Digestive activity is regulated in three sequential, partially overlapping phases — cephalic, gastric, and intestinal — defined not by anatomical location alone but by the *site of the stimulus* that initiates the regulatory response, so that secretion is anticipated, matched to, and then adjusted after the arrival of luminal contents. The cephalic phase is triggered by chemoreceptor and mechanoreceptor input from the mouth and nasal cavity (and by cognitive anticipation of food) and acts through parasympathetic vagal efferents; the gastric phase is triggered by gastric distension and by luminal chemical constituents such as peptides and amino acids, acting through vagovagal local reflexes together with the paracrine/endocrine mediators gastrin and histamine; the intestinal phase is triggered by digestion products — proteins, fats, and carbohydrates — entering the small intestine and acts through combined neural and hormonal signals directed to the pancreas and intestine. This scheme belongs to gastrointestinal physiology within systems physiology, and it provides the temporal control framework linking the GI system's core functions (secretion, motility) to the three signaling modalities — neural, hormonal, and local reflex — that operate throughout the tract.
Digestive activity is regulated in three sequential, partially overlapping phases — cephalic, gastric, and intestinal — defined not by anatomical location alone but by the *site of the stimulus* that…