Intestinal Reflexes

There are several intestinal reflexes that are controlled locally, by means of the enteric nervous system and paracrine regulators,

The inhibitory neural and endocrine mechanisms during the intestinal phase prevent the further passage of chyme from the stomach to the duodenum. This gives the duodenum time to process the load of chyme received previously. Since secretion of the enterogastrone is stimulated by fat in the chyme, a breakfast of bacon and eggs takes longer to pass through the stomach—and makes one feel "fuller" for a longer time—than does a breakfast of pancakes and syrup.

Substance P

ACh

Motor neurons

w

Interneurons

r Sensory

neurons

Lumen | Mucosa and Circular Longitudinal submucosa | muscle muscle

■ Figure 18.31 The enteric nervous system. Peristalsis is produced by local reflexes involving the enteric nervous system. Notice that the enteric nervous system consists of motor neurons, interneurons, and sensory neurons. The neurotransmitters that stimulate smooth muscle contraction are indicated with a ©, while those that produce smooth muscle relaxation are indicated with a Q. (NO = nitric oxide; VIP = vasoactive intestinal peptide.)

and extrinsically through the actions of the nerves and hormones previously discussed. These reflexes include:

1. the gastroileal reflex, in which increased gastric activity causes increased motility of the ileum and increased movements of chyme through the ileocecal sphincter;

2. the ileogastric reflex, in which distension of the ileum causes a decrease in gastric motility;

3. the intestino-intestinal reflexes, in which overdistension of one intestinal segment causes relaxation throughout the rest of the intestine.

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