Integrative Neurophysiology of the Lobster Cardiac Ganglion

The synchronized bursts of impulses produced by the nine neuronsof the isolated Homarus cardiac ganglion are usually initiatedby Cell 7. Activity in all other cells commences with very shortlatency thereafter. Impulses in most cells originate in triggerzones located 1–2 mm from the cell body, but the firstseveral impulses in Cells 8 and 9 frequently originate in distaltrigger zones some distance from the somata. Large cells fireat a high initial frequency, dropping rapidly to a low frequencyplateau. Small cells exhibit a more tonic behavior and fireat intermediate rates. More anterior small cells tend to firefaster than more posterior ones. The major synaptic interactionsare the impulse-mediated excitatory ones from small cells tolarge cells, and possibly to more anterior small cells. Thereare weak interactions from large cells back onto small cells,and very specific interactions from Cells 1 and 2 onto 3_A, 4_A,5_A, and 3_B 4_B 5_B respectively. The large discrete EPSPs generatedin large cells by small cell impulses appear to be the explanationfor "discrete positioning" in large-cell firing patterns. Inthis situation, large-cell impulses only fire at discrete timesduring the burst, regardless of the actual large-cell pattern. The overall view is of a two-layered neural system in whichthe small cells possess an endogenous oscillatory driver potential,synchronized by synaptic and electrotonic interactions, anddriving a train of impulses in each cell. This activates excitatorysynapses on the large cells, which combined with a triggereddriver potential in each large cell, produces synchronized trainsof motor impulses which activate the heart muscle, causing theheartbeat.