Movements Of Gastrointestinal Tract Notes

Mastication

Mastication or chewing is the first mechanical process in the Gl tract by which the food substances are torn or cut into small particles and crushed or ground into a soft bolus.

The significance of mastication:

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

Read And Learn More: Medical Physiology Notes

1. Muscles And The Movements Of Mastication

• Muscles of mastication:
  • Masseter muscle
  • Temporal muscle
  • Pterygoid muscles
  • Buccinator muscle.
• Movements involved in mastication:
  • Opening and closure of the mouth
  • Rotational movements of the jaw
  • Protraction and retraction of the jaw.

Control Of Mastication: The action of mastication is mostly a reflex process. It is carried out voluntarily also. The center for mastication is situated in the medulla and cerebral cortex. The muscles are supplied by the mandibular division of the V cranial (trigeminal) nerve.

Deglutition

Deglutition Definition: Deglutition or swallowing is the process by which food is passed from the mouth into the stomach.

Stages of Deglutition:

Deglutition occurs in three stages:

• The oral stage is when food enters the pharynx from the mouth
• The pharyngeal stage is when food enters the esophagus from the pharynx
• The esophageal stage is when food enters the stomach from the esophagus.

1. Oral Stage Or First Stage:

In the oral stage,, food enters the pharynx from the mouth. The oral stage of deglutition is a voluntary stage. In this stage of swallowing, the bolus passes through the oral cavity into the pharynx by means of a series of actions such as:

• The bolus is placed over the posterodorsal surface of the tongue. It is called the preparatory position
• The anterior part of the tongue is retracted and depressed
• The posterior part of the tongue is elevated and retracted against the hard palate. This pushes the bolus backward into the pharynx
• The forceful contraction of the tongue against the palate produces a positive pressure in the posterior part of the oral cavity. This pressure in the oral cavity also pushes the food into the pharynx.

2. Pharyngeal Stage Or Second Stage:

• The pharyngeal stage is an involuntary stage. In this stage, the bolus is pushed from the pharynx into the esophagus. The pharynx is a common passage for food and air. It divides into the larynx and esophagus.
• Larynx lies anteriorly and continues as respiratory passage. Esophagus lies behind the larynx and continues as the Gl tract. Since the pharynx communicates with the mouth, nose, larynx, and esophagus, during this stage of deglutition, the bolus from the pharynx can enter into four paths:

1. It can come back into the mouth
2. It can go upwards into the nasopharynx
3. It can move forwards into the larynx
4. It can move downwards into the esophagus.

However, due to various coordinated movements, the bolus is made to enter only into the esophagus. The entrance of bolus through other paths is prevented as follows:

1. Back into Mouth: Return of bolus back into the mouth is prevented by:

• The position of the tongue against the soft palate (roof of the mouth)
• The high intraoral pressure developed by the movement of the tongue.

2. Upward into Nasopharynx: The movement of the bolus into the nasopharynx from the pharynx is prevented by the elevation of the soft palate along with its extension called the uvula.

3. Forward into Larynx: The movement of the bolus into the larynx is prevented by the following actions:

• Approximation of the vocal cords
• Forward and upward movement of the larynx
• The backward movement of the epiglottis to seal the opening of the larynx (glottis)
• All these movements arrest respiration for a few seconds. It is called deglutition apnea.

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

4. Entrance of Bolus into Esophagus: Since the other three paths are closed for the bolus, it. has to pass only through the esophagus. It occurs by the combined effects of various factors:

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

All the factors mentioned above act together so that, the bolus moves easily into the esophagus. The whole process takes place within 1-2 seconds. And this process is purely involuntary.

3. Esophageal Stage Or Third Stage:

• In the esophageal stage, food enters the stomach from the esophagus. It is also an involuntary stage. The function of the esophagus is to transport the bolus from the pharynx to the stomach. The movements of the esophagus are specifically organized for this function and the movements are called peristaltic waves.
• Peristalsis means a wave of contraction followed by the wave of relaxation of muscle fibers of Gl which travel in an aboral direction (away from the mouth). Dy type of movement, the contents are propelled down the G! tract.

When the bolus reaches the esophagus, the peristaltic waves are initiated. Usually, two types of peristaltic contractions are produced in the esophagus.

1. Primary peristaltic contractions
2. Secondary peristaltic contractions.

1. Primary Peristaltic Contractions:

• When the bolus reaches the upper part of the esophagus, the peristalsis starts. This is known as primary peristalsis. After origin, the peristaltic contractions pass down through the rest of the esophagus propelling the bolus towards the stomach.
• The pressure developed during the primary peristaltic contractions is important to propel the bolus. Initially, the pressure becomes negative in the upper part of the esophagus.
• This is due to the stretching of the closed esophagus by the elevation of the larynx. But, immediately the pressure becomes positive and increases up to IQ- 15 cm of H₂O.

2. Secondary Peristaltic Contractions:

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

Role of Lower Esophageal Sphincter:

• The distal 2-5 cm of esophagus acts like a sphincter and it is called lower esophageal sphincter. It is constricted always. When the bolus enters this part of the esophagus, this sphincter relaxes so that the contents enter the stomach.
• After the entry of bolus into the stomach, the sphincter constricts and closes the lower end of the esophagus. The relaxation and constriction of the sphincter occur in sequence with the arrival of peristaltic contractions of the esophagus.

4. Deglutition Reflex;

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

• Stimulus: When the bolus enters the oropharyngeal region, the receptors present in this region are stimulated.
• Afferent Fibers: Afferent impulses from the oropharyngeal receptors pass via the glossopharyngeal nerve fibers to the deg¬lutition center.
• Center: The deglutition center is at the floor of the fourth ventricle in the medulla oblongata of the brain.
• Efferent Fibers: The impulses from the deglutition center travel through glossopharyngeal and vagus nerves (parasympathetic motor fibers) and reach the soft palate, pharynx, and esophagus. The glossopharyngeal nerve is concerned with the pharyngeal stage of swallowing. The vagus nerve is concerned with the esophageal stage.
• Response: The reflex causes upward movement of the soft palate to close the nasopharynx and upward movement of the larynx to close the respiratory passage so that the bolus enters the esophagus. Now the peristalsis occurs in the esophagus pushing the bolus into the stomach.

5. Applied Physiology:

1. Dysphagia: Dysphagia means difficulty in swallowing. The causes of dysphagia are:

• Mechanical obstruction of the esophagus due to tumor, strictures, diverticular hernia (outpouching of the wall), etc.
• Decreased movement of the esophagus due to neurological disorders such as Parkinsonism
• Muscular disorders lead to difficulty in swallowing during the oral stage or esophageal stage.

2. Esophageal Achalasia:

Esophageal achalasia or achalasia is a neuromuscular disease characterized by the accumulation of food substances in the esophagus. It is due to the failure of the lower esophageal (cardiac) sphincter to relax during swallowing. The accumulated food substances cause dilatation of the esophagus. The features of this disease are:

• Dysphagia
• Chest pain
• Weight loss
• Cough.

3. Gastroesophageal Reflux Disease (GERD):

CSfRD is a disorder characterized by regurgitation of ao-diC gastric content through the esophagus. The regurgitated gastric content flows into the pharynx or mouth. Regurgitation is due to the weakness or incompetence (failure to constrict) of the lower esophageal sphincter. The features of this disease are:

• Heartburn or pyrosis (painful burning sensation in the chest due to regurgitation of acidic gastric content into the esophagus)
• Esophagitis (inflammation of the esophagus)
• Dysphagia
• Cough and change of voice
• Esophageal ulcers or cancer (in chronic cases).

Movements Of Stomach

The activities of the smooth muscles of the stomach increase during gastric digestion (when the stomach is filled with food) and when the stomach is empty. Different types of movements occur in the stomach. The movements of the stomach area:

1. Hunger contractions
2. Receptive relaxation.
3. Peristalsis

1. Hunger Contractions:

• Hunger contractions are the movements of an empty stomach. These contractions are related to the sensations of hunger. Hunger contractions are the peristaltic waves superimposed over the contractions of gastric smooth muscle as a whole.
• This type of peristaltic wave is different from the digestive peristaltic contractions. The digestive peristaltic contractions usually occur in the body and pyloric parts of the stomach. But, the peristaltic contractions of an empty stomach involve the entire stomach.

Hunger contractions are of three types.

• Type 1 Hunger Contractions:
  • Type 1 hunger contractions are the first contractions to appear in the empty stomach when the tone of the gastric muscles is low. Each contraction lasts for about 20 seconds.
  • The interval between contractions is about 3-4 seconds. The tone of the muscles does not increase between the contractions. The pressure produced by these contractions is about 5 cm of H₂O.
• Type 2 Hunger Contractions:
  • Type 2 hunger contractions appear when the tone of the stomach is stronger. The tone increases in the stomach if the food intake is postponed even after the appearance of type 1 contractions.
  • Each of the type 2 contractions lasts for 20 seconds like type 1 contractions. But the pause between the contractions is decreased. The pressure produced by these contractions is 10-15 cm of H₂O.
• Type 3 Hunger Contractions:
  • Type 3 hunger contractions are like incomplete tetanus. These contractions appear when the hunger becomes severe and the tone increases to a great extent.
  • Type 3 hunger contractions are rare in men as the food is taken usually before the appearance of these contractions. Type 3 contractions last for 1-5 minutes. The pressure produced by these contractions increases to 10-20 cm of H₂O.
  • When the stomach is empty, type 1 contractions occur first followed by type 2 contractions. If food intake is still postponed, then type 3 contractions appear and, as soon as food is consumed, hunger contractions disappear.

2. Receptive Relaxation:

• Receptive relaxation is the relaxation of the upper portion of the stomach when a bolus enters the stomach from the esophagus. It involves the fundus and upper part of the body of the stomach.
• Its significance is to accommodate the food easily without much increase in pressure inside the stomach. This process is called the accommodation of the stomach.

3. Peristalsis Of the Stomach:

• When the food enters the stomach, the peristaltic contraction or peristaltic wave appears with a frequency of 3 per minute. It starts from the lower part of the body of the stomach and passes through the pylorus to the pyloric sphincter.
• Initially, the contraction appears as a slight indentation on the greater and lesser curvatures and travels toward the pylorus. The contraction becomes deeper while traveling. Finally, it ends with the constriction of the pyloric sphincter.
• Some of the waves disappear before reaching the sphincter. Each peristaltic wave takes about one minute to travel from the point of origin to the point of ending.
• This type of peristaltic contraction is called digestive peristalsis because it is responsible for the grinding of food particles and mixing them with gastric juice for digestive activities.

Filling And Emptying Of Stomach

1. Filling Or Stomach:

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

2. Emptying Of Stomach: Gastric emptying is the process by which the chyme from the stomach is emptied into the intestine. The food that is swallowed, enters the stomach and remains there for about 3 hours. During this period, digestion takes place. The partly digested food becomes the chyme.

Chyme:

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

Gastric emptying is influenced by various factors of gastric content and food. The factors which affect gastric emptying are:

1. Volume of Gastric Content: For any type of meal, gastric emptying is directly proportional to the volume. If the content of the stomach is more, a large amount is emptied into the intestine rapidly.

2. Consistency of Gastric Content: Emptying of the stomach depends upon the consistency (degree of density) of the contents. Liquids, particularly inert liquids (which do not stimulate the stomach) leave the stomach rapidly. Water is emptied into the intestine as soon as it is swallowed. But, the solids move out of the stomach only after being converted into fluid or semifluid. Normally, the undigested solid particles are not easily emptied.

3. Chemical Composition: The chemical composition of the food also plays an important role in the emptying of the stomach. The carbohydrates leave the stomach more rapidly than proteins. The proteins leave the stomach more rapidly than the fats. Thus, the fatty food remains in the stomach for a longer period.

4. pH of the Gastric Content: Gastric emptying is directly proportional to the pH of the chyme. Thus, the chyme with low pH is emptied slowly from the stomach.

5. Osmolar Concentration of Gastric Content: The gastric content, which is isotonic to blood, leaves the stomach more rapidly than the hypotonic or hypertonic content.

3. Regulation Of Gastric Emptying: Gastric emptying is regulated by nervous and hormonal factors.

Nervous Factor: The nervous factor, which regulates the emptying of the stomach, is the intragastric reflex.

Enterogastric reflex:

• Enterogastric reflex is the reflex that inhibits gastric emptying. This reflex is elicited by the presence of chyme in the duodenum which prevents further emptying of the stomach. The enterogastric reflex occurs through three mechanisms.
• The presence of chyme in the duodenum causes the generation of nerve impulses which are transmitted to the stomach by the intrinsic nerve fibers of Gl tract. After reaching the stomach these impulses inhibit emptying
• Impulses from the duodenum pass via extrinsic sympathetic fibers to the stomach and inhibit emptying
• Impulses from the duodenum travel through afferent vagal fibers to the brainstem. Normally, brainstem neurons send excitatory impulses to the stomach through efferent vagal fibers and stimulate gastric emptying. However, the impulses from the duodenum inhibit these brainstem neurons and thereby inhibit gastric emptying.

Factors which initiate intragastric reflex:

• Duodenal distension
• Irritation of the duodenal mucosa
• The acidity of the chyme
• The osmolality of the chyme
• Breakdown products of proteins and fats.

Hormonal Factors:

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

• VIP
• GIP
• Secretin
• Cholecystokinin
• Somatostatin
• Peptide YY

Applied Physiology-Abnormal Gastric Emptying

1. Gastric Dumping Syndrome:

• Gastric dumping syndrome or rapid gastric emptying is a condition characterized by a series of upper abdominal symptoms. It is due to the rapid or quick dumping of undigested food from stomach into the jejunum. It occurs in patients following partial removal of the stomach (gastrectomy) or gastric bypass surgery (gastroenterostomy).
• The rapid gastric emptying may begin immediately after taking meals (early dumping) or about a few hours after taking meals (late dumping).

Gastric Dumping Syndrome Causes:

• Gastric surgery
• Zollinger-Ellison syndrome (rare disorder due to severe peptic ulcer and gastrin-secreting tumor in the pancreas).

Symptoms of early dumping:

• Nausea and vomiting
• Bloating (increase in abdominal volume with a feeling of abdominal fullness and tightness)
• Diarrhea
• Sweating and weakness
• Fatigue and dizziness
• Fainting and palpitations (sensation of heartbeat).

Symptoms of late dumping:

• Hypoglycemia
• Sweating and weakness
• Dizziness.

2. Gastroparesis: It is a symptomatic chronic disorder characterized by delayed gastric emptying. It usually occurs as a secondary disorder precipitated by a primary cause.

Gastroparesis Causes:

• Diabetes mellitus
• Postsurgical complications
• Motility disorder
• Gastric infection
• Metabolic and endocrine disorder
• Decrease in myenteric ganglia (rare).

Gastroparesis Symptoms:

• Early satiety (feeling full with a small quantity of food)
• Nausea
• Vomiting
• Bloating
• Upper abdominal discomfort.

Vomiting

Vomiting or emesis is the abnormal emptying of the stomach and upper part of the intestine through the esophagus and mouth.

1. Causes Of Vomiting

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

2. Mechanism Of Vomiting:

Nausea: Vomiting is always preceded by nausea. Nausea is an unpleasant sensation that induces the desire for vomiting. It is characterized by the secretion of a large amount of saliva containing more amount of mucus.

Retching: Strong involuntary movements in the Gl tract start even before actual vomiting and intensify the feeling of vomiting. This condition is called retching (trying to vomit). And, vomiting occurs a few minutes after this.

Act of Vomiting: The act of vomiting involves a series of movements that takes place in the Gl tract. The sequence of events is:

1. Beginning of antiperistalsis which runs from the ileum towards the mouth through the intestine pushing the intestinal contents into the stomach within a few minutes. The velocity of the antiperistalsis is about 2-3 cm/ second
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 the soft palate
6. Contraction of the diaphragm and abdominal muscles with a characteristic jerk resulting in elevation of intra-abdominal pressure
7. Compression of the stomach between the diaphragm and abdominal wall leads to rise in intragastric pressure
8. Simultaneous relaxation of the lower esophageal sphincter, esophagus, and upper esophageal sphincter
9. Forceful expulsion of gastric contents (vomitus) through the esophagus, pharynx, and mouth.

All the movements during the act of vomiting throw the vomitus (materials ejected during vomiting) to the exterior through the mouth. Some of the movements play important roles by preventing the entry of vomitus through other routes and thereby preventing the adverse effect of the vomitus on many structures.

Such movements are:

1. Closure of glottis and cessation of breathing prevents the entry of vomitus into the lungs
2. Elevation of the 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 the dilatation of the throat which allows free exit of vomitus.

Vomiting Reflex:

• Vomiting is a reflex act. The sensory impulses for vomiting arise from the irritated or distended part of the Gl tract or other organs and are transmitted to the vomiting center through the vagus and sympathetic afferent fibers.
• The vomiting center is situated bilaterally in the medulla oblongata near the nucleus tractus solitarius. Motor impulses from the vomiting center are transmitted through 5, 7, 9, 10 <and 12 cranial nerves to the upper part of Gl tract; and through spinal nerves to the diaphragm and abdominal muscles. –

Center for Vomiting during Motion Sickness and Vomiting Induced by Drugs:

• The center for vomiting during motion sickness and- vomiting induced by drugs such as morphine, ‘1 apomorphine, etc. is on the floor of the fourth ventricle.
• This K area is called the chemoreceptor trigger zone. During motion sickness, the afferent impulses from the vestibular apparatus reach the vomiting center through this zone.

Center for Psychic Stimuli-Induced Vomiting: The center for vomiting due to psychic stimuli such as nauseating odor, sight or noise is in the cerebral cortex.

Movements Of Small Intestine

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

Four types of movements occur in the small intestine:

1. Mixing movements:

• Segmentation movements
• Pendular movements

2. Propulsive movements:

• Peristaltic movements
• Peristaltic rush

3. Peristalsis in fasting – Migrating motor complex

4. Movements of villi.

1. Mixing Movements: The mixing movements of the small intestine are responsible for the proper mixing of chyme with digestive juices like pancreatic juice, bile, and intestinal juice. The mixing movements of the small intestine are segmentation contractions and pendular movements.

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

• The contractions occur at regularly spaced intervals along a section of the intestine. The segment of the intestine Involved in each contraction is about 1-5 cm long. The segments of the intestine in between the contracted segments are relaxed.
• The length of the relaxed segments is the same aslant of the contracted segments. These alternate segments of contraction and relaxation give the appearance of rings resembling the chain of sausages.
• After some time, the contracted segments are relaxed and the relaxed segments are contracted. Therefore, the segmentation contractions chop the chyme many times. This helps in the mixing of chyme with digestive juices.

Pendular Movement:

• Pendular movement is the sweeping movement of the small intestine resembling the movements of the pendulum of a clock. Small portions of the intestine (loops) sweep forward and backward or upward and downward.
• It is a type of mixing movement noticed only by close observation. It helps in the mixing of chyme with digestive juices.

2. Propulsive Movements: Propulsive movements are the movements of the small intestine that push the chyme in the aboral direction through the intestine. The propulsive movements are peristaltic movements and peristaltic rush.

Peristaltic Movements:

• Peristalsis is defined as the wave of contraction followed by a wave of relaxation, which travels in an aboral direction. The stimulation of smooth muscles of the intestine initiates the peristalsis.
• It travels from the point of stimulation in both directions. But under normal conditions, the progress of contraction in an oral direction is inhibited quickly and the contractions disappear. Only the contraction that travels in an aboral direction persists.

Starling’s law of the intestine:

• Depending upon the direction of the peristalsis, the “Law of intestine” was put forth by Starling. According to the law of the intestine, the response of the intestine for a local stimulus consists of a contraction of smooth muscle above and relaxation below the stimulated area.
• The peristaltic contractions start at any part of the intestine and travel towards the anal end at a velocity of 1-2 cm/sec. The contractions are always weak and usually disappear after traveling for a few centimeters. Because of this, the average movement of chyme through the small intestine is very slow, and the average velocity of movement of the chyme is less than 1 cm/sec. So the chyme requires several hours to travel from the duodenum to the end of the small intestine.
• The peristaltic waves in the small intestine increase to a great extent immediately after a meal. This is because of the gastroenteric reflex which is initiated by the distension of the stomach. The impulses for this reflex are transmitted from the stomach along the wall of the intestine via myenteric plexus.

Peristaltic Rush:

• Sometimes, the small intestine shows a powerful peristaltic contraction. It is caused by excessive irritation of intestinal mucosa or extreme distention of the intestine. This type of powerful contraction begins in the duodenum and passes through the entire length of the small intestine and reaches the ileocecal valve within a few minutes.
• This is called a peristaltic rush or rush waves. The peristaltic rush sweeps the contents of the intestine into the colon. Thus, it relieves the small intestine of either irritants or excessive distention.

3. Peristalsis In Fasting – Migrating Motor Complex:

• It is a type of peristaltic contraction, which occurs in the stomach and small intestine during periods of fasting for several hours. It is different from regular peristalsis because a large portion of the stomach or intestine is involved in the contraction.
• The contraction extends to about 20-30 cm of the stomach or intestine. This type of movement occurs once in every 114-2 hours. If stories and a moderately active peristalsis in the body of Potomac:’ unci rune through the entire length of small imcsqno. H has ce a velocity of 6-12 cm/min. Thus, it takes about 1C to reach the colon after taking origin from the stomach

Significance of Peristalsis in Fasting: The migrating motor complex sweeps the excess digestive secretions into the colon and prevents the accumulation of the secretions in the stomach and intestine. It also sweeps the residual indigested materials into the colon.

4. Movements Of Villi:

• The intestinal villi also show movements simultaneously with intestinal movements. It is because of the extension of smooth muscle fibers of the intestinal wall into the villi.
• The movements of villi are shortening and elongation, which occur alternatively and help in emptying lymph from the central lacteal into the lymphatic system. The surface area of villi is increased during elongation. This helps the absorption of digested food particles from the lumen of the intestine.
• Movements of villi are caused by local nervous reflexes, which are initiated by the presence of chyme in the small intestine. The hormone secreted from small intestinal mucosa called villi kinin is also believed to play an important role in increasing the movements of villi.

Movements Of Large Intestine

Usually, the large intestine shows sluggish movements. Still, these movements are important for mixing, propulsive and absorptive functions. The large intestine shows two types of movements:

1. Mixing movements – Segmentation contractions
2. Propulsive movements – Mass peristalsis.

1. Mixing Movements – Segmentation Contractions: Large circular constrictions, which appear in the colon, are called mixing segmentation contractions. The contractions occur at regular distances in the colon. The length of the portion of the colon involved in each contraction is nearly about 2.5 cm.

2. Propulsive Movements – Mass Peristalsis: Mass peristalsis or mass movement propels the feces from the colon towards the anus. Usually, this movement occurs only a few times every day. The duration of the mass movement is about 10 minutes in the morning before or after breakfast. This is because of neurogenic factors like the gastrocolic reflex (see below) and parasympathetic stimulation.

Decation

The voiding of feces is known as defecation. Feces is formed in the large intestine and stored in the sigmoid colon. By the influence of an appropriate stimulus, it is expelled out through the anus. This is prevented by tonic constriction of anal sphincters in the absence of the stimulus.

1. Defecation Reflex:

• The mass movement drives the feces into the sigmoid or pelvic colon. In the sigmoid colon, the feces is stored. The desire for defecation occurs when some feces enter the rectum due to the mass movement. Usually, the desire for defecation is elicited by an increase in the intrarectal pressure to about 20-25 cm H20.
• The usual stimulus for defecation is the intake of liquid like coffee or tea or water. But it differs from person to person.

Act of Defecation:

• The act of defecation is preceded by voluntary efforts like assuming an appropriate posture, voluntary relaxation of the external sphincter, and the compression of abdominal contents by voluntary contraction of abdominal muscles.
• Usually, the rectum is empty. During the development of mass movement, the feces is pushed into the rectum and the defecation reflex is initiated. The process of defecation involves the contraction of the rectum and relaxation of internal and external anal sphincters.
• The internal anal sphincter is made up of smooth muscle and it is innervated by parasympathetic nerve fibers via pelvic nerve. The external anal sphincter is composed of skeletal muscle and it is controlled by somatic nerve fibers, which pass through the pudendal nerve. The pudendal nerve always keeps the external sphincter constricted and the sphincter can relax only when the pudendal nerve is inhibited.

Gastrocolic Reflex:

• Gastrocolic reflex is the contraction of the rectum followed by a desire for defecation caused by distention of the stomach by food. It is mediated by intrinsic nerve fibers of Gl tract.
• This reflex causes only a weak contraction of the rectum. But, it initiates a defecation reflex.

2. Pathway For Defecation Reflex:

• When the rectum is distended due to the entry of feces by mass movement, sensory nerve endings are stimulated. The impulses from the nerve endings are transmitted vm afferent fibers of the pelvic nerve to the defecation cento; situated in sacral segments (center) of the spinal cord.
• The center, in turn, sends motor impulses to the descending colon, sigmoid colon, and rectum via efferent nerve fibers of the pelvic nerve. The motor impulses cause strong contraction of descending colon, sigmoid colon, and rectum and relaxation of the internal sphincter.
• Simultaneously, voluntary relaxation of the external sphincter occurs. It is due to the inhibition of the pudendal nerve by impulses arising from the cerebral cortex.
• Failure of voiding of feces is called constipation.

Evacuation Of Gases From Gastrointestinal Tract

Normally gas accumulates in the Gl tract either because of the entrance of outside air or the production of gases in the body. Accordingly, the gases accumulated in Gl tract are classified into two groups:

1. Exogenous gases
2. Endogenous gases

1. Exogenous Gases: Exogenous gases form about 90% of accumulated gases. These gases enter the Gl tract either by swallowing through the mouth or drinking carbonated beverages.

2. Endogenous Gases: Endogenous gases form about 10% of accumulated gases. These gases are produced by the digestion of foodstuffs and the interaction between bacteria and foodstuffs in the intestine.

1. Evacuation Of Accumulated Gases: Evacuation of accumulated gases usually occurs by two processes:

1. Belching
2. Flatulence.

1. Belching:

• Belching is the process by which the gas accumulated in the stomach is expelled through the mouth. It is also called burping. It occurs because of inflation (distention) of the stomach by swallowed air. The distention of the stomach causes abdominal discomfort, and the belching expels the air and relieves the discomfort.
• Most of the gas accumulated in the stomach is expelled through the mouth. Only a small amount enters the intestine.

Causes for Accumulation of Gases in Stomach:

1. Aerophagia: Swallowing large amounts of air due to gulping food or drink too rapidly
2. Drinking carbonated beverages
3. During some emotional conditions like anxiety lot of air enters the stomach through the mouth.

Act of Belching:

Belching is not a simple act and it requires the coordination of several activities such as:

1. Closure of the larynx which prevents entry of liquid or food with the air from the stomach into the lungs
2. Elevation of the larynx and relaxation of the upper esophageal sphincter. It allows the exit of air through the esophagus more
3. Opening of lower esophageal sphincter
4. Descent of diaphragm which increases abdominal pressure end decreases intrathoracic pressure.

All these activities be responsible for the expulsion of air from the stomach to the exterior via the esophagus.

2. Flatulence:

Flatulence is the production of a mixture of intestinal gases. The mixture of gases is known as flatus (in Latin flatus = wind). The expulsion of flatus through the anus under pressure is called farting or passing gas. Farting is associated with disagreeable odor (due to odorous gases and sound (due to the vibration of the anal sphincter).

Quantity of Flatus: Average flatus released by humans is about 500-1500 mL per day with 10-25 episodes throughout the day.

Source of Gases in Intestine:

Flatulence is the mixture of gases present in the intestine. The excess flatulence by swallowed air is rare. The common sources for flatulence are:

• The bacterial action on undigested sugars and polysaccharides (example starch, cellulose)
• Digestion of some flatulence-producing foodstuffs. The common flatulence-producing foodstuffs are cheese, yeast in bread, oats, onion, beans, cabbage, milk, etc.

Constituents of Flatus: The major constituents of flatus:

1. Swallowed non-odorous gases

• Nitrogen (a major constituent)
• Oxygen

2. Nonodorous gases produced by microbes

• Methane
• Carbon dioxide
• Hydrogen

3. Odorous materials such as

• Low molecular weight fatty acids like butyric acid
• Reduced sulfur compounds (hydrogen sulfide and carbonyl sulfide)