Significances of Mastication

1. Breakdown of foodstuffs into smaller particles.
2. Mixing of saliva with food substances thoroughly.
3. Lubrication and moistening of dry food by saliva so that the bolus can be easily swallowed.
4. Appreciation of taste of the food.

Muscles of Mastication

Movements of Mastication

1. Opening and closure of mouth.
2. Rotational movements of jaw.
3. Protraction and retraction of jaw.

Control of Mastication

• Action of mastication is mostly a reflex process.
• It can be carried out voluntarily also.
• Center for mastication is situated in medulla and cerebral cortex.
• Muscles of mastication are supplied by mandibular division of trigeminal nerve (V cranial nerve).

Stages of Deglutition

I. Oral Stage or First Stage

Oral stage is a voluntary stage. During this stage, bolus from oral cavity passes into the pharynx by means of a series of actions.

Sequence of Events During Oral Stage

1. Bolus is placed over posterodorsal surface of tongue. This is called preparatory position.
2. Anterior part of tongue is retracted and depressed.
3. Posterior part of tongue is elevated and retracted against hard palate. This pushes bolus backwards into pharynx.
4. Forceful contraction of tongue against the palate produces a positive pressure in posterior part of oral cavity. This pressure in oral cavity also pushes the food into pharynx.

II. Pharyngeal Stage or Second Stage

Pharyngeal stage is an involuntary stage. During this stage, bolus is pushed from pharynx into the esophagus.

Since pharynx communicates with mouth, nose, larynx and esophagus, during this stage of deglutition the bolus from pharynx can enter into four paths:

1. Back into mouth.
2. Upward into nasopharynx.
3. Forward into larynx.
4. Downward into esophagus.

1. Prevention of Movement of Bolus Back into Mouth

• Return of bolus back into the mouth is prevented by position of tongue against the soft palate (roof of the mouth) and high intraoral pressure, developed by movement of tongue.

2. Prevention of Movement of Bolus Upward into Nasopharynx

• Movement of bolus into nasopharynx from pharynx is prevented by elevation of soft palate along with its extension called uvula.

3. Prevention of Movement of Bolus Forward into Larynx

Movement of bolus into larynx is prevented by:

1. Approximation of the vocal cords.
2. Forward and upward movement of larynx.
3. Backward movement of epiglottis to seal the opening of the larynx (glottis).

Deglutition Apnea

Apnea refers to temporary arrest of breathing. Deglutition apnea or swallowing apnea is the arrest of breathing during deglutition.

Choking

Choking is the inability to breathe due to obstruction or compression of respiratory passage. Sometimes during the second stage of swallowing, solid food particles may enter larynx resulting in obstruction and choking. However, it may be prevented automatically by gag reflex.

4. Movement of Bolus into Esophagus

Since the other three paths are closed for the bolus, it must pass only through esophagus.

Movement of bolus into esophagus occurs by combined effects of various factors:

1. Upward movement of larynx stretches the opening of esophagus.
2. Simultaneously, upper 3 to 4 cm of esophagus relaxes.
3. This part of esophagus is formed by cricopharyngeal muscle and is called upper esophageal sphincter or pharyngoesophageal sphincter.
4. At the same time, peristaltic contractions start in pharynx due to contraction of pharyngeal muscles.
5. Elevation of larynx also lifts the glottis away from the food passage.

III. Esophageal Stage or Third Stage

It is also an involuntary stage. During esophageal stage food from esophagus enters the stomach. Esophagus forms the passage for movement of bolus from pharynx to stomach.

Peristalsis

Movements of esophagus called peristaltic waves are specifically organized for this function. Peristalsis means a wave of contraction followed by a wave of relaxation of muscle fibres of GI tract, which travel in aboral direction (away from mouth). By this type of movement, the contents are propelled down along GI tract.

1. Primary Peristaltic Contractions

When bolus reaches upper part of esophagus, peristalsis starts. This is known as primary peristalsis. After origin, the peristaltic contractions pass down through the rest of esophagus, propelling the bolus towards stomach.

2. Secondary Peristaltic Contractions

If the primary peristaltic contractions are unable to propel the bolus into stomach, secondary peristaltic contractions appear and push the bolus into stomach. Secondary peristaltic contractions are induced by distention of upper esophagus by the bolus. After origin, secondary peristaltic contractions pass down like primary contractions, producing a positive pressure.

Role of Lower Esophageal Sphincter

• Lower esophageal sphincter is constricted always.
• When bolus enters this part of esophagus, the sphincter relaxes so that the contents enter stomach.
• After entry of bolus into the stomach, this sphincter constricts and closes the lower end of esophagus.
• Relaxation and constriction of lower esophageal sphincter occur in sequence with arrival of peristaltic contractions of esophagus.

Gag Reflex

Initiation

It is initiated by touch of a wisp of cotton or any other object such as solid food particle at roof of mouth, back of tongue, uvula, tonsils or back of throat.

It occurs by mass contraction of the posterior oral and pharyngeal muscles and elevation of the soft palate.

Significance of Gag Reflex

Gag Reflex in Babies

• Gag reflex is very active in babies below 6 months.
• Forceful gag makes the baby to vomit.
• Thus, in babies, the gag reflex prevents swallowing of hard and solid food and its entry into respiratory passage.
• After 6 months, when the baby learns to swallow the food properly, the gag reflex diminishes enabling the baby to swallow chunky or solid food.

Deglutition Reflex

Though the beginning of swallowing is a voluntary act, later it becomes involuntary and it is carried out by a reflex action called deglutition reflex. This reflex occurs during pharyngeal and esophageal stages.

Sequences of Deglutition Reflex

1. When bolus enters oropharyngeal region, receptors present in this region are stimulated.
2. Afferent impulses from oropharyngeal receptors pass via glossopharyngeal nerve fibres to the deglutition center situated at the floor of fourth ventricle in medulla oblongata of brain.
3. Efferent impulses from deglutition center travel through glossopharyngeal and vagus nerves (parasympathetic motor fibres) and reach soft palate, pharynx and esophagus.
4. Glossopharyngeal nerve is concerned with pharyngeal stage of swallowing. Vagus nerve is concerned with esophageal stage.
5. The reflex causes upward movement of soft palate to close nasopharynx and upward movement of larynx to close respiratory passage so that bolus enters esophagus.
6. Then, peristalsis occurs in esophagus, pushing the bolus into stomach.

Swallowing in Infants

Swallowing hard and solid food by infants is prevented by gag reflex or pharyngeal reflex. Gag reflex prevents choking.

Activities of smooth muscles of stomach increase during gastric digestion when stomach is filled with food and when the stomach is empty.

1. Hunger Contractions

Hunger contractions are the movements of empty stomach. Such contractions are related to sensations of hunger.

Hunger contractions are strong peristaltic contractions associated with hunger pain. This type of peristaltic contraction is different from digestive peristaltic contractions. Digestive peristaltic contractions usually occur in body and pyloric parts of the stomach, but hunger contractions of empty stomach involve the entire stomach.

Types of Hunger Contractions

Hunger contractions are of three types. When the stomach is empty, type I contractions occur first, followed by type II contractions. If food intake is still postponed, type III contractions appear. As soon as food is consumed, hunger contractions disappear.

2. Receptive Relaxation

Receptive relaxation is the relaxation of upper portion of stomach when bolus enters the stomach from esophagus. Its significance is to accommodate the food easily without much increase in pressure inside stomach.

3. Peristalsis of Stomach

When food enters the stomach, peristaltic contraction or peristaltic wave appears with a frequency of 3/min.

• It starts from lower part of the body of stomach.
• It passes through pylorus till the pyloric sphincter.
• Initially, the contraction appears as a slight indentation on greater and lesser curvatures and travels towards pylorus.
• Later, the contraction becomes deeper while travelling.
• Finally, the contraction ends with constriction of pyloric sphincter.
• Some waves disappear before reaching the sphincter.
• Each peristaltic wave takes about one minute to travel from point of origin to point of ending.

Filling and Emptying of Stomach

Filling of Stomach

While taking food, it arranges itself in the stomach in different layers.

• First eaten food is placed against greater curvature in fundus and body of the stomach.
• Successive layers of food particles lie nearer the lesser curvature until the last portion of food eaten lies near upper end of lesser curvature, adjacent to cardiac sphincter.
• Liquid remains near lesser curvature and flows towards pyloric end of stomach along a V-shaped groove called magenstrasse.
• This groove is formed by smooth muscle.
• If a large quantity of fluid is taken, it flows around entire food mass and is distributed over the interior part of stomach, between wall of stomach and food mass.

Emptying of Stomach

Gastric emptying is a process during which chyme from stomach is emptied into intestine. Food that is swallowed enters stomach and remains there for about 3 hours. During this period, digestion takes place. Partly digested food becomes the chyme.

• Acid chyme is emptied from stomach into intestine slowly with the help of peristaltic contractions.
• It takes about 3 to 4 hours for emptying of the chyme.
• This slow emptying is necessary to facilitate the final digestion and maximum (about 80%) absorption of digested food materials from small intestine.
• Gastric emptying occurs due to peristaltic waves in the body and pyloric part of stomach and simultaneous relaxation of pyloric sphincter.

Factors Affecting Gastric Emptying

Regulation of Gastric Emptying

Gastric emptying is regulated by nervous and hormonal factors.

Nervous Factor: Enterogastric Reflex

The nervous factor which regulates emptying of stomach is the enterogastric reflex. This reflex inhibits gastric emptying. It is elicited by presence of chyme in duodenum, which prevents further emptying of stomach.

Mechanism of Enterogastric Reflex

1. Presence of chyme in duodenum causes generation of nerve impulses which are transmitted to stomach by intrinsic nerve fibres of GI tract. After reaching the stomach, the impulses inhibit emptying.
2. Impulses from duodenum pass via extrinsic sympathetic fibres also to stomach and inhibit emptying.
3. Some impulses from duodenum travel through afferent vagal fibres to the brainstem. Normally, brainstem neurons send excitatory impulses to stomach through efferent vagal fibres and stimulate gastric emptying. However, impulses from duodenum inhibit the brainstem neurons and thereby inhibit gastric emptying.

Factors Which Initiate Enterogastric Reflex

1. Duodenal distension.
2. Irritation of duodenal mucosa.
3. Acidity of the chyme.
4. Osmolality of the chyme.
5. Breakdown products of proteins and fats.

Hormonal Factors Regulating Gastric Emptying

When an acid chyme enters the duodenum, duodenal mucosa releases some hormones which enter the blood and inhibit the motility of stomach.

Causes of Vomiting

1. Presence of irritating contents in GI tract.
2. Mechanical stimulation of pharynx.
3. Pregnancy.
4. Excess intake of alcohol.
5. Nauseating sight, odor or taste.
6. Unusual stimulation of labyrinthine apparatus, as in sea sickness, air sickness, car sickness and swinging.
7. Abnormal stimulation of sensory receptors in organs like kidney, heart, semicircular canals and uterus.
8. Drugs like antibiotics, opiates, etc.
9. Any GI disorder.
10. Acute infection like urinary tract infection, influenza, etc.
11. Metabolic disturbances like carbohydrate starvation and ketosis during pregnancy, acidosis during diabetes and uremia.

Mechanism of Vomiting

Nausea

Vomiting is always preceded by nausea. Nausea is an unpleasant sensation of discomfort in abdomen with urge to vomit.

Nausea is characterized by secretion of large amount of saliva containing more amount of mucus.

Retching

Strong involuntary movements in GI tract start before actual vomiting and intensify the feeling of vomiting. This condition is called retching (try to vomit). Vomiting occurs few minutes after this.

Act of Vomiting — Sequence of Events

1. Beginning of antiperistalsis which runs from ileum towards mouth through intestine and pushes intestinal contents into stomach within few minutes.
2. Deep inspiration followed by temporary cessation of breathing.
3. Closure of glottis.
4. Upward and forward movement of larynx and hyoid bone.
5. Elevation of soft palate.
6. Contraction of diaphragm and abdominal muscles with a characteristic jerk resulting in elevation of intra-abdominal pressure.
7. Compression of stomach between diaphragm and abdominal wall leading to rise in intragastric pressure.
8. Simultaneous relaxation of lower esophageal sphincter, esophagus and upper esophageal sphincter.
9. Forceful expulsion of gastric contents (vomitus) through esophagus, pharynx and mouth.

Movements Preventing Entry of Vomitus Through Other Routes

1. Closure of glottis and cessation of breathing prevent entry of vomitus into the lungs.
2. Elevation of soft palate prevents entry of vomitus into the nasopharynx.
3. Larynx and hyoid bone move upward and forward and are placed in this position rigidly. This causes dilatation of throat which allows free exit of vomitus.

Vomiting Reflex

Vomiting is a reflex act. Receptors for nausea or vomiting are chemoreceptors and stretch receptors in GI tract. Receptors are stimulated by irritation or distention of any part of GI tract or other organs.

• Sensory impulses from receptors are transmitted to vomiting center through vagus and sympathetic fibres.
• Vomiting center is situated in medulla oblongata near the nucleus tractus solitarius.
• Motor impulses from vomiting center are transmitted through V, VII, IX, X and XII cranial nerves to the upper part of GI tract; and through spinal nerves to diaphragm and abdominal muscles.

Movements of small intestine are essential for mixing the chyme with digestive juices, propulsion of food and absorption.

I. Mixing Movements

Mixing movements of small intestine are responsible for proper mixing of chyme with digestive juices such as pancreatic juice, bile and intestinal juice.

1. Segmentation Contractions

Segmentation contractions are the common type of movements of small intestine, which occur regularly or irregularly but in a rhythmic fashion. They are also called rhythmic segmentation contractions.

• Contractions occur at regularly spaced intervals along a section of intestine.
• Segment of intestine involved in each contraction is about 1 to 5 cm long.
• Segments of intestine between the contracted segments are relaxed.
• Length of relaxed segments is the same as that of the contracted segments.
• The alternate segments of contraction and relaxation give appearance of rings resembling the chain of sausages.
• After sometime, contracted segments are relaxed and the relaxed segments are contracted.
• Segmentation contractions chop the chyme many times and help in mixing chyme with digestive juices.

2. Pendular Movement

Pendular movement is the sweeping movement of small intestine resembling movements of pendulum of a clock.

• Small portions of intestine (loops) sweep forward and backward or upward and downward.
• It helps in mixing of chyme with digestive juices.

II. Propulsive Movements

Propulsive movements are movements of small intestine which push the chyme in aboral direction through intestine.

1. Peristaltic Movements

Peristalsis travels from the point of stimulation in both directions. Under normal conditions, progress of contraction in an oral direction is inhibited quickly and the contractions disappear. Only the contraction that travels in an aboral direction persists.

2. Peristaltic Rush

Sometimes, small intestine shows a powerful peristaltic contraction. It is caused by excessive irritation of intestinal mucosa or extreme distention of the intestine.

• This powerful contraction begins in duodenum.
• It passes through the entire length of small intestine.
• It reaches the ileocecal valve within a few minutes.
• It is called peristaltic rush or rush waves.
• It sweeps the contents of intestine into colon.
• It relieves the small intestine of either irritants or excessive distention.

3. Peristalsis in Fasting — Migrating Motor Complex

Peristalsis in fasting or migrating motor complex is a type of peristaltic contraction which occurs in stomach and small intestine during periods of fasting for several hours.

• It differs from regular peristalsis because a large portion of stomach or intestine is involved in this contraction.
• Contraction extends to about 20 to 30 cm of the stomach or intestine.
• This type of movement occurs once in every 1½ to 2 hours.

Significance of Peristalsis in Fasting

• Sweeps excess digestive secretions into colon and prevents accumulation of secretions in stomach and intestine.
• Sweeps residual undigested materials into colon.

4. Movements of Villi

• Intestinal villi show movements simultaneously along with intestinal movements.
• Movements of villi are shortening and elongation, which occur alternatively.
• They help in emptying lymph from the central lacteal into the lymphatic system.
• Surface area of villi is increased during elongation.
• This helps absorption of digested food particles from the lumen of intestine.
• Movements of villi are caused by local nervous reflexes, which are initiated by the presence of chyme in small intestine.

Large intestine shows sluggish movements. Still, these movements are important for mixing, propulsive and absorptive functions.

Types of Movements

I. Mixing Movements — Segmentation Contractions

Large circular constrictions which appear in the colon are called mixing segmentation contractions.

• Contractions occur at regular distance in colon.
• Length of the portion of colon involved in each contraction is nearly about 2.5 cm.

II. Propulsive Movements — Mass Peristalsis

Mass peristalsis or mass movement propels the feces towards anus.

• Usually, this movement occurs only a few times every day.
• Duration of mass movement is about 10 minutes.
• It usually occurs in the morning before or after breakfast.
• It is because of neurogenic factors like gastrocolic reflex and parasympathetic stimulation.

Defecation Reflex

• Mass movement drives the feces into sigmoid or pelvic colon.
• In the sigmoid colon, feces is stored.
• Defecation occurs when some quantity of feces reaches rectum due to mass movement.
• Usually, the stimulus for defecation is elicited by an increase in intrarectal pressure to about 20 to 25 cm H₂O.
• Usual stimulus for defecation is intake of liquid, coffee or tea or water.
• The stimulus differs from person to person.

Act of Defecation

Act of defecation is preceded by voluntary effort, assuming an appropriate posture, voluntary relaxation of external sphincter and compression of abdominal contents by voluntary contraction of abdominal muscles.

• Usually, rectum is empty.
• During development of mass movement, feces is pushed into rectum and the defecation reflex is initiated.
• Process of defecation involves contraction of rectum and relaxation of internal and external anal sphincters.

Gastrocolic Reflex

Gastrocolic reflex is the contraction of rectum followed by desire for defecation caused by distention of stomach by food.

• It is mediated by intrinsic nerve fibres of GI tract.
• This reflex causes only a weak contraction of rectum.
• But it initiates defecation reflex.

Pathway for Defecation Reflex

1. When rectum is distended due to entry of feces, sensory nerve endings are stimulated.
2. Impulses from the nerve endings are transmitted via afferent fibres of pelvic nerve to defecation center situated in sacral segments of spinal cord.
3. Defecation center sends motor impulses through descending colon, sigmoid colon and rectum via efferent fibres of pelvic nerve.
4. Motor impulses cause contraction of descending colon, sigmoid colon and rectum and relaxation of internal sphincter.
5. Simultaneously, voluntary relaxation of external sphincter occurs due to inhibition of pudendal nerve by impulses arising from cerebral cortex.

Components of Gut-Brain Axis

1. Central nervous system.
2. Autonomic nervous system.
3. Enteric nervous system.
4. Hypothalamic-pituitary-adrenal axis.
5. Neuroendocrine system.
6. Immune system.
7. Gut microbiota and microbiome.

Gut Microbiota and Microbiome

Gut microbiota refers to microorganisms such as bacteria, viruses and fungi occupying the gut.

Microbiome is defined as all microorganisms in the gut along with their genetic materials. Genetic material of microbiome is many times greater than genome of whole body.

Gut Microbiota and Brain

Communication between gut microbiota and brain involves various factors such as afferent and efferent pathways which pass through vagus nerve and autonomic nervous system, hypothalamo-pituitary-adrenal axis and immune system.

This communication regulates all aspects of homeostasis.

Involvement of Nervous System

• Chemical mediators released by microbiota stimulate the sensory nerve endings in gastrointestinal tract.
• Impulses from here are carried by afferent fibres of vagus and spinal nerve to brain, to hypothalamus and limbic system via brainstem.
• Hypothalamus and limbic system in turn influence the activities of gut through descending efferent nerve fibres.

Involvement of Gastrointestinal Hormones

Metabolic products from microorganisms stimulate the enteroendocrine cells which release various GI hormones particularly such as:

• Glucagon-like polypeptide-1.
• Glucagon-like polypeptide-2.
• Neuropeptide Y.
• Peptide YY.

All the hormones influence the intrinsic pathway (enteric nervous system) via myenteric (Auerbach’s) nerve plexus and submucus (Meisner’s) nerve plexus and regulate the movements and secretory activities of gastrointestinal tract.

Involvement of Immune System

Gut microbiota also helps the body in defense against disease-inducing invading organisms.

Microorganisms of gut stimulate the immune system which rapidly challenges the pathogens by means of bacterial antagonism (inhibition of one type of bacteria by another type).