Abstract: The nervous system and the gastrointestinal tract work together in a number of ways both during normal health and during stress. The enteric nervous system is a division of the somatic nervous system that regulates gastrointestinal functions like motility and secretions at both health and under stress. During normal health, the enteric nervous system helps with motility of food (digestion) by stimulating neuroreceptors which secrete hormones to help food move down the gastrointestinal tract to be excreted. Under stress, the enteric nervous system decreases motility and gastric and can cause problems like diarrhoea, heart burn and vomiting.
Many systems in the body work together to keep the body working efficiently so that we can complete tasks throughout our lives. One such example is the gastrointestinal working with the nervous system or more precisely the enteric nervous system. The enteric nervous system is a division of the autonomic nervous system along with the sympathetic and parasympathetic nervous systems. The enteric nervous system is a network of nerves located in the wall of the gastrointestinal tract and can function independently of the brain. The ENS has as many neurons as the spinal cord and also contains all the neurotransmitters found in the brain, making it very complex and because of this complexity is often referred to as the second brain1.
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The gastrointestinal tract is the term given to the passageway from the mouth to the anus. The gastrointestinal tract is part of the digestive system which also consists of the pancreas, kidney and salivary glands. Gastrointestinal functions are regulated by the enteric nervous system. These functions include; motility, secretions and the immune system2. Even though the ENS can function independently of the brain, it is usually monitored by the parasympathetic and sympathetic nerve fibres which are connected to its three plexuses3. The enteric nervous system is comprised of both intrinsic and extrinsic innervations. Intrinsic innervations are stimulation from within the GI tract (the enteric nervous system) while extrinsic innervations are stimulations from out of the GI tract2.
The enteric nervous system is comprised of two networks or plexuses of neurons, the myenteric (or Auerbach's) plexus and the submucous (or Meissner's) plexus. The submucous plexus innervates cells in the epithelial layer of the muscularis mucosa (a smooth muscle found in most parts of the gastrointestinal tract). The myenteric plexus is the major nerve supply to the gastrointestinal tract and controls motility in the gastrointestinal tract3. Gut motility is the term given to stretching and contractions of muscles in the gastrointestinal tract2.
Acids and enzymes produced by the gastric mucosa can be controlled by the CNS, regulated by hormones in the digestive tract and regulated by short reflexes in the enteric nervous system1. For the purpose of this essay, only the controlling by the CNS and the regulation with the enteric nervous system will be studied.
During normal health the regulation of gut motility and gastric secretions in the gastrointestinal tract is done in three stages, the cephalic stage, the gastric phase and the intestinal phase1.
The cephalic phase begins when the cerebral cortex in the hypothalamus recognises the smell, sight or thought of food. At this stage a signal is sent to the medulla oblongata which in turn sends a signal to the submucosal plexus. The submucosal plexus increases the secretion from gastric glands which in turn helps the motility of the food down the oesophagus4. In the lower oesophagus the peristalsis is mostly independent of the nervous system so momentarily doesn't need the nervous system.
The food then enters the stomach, the gastric phase is then initiated by three common stimuli; increase of pH, presence of undigested food in the stomach and swelling of the stomach. Stretch receptors in the stomach wall are stimulated while chemoreceptor's also triggers short reflexes coordinated in the two plexuses1. In the presence of stimuli (proteins, alcohol in small doses and caffeine) chemoreceptors enhance gastric secretion which in turns maintains the motility. The gastric phase usually continues for three to four hours. Then gastrin in the stomach stimulates weakcontractions in the intestinal tract and muscularis externa of the stomach1. These contractions slowly start to intensify and after an hour chyme (partly digested food) is made and is mixing around in the stomach. Once the chyme has been sufficiently mixed it enters the small intestine and the intestinal phase begins.
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The intestines normal functions of secretion, digestion and absorption must be monitored to ensure that they are continuing with sound efficiency so that the body can work efficiently over. Regulation of these processes is done during the intestinal phase. When the chyme starts to leave the stomach, swelling of the stomach decreases and stimulation of stretch receptors also decreases. At the same time swelling in the duodenum triggers stretch receptors and chemoreceptors which in turn triggers the enterogastric reflex. Upon stimulation of the reflex, both gastric production and gastric contractions are both momentarily halted. To prevent a build up of chyme in the duodenum gastric contractions decrease in strength and frequency and no more chyme is discharged into the duodenum1. This allows the duodenum to deal with the arriving acid before the next gastric contraction. Mucus production is also enabled at local reflexes. Mucus helps with protecting the intestinal lining from the harmful acid and enzymes arriving from the stomach1.
Overall, the enterogastric reflex inhibits gastric secretion and motility. After the chyme is processed, the enterogastric reflex diminishes and the stomach returns to its normal state until the next cephalic phase.
Stress is a major component in humans that threatens our wellbeing. Stress may be physical or physiological. Physical stress acts both internally and externally while physiological stress only acts externally making this the main difference between the two different types5. Stress is usually regulated by the emotional motor system, a network of structures that integrate the autonomic, neuroendocrine and pain response stressors associated with anger, anxiety and other emotions5.
Stress affects both gastrointestinal motility and visceral perception. When a physical challenge is presented to a person the higher brain begins to ready the person for either 'fight or flight'. The brain also tells the enteric nervous system (gut brain) to shut down so that other more important processes can be completed at higher efficiency. This may be true for physical challenges but not always true for other types of stress.
Research has shown that when acute stress (both physical and physiological stress) is applied to a healthy person, gastrointestinal motility (how fast food moves through the oesophagus) is affected5. Fear is an example of acute stress and can alter gastrointestinal motility in the vagus nerve and nerves in the oesophagus. When stress is applied, oesophagus motility is decreased; this is why people have difficulty swallowing when faced with a challenge5. When presented with a less intense fear like public speaking, the gastrointestinal motility may slow or be disrupted which leads to abdominal pain.
Another common example of stress on the body is exercise. Studies have shown that both aerobic (walking, cycling, running etc.) and resistance (weights etc) improves muscle mass and decreases fat mass. Regular exercise also protects us from diseases such as sclerosis, hypertension and type 2 diabetes6. Unfortunately, exercise can have the inverse effect on the gastrointestinal tract. Exercise usually leads to distress in the gastrointestinal tract when; the exercise is vigorous, the person is training in a hot environment and training without proper hydration6.
Usually when exercising in a hot environment, more water is lost from the skin. When a person is exercising, gastric emptying is slowed (almost to a stop). This decreased gastric emptying is further slowed by hypertonic carbohydrate beverages (such as Gatorade) given after running. The most common gastrointestinal diseases caused by exercise are heartburn, nausea, vomiting, diarrhoea and constipation6. Decreased gastric emptying is due to the body needing more energy for other processes to run at higher efficiency.
The gastrointestinal tract and the nervous system work together in unique ways. The gastrointestinal tract is controlled by a part of the autonomic nervous system called the enteric system (a system of nerves in the wall of the gastrointestinal tract). One of the most important processes that the enteric nervous system helps the gastrointestinal tract with is, gastrointestinal motility or movement of food through the gastrointestinal tract. Digestion is done in three stages, the cephalic stage, the gastric phase and the intestinal phase. The enteric nervous system helps by controlling the stimulation of chemoreceptors and stretch receptors which controls secretions. This secretion increases the speed of motility. Although when stress, such as fear, is applied to the gastrointestinal tract, the gastric emptying is slowed and motility on the whole is slowed. Unfortunately, this can lead to gastrointestinal diseases. Gastrointestinal diseases and problems like heartburn, vomiting etc, are also common during strenuous exercise and are fairly common in long distance runners. Overall, the enteric nervous system is essential for the gastrointestinal tract to function normally.
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