Studies of Behavioral State in Aplysia.

This paper reviews a series of studies on the neural organization and the cellular mechanisms underlying behavioral states; in these studies, feeding behavior in Aplysia was used as a model system. Feeding in Aplysia has similarities to motivated behaviors in other animals and is modulated by a number of interesting state variables, including arousal. Food-induced arousal manifests itself in two categories of feeding behavior: (1) appetitive responses (e.g., head-up feeding posture and directed head turning), which orient the animal to potential goal objects such as food; and (2) consummatory responses (biting, swallowing), which obtain the goal object. The consummatory responses are rhythmic and relatively stereotyped, whereas the appetitive responses are highly variable. Our evidence suggests that one consummatory response, biting, appears to be controlled by command elements in the cerebral-ganglion, such as neuron CBI-2, which are capable of driving the behavior. One component of the appetitive behavior, head lifting, may be controlled (at least in part) by another cerebral neuron, C-PR. C-PR, however, affects numerous systems in the animal, but all the systems affected seem to be involved in the food-induced arousal state of the animal. We postulate that C-PR is, in some ways, analogous to command neurons that evoke behaviors. The C-PR, however, not only evokes a behavior, but also evokes a central motive state which aids in insuring that behavior is efficiently expressed.