Mastication .
Mastication or chewing is the first mechanical process in the gastrointestinal (GI) tract, by which the food substances are torn or cut into small particles and crushed or ground into a soft bolus.
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 & Movement of Mastication .
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| Muscles of Mastication . |
Muscles of Mastication
[1]. Masseter muscle
[2]. Temporal muscle
[3]. Pterygoid muscles
[4]. Buccinator muscle.
Movements of Mastication
[1]. Opening and closure of mouth
[2]. Rotational movements of jaw
[3]. Protraction and retraction of jaw.
Control of Mastication .
[1]. Action of mastication is mostly a reflex process. It is carried out voluntarily also.
[2]. The center for mastication is situated in medulla and cerebral cortex.
[3]. Muscles of mastication are supplied by mandibular division of 5th cranial (trigeminal) nerve.
Deglutition .
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| Stages of deglutition. A. Preparatory stage; B. Oral stage; C. Pharyngeal stage; D. Esophageal stage . |
Deglutition or swallowing is the process by which food moves from mouth into stomach.
Stages of Deglutition .
Deglutition occurs in three stages:
[1]. Oral stage, when food moves from mouth to pharynx .
[2]. Pharyngeal stage, when food moves from pharynx to esophagus .
[3]. Esophageal stage, when food moves from esophagus to stomach.
Oral Stage or First Stage .
Oral stage of deglutition is a voluntary stage. In this stage, the bolus from mouth passes into pharynx by means of series of actions.
Sequence of Events during Oral Stage .
[1] . Bolus is placed over postero-dorsal surface of the tongue. It is called the preparatory position
[2] . Anterior part of tongue is retracted and depressed.
[3] . Posterior part of tongue is elevated and retracted against the hard palate. This pushes the bolus backwards into the pharynx
[4] . Forceful contraction of tongue against the palate produces a positive pressure in the posterior part of oral cavity. This also pushes the food into pharynx .
Pharyngeal Stage or Second Stage .
[1]. Pharyngeal stage is an involuntary stage.
[2]. In this stage, the bolus is pushed from pharynx into the esophagus.
[3]. Pharynx is a common passage for food and air. It divides into larynx and esophagus.
[4]. Larynx lies anteriorly and continues as respiratory passage.
[5]. Esophagus lies behind the larynx and continues as GI tract.
[6]. Since pharynx communicates with mouth, nose, larynx and esophagus, during this stage of deglutition, bolus from the pharynx can enter into four paths:
1. Back into mouth
2. Upward into nasopharynx
3. Forward into larynx
4. Downward into esophagus.
However, due to various coordinated movements, bolus is made to enter only the esophagus.
1. Back into Mouth .
Return of bolus back into the mouth is prevented by:
[1]. Position of tongue against the soft palate (roof of the mouth)
[2]. High intraoral pressure, developed by the movement of tongue.
2. Upward into Nasopharynx .
Movement of bolus into the nasopharynx from pharynx is prevented by elevation of soft palate along with its extension called uvula.
3. Forward into Larynx .
Movement of bolus into the larynx is prevented by the following actions:
[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) .
[4]. All these movements arrest respiration for a few seconds. It is called deglutition apnea.
Deglutition apnea
[1]. Apnea refers to temporary arrest of breathing.
[2]. Deglutition apnea or swallowing apnea is the arrest of breathing during pharyngeal stage of deglutition.
4. Entrance of Bolus into Esophagus .
As the other three paths are closed, the bolus has to pass only through the esophagus. This occurs by the 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. This part of esophagus is formed by the cricopharyngeal muscle and it is called upper esophageal sphincter or pharyngoesophageal sphincter
[3]. At the same time, peristaltic contractions start in the pharynx due to the contraction of pharyngeal muscles
[4]. Elevation of larynx also lifts the glottis away from the food passage.
All the factors mentioned above act together so that, bolus moves easily into the esophagus. The whole process takes place within 1 to 2 seconds and this process is purely involuntary.
Esophageal Stage or Third Stage .
[1]. Esophageal stage is also an involuntary stage. In this stage, food from esophagus enters the stomach.
[2]. Esophagus forms the passage for movement of bolus from pharynx to the stomach.
[3]. Movements of esophagus are specifically organized for this function and the movements are called peristaltic waves.
[4]. Peristalsis means a wave of contraction, followed by the wave of relaxation of muscle fibers of GI tract, which travel in aboral direction (away from mouth).
[5]. By this type of movement, the contents are propelled down along the GI tract.
[6]. When bolus reaches the esophagus, the peristaltic waves are initiated.
[7]. Usually, two types of peristaltic contractions are produced in esophagus.
1. Primary peristaltic contractions .
2. Secondary peristaltic contractions.
1. Primary Peristaltic Contractions .
[1]. When bolus reaches the upper part of esophagus, the peristalsis starts. This is known as primary peristalsis.
[2]. After origin, the peristaltic contractions pass down through the rest of the esophagus, propelling the bolus towards stomach.
[3]. Pressure developed during the primary peristaltic contractions is important to propel the bolus.
[4]. Initially, the pressure becomes negative in the upper part of esophagus. This is due to the stretching of the closed esophagus by the elevation of larynx.
[5]. But immediately, the pressure becomes positive and increases up to 10 to 15 cm of H2O.
2. Secondary Peristaltic Contractions .
[1]. If the primary peristaltic contractions are unable to propel the bolus into the stomach, the secondary peristaltic contractions appear and push the bolus into stomach.
[2]. Secondary peristaltic contractions are induced by the distention of upper esophagus by the bolus. After origin, these contractions pass down like the primary contractions, producing a positive pressure.
Role of Lower Esophageal Sphincter .
[1]. Distal 2 to 5 cm of esophagus acts like a sphincter and it is called lower esophageal sphincter. It is constricted always.
[2]. When bolus enters this part of the esophagus, this sphincter relaxes so that the contents enter the stomach.
[3]. After the entry of bolus into the stomach, the sphincter constricts and closes the lower end of esophagus.
[4]. The relaxation and constriction of sphincter occur in sequence with the arrival of peristaltic contractions of esophagus.
Deglutition Reflex .
[1]. Though the beginning of swallowing is a voluntary act, later it becomes involuntary and is carried out by a reflex action called deglutition reflex.
[2]. 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 deglutition center.
Center .
Deglutition center is at the floor of the fourth ventricle in medulla oblongata of brain.
Efferent Fibers .
[1]. Impulses from deglutition center travel through glossopharyngeal and vagus nerves (parasympathetic motor fibers) and reach soft palate, pharynx and esophagus.
[2]. The glossopharyngeal nerve is concerned with pharyngeal stage of swallowing. The vagus nerve is concerned with esophageal stage.
Response .
[1]. The reflex causes upward movement of soft palate to close nasopharynx and upward movement of larynx, to close respiratory passage so that bolus enters the esophagus.
[2]. Now the peristalsis occurs in esophagus, pushing the bolus into stomach.
Movement of Stomach .
[1]. Activities of smooth muscles of stomach increase during gastric digestion (when stomach is filled with food) and when the stomach is empty.
[2]. Types of movements in stomach
1. Hunger contractions .
2. Receptive relaxation .
3. Peristalsis.
1. Hunger Contractions .
[1]. Hunger contractions are the movements of empty stomach. These contractions are related to the sensations of hunger.
[2]. Hunger contractions are the peristaltic waves superimposed over the contractions of gastric smooth muscle as a whole.
[3]. This type of peristaltic waves is different from the digestive peristaltic contractions.
[4]. The digestive peristaltic contractions usually occur in body and pyloric parts of the stomach. But, peristaltic contractions of empty stomach involve the entire stomach.
[5]. Hunger contractions are of three types:
Type I Hunger Contractions .
[1]. Type I hunger contractions are the first contractions to appear in the empty stomach, when the tone of the gastric muscles is low.
[2]. Each contraction lasts for about 20 seconds. The interval between contractions is about 3 to 4 seconds.
[3]. Tone of the muscles does not increase between contractions.
[4]. Pressure produced by these contractions is about 5 cm of H2 O.
Type II Hunger Contractions .
[1]. Type II hunger contractions appear when the tone of stomach is stronger.
[2]. Tone increases in stomach if food intake is postponed, even after the appearance of the type I contractions.
[3]. Each of the type II contractions lasts for 20 seconds like type I contractions. But the pause between the contractions is decreased.
[4]. Pressure produced by these contractions is 10 to 15 cm of H2 O.
Type III Hunger Contractions .
[1]. Type III hunger contractions are like incomplete tetanus. These contractions appear when the hunger becomes severe and the tone increases to a great extent.
[2]. Type III hunger contractions are rare in man as the food is taken usually before the appearance of these contractions. These contractions last for 1 to 5 minutes.
[3]. The pressure produced by these contractions increases to 10 to 20 cm of H2 O.
[4]. When the stomach is empty, the type I contractions occur first, followed by type II contractions.
[5]. If food intake is still postponed, then type III contractions appear and as soon as food is consumed, hunger contractions disappear.
2. Receptive Relaxation .
[1]. Receptive relaxation is the relaxation of the upper portion of the stomach when bolus enters the stomach from esophagus.
[2]. It involves the fundus and upper part of the body of stomach.
[3]. Its significance is to accommodate the food easily, without much increase in pressure inside the stomach. This process is called accommodation of stomach.
3. Peristalsis .
[1]. When food enters the stomach, the peristaltic contraction or peristaltic wave appears with a frequency of 3 per minute.
[2]. It starts from the lower part of the body of stomach, passes through the pylorus till the pyloric sphincter.
[3]. Initially, the contraction appears as a slight indentation on the greater and lesser curvatures and travels towards pylorus.
[4]. The contraction becomes deeper while traveling. Finally, it ends with the constriction of pyloric sphincter.
[5]. Some of the waves disappear before reaching the sphincter.
[6]. Each peristaltic wave takes about one minute to travel from the point of origin to the point of ending.
[7]. 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 of Stomach .
[1]. While taking food, it arranges itself in the stomach in different layers.
[2]. The first eaten food is placed against the greater curvature in the fundus and body of the stomach .
[3]. The successive layers of food particles lie nearer, the lesser curvature, until the last portion of food eaten lies near the upper end of lesser curvature, adjacent to cardiac sphincter.
[4]. 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.
[5]. But, if a large quantity of fluid is taken, it flows around the entire food mass and is distributed over the interior part of stomach, between wall of the stomach and food mass.
Emptying of Stomach .
[1]. Gastric emptying is the process by which the chyme from stomach is emptied into intestine.
[2]. Food that is swallowed enters the stomach and remains there for about 3 hours.
[3]. During this period, digestion takes place. Partly digested food in stomach becomes the chyme.
Chyme .
[1]. Chyme is the semisolid mass of partially digested food that is formed in the stomach. It is acidic in nature.
[2]. Acid chyme is emptied from stomach into the intestine slowly, with the help of peristaltic contractions.
[3]. It takes about 3 to 4 hours for emptying of the chyme.
[4]. This slow emptying is necessary to facilitate the final digestion and maximum (about 80%) absorption of the digested food materials from small intestine.
[5]. Gastric emptying occurs due to the peristaltic waves in the body and pyloric part of the stomach and simultaneous relaxation of pyloric sphincter.
[6]. Gastric emptying is influenced by various factors of the gastric content and food.
Factors Affecting Gastric Emptying .
1. Volume of gastric content .
[1]. For any type of meal, gastric emptying is directly proportional to the volume.
[2]. If the content of stomach is more, a large amount is emptied into the intestine rapidly.
2. Consistency of gastric content .
[1]. Emptying of the stomach depends upon consistency (degree of density) of the contents.
[2]. Liquids, particularly the inert liquids like water leave the stomach rapidly.
[3]. Solids leave the stomach only after being converted into fluid or semifluid.
[4]. Undigested solid particles are not easily emptied.
3. Chemical composition .
[1]. Chemical composition of the food also plays an important role in the emptying of the stomach. Carbohydrates are emptied faster than the proteins.
[2]. Proteins are emptied faster than the fats. Thus, the fats are emptied very slowly.
4. pH of the gastric content .
Gastric emptying is directly proportional to pH of the chyme.
5. Osmolar concentration of gastric content .
Gastric content which is isotonic to blood, leaves the stomach rapidly than the hypotonic or hypertonic content.
Regulation of Gastric Emptying .
Gastric emptying is regulated by nervous and hormonal factors.
Nervous Factor .
Nervous factor which regulates the emptying of stomach is the enterogastric reflex.
Enterogastric Reflex .
[1]. Enterogastric reflex is the reflex that inhibits gastric emptying.
[2]. It is elicited by the presence of chyme in the 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 the intrinsic nerve fibers of GI tract. After reaching the stomach, these impulses inhibit emptying.
[2] . Impulses from duodenum pass via extrinsic sympathetic fibers to stomach and inhibit emptying.
[3] . Some impulses from duodenum travel through afferent vagal fibers to the brainstem.
Normally, brainstem neurons send excitatory impulses to stomach through efferent vagal fibers and stimulate gastric emptying. However, the impulses from duodenum inhibit these brainstem neurons and thereby inhibit gastric emptying.
Factors which initiate enterogastric reflex .
[1] . Duodenal distension
[2] . Irritation of the duodenal mucosa
[3] . Acidity of the chyme
[4] . Osmolality of the chyme
[5] . 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 stomach.
Hormones inhibiting gastric motility and emptying .
[1] . Vasoactive intestinal peptide (VIP)
[2] . Gastric inhibitory peptide (GIP)
[3] . Secretin
[4] . Cholecystokinin
[5] . Somatostatin
[6] . Peptide YY .
Vomiting.
Vomiting or emesis is the abnormal emptying of stomach and upper part of intestine through esophagus and mouth.
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 the case of sea sickness, air sickness, car sickness or swinging
[7] . Abnormal stimulation of sensory receptors in other organs like kidney, heart, semicircular canals or 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 (pregnancy), uremia, ketoacidosis (diabetes) and hypercalcemia.
Mechanism of Vomiting .
Nausea
[1]. Vomiting is always preceded by nausea.
[2]. Nausea is unpleasant sensation which induces the desire for vomiting.
[3]. It is characterized by secretion of large amount of saliva containing more amount of mucus.
Retching
[1]. Strong involuntary movements in the GI tract which start even before actual vomiting.
[2]. These movements intensify the feeling of vomiting.
[3]. This condition is called retching (try to vomit) and vomiting occurs few minutes after this.
Act of Vomiting
Act of vomiting involves series of movements that takes place in GI tract.
Sequence of events
[1] . Beginning of antiperistalsis, which runs from ileum towards the mouth through the intestine, pushing the intestinal contents into the stomach within few minutes. Velocity of the antiperistalsis is about 2 to 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 soft palate .
[6] . Contraction of diaphragm and abdominal muscles with a characteristic jerk, resulting in elevation of intra-abdominal pressure .
[7] . Compression of the 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 during act of vomiting throw the vomitus (materials ejected during vomiting) to the exterior through mouth.
Some of the movements play important roles by preventing the entry of vomitus through other routes and thereby prevent the adverse effect of the vomitus on many structures.
Such movements are:
[1] . Closure of glottis and cessation of breathing prevents 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 the dilatation of throat, which allows free exit of vomitus.
Vomiting Reflex .
[1]. Vomiting is a reflex act. Sensory impulses for vomiting arise from the irritated or distended part of GI tract or other organs and are transmitted to the vomiting center through vagus and sympathetic afferent fibers.
[2]. Vomiting center is situated bilaterally in medulla oblongata near the nucleus tractus solitarius.
[3]. Motor impulses from the 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.
Center for Vomiting during Motion Sickness and Vomiting Induced by Drugs .
[1]. Center for vomiting during motion sickness and vomiting induced by drugs such as morphine, apomorphine, etc. is on the floor of fourth ventricle.
[2]. This area is called chemoreceptor trigger zone.
[3]. During motion sickness, the afferent impulses from vestibular apparatus reach vomiting center through this zone.
Center for Psychic-stimuli-induced Vomiting .
Center for vomiting due to psychic stimuli such as nauseating odor, sight or noise is in cerebral cortex.
Movements of Small Intestine .
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| Movements of small intestine . |
Movements of small intestine are essential for mixing the chyme with digestive juices, propulsion of food and absorption.
Types of Movements of Small Intestine .
Movements of small intestine are of four types:
1. Mixing movements:
i. Segmentation movements
ii. Pendular movements.
2. Propulsive movements:
i. Peristaltic movements
ii. Peristaltic rush.
3. Peristalsis in fasting – migrating motor complex .
4. Movements of villi.
1. 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. The mixing movements of small intestine are segmentation contractions and pendular movements.
i. Segmentation Contractions .
[1]. Segmentation contractions are the common type of movements of small intestine, which occur regularly or irregularly, but in a rhythmic fashion. So, these movements are also called rhythmic segmentation contractions.
[2]. The contractions occur at regularly spaced intervals along a section of intestine. The segment of the intestine involved in each contraction is about 1 to 5 cm long.
[3]. The segments of intestine in between the contracted segments are relaxed. The length of the relaxed segments is same as that of the contracted segments.
[4]. These alternate segments of contraction and relaxation give appearance of rings, resembling the chain of sausages. After sometime, the contracted segments are relaxed and the relaxed segments are contracted .
[5]. Therefore, the segmentation contractions chop the chyme many times. This helps in mixing of chyme with digestive juices.
ii. Pendular Movement .
[1]. Pendular movement is the sweeping movement of small intestine, resembling the movements of pendulum of clock.
[2]. Small portions of intestine (loops) sweep forward and backward or upward and downward.
[3]. It is a type of mixing movement, noticed only by close observation.
[4]. It helps in mixing of chyme with digestive juices.
2. Propulsive movements .
[1]. Propulsive movements are the movements of small intestine which push the chyme in the aboral direction through intestine.
[2]. The propulsive movements are peristaltic movements and peristaltic rush.
i. Peristaltic Movements .
[1]. Peristalsis is defined as the wave of contraction followed by wave of relaxation of muscle fibers. In GI tract, it always travels in aboral direction.
[2]. Stimulation of smooth muscles of intestine initiates the peristalsis. It travels from point of stimulation in both directions.
[3]. But under normal conditions, the progress of contraction in an oral direction is inhibited quickly and the contractions disappear.
[4]. Only the contraction that travels in an aboral direction persists.
Starling’s law of intestine .
[1]. Depending upon the direction of the peristalsis, ‘Law of intestine’ was put forth by Starling.
[2]. According to the law of intestine, the response of the intestine for a local stimulus consists of a contraction of smooth muscle above and relaxation below the stimulated area.
[3]. Peristaltic contractions start at any part of the intestine and travel towards anal end, at a velocity of 1 to 2 cm/sec.
[4]. The contractions are always weak and usually disappear after traveling for few centimeter. Because of this, the average movement of chyme through small intestine is very slow and the average velocity of movement of the chyme is less than 1 cm/ sec.
[5]. So, the chyme requires several hours to travel from duodenum to the end of small intestine.
[6]. Peristaltic waves in small intestine increase to a great extent immediately after a meal. This is because of gastroenteric reflex, which is initiated by the distention of stomach.
[7]. Impulses for this reflex are transmitted from stomach along the wall of the intestine via myenteric plexus.
ii. Peristaltic Rush .
[1]. Sometimes, the small intestine shows a powerful peristaltic contraction. It is caused by excessive irritation of intestinal mucosa or extreme distention of the intestine.
[2]. This type of powerful contraction begins in duodenum and passes through entire length of small intestine and reaches the ileocecal valve within few minutes. This is called peristaltic rush or rush waves.
[3]. Peristaltic rush sweeps the contents of intestine into the colon. Thus, it relieves the small intestine off either irritants or excessive distention.
3. Peristalsis In fasting – Migrating Motor Complex .
[1]. Migrating motor complex is a type of peristaltic contraction, which occurs in stomach and small intestine during the periods of fasting for several hours. It is also called migrating myoelectric complex. [2]. It is different from the regular peristalsis because, a large portion of stomach or intestine is involved in the contraction.
[3]. The contraction extends to about 20 to 30 cm of stomach or intestine. This type of movement occurs once in every 1½ to 2 hours.
[4]. It starts as a moderately active peristalsis in the body of stomach and runs through the entire length of small intestine.
[5]. It travels at a velocity of 6 to 12 cm/min. Thus, it takes about 10 minutes to reach the colon after taking origin from the stomach.
Significance of Peristalsis in Fasting .
[1]. Migrating motor complex sweeps the excess digestive secretions into the colon and prevents the accumulation of the secretions in stomach and intestine.
[2]. It also sweeps the residual indigested materials into colon.
4. Movement of Villi .
[1]. Intestinal villi also show movements simultaneously along with intestinal movements. It is because of the extension of smooth muscle fibers of the intestinal wall into the villi.
[2]. Movements of villi are shortening and elongation, which occur alternatively and help in emptying lymph from the central lacteal into the lymphatic system.
[3]. The surface area of villi is increased during elongation. This helps absorption of digested food particles from the lumen of intestine.
[4]. Movements of villi are caused by local nervous reflexes, which are initiated by the presence of chyme in small intestine.
[5]. Hormone secreted from the small intestinal mucosa called villikinin is also believed to play an important role in increasing the movements of villi.
Movement of Large Intestine .
[1]. Usually, the large intestine shows sluggish movements.
[2]. Still, these movements are important for mixing, propulsive and absorptive functions.
Types of Movements of Large Intestine .
Movements of large intestine are of two types:
1. Mixing movements: Segmentation contractions .
2. Propulsive movements: Mass peristalsis.
1. Mixing Movements – Segmentation Contractions .
[1]. Large circular constrictions, which appear in the colon, are called mixing segmentation contractions.
[2]. These contractions occur at regular distance in colon.
[3]. Length of the portion of colon involved in each contraction is nearly about 2.5 cm.
2. Propulsive Movements – Mass Peristalsis .
[1]. Mass peristalsis or mass movement propels the feces from colon towards anus. Usually, this movement occurs only a few times every day.
[2]. Duration of mass movement is about 10 minutes in the morning before or after breakfast. This is because of the neurogenic factors like gastrocolic reflex and parasympathetic stimulation .
Defecation .
[1]. Voiding of feces is known as defecation.
[2]. Feces is formed in the large intestine and stored in sigmoid colon. By the influence of an appropriate stimulus, it is expelled out .through the anus.
[3]. This is prevented by tonic constriction of anal sphincters, in the absence of the stimulus.
Defecation Reflex .
[1]. Mass movement drives the feces into sigmoid or pelvic colon. In the sigmoid colon, the feces is stored.
[2]. The desire for defecation occurs when some feces enters rectum due to the mass movement.
[3]. Usually, the desire for defecation is elicited by an increase in the intrarectal pressure to about 20 to 25 cm H2 O.
[4]. Usual stimulus for defecation is intake of liquid like coffee or tea or water. But it differs from person to person.
Act of Defecation .
[1]. Act of defecation is preceded by voluntary efforts like assuming an appropriate posture, voluntary relaxation of external sphincter and the compression of abdominal contents by voluntary contraction of abdominal muscles. Usually, the rectum is empty.
[2]. During the development of mass movement, the feces is pushed into rectum and the defecation reflex is initiated.
[3]. The process of defecation involves the contraction of rectum and relaxation of internal and external anal sphincters.
[4]. Internal anal sphincter is made up of smooth muscle and it is innervated by parasympathetic nerve fibers via pelvic nerve.
[5]. External anal sphincter is composed of skeletal muscle and it is controlled by somatic nerve fibers, which pass through pudendal nerve.
[6]. Pudendal nerve always keeps the external sphincter constricted and the sphincter can relax only when the pudendal nerve is inhibited.
Gastrocolic Reflex .
[1]. Gastrocolic reflex is the contraction of rectum, followed by the desire for defecation caused by distention of stomach by food.
[2]. It is mediated by intrinsic nerve fibers of GI tract. This reflex causes only a weak contraction of rectum. But, it initiates defecation reflex.
Pathways for Defecation Reflex .
[1]. When rectum is distended due to the entry of feces by mass movement, sensory nerve endings are stimulated.
[2]. Impulses from the nerve endings are transmitted via afferent fibers of pelvic nerve to the defecation center, situated in sacral segments (center) of spinal cord.
[3]. The center in turn, sends motor impulses to the descending colon, sigmoid colon and rectum via efferent nerve fibers of pelvic nerve.
[4]. Motor impulses cause strong contraction of descending colon, sigmoid colon and rectum and relaxation of internal sphincter.
[5]. Simultaneously, voluntary relaxation of external sphincter occurs. It is due to the inhibition of pudendal nerve, by impulses arising from cerebral cortex .
Evacuation of Gases from Gastrointestinal Tract .
[1]. Normally, gas accumulates in the GI tract either because of entrance of outside air or production of gases in the body.
[2]. Accordingly, the gases accumulated in GI tract are classified into two groups:
1. Exogenous gases
2. Endogenous gases.
1. Exogenous Gases .
[1]. Exogenous gases form about 90% of accumulated gases.
[2]. These gases enter the GI tract either by swallowing through mouth or drinking carbonated beverages.
2. Endogenous Gases .
[1]. Endogenous gases form about 10% of accumulated gases.
[2]. These gases are produced by digestion of food stuffs and interaction between bacteria and food stuffs in the intestine.
Evacuation of Accumulated Gases .
Evacuation of accumulated gases usually occurs by two processes:
1. Belching
2. Flatulence.
Bleaching .
[1]. Belching is the process by which the gas accumulated in stomach is expelled through mouth. It is also called burping.
[2]. It occurs because of inflation (distention) of stomach by swallowed air.
[3]. The distention of the stomach causes abdominal discomfort and the belching expels the air and relieves the discomfort.
[4]. Most of the gas accumulated in stomach is expelled through 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 the food or drink too rapidly
[2]. Drinking carbonated beverages
[3]. During some emotional conditions like anxiety lot of air enters the stomach through mouth.
Act of Belching .
Belching is not a simple act and it requires the coordination of several activities such as:
[1] . Closure of larynx, which prevents entry of liquid or food with the air from stomach into the lungs .
[2] . Elevation of larynx and relaxation of upper esophageal sphincter. It allows exit of air through esophagus more easily.
[3] . Opening of lower esophageal sphincter.
[4] . Descent of diaphragm, which increases abdominal pressure and decreases intrathoracic pressure.
All these activities are responsible for the expulsion of air from stomach to the exterior via esophagus.
Flatulence .
[1]. Flatulence is the production of a mixture of intestinal gases. The mixture of gases is known as flatus (in Latin, flatus = wind).
[2]. Expulsion of flatus through anus under pressure is called farting or passing gas.
[3]. Farting is associated with disagreeable odor (due to odorous gases) and sound (due to vibration of anal sphincter).
Quantity of Flatus .
Average flatus released by human is about 500 to 1500 mL per day, with 10 to 25 episodes throughout the day.
Source of Gases in Intestine .
Flatulence is the mixture of gases present in the intestine. Flatulence by swallowed air is rare.
Common sources of gases in flatulence are:
1. Bacterial action on undigested sugars and polysaccharides (e.g. starch, cellulose)
2. Digestion of some flatulence producing food stuffs such as cheese, yeast in bread, oats, onion, beans, cabbage, milk, etc.
Constituents of Flatus .
Major constituents of flatus:
1. Swallowed non-odorous gases
i. Nitrogen (major constituent)
ii. Oxygen
2. Non-odorous gases produced by microbes
i. Methane
ii. Carbon dioxide
iii. Hydrogen
3. Odorous materials such as
i. Low molecular weight fatty acids like butyric acid
ii. Reduced sulfur compounds (hydrogen sulfide and carbonyl sulfide).
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