17.8:
Hormonal Regulation
Digestion involves the neuroendocrine system, with the brain monitoring and releasing digestive hormones at distinct periods throughout the process.
In the stomach, during the gastric phase, when proteins are detected, glands release two hormones, gastrin and histamine, which stimulate the production of hydrochloric acid and increase gastric juices that break down food into chyme.
The arrival of chyme in the duodenum causes the release of several hormones, which shut down the digestive process in the stomach and will begin pacing the movement of chyme through the rest of the system.
For instance, gastric inhibitory peptide, or GIP, slows stomach churning. Secretin and cholecystokinin, or CCK, then induce the sphincter between the stomach and duodenum to close, limiting the release of chyme.
Secretin also inhibits gastric juice production and stimulates the release of bicarbonates from the pancreas to neutralize the acidity of the chyme, and CCK stimulates digestive enzymes from the pancreas and bile from the gallbladder.
Lastly, peptide YY is a hormone released by the ileum, the end of the small intestine, and the colon. It slows the movement of chyme into the colon so that water and electrolytes can be readily absorbed. Peptide YY is also an I'm full signal sent to the brain to halt the eating process.
17.8:
Hormonal Regulation
Hormones regulate a significant portion of digestion through activation of the neuroendocrine system. The neuroendocrine system of digestion contains many different hormones all with multiple functions that are both, directly and indirectly, involved in digestion.
The Process
Starting in the stomach, when proteins are detected by sensory neurons of the enteric nervous system, the pyloric gland is stimulated to release gastrin. In turn, this hormone induces the release of histamine. Combined, they initiate the production of hydrochloric acid which facilitates digestion—turning food into chyme. When the pH of the stomach becomes more acidic, a negative feedback loop halts the production of both hormones.
The chyme then moves to the duodenum, where several hormones are released—each with multiple functions. Some inhibit digestion in the stomach. Gastric inhibitory peptide (GIP) slows stomach churning. Secretin inhibits gastric juice production and, along with cholecystokinin (CCK), induces the pyloric sphincter between the stomach and duodenum to close. This limits the volume of chyme in the duodenum, pacing the rate of digestion.
Once the chyme is in the duodenum, secretin prompts the release of bicarbonate from the pancreas. This reduces the acidity of the chyme, protecting the sensitive lining of the duodenum and setting up an optimal environment in which digestive enzymes can function. Digestive enzymes and bile are released from the pancreas and gallbladder when stimulated by CCK, allowing digestion to continue through the small intestine.
At the end of the small intestine, in the ileum, another hormone is released: peptide YY (PYY) just as chyme is passing into the large intestine. This hormone is released over time, peaking about 1-2 hours after eating. Its function is to slow the passage of chyme into the large intestine so water and electrolytes can be maximally absorbed. It also serves as a satiety signal to the brain, indicating to the organism to stop eating. PYY may play a role in obesity. Low levels of PYY have been observed in obese compared to non-obese individuals. A synthetic analog of PYY is being investigated as a possible treatment for obesity.
Suggested Reading
Ye, Lihua, and Rodger A. Liddle. “Gastrointestinal Hormones and the Gut Connectome.” Current Opinion in Endocrinology, Diabetes, and Obesity 24, no. 1 (February 2017): 9–14. [Source]
Kairupan, Timothy Sean, Haruka Amitani, Kai-Chun Cheng, Joshua Runtuwene, Akihiro Asakawa, and Akio Inui. “Role of Gastrointestinal Hormones in Feeding Behavior and Obesity Treatment.” Journal of Gastroenterology 51, no. 2 (February 1, 2016): 93–103. [Source]