Neural and Hormonal Control of Digestion: Taste and smell are related. What happens when we walk past a fast-food stall and catch a whiff of samosas and pani puris. Immediately we feel hungry. Why is that? It is because of the neural reflex, which triggers and causes salivation because of the smell of food. The regulation of our digestive system is controlled by our neural system and hormones. Every phase of digestion, from ingestion to egestion, has neurological and hormonal control, which ensures proper coordination and smooth regulation of digestion. Read further and learn intriguing details about neural and hormonal control of digestion.

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Regulation of Digestion

The entire digestion process involves six main steps: ingestion, propulsion (swallowing and movement of food through the alimentary canal), mechanical or physical digestion, chemical digestion, absorption, and excretion.

Each step of digestion is under neural and hormonal regulation. The regulation allows communication between different parts of the digestive tract, and it ensures sufficient secretions of enzymes in the presence of food. Regulation helps avoid over or under secretions of digestive juice. Neural and Hormonal are two kinds of digestive regulations.

Regulation of Digestion: Classification

The entire regulation of digestion can be studied in three different phases: The cephalic phase, the gastric phase and the intestinal phase.

1. The cephalic phase is controlled by the neural response to the stimulus provided by food. The body responds to food before it enters the mouth. Secretion of gastric juice can be triggered by mere sight, sense, smell, and even thought of food resulting in salivation and secretion of gastric juices. Salivation and release of gastric juice prepare the stomach to receive food by the central nervous system.
2. The gastric phase includes local, hormonal, and neural responses. It begins when the food reaches the stomach and extends the stimulus received during the cephalic phase. Physiological aspects that stimulate the gastric phase are stomach distention, a drop in the pH of the gastric contents, and the presence of undigested food. These responses cause substantial contractions and secretions.
3.  When chyme enters the small intestine, digestive secretions are triggered, and the intestinal phase begins. This phase regulates the rate at which the stomach empties. When chyme enters the small intestine, it stimulates the secretion of various hormones like secretin, CCK, and GIP and neurological reactions like inhibition of Gastric secretion and gastric motility that coordinate the activity of different organs.

Neural Control of Digestion

For proper coordination of different parts and the action of the gastrointestinal tract requires neurological control. The GI tract is regulated by two sets of nervous systems: extrinsic and intrinsic.

1. The intrinsic or enteric nervous system consists of the Meissner’s plexus, located in the submucosa, and the Auerbach’s plexus, located in the muscular layer. Most gastrointestinal activities, such as secretion and motility, are controlled by the enteric nervous system.
2. The extrinsic innervation of the GI tract comprises parasympathetic and sympathetic nerves. They can alter the activity of the intrinsic nervous system in response to reflex activity originating in the GIT or elsewhere in the body.

Apart from these, the stimulation of the vagus nerve causes saliva production in response to the sight, taste and smell of food.

Types of Reflexes

As discussed above, stimulated receptors produce different kinds of reflexes that are discussed below:
1. Long Reflexes: These are caused by the central nervous system, which involves sending information to the brain. The source of this information can be the digestive system, or external; like an emotional response, smell of food, hearing someone talk about food, seeing pictures of food etc. The reflexes generated due to external factors are also called feedforward reflexes. Long reflexes can also cause GI responses, like the “feeling butterflies in the stomach” when nervous.

2. Short Reflexes: These are caused by the enteric nervous system (ENS), also known as the digestive brain. Short reflexes help to regulate motility, secretion, and growth.

3. Reflexes From the Gastrointestinal System (GI)

There are three main types of Gastrointestinal Reflex, which are discussed below:

A. The Enterogastric Reflex: These are caused by the pH level in the duodenum (3–4) or the stomach (1.5). Stimulation of intragastric reflex checks secretion of gastrin from G- cells in the antrum of the stomach. It also inhibits gastric motility and the secretion of HCl.

B. The Gastrocolic Reflex: These are physiological reflexes that control peristalsis, the motility of the GI tract. Food that distends the stomach increases the motility of the colon. This reflex causes the urge to defecate undigested material to create space for food in the stomach.

C. The Gastroileal Reflex: These reflexes work in coordination with the gastrocolic reflex to stimulate the urge to defecate by the opening of the ileocecal valve.

Hormonal Control of Digestion

Hormonal control is crucial in the digestive process. These hormones are released by specialised epithelial cells, called endocrinocytes, present in the mucosal epithelium of the stomach and small intestine. The digestive hormones are released in the bloodstream, which reaches their target site and acts on the target organ.

These hormones play a crucial role in controlling food intake and energy expenditure. The GI tract is the largest endocrine organ in the human body. These hormones regulate the process of digestion along with appetite and influence the pleasure of eating.

The release of hormones is regulated by stimulus and the nervous system. The list of digestive hormones and their functions are given below:

S.No	Hormone	Stimulus	Site	Action
1	Gastrin	Distention of stomach, Presence of Peptide in food	Produced in G-cells of the lining of the stomach.	Stimulates the release of HCl and Pepsin. Stimulates gastric motility.
2	Cholecystokinin (CCK-PZ)	Presence of Peptide and Fatty acid in food	Produced in I cell of the lining of the duodenum	Stimulates secretion of pancreatic juice, Inhibits secretion of gastric juice Increase motility of the colon
3	Secretin	Presence of Acid and peptides in food	Produced in S cells in the duodenum	Stimulate secretion of bile juice, Inhibits secretion of gastric juice, Augments action of CCK
4	GIP	Presence of Peptide, Fatty acid or Glucose in food	Produced in K cell of the intestine	Stimulates secretion of insulin from pancreases.
5	Enterogastrone (Gastric Inhibitory Peptide: GIP)	Presence of Peptide	Duodenum	Inhibits gastric secretion and motility
6	Duocrinin	Presence of food in the intestine	Duodenum	Stimulates the Brunner’s glands to release mucus and enzyme
7	Enterocrinin	Presence of food in the intestine	Small intestine	Stimulates the crypts of Lieberkuhn to release enzymes
8	Vasoactive Intestinal Peptide (VIP)	In the presence of lipopolysaccharide (LPS) and proinflammatory cytokines	Small intestine and stomach	Dilates peripheral blood vessels of GI tract
9	Villikinin	Food in the small intestine	Small intestine	Accelerates movement of villi
10	Somatostatin	Presence of peptide	Delta cells of Islet of Langerhans	Inhibits secretion of insulin and glucagon

Summary

The entire digestion process involves six main steps: ingestion, propulsion (swallowing and movement of food through the alimentary canal), mechanical or physical digestion, chemical digestion, absorption, and defecation. Each step of digestion is under neural and hormonal regulation. The regulation allows communication between different parts of the digestive tract, ensuring sufficient secretions when food is present. Neural control requires signalling. The entire process of digestion is controlled mainly by the parasympathetic nervous system and some local reflexes. These nerves are connected to our central nervous system. The entire alimentary canal has a lot of sensory receptors in its walls, which help regulate digestion. Each step is controlled by a different part of the sensory system. Stimulated receptors produce various reflexes like a long reflex, short reflex, and other gastrointestinal (GI) reflexes. These reflexes regulate the release of hormones, peristalsis and emotional responses as well. Hormones are released by specialised epithelial cells, called endocrinocytes, present in the mucosal epithelium of the stomach and small intestine. Each hormone has a different stimulus, which activates or inhibits the release of digestive juices.

Frequently Asked Questions (FAQs)

Q.1. What hormone slows digestion?
 Ans: Cholecystokinin (CCK) slows digestion.

Q.2. How does the nervous system control the digestive system?
Ans: Nervous system controls digestion with the help of neurons located in the walls of the alimentary canal.

Q.3. What is the importance of neural control within the digestive system?
Ans: Neural control within the digestive system is also called the enteric nervous system (ENS) or digestive brain. It regulates motility, secretion and growth.

Q.4. What are the three main hormones that regulate digestion?
Ans: Gastrin, CCK-PZ and secretin are three main hormones that regulate digestion.

Q.5. Which are the main regulators of digestion?
Ans: Nervous system and endocrine system are the main regulators of the digestive system.

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