The Digestive System

1. The Digestive System Gastrointestinal tract Physiology Dr. Suaad M. Ghazi MBChB, MSc, PhD

2. Objectives of lecture 2 • Describe the mechanisms of chewing and swallowing. • Describe the composition and functions of saliva, and explain how salivation is regulated.

3. Mastication (chewing): The teeth is designed for chewing. The incisors providing a strong cutting action and the molars for grinding action. The chewing reflex is controlled by nuclei in the medulla and cerebral cortex. Most of the muscles of chewing are innervated by the motor branch of the 5th cranial nerve (trigeminal N).

4. A bolus of food in the mouth →Voluntary contraction of the muscles of chewing→ compresses the bolus against the linings of the mouth and push the food to come in contact with the buccal receptors → causes reflex inhibition of the muscles of mastication → allows the lower jaw to drop. The drop in turn initiates a stretch reflex of the jaw muscles (the masseter, medial pterygoid, and temporalis muscles) → leads to rebound contraction. This automatically raises the jaw to cause closure of the teeth. but again, it also compresses the bolus against the linings of the mouth and push the food to come in contact with the buccal receptors, which inhibit the jaw muscles once again and allowing the jaw to drop and rebound another time, and this is repeated again and again.

5. 5th CN Cerebral cortex - + Muscles of mastication Integration center (medulla) - + + 5th CN Contraction 5th CN Stretch Relaxation Food in contact with buccal receptors Chewing (mastication) reflex

7. The importance of chewing: • Breaks down the indigestible cellulose membranes. • Increases the surface area of the food →↑ rate of digestion by the digestive enzymes. • Grinding the food to a very fine particulate → Prevents excoriation of the GIT surface. • Makes the food easy to swallow. • Mixes the food with the salivary gland secretions which initiates : - the process of starch digestion ( by salivary amylase ) . - much less extent of lipid digestion ( by lingual lipase ) . - lubricate and soften the bolus of food → easier to swallow. • Brings the food into contact with taste receptors. • Releases the odors from the food particles → stimulate olfactory receptors →↑ pleasure of eating and initiate gastric secretions.

8. Saliva Salivary glands 500-1500 ml daily of saliva • Parotid glands  25% • Submandibular glands  70% • Sublingual glands  5% • Saliva is: • Hypotonic • Contains high K & HCO3-, low Na & Cl ions • pH 6.0-7.4

9. The structure of each gland is similar to a bunch of grapes. • The acinus is lined with acinar cells and secretes an initial saliva which is a plasma-like solution (isotonic) containing amylase and/or mucin. • A branching duct system is lined with columnar epithelial cell, which modify the initial saliva. • As the primary secretion flows through the ducts, Na+ actively reabsorbs from the initial saliva in an exchange to K+ by Na+-K+ countertransport mechanism which is controlled by hormone aldosterone. • The excess Na reabsorption over that of K secretion creates negativity in the salivary ducts, and this causes Cl ions to be reabsorbed passively. • HCO3- ions are secreted actively by the ductal epithelium into the lumen of the duct.

10. Na reab. Na-K countertransport Reabsorbed passively Secreted actively by ductal epithelium into lumen of duct Ducts are relatively impermeable to water

12. The composition of saliva varies with the salivary flow rate: • [1] At the lowest flow rates, saliva has : • the lowest osmolarity • lowest Na+, and Cl-concentrations • highest K+ and HCO3- ion concentration. • [2] At the highest flow rates, saliva is : • most like the initial secretion from the acinus and its composition is closet to that of plasma.

13. Types of salivary secretions: Serous (watery) secretion (containing alpha amylase) which is an enzyme for digesting starches & electrolyte. Mucous secretion (containing mucin) glycoprotein for lubricating purposes. • Parotid glands  Serous secretion • Submandibular glands • Sublingual glands • Buccal glands  Mucus Serous & mucus secretion 

14. Serous secretion parotid gland (25%) parotid duct sublingual gland (5%) & Submandibular gland (70%) Serous & mucus secretion

16. Control of salivary secretion

17. Salivatory nuclei are excited or inhibited by: [A] Signals from the mouth: Tasteespecially the sour taste-excitatory, bitter taste-inhibitory.tactilestimuli (especially smooth objects-excitatory) from the tongue and other areas of the mouth. [B] Signals from higher centers of CNS: For instance, when a person smells or see favorite foods, salivation is greater than when disliked food is smelled or seen. [C] Signals from GIT: Salivation also occurs in response to reflex originating in the stomach and upper intestine particularly when very irritating foods are swallowed or when a person is nauseated. The swallowed saliva may help to remove the irritating factor in the GIT by diluting or neutralizing the irritant substances.

18. Salivary glands Sympathetic and parasympathetic responses are not antagonistic Parasympathetic system has the dominant role – continuous discharge 1. Increased parasympathetic stimulation produces a large volume of watery salivarich in enzymes, high in electrolytes (K and bicarbonate) but low in proteins. 2. Increased sympathetic stimulation produces a smaller volume of thick salivarich in mucus, low in electrolytes (K and bicarbonate).

19. Saliva production is decreased • By inhibition of the parasympathetic nervous system • sleep • dehydration • fear • anticholinergic drugs (atropine). • Under basal conditions, saliva is almost entirely of the mucus type and is secreted all the time except during sleep when the secretion become verylittle.

20. Functions of saliva • Protection of mouth  by cooling hot food •  by maintaining healthy oral tissues. •  by washing away the bacteria & food particles •  by salivary factors that kill bacteria thiocyanate ions, proteolytic enzymes, and antibodies (IgA). • 2.Digestion  by alpha amylase (starch digestion) • 3.Lubrication of food  making swallowing easier, moisten the mouth, facilitating speech & neutralizes any gastric acid. Saliva prevents harmful effects of bacteria

21. Pathophysiological conditions associated with saliva secretion: Xerostomia (dry mouth) is associated with chronic ulceration’s of the buccal mucosa and with dental caries. Saliva dissolves and washes out food particles from between teeth. Congenital xerostomia- absence of saliva. Sjogren's syndrome- atrophy of the glands and decreased saliva production.In cystic fibrosis, salivary sodium, calcium and protein are elevated. Digitalis drugs cause increased calcium and potassium concentrations in saliva. Addison's disease- sodium concentrations are increased. Cushing's syndrome- sodium concentrations are decreased as they are in primary aldosteronism and during pregnancy. Tumors - Excessive salivation is observed with tumors of the mouth or esophagus and with Parkinson's disease.

22. Swallowing (deglutition) • Oral (voluntary) stage. • Pharyngeal (involuntary) stage. • Esophageal (involuntary)stage. • Relaxation of lower esophageal sphincter.

23. Food voluntarily squeezed posteriorly to oropharynx by pressure of tongue upward and backward against hard palate, soft palate and back of mouth into the pharynx. From here swallowing reflux becomes automatic.

24. Pharyngeal (involuntary) stage of swallowing: The bolus of food is pushed backward in the mouth stimulates swallowing receptor areas around the opening of the pharynx impulses from these pass to medulla oblongata through the sensory portions of ( 5th and 9th ) nerves to initiate a series of automatic pharyngeal muscular contractions by neuronal areas collectively called swallowing (or deglutition) centerdistributed throughout the reticular substance of the medulla and lower portion of the pons. The motor impulses from the swallowing center to the pharynx and upper esophagus that cause swallowing are transmitted by the ( 5th , 9th , 10th , 12thcranial nerves and few of the superior cervical nerves ) The swallowing center specifically inhibits the respiratory center ofthe medulla during swallowing, halting respiration at any point in its cycle to allow swallowing to proceed.

25. In order: 1. The soft palate is pulled upward closed pos. nares. 2. Approximation of the palatopharyngeal folds pulled medial ward form sagital slit. 4. Upward movement of larynx stretches opening of oesophagus. Upper 3 - 4 cm called UES (striated muscle) relaxes allowing food to move from pos. pharynx into upper oesophagus. UES between swallow tonically contracted preventing air from going into oesophagus. 3. Approximation of vocal cords and pulling the hyoid bone and larynx upward and anteriorly. Swing the epiglottis backward to prevent passage of food into the trachea to keep swallowed material out of the airways. 5. Larynx is raised and UES is relaxed, superior constrictor muscle of pharynx contracts. Rapid peristalses wave downward which propels the food into oesophagus.

26. UES • Primary peristaltic wave (vagal reflexes) continuation begins in pharynx to stomach through esophagus. Reflexes transmitted through vagal afferent from eso. to medulla and back to eso. through vagal efferent fibers. • Secondary peristaltic wave (Enteric reflexes) initiated from distention of eso. by retained food if the primary peristaltic wave fails to move all the food that has entered eso. into the stomach.

27. Step 4 Relaxation of lower esophageal sphincter As the esophageal peristaltic wave passes toward the stomach, the lower esophageal sphincter relaxes. This relaxation is vagally mediated.

28. The lower esophageal sphincter is not anatomically separate identifiable muscles.  The fundus of the stomach and lower esophageal sphincter extending about 2-5 cm above its junction with the stomach both relax during a swallow while the bolus of food is still higher in the esophagus.  This phenomenon is called receptive relaxation.  Receptive relaxation is vagally mediated.  Nitric oxide is the neurotransmitter thought to mediate receptive relaxation at the smooth muscle cell allowing food to pass to the stomach.

29. diaphragm Lower Esophageal Sphincter (Gastro esophageal Sphincter) Prevention of reflux of the gastric contents is achieved by: [1] The tonic contraction of LES. [2] Subdiaphragmatic oblique entrance of esophagus to the stomach (valve-like mechanism).

30. Neural control of the LES Sympathetic Vagal excitatory Vagal inhibitory ACh (N) + + + Enteric Nervous System NE - ACh (M) Nitric oxide + Muscle cells of the LES

31. NEURAL PATHWAYS CONTRIBUTING TO CONTROL OF LES TONE • Vagal efferent nerve fibers control contraction and relaxation of the LES. • Vagal excitatory fibers synapse on post-ganglionic, enteric motor neurons that release acetylcholine within the musculature of the LES. • Vagal inhibitory fibers synapse on enteric inhibitory motor neurons that release nitric oxide (NO). These neurons may also utilize ATP and VIP as inhibitory transmitters, but NO is the primary inhibitory neurotransmitter. Sympathetic neurons also provide excitatory input to the LES via excitatory motor neurons.

32. Gastroesophageal reflux initiates a cycle of increased esophageal acid exposure

33. GASTROESOPHAGEAL REFLUX INITIATES A CYCLE OF INCREASED ACID EXPOSURE The relationship between gastroesophageal reflux and altered pressures in the distal esophagus and lower esophageal sphincter is difficult to clarify. Do the abnormal pressures occur as a result of the chronic reflux or are they a primary event? A reasonable hypothesis at this point is that incompetence of the LES (probably initially produced by transient spontaneous relaxations) leads to reflux and prolonged esophageal mucosal acid contact causing esophagitis. The esophagitis results in chronic injury to the distal esophagus affecting the peristaltic function and causing frequent ineffective peristalsis which prolongs the acid contact secondary to reflux. The esophagitis may also cause chronic decreases in LES pressure, resulting in a damaging cycle of events that combine to perpetuate the reflux injury.

34. Disorders of swallowing: • Esophageal reflux: Reflux of stomach acid to the esophagus causes esophageal pain (heartburn) and may lead to esophagitis. • Intragastric pressure is usually greater than atmospheric pressure. • The pressure gradient between the lumens of the stomach and esophagus would tend to facilitate the reflux of food from the stomach into the esophagus. • (LES) normally prevents this and mediates the unidirectional transfer of food. • Largest pressure gradient from stomach to esophagus would occur during inspiration. • An increase in intra-abdominal pressure will increase both intragastric pressure and the closing pressure for the LES by approximately the same amount. Therefore, the closing pressure of the LES will always exceed intragastric pressure by 30-40 mmHg.

36. A number of things can increase intraabdominal pressure : • Ingestion of a very large meal. • Production of intestinal gas by bacterial metabolism. • Pregnancy. • An abdominal mass such as a tumor. • Straining (contraction of abdominal muscles) against a closed glottis. • Etc. • Latter maneuver increases intraabdominal pressure relative to atmospheric pressure, and just as it facilitates defecation, it will also facilitate gastro-esophageal reflux if the LES is not contracted.

37. Esophageal reflux may occur : • [A] If the intragastric pressure rises high enough to force the lower esophageal sphincter open. • [B] If the lower esophageal sphincter is unable to maintain its normal tone. • [C] If the lower esophageal sphincter is forced through the diaphragm and into the thoracic cavity as in hiatus hernia. • In this situation (The same may occur during pregnancy) : • Intra-abdominal pressure no longer contributes to the closing pressure of the LES. • Negative intrathoracic pressure would reduce the closing pressure. • In either case, the low intrathoracic pressure compared to the high intra-abdominal pressure, causes LES to expand, allowing reflux to occur.

38. Belching (eructation): Following a heavy meal or the ingestion of large amounts of gas (e.g., from carbonated beverages), the gas bubble that is usually in the fundus of the stomach is displaced to the cardia. When lower esophageal sphincter relaxes during the swallowing process, gas enters the esophagus and is regurgitated.

39. Dysphagia: Difficulty in swallowing. Persons with dysphagia usually report choking, coughing, or an abnormal sensation of food sticking in the back of the throat or upper chest when they swallow. If swallowing is painful, it is referred to as odynophagia. Dysphagia can result from altered nerve function or from disorders such as: • Narrowing of the esophagus. • Lesions of the central nervous system (CNS), such as a stroke, often involve the cranial nerves that control swallowing. • Strictures and cancer of the esophagus and strictures resulting from scarring can reduce the size of the esophageal lumen and make swallowing difficult.

40. Achalasia: It is a neuromuscular disorder of the lower two-thirds of the esophagus that leads to absence of peristalsis and failure of the lower esophageal sphincter to relax. Food accumulates above this sphincter, taking hours to enter the stomach and dilating the esophagus.