17.7:
Neural Regulation
During the first part of digestion, the cephalic phase, the brain responds to related stimuli such as the visual or olfactory information about food before it even enters the mouth.
Such condition control is designed to prepare the digestive system by triggering neural impulses in the cranial nerves that innovates salivary glands to produce more saliva. The vagus nerve is also activated and increases the production of gastric juices in the stomach.
Once food is swallowed, digestion becomes an unconditioned reflex governed by a branch of the autonomic nervous system called the enteric nervous system or ENS, which governs smooth muscle movements that churn and propel food along the digestive track, from the esophagus to the anus.
The arrival of food in the stomach during the gastric phase stretches the walls and the distention is detected by mechanoreceptors in the ENS. Other sensory receptors, chemoreceptors, detect changes in pH in the stomach as the food is turned into chyme, a liquid mixture of food and gastric juices.
During the intestinal phase, when chyme moves into the duodenum, the beginning of the small intestine, similar receptors are activated. In both gastric and intestinal phases, activation of ENS receptors is connected to the secretion of digestive hormones, establishing a neuroendocrine relationship in digestion.
17.7:
Neural Regulation
Digestion begins with a cephalic phase that prepares the digestive system to receive food. When our brain processes visual or olfactory information about food, it triggers impulses in the cranial nerves innervating the salivary glands and stomach to prepare for food.
The cephalic phase is a conditioned or learned response to familiar foods. Our appetite or desire for a particular food modifies the preparatory responses directed by the brain. Individuals may produce more saliva and stomach rumblings in anticipation of apple pie than of broccoli. Appetite and desire are products of the hypothalamus and amygdala—brain areas associated with visceral processes and emotion. After the cephalic phase, digestion is governed by the enteric nervous system (ENS) as an unconditioned reflex. Individuals do not have to learn how to digest food; it happens regardless of whether it is apple pie or broccoli.
The ENS is unique in that it functions (mostly) independent of the brain. About 90% of the communication are messages sent from the ENS to the brain rather than the other way around. These messages give the brain information about satiety, nausea, or bloating.
The ENS, as part of the peripheral nervous system, is also unique in that it contains both motor and sensory neurons. For example, the ENS directs smooth muscle movements that churn and propel food along the digestive tract—from the esophagus to the anus. The brain, though, directs the skeletal muscles that perform conscious processes like swallowing and defecation.
Sensory ENS neurons detect changes in the stomach and intestine. Mechanoreceptors detect stretching and distension of the stomach and duodenum linings when food enters these cavities. Chemoreceptors then detect changes in the chemical composition of the chyme—like pH levels and the presence of proteins and fats. This information is used to propel each step in digestion and to coordinate with the endocrine system to release digestive hormones.
The ENS contains 200-600 million neurons and is sometimes referred to as the “little brain.”.It uses many of the same neurotransmitters as the central nervous system (CNS) and brain. Because of this, neurons in the ENS are susceptible to the same neurological insults as brain neurons, creating a link between gut disorders and neurological disorders. For example, in people with Parkinson’s disease, neurons in both the ENS and brain show similar disease symptoms such as alpha-synuclein inclusions and possibly Lewy bodies.
Suggested Reading
Rao, Meenakshi, and Michael D. Gershon. “The Bowel and beyond: The Enteric Nervous System in Neurological Disorders.” Nature Reviews. Gastroenterology & Hepatology 13, no. 9 (September 2016): 517–28. https://doi.org/10.1038/nrgastro.2016.107.
Mittal, Rahul, Luca H. Debs, Amit P. Patel, Desiree Nguyen, Kunal Patel, Gregory O’Connor, M’hamed Grati, et al. “Neurotransmitters: The Critical Modulators Regulating Gut-Brain Axis.” Journal of Cellular Physiology 232, no. 9 (September 2017): 2359–72. [Source]