The controls of eating: a shift from nutritional homeostasis to behavioral neuroscience.

The experimental analysis of the controls of eating has undergone a paradigmatic shift in the past decade. Instead of seeing meals as a problem of how intake serves metabolism and nutritional homeostasis, meals are now seen as a problem in behavioral neuroscience. The major developments underlying this significant change are behavioral, neurological, and theoretical. Behavioral analysis has shown that a central pattern generator in the caudal brainstem organizes eating movements and that the size of a liquid meal is determined by the number and size of clusters of licking. Neurologic analysis has shown that eating is under orosensory positive-feedback control and postingestive, preabsorptive, negative-feedback control. These feedback controls are activated by food ingested during a meal. The sensory information of the feedbacks is carried by afferent fibers that project to the caudal brainstem. The new theory is based on the fact that the feedback controls are stimulated by food acting directly on mucosal receptors along the gastrointestinal tract, from the mouth to the end of the small intestine. Thus, they are referred to as direct controls, and the caudal brainstem is sufficient for organizing their action. All other controls, such as metabolic, rhythmic, and ecologic, that do not contact the mucosal receptors are indirect controls. Indirect controls act by modulating the potency of the central effects of the direct controls, and they require the forebrain and its reciprocal connections with the caudal brainstem for their control of eating and meal size.