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Fig. 4. Extracellular recording from cardiac ganglion and dorsal nerve (Hoina- 

 rus). A, before cutting dorsal nerve; b, after cutting dorsal nerve. CG, cardiac 

 ganglion; DN, dorsal nerve; electrode positions indicated in diagram. Arrows 

 represent accelerator (small) and inhibitor impulses (large) reaching ganglion 

 from opposite side of body. Sixty-cycle ripple on DN trace. 



