How are the Endocrine and Nervous system involved in regulating digestion?

In this essay, I will describe how the endocrine system and the nervous system regulate digestion by discussing what happens when the body anticipates food and increases the rate of digestion (for example before eating), what happens during digestion, and what happens after digestion. I will also mention the roles of specific neurotransmitters and hormones during regulation.

The endocrine system and the nervous system

The enteric nervous system (ENS) consists of nerves within the gastrointestinal tract. These nerves can detect food when the walls of the tract are stretched, causing the release of substances that can increase or delay the movement of food or the production of digestive juices (NIH, 2017). Signals to control when the gut contracts and relaxes to push food through the intestines are also released by the ENS (NIH, 2017).

The ENS also detects the change in the chemical composition of food, stimulates or inhibits smooth muscle contraction and the glandular secretions in the digestive system, controls peristaltic and mixing movements, as well as the blood flow to the digestive tract (Vanputte, 2020). Two major neurotransmitters associated with the ENS are acetylcholine which stimulates digestive tract motility and secretions, and norepinephrine which inhibits digestive tract motility and secretions (Vanputte, 2020).

Another important ENS neurotransmitter – serotonin, also stimulates digestive tract motility, 95% of serotonin is secreted by endocrine cells within the digestive tract wall. Since serotonin is used in cancer therapies and antidepressants, it can also affect the digestive tract. Serotonin binds to serotonin receptors on the sensory terminals of the vagus nerve stimulating the vomiting centre of the brain, resulting in nausea and vomiting during chemotherapy and radiotherapy (Vanputte, 2020).

The majority of the ENS’s ganglia are found in the myenteric and submucosal plexuses. The myenteric plexus continually extends from the upper oesophagus to the internal anal sphincter (Furness et al, 2014). Submucosal ganglia and connecting fibre bundles form plexuses in both the intestines (Furness et al, 2014). The connections between the ENS and CNS are carried by the vagus and pelvic nerves (Furness et al, 2014). The environment of the stomach by controlling contractile activity and acid secretion, is mainly monitored by the CNS through vago-vagal reflexes (Furness et al, 2014). Defecation is also controlled by the CNS, via the defecation centres in the lumbosacral spinal cord (Furness et al, 2014).

The autonomic nervous system consists of the parasympathetic nervous system (PNS) and the sympathetic nervous system (SNS) (Vanputte, 2020). The PNS, which controls the rest-and-digest response, aids digestion; whereas the SNS, which controls the flight-or-fight response, inhibits digestion (Vanputte, 2020).

The endocrine system consists of and glands, made of specialised endocrine cells (Vanputte, 2020), which travel to their target tissue via the bloodstream to stimulate a specific response (Vanputte, 2020). In this case, it produces hormones that control when to start and stop digestion (Reference, 2020).

Regulation

Regulation can be split into three phases: the cephalic phase – the first phase of ingestion when the body is preparing for food; the gastric phase – the food arrives into the stomach and is processed by the stimulation from the cephalic phase as well as gastric acids and enzymes; and the intestinal phase – which starts when chyme (the mixture of partially digested foods and stomach juices (Fowler, et al., 2016)) enters the small intestine and triggers hormonal and neural pathways that coordinate the intestinal tract, pancreas, liver and gallbladder (Fowler, et al., 2016).

Cephalic phase

Ghrelin, a hormone produced in the stomach by the endocrine system (figure 1), signals hunger to the brain, increasing appetite (Science Learning Hub, 2019). The central nervous system (CNS) can affect the ENS by decreasing or increasing its activity, for example when a reflex is activated by the sight, smell, or taste of food, the PNS extends to the digestive tract through the vagus nerve which increases salivation and pancreatic juices (Vanputte, 2020). Gastrin, released by the G cells in the stomach by the endocrine system (Fowler, et al., 2016), stimulates the release of gastric juice consisting of hydrochloric acid (HCL) and pepsin in the stomach (Science Learning Hub, 2019). HCL and pepsin aid in the digestion of proteins (Fowler, et al., 2016). Once the stomach has been emptied, the acidic environment is no longer needed therefore a hormone called somatostatin, released by the endocrine system, stops the release of HCL, which is an example of negative feedback (a mechanism used to maintain homeostasis by reversing change and bringing the internal environment back to its original state (Vanputte, 2020)) (Fowler, et al., 2016).

Gastric phase

To neutralise the acidic chyme from the stomach, secretin (figure 1) released in the duodenum, stimulates alkaline secretions from the pancreas (Fowler, et al., 2016) as well as aiding the emptying of the stomach (Science Learning Hub, 2019). Secretin works conjunctively with cholecystokinin (CCK), which is also produced in the duodenum (Fowler, et al., 2016). The production of pancreatic juices is stimulated by CCK which, reduces appetite, stimulates bile release from the gall bladder and slows down the emptying from the stomach (Science Learning Hub, 2019).

Intestinal phase

Food is slowed down in the gut due to Peptide YY (PYY) release (figure 1), which is produced at the end of the small intestine (the ileum) and the large intestine (Science Learning Hub, 2019). This leads to increased digestion efficiency and nutrient absorption after eating (Science Learning Hub, 2019). Gastric emptying and appetite are inhibited by glucagon-like peptide 1 (GLP-1) produced in the small intestine and colon; it also stimulates the release of insulin (Science Learning Hub, 2019).

A schematic diagram of the digestive system and the specific hormones associated with each organ.

Conclusion

Neurotransmitters are released by the ENS and PNS to increase the rate of digestion whereas the SNS inhibits the rate of digestion. The ENS can release digestive juices as well as send signals to control when the gut contracts and relaxes to push food through the intestines. The main neurotransmitters (such as acetylcholine, norepinephrine and serotonin) are released by the ENS to regulate the rate of digestion. The vomiting centre in the brain can be triggered by serotonin. The ENS also detects the change in the chemical composition of food, controls peristaltic movements, as well as the blood flow to the digestive tract.

The endocrine system aids regulation by releasing hormones. Regulation is split into 3 phases: the cephalic phase – where ghrelin, gastrin, pepsin and somatostatin are released; the gastric phase – where secretin and cholecystokinin are released and the intestinal phase – where PYY, GLP-1, and insulin are released. The CNS monitors the state of the stomach by controlling contractile activity and acid secretion, through vago-vagal reflexes. The CNS also controls defecation.

Reference-list:

Fowler, S., Roush, R., Wise, J. and College, O., 2016. Digestive System Regulation. In: C., Molnar and J., Gair eds. 2021. Concepts of Biology. Canada: OpenStax. Ch.15.4.

Furness, J.B., Callaghan, B.P., Rivera L.R., Cho, H., 2014. The enteric nervous system and gastrointestinal innervation: integrated local and central control. Advances in experimental medicine and biology, [e-journal] 817(1), pp.39-71. Abstract only. Available through: PubMed library website [Accessed 23 April 2021]

NIH, 2017. Your Digestive System & How it Works. NIDDK, [online] Available at: [Accessed 20 April 2021]

Reference, 2020. How Do the Digestive and Endocrine Systems Work Together? Reference [online] [Accessed 20 April 2021]

Science Learning Hub, 2019. Hormonal control of digestion. Science Learning Hub, [online] Available at: [Accessed 20 April 2021]

VanPutte, C., Regan, J., Russo, A., Seeley, R., Stephens, T. and Tate. P., 2020. Seeley’s Anatomy and Physiology, 12th ed. New York: McGraw-Hill Education.