A number of peptides are classed as neurotransmitters in the gastrointestinal tract (Fig. 19.4). These are released from enteric nerves and have a local role; they include calcitonin-gene-related peptide (CGRP), galanin, bombesin, and gastrin-releasing peptides, and do not have a clear physiological function in the gastrointestinal tract, nor do they produce a specific gastrointestinal syndrome when secreted in excess.
