MASTICATION, DEGLUTITION, DIGESTION &ASSIMILATION

Dr. Neelam Bhandari

P.G STUDENT 1st year

Dept. of prosthodontics

Mastication & Deglutition

Mastication:Processes involved in food

preparation, including moving unchewed food onto the grinding surface of the teeth, chewing, it, and mixing it with saliva in preparation for swallowing

Deglutition:swallowing

CHEWING CYCLE

Duration: 0.6 to 1sec depending on food type

Murphy summarised 6 phases in chewing cycle:

CentricOcclusio

n

Preparatory

Phase

Food ContactPhase

Crushing

Phase

Tooth contactPhase

Grinding

Phase

Tongue positions food inoral cavity. Mandible deviates towards chewing side

Sensory receptors triggereddue to food contact

Starts with high velocity, slows as food gets crushed

All reflex muscularadjustments fortooth contact made

Grinding movementat occlusal tables

Mandible return to single terminal point

The coordination of occlusal contacts, jaw motion and tongue movement during mastication require a very intricate control system.

Involves guiding influences from:

◦Teeth and supporting structures◦TMJ◦Masticatory muscles◦Higher centres in brain

MASTICATORY APPARATUS

Teeth Specialized occlusal form Opposing contacts Contours Sluiceways

Tongue Level w.r.t occlusal plane

Muscles of mastication◦Temporalis, Masseter, Lat. Pterygoid◦Medial Pterygoid,

DEGLUTITIONSwallowing is a reflex response.

It involves coordination of nearly 20 different muscles with motor neurons distributed from mesencephalic to posterior medullary levels.

Neurons responsible for coordination form the swallowing center – neuronal groups fire automatically in a particular sequence to achieve a necessary pattern of muscle activity to produce swallowing.

Swallowing is initiated by the voluntary action of collecting the oral contents on the tongue and propelling them backwards into the pharynx.

This starts a wave of involuntary contraction in pharyngeal muscles that pushes the material into oesophagus.

Inhibition of respiration and glottic closure are part of reflex response.

PERISTALSISReflex response that is initiated

when the gut wall is stretched by the contents of the lumen – from oesophagus to rectum.

Stretch initiates circular contraction behind the stimulus and area of relaxation in front of it.

Rate: 2-25 cm/sec

Stages of Duglutition

Lower Esophageal Sphincter

DIGESTION

Digestion is the breakdown of food into smaller components that can be more easily absorbed and assimilated by the body.

These smaller substances are absorbed through the small intestine into the blood stream.

Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion.

The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules the body can use.

In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication and the wetting contact of saliva.

Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food.

After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. 

Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin.

At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis. This allows the mass of food to further mix with the digestive enzymes.

After some time (typically 1–2 hours in humans, 4–6 hours in dogs, 3–4 hours in house cats), the resulting thick liquid is called chyme.

When the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas, and then passes through the small intestine, in which digestion continues.

When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine.

Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 - 6.9.

Some vitamins, such as biotin and vitamin K  produced by bacteria in the colon are also absorbed into the blood in the colon.

Waste material is eliminated from the rectum during defecation.

Phases of gastric secretion

Cephalic phase - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion.

Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine occurs.

Gastric secretion at this phase rises to 40% of maximum rate.

Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH.

As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach.

This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete hcl, which lowers the pH to the desired pH of 1-3.

Acid release is also triggered by acetylcholine and histamine.

Intestinal phase - This phase has 2 parts, the excitatory and the inhibitory.

Partially digested food fills the duodenum. This triggers intestinal gastrin to be released.

Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.

PARTS OF GIT

Oral cavity

In humans, digestion begins in the Mouth, where food is chewed. Saliva is secreted in large amounts in the oral cavity, and is mixed with the chewed food by the tongue. Saliva cleans the oral cavity, moistens the food, and contains digestive enzymes such as salivary amylase.

An additional enzyme, lingual lipase, hydrolyzes long-chain triglycerides into partial glycerides and free fatty acids.

Swallowing transports the chewed food into the esophagus. The mechanism for swallowing is coordinated by the swallowing center.

PharynxThe pharynx is the part of the neck and

throat situated behind the mouth and nasal cavity, and cranial, or superior, to the esophagus.

It is part of the digestive system and respiratory system.

A flap of connective tissue, the epiglottis closes over the trachea when food is swallowed to prevent choking or asphyxiation.

Esophagus

The esophagus is a narrow muscular tube about 20-30 centimeters long, which starts at the pharynx , passes through the thoracic diaphragm, and ends at the cardiac orifice of the stomach. The wall of the esophagus is made up of two layers of smooth muscles.

The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles.

Stomach

The stomach is a small, 'J'-shaped pouch with walls made of thick, distensible muscles, which stores and helps break down food.

Food enters the stomach through the cardiac orifice where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins.

Food in the stomach is in semi-liquid form, which upon completion is known as chyme.

When the chyme reaches the opening to the duodenum known as the pylorus, contractions "squirt" the food back into the stomach through a process called retropulsion, which exerts additional force and further grinds down food into smaller particles. 

Gastric emptying is the release of food from the stomach into the duodenum; the process is tightly controlled with liquids being emptied much more quickly than solids.

Small intestine

It has three parts: the Duodenum, Jejunum and Ileum. After being processed in the stomach, food is passed

to the small intestine via the pyloric sphincter. Here it is further mixed with three different liquids:

Bile, which emulsifies fats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids.

Pancreatic juice made by the pancreas, which secretes enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen .

Intestinal juice contains enzymes such as enteropeptidase, erepsin, trypsin, chymotrypsin,  maltase,  lactase and sucrase .

The pH level increases in the small intestine as all three fluids are alkaline.

A more basic environment causes more helpful enzymes to activate and begin to help in the breakdown of molecules such as fat globules.

The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices.

Large intestine

After the food has been passed through the small intestine, the food enters the large intestine.

Within it, digestion is retained long enough to allow fermentation due to the action of gut bacteria, which breaks down some of the substances that remain after processing in the small intestine.

some of the breakdown products are absorbed.

In general, the large intestine is less vigorous in absorptive activity.

In humans, the large intestine is roughly 1.5 meters long, with three parts: the cecum , the colon and the rectum.

The large intestine absorbs water from the chyme and stores feces until it can be egested.

Protein digestion

Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids.

Fat digestion

Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides.

However fats are mainly digested in the small intestine.

Pancreatic lipase from the pancreas and bile from the liver which helps in the emulsification of fats for absorption fatty acids. 

Complete digestion of one molecule of fat (a triglyceride) results in 3 fatty acid molecules and one glycerol molecule.

Carbohydrate digestion In humans, dietary starches are composed

of glucose units arranged in long chains called amylose, a polysaccharide. During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase. This results in simple sugars glucose and maltose that can be absorbed by the small intestine.

Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose. Glucose and galactose can be absorbed by the small intestine.

Sucrase is an enzyme that breaks down the disaccharide sucrose. Sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the small intestine.

Digestive hormones

There are at least five hormones that aid and regulate the digestive system in mammals.

Gastrin Secretin Cholecystokinin (CCK) Gastric inhibitory peptide (GIP) Motilin 

ASSIMILATIONCarbohydrate Absorption:

Hexoses are rapidly absorbed by the wall of small intestine (before the meal reaches terminal ileum).

Its transport is dependent on Na+ in intestinal lumen

This is because Na+ and Glucose share same cotransporter, or symport:

Sodium Dependent Glucose Transporter (SGLT)

SGLT1 uptake of dietary glucose from gut SGLT2 transport glucose out of the renal tubules

Glucose is transported by Glucose Transporter (GLUT 2) into interstitium and to capillaries.

Transport via GLUT is secendory active transport as the energy for this is provided indirectly by active transport of Na+ out of the cell.

Protein AbsorptionAt least 7 transport systems transport

amino acids into enterocytes:◦5 require Na+ and transport AA with Na+◦2 of these 5 also require Chloride ion.◦2 systems transport independent of Na+

Di- and Tripeptides are transported into enterocytes by a system called PepT1◦Peptide Transporter 1◦This requires H+ instead of Na+

AA released from peptides plus AA absorbed from brush border are transported out of enterocytes.

They enter hepatic portal blood from there.

Absorption is rapid in duodenum and jejunum but slow in ileum

Only 2-5% protein in small intestine escape digestion and absorption.

This too is digested by bacteria in colon.

Fat Absorption

Traditionally fats were thought to enter enterocytes by passive diffusion but now role of carriers is known.

There are also carriers that export lipids back into lumen.

Fate of Fatty Acids depend on their size:

Less than 10-12 carbon atoms Pass through enterocyte unmodified

Actively transported to portal Blood

More than 10-12 Carbon atoms Re esterified to triglycerides in enterocytes

Cholesterol EstersCoated with a layer of proteinCholesterol and phospholipid

CHYLOMICRONS

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