Digestive System Exam 4

The approach of the bolus triggers the opening of lower esophageal sphincter. The bolus then continues into the stomach.

Bolus enters the stomach

Begins with the compression of the bolus against the hard plate. Subsequent retraction of the tongue then forces the bolus into oropharynx and assists in the elevation of the soft palate, thereby sealing off the nasopharynx. Once the bolus enters the oropharynx, reflex responses begin and the bolus is moved toward the stomach.

Buccal Phase

The gastroenteric reflex stimulates motility and secretion along the entire small intestine. The gastroileal reflex triggers the opening of the ileocecal valve allowing materials to pass from the small intestines into the large. Ileocecal valve controls the passage of material into the large intestines.

Central Gastric Reflexes

Released when chyme enters the duodenum and stimulates mucin production by the submucosal glands

Enterocrinin

Begins as the contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus. Once in the esophagus, the bolus is pushed toward the stomach by peristalsis.

Esophageal Phase

Transport of materials to the stomach

Esophagus

Storage and concentration of bile

Gallbladder

Secreted when fats and carbs enter SI; the inhibition of gastric activity is accompanied by stimulation of insulin release at pancreatic islets; also stimulates duodenal gland activity, lipid synthesis in adipose tissue, and increasing glucose use by skeletal muscles.

Gastric Inhibitory Peptide

Begins with the arrival food in the stomach and builds on the stimulation provided during the cephalic phase; phase may continue for three to four hours while the acid and enzymes process the ingested materials. The stimuli that initiate the gastric phase are distension of stomach, increase in ph of gastric contents, and the presence of undigested materials in the stomach (proteins, peptides).

Gastric Phase

Secreted by G cells in the duodenum when exposed to incomplete digested proteins; gastrin promotes increased stomach motility and stimulating the production of gastric acids and enzymes

Gastrin

The digestive tract produces at least 18 hormones that affect almost every aspect of digestion, and some of them also effect the activities of other systems. These hormones are peptides produced by enteroendocrine cells, endocrine cells in epithelium of the digestive tract.

Hormonal Control Mechanisms

Neural stimulation and the presence of peptides and amino acids in chyme stimulation the secretion of hormone gastric by G cells. Gastrin levels in bloodstream to parietal and chief cells, whose increased secretions reduce the pH of the gastric juice. Gastrin also stimulates gastric motility.

Hormonal response:

Gastric secretion begins when chyme first enters the small intestine. The function of the intestinal phase is to control the rate of gastric emptying to ensure that the secretory, digestive, absorptive functions of the small intestines can proceed with reasonable efficiency. The arrival of chyme in small intestine also triggers other neural and hormonal events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder.

Intestinal Phase

Secretion of bile, storage of nutrients

Liver

Are the primary stimulus for digestion. They coordinate the response changes in the pH of the contents of the lumen, physical distortion of the wall of digestive tract, or the presence of chemicals

Local Factors

Local response, neural response, and hormonal response.

Mechanisms of the gastric phase:

The stimulation of stretch receptors and chemoreceptors triggers short reflexes coordinated in the submucosal and myenteric plexuses. This in turn activates stomach's secretory cells. The stimulation of the myenteric plexuses produces powerful contractions called mixing waves in muscularis externa.

Neural response:

Begins at the bolus comes into contact with the palatoglossal and palatopharyngeal arches and the posterior pharyngeal wall. Elevation of the larynx and folding of the epiglottis direct the bolus past the closed glottis. At the same time, the uvula and soft palate block passage back to the nasopharynx.

Pharyngeal Phase

Muscular propulsions of materials into the esophagus

Pharynx

Secretion of lubricating fluid containing enzymes that break down carbohydrates

Salivary Glands

Released when chyme arrives in duodenum; increases the secretion of bile and buffers which then increase the pH of the chyme; it also reduces gastrin motility

Secretin

Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.

Small Intestine

Mechanical processing by chewing

Teeth

Assists mechanical processing with teeth, sensory glands

Tongue

Stimulates the secretion of intestinal glands, dilates regional capillaries, and inhibits acid production in the stomach; provides an efficient mechanism for removing absorbed nutrients.

Vasoactive Intestinal Peptide

Distension of the gastric walls stimulates a release of histamine in the lamina propria, which binds to receptors on the parietal cells and stimulates acid secretion

Local response:

Short reflexes are triggered by chemoreceptors or stretch receptors in the walls of the digestive tract; the controlling of neurons are located in the myenteric plexus. Reflexes are called myenteric reflexes. Long reflexes involving interneurons and motor neurons in the CNS provide higher level of control over digestive and glandular activities, controlling large scale peristalsis. - involve parasympathetic motor fibers in glossopharyngeal, vagus and pelvis nerves that synapse in the myenteric plexus.

Neural Control Mechanism

Chyme leaving the stomach decreases the distension in the stomach, reducing the stimulation of stretch receptors. Distension of duodenum by chyme stimulates stretch receptors and chemoreceptors that trigger enterogastric reflex which inhibits both gastrin production and gastric contractions and stimulates contraction of the pyloric sphincter that prevents further discharge of chyme. At the same time, local reflexes at the duodenum stimulate mucus production, which can help protect the duodenal lining from the arriving acid and enzymes.

Neural response of intestinal phase:

Ingestion, mechanical processing with accessory organs (teeth and tongue), moistening, mixing with salivary secretions

Oral Cavity

Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones

Pancreas

Chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions

Stomach

Occurs as the gastroesophageal sphincter relaxes and the stomach regurgitates its contents through the esophagus and pharynx and out through the mouth.

Vomiting (emesis)

Occurs in response to irritation of fauces, pharynx, esophagus, stomach, or proximal segment of the small intestine. These sensations are relayed to the vomiting center of the medulla oblongata which coordinates motors responses. In preparation for vomiting, the pyloric sphincter relaxes and the contents of the duodenum are discharged back into the stomach by strong peristaltic waves that travel toward stomach.

Vomiting reflex

Two central reflexes, the gastroenteric reflex and gastrolieal reflex, are also triggered by the simulation of stretch receptors in the stomach walls as it fills. These reflexes accelerate movement along the small intestines while the enterogastric reflex controls the rate of chyme entry into the duodenum.

Central Reflexes

Begins gastric secretion begins when you see, smell, taste, or think of food. This phase, which is directed by the CNS, prepares the stomach to receive food. The neural output proceeds by way of the parasympathetic division of the ANS. The vagus nerves innervate mucous cells, chief cells, parietal cells, and G cells of the stomach. In response to stimulation, the production of gastric juice speeds up, reaching rate of about 500ml/h, or two cups per house. Emotional states can exaggerate or inhibit the cephalic phase.

Cephalic Phase (only last minutes)

Oral Cavity- carbs (salivary amylase), lipids (lingual lipase), proteins (pepsin) Stomach- stimulus is the anticipation or arrival of food, hormone is gastrin, and the proenzyme released is pepsinogen by chief cells, activated to pepsin by HCI; carbs (disaccharides and trisaccharides), proteins (pepsin--> polypeptides SI stimulus is the arrival of chyme in duodenum, hormone is CCK, proenzymes released are chymotrypsinogen, procarboxypeptides, proelastase, trypsinogen. Enteropeptidase activates trypsin which activates other enzymes; Lipids (bile sals and pancreatic lipase, and monoglycerides, fatty acids in micelles) proteins (trypsin chymotrypsin elastase and carboxypeptidase, short amino acids. Intestinal mucosa carbs (lactase, maltase, sucrase) through brush border (lining of small intestine) by facilitated diffusion and cotransport --> monosaccharides; lipids through brush border by diffusion --> monoglycerides, fatty acids--> triglycerides ---> chylomicrons; proteins (dipeptidases) through brush border by facilitated diffusion and cotransport --> amino acids Route to bloodstream- Carbs and amino acids absorbed and transported by intestinal capillaires. Lipids from chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by thoracic duct; carbs (facilitated diffusion--> monosaccarides) lipids (excytosis --> chylomicrons, proteins (facilitated diffusion and cotransport --> amino acids.

Chemical Events of Digestion

Secreted when cyme arrives in duodenum, especially when the chyme contains lipids and partially digested proteins. In pancreas, CCK accelerates production and secretion of all types of digestive enzymes; also causes a relaxation of hepatopancreatic sphincter and contraction of gallbladder, resulting in the ejection of bile and pancreatic juice in duodenum. High CCK= inhibits gastric activity, and appears to have CNS effects that reduce the sensation of hunger.

Cholecystokinin

The arrival of chyme in the duodenum triggers hormonal responses such as the arrival of lipids and carbohydrates stimulate the secretion of CCK and GIP; a decrease in pH below 4.5 stimulates the secretion of secretin; partially digested proteins in the duodenum stimulate G cells that secrete gastrin, which circulates to the stomach and speed gastric processing.

Hormonal Responses of intestinal phase:

Dehydration and compaction of indigestible materials in preparation for elimination

Large Intestine