Neuronal network triggering a fixed action pattern.

Bursts of impulses in groups of brain cells of the nudibranch Tritonia trigger prolonged swimming that is identical to the natural escape response. The cells in which the activity occurs form two bilaterally symmetrical groups of at least 30 cells in each pleural ganglion. These neurons are interconnected by pathways that have a low electrical resistance, both within a ganglion and across the brain. Together they form a network that determinies whether a swimming escape response will occur or not by filtering out weak neural activity yet responding with a burst of impulses to intensive specific input to either group.