The digestive system is composed of the gastro-intestinal tract (digestive tract), three pairs of salivary glands, the liver and the pancreas.
GUT - STRUCTURE
The outermost layer of the gut is formed by peritoneum and is called the serosal layer.
The second layer is formed of the intestinal smooth muscles (the outer longitudinal and the inner circular).
The submucous coat forms the third layer (is composed of areolar tisues, lymphatics, blood vessels and nerves).
The innermost layer is the mucous membrane.
Extension of the mucous membrane towards the lumen forms the finger-like projections in small intestine, called villi.
The reverse extension produces the exocrine glands which drain into the lumen of the gut and are present in various forms throughout the gut, (gastric glands, intestinal glands, etc).
DESTINY OF THE FOOD
The digestive system receives the food composed of proteins, lipids, carbohydrates, vitamins, minerals and water.
Of them the proteins, lipids and carbohydrates require digestion but all need to be absorbed from the gut.
DESTINY OF THE FOOD
The food we take is placed first in the mouth and then chewed to make it suitable for swallowing by mixing with saliva .
The food is then formed into bolus , which is swallowed with the help of tongue, pharynx, palate, and passed through the oesophagus to the stomach which also acts as a temporary reservoir for food.
In stomach the bolus is mixed with the gastric juice and is pulverised by the movements of stomach (converted into chyme ).
DESTINY OF THE FOOD
In the small intestine the chyme mixes with the secretions from liver ( bile ) and pancreas ( pancreatic juice ). The succus entericus also mixes with the chyme by the agitation produced by the movements of small intestine and the process of digestion continues.
After digestion, extensive absorption occurs in the small intestine and whatever remains at the end is delivered to large intestine (colon).
In colon there is absorption of water and electrolytes along with other materials and the chyme is converted into a semisolid mass . This is stored in pelvic colon. During defaecation this semisolid material is passed out as stool through rectum and anal canal.
The GI system receives blood supply from the different arteries and the venous drainage pass mostly through the liver (portal circulation).
Due to this arrangement all the materials absorbed from the gut are presented to the liver, the main organ of assimilaltion .
The digestive system is supplied by the extrinsic and intrinsic nerves of the gut.
The extrinsic nerves come from the sympathetic and the parasympathetic subdivisions of the autonomic nervous system (the parasympathetic is stimulatory and sympathetic is inhibitory to the digestive system).
The intrinsic nerves forming the “enteric nervous system” (the third nervous system). The enteric nervous system is composed of about 100 million of neurons. These neurons are arranged in two plexuses.
-The first one is the myenteric (Auerbach's) plexus situated in between the circular and longitudinal muscle layer. It receives connections from the extrinsic nerves and is mainly motor in function.
-The second one is the submucous (Meissner's) plexus in the submucous layer, cells of which are mainly sensory in function.
TYPE OF THE NEURONS
SENSORY NEURONS - receive information from sensory receptors in the mucosa and muscle. Sensory receptors in muscle respond to stretch and tension. Collectively, enteric sensory neurons compile a comprehensive battery of information on gut contents and the state of the gastrointestinal wall.
MOTOR NEURONS - control gastrointestinal motility and secretion, and absorption. Acts directly on a large number of effector cells, including smooth muscle, secretory cells (chief, parietal, mucous, enterocytes, pancreatic exocrine cells) and gastrointestinal endocrine cells.
INTERNEURONS - are largely responsible for integrating information from sensory neurons and providing it to enteric motor neurons.
LOCAL AND LONG REFLEXES
The enteric nervous system is responsible for the motor and secretory activities through local reflexes . The enteric nervous system also mediates most of the actions of the extrinsic nerves. The extrinsic nerves have direct motor and secretory activities and they also mediate the long reflexes (e.g., long vagovagal reflex).
Structure of local reflex : Receptors in the mucous membrane —> afferent nerve —> intrinsic plexuses —> efferent nerve —> secretory and motor activities.
Structure of long reflex : Receptors in the mucous membrane —> afferent extrinsic nerves —> brain —> efferent extrinsic nerves —> secretory and motor activities.
The hormones of the GI tract are gastrin, cholecystokinin-pancreozymin (CCK-PZ), secretin, motilin, vasoactive intestinal polypeptide (VIP), gastric inhibitory peptide (GIF), bombesin, enteroglucagon, somatostatin, neurotensin, gastrinreleasing peptide (GRP), opioid peptides, substances P, glicentin.
Many of these GI hormones are actually the neurotransmitters of the enteric nervous system. The rest are secreted from the endocrinal cells of the gut.
Gastrin is secreted from the G cells (gastrin cells) present in the pyloric glands. It is also secreted from the duodenum and from the pancreas (Islets of Langerhans).
Gastrin, a polypeptide hormone, is found in various forms. According to the number of amino acids present, these are named as: minigastrin, gastrin, big gastrin and big big gastrin. All these forms of gastrin may be present again in sulphated and non-sulphated forms and are respectively called gastrin II and gastrin I. All these forms of gastrin are present in equal concentration in blood and in the tissues.
Gastrin secreted from the G cells reach their site of action (gastric glands) through blood like a true hormone. After the action is over gastrin is degraded in kidney and in the small intestine.
Though the main function of gastrin is to stimulate gastric acid secretion it has many other actions as well.
It stimulates secretion of pepsin and also of intrinsic factor.
Gastrin also helps in the secretion of water and electrolytes from the liver and pancreas.
Gastrin is stimulatory to the movements of stomach so also to the peristalsis in the small intestine, at the same time it prevents oesophageal reflux due to contraction of lower oesophageal sphincter.
It increases blood flow in stomach. It helps to maintain the health of the gastric mucous membrane.
It has another important function, i.e., increasing insulin secretion against a meal.
Regulation of gastrin secretion is achieved by multiple factors. The products of protein digestion, particularly amino acids like phenylalanine, arginine, etc., are strong stimulants.
Vagal stimulation increases gastrin secretion so also by local nerves as a result of distension of the stomach by food. Acetylcholine and bombesin are also stimulatory.
The Ca concentration of stomach content increases gastrin secretion.
Secretion of gastrin is inhibited by acidity in the stomach. If the pH of stomach contents is below 2.5, gastrin secretion is inhibited. Probably the acid acts as a feedback inhibition on the G cells. This inhibition of gastrin helps to protect the stomach from high acidity and also maintains a suitable pH for pepsin activity.
This hormone of the GI tract has multiple actions. The CCK-PZ is a polypeptide hormone and is found various forms with varying numbers of amino acids (CCK-58, CCK-39, CCK-33, CCK-12, CCK-8, etc).
The CCK-PZ is responsible contraction of gall bladder and expulsion of bile into intestine. It stimulates pancreatic secretion rich in enzym increases secretion of enterokinase and also potentiates the action of secretin on pancreas.
It regulates GI movements by way of contraction of pyloric sphincter and inhibition of gastric motility and emptying. In this way entry of chyme into the duodenum is controlled by it so that digestion can occur properly. CCK-PZ stimulates secretion of calcitonin and glucagon. It is an important mediator of satiety in response to a meal.
Secretion of CCK-PZ is stimulated by the products of digestion of protein and fat. This occurs in a positive feedback manner as follows: Products of digestion —> the CCK-PZ —> enzyme secretion from pancreas —> formation of more products of digestion —> more CCK-PZ secretion.
This continues as long as there is food in the intestine. When food is absent there are no products of digestion in the intestine and secretion of CCK-PZ automatically stops .
It is secreted from the cells of the crypts in the upper small intestine. It is a polypeptide with 21 amino acids and the whole molecule is necessary for its action.
The main function of secretin is secretion of bicarbonate and other electrolytes from the early generations of ducts of pancreas and liver, i.e., the fluid part of pancreatic juice and bile. It inhibits gastric secretion and emptying. Secretion of bicarbonate into the intestine and inhibition of acid secretion from stomach are achieved by it.
Thus it helps to maintain a proper pH in the intestine which is very much important for digestion. Secretin also augments the actions of gastrin and CCK-PZ on pancreas and liver.
It is an important local hormone secreted at various sites of the body and also from the GI tract. It is a polypeptide hormone with 14 amino acids and is also found in other forms.
Somatostatin inhibits gastrin secretion and thereby acid secretion. Here in the stomach its action is mostly paracrine in nature. It is also found in other parts of the GI tract. It is inhibitory to GI movements including contraction of stomach and gall bladder.
Thus somatostatin from the D cells of islets of Langerhans is an important paracrine agent.
Vaso-active intestinal polypetide (VIP)
It is a polypeptide hormone with 28 amino acids. It is secreted from the GI tract and also from the intrinsic neurons of the gut.
It is inhibitory to gastric secretion and motility but stimulatory to water and electrolyte secretion from the small intestine. It also stimulates bicarbonate secretion from the pancreas. It causes relaxation of smooth muscles and dilatation of blood vessels.
Gastric inhibitory peptide (GIP)
It is a polypeptide with 43 amino acids. It is secreted from the upper part of the small intestine.
It is the main mediator of insulin secretion after a meal.
Contrary to its name it has not much inhibitory effect on stomach in physiological concentration.
The gastric inhibitory effect is found only with a high dose.
It is secreted from duodenal mucosa. It has 22 amino acids.
It is involved in movements of gut as it is found to cause contraction of smooth muscles of intestine.
The glucagon is also secreted from GI tract and is called enteroglucagon.
Its secretion stops when the intestinal lumen becomes alkaline due to increased bicarbonate secretion.
INGESTION OF FOOD
INGESTION OF FOOD : CHEWING
After food is placed into the mouth, it is cut and ground into smaller pieces by chewing (mastication).
Although chewing is a voluntary act, it is coordinated by chewin reflex centers ( brain stem ) that facilitate the opening and closing of the jaw.
INGESTION OF FOOD : CHEWING
When the month opens, stretch receptors in the jaw muscles initiate a reflex contraction of the masseter, medial pterygoid, and temporalis muscles, causing the mouth to close.
When the mouth closes, food comes into contact with buccal receptors, eliciting a reflex contraction of the digastric and lateral pterygoid muscles, causing the mouth to open.
When the jaw drops, the stretch reflex causes the entire cycle to be repeated. The tongue contributes to the grinding process by positioning the food between the upper and lower teeth.
FUNCTION OF CHEWING
1. Chewing breaks food into smaller pieces, which makes it easier for the food to be swallowed; breaks off the indigestible cellulose coatings of fruits and vegetables; increases the surface area of the food particles, making it easier for them to be digested by the digestive enzymes.
2. Chewing mixes the food with salivary gland secretions, which initiates the process of starch digestion by salivary amylase; initiates the process of lipid digestion by lingual lipase; lubricates and softens the bolus of food, making it easier to swallow.
FUNCTION OF CHEWING
3. Chewing brings food into contact with taste receptors and releases odors that stimulate the olfactory receptors. The sensations generated by these receptors increase the pleasure of eating and initiate gastric secretions.
LUBRICATION OF FOOD BY SALIVA
1. The parotid glands, located near the angle of the jaw, are the largest glands. They secrete a watery fluid.
2. The submandibular and sublingual glands secrete a fluid that contains a higher concentration of proteins and so is more viscous.
3. Smaller glands are located throughout the oral cavity. Those in the tongue secrete lingual lipase.
COMPOSITION OF SALIVA
The salivary glands secrete a relatively high volume of fluid (0.5-1 L/day) containing electrolytes and proteins.
Saliva is secreted into the salivary ducts by acinar cells that line the beginning of the salivary duct.
Salivary flow rates and enzymatic secretions are increased by parasympathetic nervous system activity.
COMPOSITION OF SALIVA
The initial electrolyte composition of saliva is similar to plasma. The ductal cells that line the tubular portions of the salivary ducts change the composition of saliva. Ductal cells reabsorb Na and Cl and secrete K and HCO 3 .
At high flow rates, there is less time for reabsorption and secretion, and, therefore, saliva contains lower concentrations of Na and Cl and higher concentrations of K.
At low flow rates there is more time for reabsorption and secretion and, therefore, saliva contains higher concentrations of Na and Cl and lower concentrations of K.
COMPOSITION OF SALIVA
HCO 3 concentration increases when salivary flow increases because HC О 3 secretion is increased when salivary glands are stimulated by the parasympathetic nervous system.
Two types of proteins are found in saliva. The enzymes alpha-amylase (ptyalin) and lingual lipase begin the process of starch and fat digestion.
Mucin is a glycoprotein that lubricates the food.
CONTROL OF SALIVARY SECRETION
Salivary secretion is controlled entirely by autonomic nervous system reflexes.
Parasympathetic nerve stimulation causes the salivary gland cells to secrete a large volume of watery fluid that is high in electrolytes but low in proteins.
Sympathetic nerve stimulation causes the salivary glands to secrete a small volume of fluid that contains a high concentration of mucus.
Salivary reflexes are elicited by the thought, aroma, or taste of food or in the presence of food within the alimentary canal.
Salivary gland metabolism and growth are both stimulated by increased ANS activity.
FUNCTIONS OF SALIVA
1. PROTECTION. Salivary secretions protect the mouth by:
- cooling hot foods;
- diluting any HCl or bile regurgitated into the mouth;
- washing food away from the teeth;
- destroying harmful bacteria within the mouth.
FUNCTIONS OF SALIVA
2. DIGESTION. Salivary secretions begin the process of starch and fat digestion.
- alpha-amylase can digest most of the ingested starches into disaccharides before they reach the small intestine; alpha- amylase is ultimately inactivated in the low pH of the stomach;
- lingual lipase begins to break down ingested fats while they are in the mouth, stomach, and upper portions of the small intestine;
3. LUBRICATION. Salivary secretions lubricate the food, making swallowing easier, and moisten the mouth, facilitating speech.
Deglutition is the process by which the food is passed into the stomach from the oral cavity. It occurs in three phases. The initiation of the process is voluntary.
BUCCAL PHASE (1st PHASE)
The food, taken in the mouth, after mastication and mixin with saliva, is converted into bolus by the movements of tongue inside the oral cavity.
The mouth is then closed and the bolus is pushed backwards. The bolus then passes over the back of the tongue towards the pharynx. During this, the nasopharynx is shut off to prevent regurgitation of food through the nose and respiration is reflexly stopped.
The nasopharynx, is shut off by upward movement of the soft palate and forward movement of the posterior pharyngeal wall.
The process of deglutition is voluntary to start with.
PHARYNGEAL PHASE (2nd PHASE)
The bolus is pushed backwards by the tongue towards the oesophagus over the epiglottis now lying horizontally on the closed laryngeal opening.
Now there is reflex relaxation of the upper end of the oesophagus which is also called the upper oesophageal sphincter and the bolus enters into the oesophagus.
During this phase respiration remains stopped.
OESOPHAGEAL PHASE (3rd PHASE)
After the bolus is placed in the upper end of oesophagus, the upper oesophageal sphincter contracts and the bolus is placed in the oesophagus. This is called primary peristalsis and can send the food bolus upto the stomach with the help of the gravity.
If it fails and the bolus remains in the oesophagus, then due to stretching by the bolus, secondary peristaltic wave starts in the wall of the oesophagus and the bolus is carried down to the stomach.
This is also helped by gravity in erect posture and relaxation of the lower end of the oesophagus which has another sphincter.
The lower oesophageal sphincter is a 2 to 6 cm segment at the lower end of the oesophagus which is tonically contracted. It opens only during deglutition due to the action of nitric oxide and VIP from the intrinsic neurons in the wall of the oesophagus. The extrinsic parasympathetic fibres also have role in opening of the this sphincter.
Sympathetic nerves and GI hormones like gastrin help in the closure of it. Lower oesophageal sphincter prevents entry of gastric contents into the oesophagus. The valve-like action of the gastro-oesophageal junction also shuts off the lower end of the oesophagus in case of increased intra-abdominal pressure. All these mechanisms prevent gastro-oesophageal reflux.
Deglutition occurs not only when we eat but also at other times and even during sleep that is without the presence of any food bolus.
Deglutition also keeps the pharyngotympanic tube opened that is why the air passengers are liberally. This helps to keep the air pressure in the middle ear cavity equal to that of atmosphere otherwise there may be rupture of the tympanic membrane.
Deglutition is initiated voluntarily. It occurs automatically affer that due to a series of well coordinated reflexes arising from the passage due to movement of the bolus. Most of these reflexes are coordinated at the deglutition centre near the respiratory centre in the brain stem through 5, 7, 9, 10 and 12th cranial nerves. Some reflexes are local, which operate through the intrinsic nerves.