Chapter 45: Nutrient Digestion and Absorption

The enteric nervous system functions as an autonomous control center that manages peristaltic contractions, secretory responses, and localized reflex pathways independent of central nervous system input, operating through excitatory signals via acetylcholine and inhibitory pathways involving nitric oxide and vasoactive intestinal peptide. The autonomic nervous system modulates these enteric functions through parasympathetic vagal projections that generally enhance digestive activity and sympathetic pathways that tend to suppress motility and secretion. Hormonal regulation occurs through multiple gastrointestinal peptides including gastrin, which stimulates gastric acid production; cholecystokinin, which coordinates pancreatic enzyme secretion and gallbladder contraction; secretin, which promotes bicarbonate release from the pancreas; motilin, which initiates coordinated muscle contractions between meals; and glucose-dependent insulinotropic peptide, which modulates insulin secretion in response to nutrient intake. Paracrine signaling through histamine and somatostatin provides fine-tuned local control over acid secretion and hormonal release. The chapter integrates these neural, endocrine, and paracrine pathways to illustrate how the gastrointestinal system adapts its digestive capacity and absorption efficiency according to meal composition and metabolic demands. Clinical applications address conditions arising from dysregulation, including delayed gastric emptying, hormone-secreting tumors, functional bowel disorders, and the pathophysiological effects of psychological stress on digestive function, demonstrating the critical importance of coordinated neuroendocrine control in maintaining healthy digestion and nutrient homeostasis.