The digestive system has a vast network of nerve fibers that are associated with both the parasympathetic and sympathetic nervous systems. During digestion the parasympathetic nervous system (PNS) tends to be the most prominent of the two systems. The primary nerve associated with the PNS for digestion is the vagus nerve. When stimulated increase in gastrointestinal activity occurs. The sympathetic nervous system on the otherhand inhibits digestion.
The neural sequence of the parasympathetic nervous system starts with the valgus nerve activating within itself motor and sensory fibers. Digestive processes begin by the activation of motor fibers which in turn stimulate the neurons of the intrinsic nervous system, located in the wall of the gut. The smaller intrinsic neurons charge the smooth muscles and gland cells, allowing for secretion and absorption of nutrients.
The sympathetic nervous system causes an inhibitory action of the secretory system and smooth muscle contractions. This is due to the constriction of the blood vessels within the digestive tract. When this situation occurs the body goes into an alert mode. The afferent sensory fibers within the vagus nerve fire off signaling the brain that there is a problem with digestion.
Intrinsic Nervous System:
There are two parts of ganglia or plexi within the intrinsic nervous system. The superficial (further most part) submucosal plexus regulates the digestive glands, and the deeper area (myenteric plexus) within the muscle is responsible for gut motility.
Not only does the plexi contain local sensory and motor neurons it also contains interneurons as well. Detection of the different types of substances in the gut are the responsibility of the sensory neurons located within the sensory chemoreceptors. When tension is developed with the digestive tract the stretch receptors are signaled. To help move the food and chyme bulk through the intestines the short effector neurons are turned on secreting the digestive aid from the glands, or help increase contraction of the smooth muscle tissue of the intestines to move the food.
There is a constant communication among the brain and plexi network. The plexi network use interneurons to communicate with each other through the superficial and deep parts of the gut.
Phases in Neural Regulation of Digestion
There are three distinct phases within the nervous system regulatory process. Cephalic (brain, mental), gastric (stomach), and intestinal.
Cephalic Stage: The olfactory system becomes heightened when blood sugar levels drop within the blood stream. The brain is then activated and hormones are secreted to begin the salivatory process to begin. The salivatory process is a lubricating system that begins in the mouth down the esophogus, to the stomach and intestines. Small amounts of digestive juice is secreted to help begin the breakdown of macronutrients that enter the mouth. Once food enters the body the stimulation of the pancreas begins to occur. Both salivatory and pancreatic digestive juices help prepare the digestive tract into receiving food.
When food enters the stomach stretch receptors will activate communicating with the brain to decipher when to stop eating. Once the food is consumed and there is a tautness on the stretch receptors then the afferent nerves are stimulated. The brain is then signaled to stop the secretory process to help restore homeostasis within the nervous system.
As food is broken down by the stomach it then passes down into the duodenum. As the duodenum is stretched this increases the secretion of the gastric juices to help further digest the food and promotes emptying of the food within the duodenum. Food passes through a chain of intestine. Food goes through the large intestine and ends up in the small intestine, where it becomes filled with acidic and fatty chyme, decreasing stomach contractions allowing for more time for intestinal dumping.
Daryl Conant, M.Ed
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