The activities of the Gastro-Intestinal Tract (GIT) are under neural and hormonal control for proper coordination amongst different parts. The gastrointestinal tract is innervated by intrinsic nerves as well as by extrinsic nerves.

The intrinsic neural system, also called the enteric neural system consists of (i) Meissner's plexus situated in the submucosa and (ii) Auerbach's plexus situated in the muscular layer. The enteric neural system controls most of the gastrointestinal functions like secretion and motility.

The extrinsic innervation of the gut consists of parasympathetic and sympathetic nerves which can modify the activity of intrinsic neural system in response to reflex activity initiated by GIT or from other body parts.

The sight, smell and the presence of food in the oral cavity can stimulate the secretion of the saliva through vagus nerve. Gastric and intestinal secretions are also stimulated by neural signals. The muscular activity of different parts of the alimentry canal can also be moderated by neural mechanisms, both local and through CNS.

Hormonal control of the secretions of digestive juice is carried out by the local hormones produced by the gastric and intestinal mucosa. These include gastrin (stimulate release of gastric juice), enterogastrone (inhibits gastric secrection and motility), secretin (decreases gastric secretion), duodocrinin (stimulate Brunner's gland) etc.

Constipation is characterised by small amounts of hard, drybowel movements usually fever than three times a week. In constipation, the faeces are retained within the rectum as the bowel movement occurs irregularly. The causes for constipation involve water deficiency, lack of roughage in diet, spasm of colon, lack of exercise, emotional stress and certain drugs.

Indigestion is the condition in which the food is not properly digested leading to a feeling of fullness. The cause of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating and spicy food consumption.

Fats are broken down by lipases with the help of bile into di and monoglycerides and further into fatty acids and glycerol in the duodenum The reaction involved in this process are as follows

$$ \begin{gathered} \text { Fats } \xrightarrow{\text { Lipases }} \text { Diglycerides } \longrightarrow \text { Monoglycerides } \\ \text { Di and Monoglycerides } \xrightarrow{\text { Lipases }} \text { Fatty acids + Glycerol. } \end{gathered}$$

1. Digestion of Roti (Carbohydrates)

(a) Digestion of Carbohydrates in the Oral Cavity

In oral cavity, the roti is mixed with saliva. The saliva contains an enzyme salivary amylase (ptyalin) which converts starch in roti into maltose, isomaltose and small dextrins called $\alpha$-dextrin. 30\% of starch is hydrolysed in the oral cavity.

Starch $\xrightarrow[\mathrm{pH} 6-8]{\text { Salivary Amylase }}$ Maltose + Isomaltose $+\alpha$-Dextrin

(b) Digestion of Carbohydrates in the Small Intestine

The passage of party digested roti from oral cavity to oesophagus and then to stomach is regulated by peristalsis (the successive waves of muscular contraction in oesophagus). The stomach stores the food for 4-5 hours. The gastric juice does not contain carbohydrate digesting enzyme.

The partially digested food is now called as chyme. In intestine, following action occurs.

(i) Action of Pancreatic Juice Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.

$$\text { Polysaccharides (starch) } \xrightarrow{\text { Amylase }} \text { Disaccharides }$$

(ii) Action of Intestinal Juice Intestinal juice contain maltase, isomaltase, sucrase (invertase), lactase and $\alpha$-dextrinase. These enzymes act on food converting it into simpler compounds like glucose, fructose, galactose, etc.

Maltose $\xrightarrow{\text { Maltase }}$ Glucose + Glucose

Isomaltose $\xrightarrow{\text { Isomaltase }}$ Glucose + Gucose

Sucrose $\xrightarrow{\text { Sucrase }}$ Glucose + Fructose

Lactose $\xrightarrow{\text { Lactase }}$ Glucose + Galactose

$\alpha$-Dextrins $\xrightarrow{\alpha \text {-dextrinose }}$ Glucose

2. Digestion of Protein

Proteins are made up of amino acids. So proteins are broken down to amino acid during the process of digestion.

Saliva does not contain any protein digesting enzyme. So, its digestion in stomach.

(a) Digestion of Protein in Stomach The stomach normally stores food for 4-5 hours. The gastric glands of the stomach secrete gastric juice. It contains HCl , proenzymes like- pepsinogen and prorennin. Various reactions in stomach are discussed below

$\underset{\text { (proenzyme) }}{\text { Pepsinogen }} \xrightarrow{\mathrm{HCl}}$ Pepsin

Proteins $\xrightarrow{\text { Pepsin }}$ Peptones and proteoses

$\underset{\text { (Proenzyme) }}{\text { Prorennin }} \xrightarrow{\mathrm{HCl}}$ Rennin

(b) Digestion of Protein in Small Intestine

(i) Action of Pancreatic Juice The enzymes trypsinogen, chymotrypsinogen and procarboxypeptidase in pancreatic juice are all concerned with the protein digestion.

Some reactions are given below

$$ \begin{gathered} \text { Trypsinogen } \xrightarrow{\text { Enterot kinase }} \text { Trypsin } \\ \text { Proteins } \xrightarrow{\text { Trypsin }} \text { Dipeptides } \\ \text { Chymotrypsinogen } \xrightarrow{\text { Trypsin }} \text { Chymotrypsin } \\ \text { Peptones } \xrightarrow{\text { Chymotrypsin }} \text { Dipeptides } \\ \text { Procarboxypeptidase } \xrightarrow{\text { Trypsin }} \text { Carboxypeptidases } \\ \text { Proteoses } \xrightarrow{\text { Carboxypeptidases }} \text { Dipeptides } \end{gathered}$$

(ii) Action of Intestinal Juice Intestinal juice contain enzymes enterokinase, amino peptidase and dipeptidase and their actions are given below

$$\begin{aligned} & \text { Peptides } \xrightarrow{\text { Amino peptidase }} \text { Amino acid } \\ & \text { Dipeptides } \xrightarrow{\text { Dipeptidase }} \text { Amino acid } \end{aligned}$$

The macromolecules that are broken down into simpler components are the products of roti and dal (carbohydrates and proteins) which are further absorbed by the villi in intestine and the rest undigested food is removed in the form of faeces.

Among various enzymatic secretions, gastric juice is released in stomach whereas the bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. Pancreatic juice and bile are released through the hepato-pancreatic duct. Gastric juice contains hydrochloric acid and proenzyme - pepsinogen and prorennin. HCl maintains a strongly acidic pH which converst these proenzymes into pepsin and rennini (in infants) respectively. These enzymes act on proteins and convert them into simpler form, peptones. The pancreatic juice contains inactive enzymes trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by an enzyme, enterokinase, (secreted by the intestinal mucosa) into active trypsin, which in turn activates the other enzymes in the pancreatic juice. The bile released into the duodenum contains bile pigments (billirubin and billiverdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases. The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or succus entericus. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner's glands) also help in this process. Various reactions involved in this process are as follows

(i) Pepsinogen $\xrightarrow{\mathrm{HCl}}$ Pepsin

Proteins $\xrightarrow{\text { Peptine }}$ Peptones

(ii) Peptones $\xrightarrow[\text { Carboxypeptidase }]{\text { Trypin/Chymotrypsin }}$ Dipeptides

Dipeptides $\xrightarrow{\text { Dipeptidases }}$ Amino acids

(iii) Carbohydrates $\xrightarrow{\text { Amylase }}$ Disaccharides

$$ \begin{aligned} & \text { Maltose } \xrightarrow{\text { Maltase }} \text { Glucose + Glucose } \\ & \text { Lactose } \xrightarrow{\text { Lactase }} \text { Glucose + Galactose } \\ & \text { Sucrose } \xrightarrow{\text { Sucrase }} \text { Glucose + Fructose } \end{aligned}$$

(iv) Fats $\xrightarrow[\text { Hydrolysed }]{\text { Lipases }}$ Diglycerides $\longrightarrow$ Monoglycerides

$$ \begin{aligned} & \text { Di and Monoglycerides } \xrightarrow{\text { Lipases }} \text { Fatty acids + Glycerol } \\ & \text { (v) Nucleic acids } \xrightarrow{\text { Nucleases }} \text { Nucleotides } \\ & \text { Nucleosides } \xrightarrow{\text { Nucleosidases }} \text { Sugars + Bases } \end{aligned}$$