Inking in Aplysia californica. I. Neural circuit of an all-or-none behavioral response

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Abstract
Inking in Aplysia is a high threshold behavior which has a steep input-output characteristic, approaching all-or-none. Gill withdrawal is a low threshold behavior with a graded stimulus-response characteristic. The neuronal circuits of the 2 behaviors were examined to determine what causes the differences in threshold and in stimulus response characteristics. The difference in thresholds for inking and gill withdrawal is attributable to 3 features of the motor cells which mediate the 2 responses: the ink gland motor cells are silent, have a high resting potential and a high threshold, while the gill motor cells are spontaneously active and have much lower thresholds; the ink gland motor cells are electrotonically coupled whereas the gill motor cells are not; and the ink gland motor cells have weak afferent input to moderate tactile stimuli, whereas the gill motor cells have powerful input. The difference in stimulus-response characteristics for inking and gill withdrawal is attributable to differences in the biophysical properties and interconnections of the ink gland and gill motor cells. Once the high threshold of the ink gland motor cells is reached, they respond in a characteristic accelerating burst pattern, whereas the gill motor neurons have a more graded input-output relationship.