3701 ## Afferent Sensory Nerve Fibers From the Gut 3702 3703 Many afferent sensory nerve fibers innervate the gut. Some of the nerve fibers have their cell bodies in the enteric nervous system, and some have them in the dorsal root ganglia of the spinal cord. These sensory nerves can be stimulated by (1) irritation of the gut mucosa, (2) excessive gut distention, or (3) the presence of specific chemical substances in the gut. Signals transmitted through the fibers can then cause excitation or, under other conditions, inhibition of intestinal movements or intestinal secretion. 3704 3705 In addition, other sensory signals from the gut go all the way to multiple areas of the spinal cord and even to the brain stem. For example, 80% of the nerve fibers in the vagus nerves are afferent rather than efferent. These afferent fibers transmit sensory signals from the gastrointestinal tract into the brain medulla, which, in turn, initiates vagal reflex signals that return to the gastrointestinal tract to control many of its functions. 3706 3707 3708 ## Gastrointestinal Reflexes 3709 3710 The anatomical arrangement of the enteric nervous system and its connections with the sympathetic and parasympathetic systems support three types of gastro-intestinal reflexes that are essential to gastrointestinal control. 3711 3712 1. Reflexes that are integrated entirely within the gut wall enteric nervous system. These reflexes include, for example, those that control gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects, and so forth. 3713 3714 2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. These reflexes transmit signals long distances to other areas of the gastrointestinal tract, such as signals from the stomach to cause evacuation of the colon (the gastrocolic reflex), signals from the colon and small intestine to inhibit stomach motility and stomach secretion (the enterogastric reflexes), and reflexes from the colon to inhibit emptying of ileal contents into the colon (the colonoileal reflex). 3715 3716 3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. These reflexes include especially the following: (1) reflexes from the stomach and duodenum to the brain stem and back to the stomach—via the vagus nerves—to control gastric motor and secretory activity; (2) pain reflexes that cause general inhibition of the entire gastrointestinal tract; and (3) defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation (the defecation reflexes). 3717 3718 3719 ## HORMONAL CONTROL OF GASTROINTESTINAL MOTILITY 3720 3721 The gastrointestinal hormones are released into the portal circulation and exert physiological actions on target cells with specific receptors for the hormone. The effects of the hormones persist even after all nervous connections between the site of release and the site of action have been severed. Table 63.1 outlines the actions of the major gastrointestinal hormones, as well as the stimuli for secretion and sites at which secretion takes place. 3722 3723 In Chapter 65, we discuss the importance of several hormones for controlling gastrointestinal secretion. Most of these same hormones also affect motility in some parts 3724 3725 3726 3727 *Table 63.1 Major Gastrointestinal Hormone Actions, Stimuli for Secretion, and Sites of Secretion* 3728 3729 3730

Hormone	Stimuli for Secretion	Site of Secretion	Actions
Gastrin	Protein Distention Nervous (Acid inhibits release)	G cells of the antrum, duodenum, and jejunum	Stimulates • Gastric acid secretion • Mucosal growth
Cholecystokinin	Protein Fat Acid	I cells of the duodenum, jejunum, and ileum	Stim [... Output truncated due to length. 2979 tokens/12048 characters removed ...]