4 Physiol Mastication Swallowing 2024.pptx

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Physiology of Mastication & Swallowing Class Year 2 Course Gastrointestinal & Hepatology Title Physiology of Mastication & Swallowing Lecturer Prof. Christopher Torren...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Physiology of Mastication & Swallowing Class Year 2 Course Gastrointestinal & Hepatology Title Physiology of Mastication & Swallowing Lecturer Prof. Christopher Torrens Presented by Dr Ebrahim Rajab Date 8th September 2024 1 Learning Outcomes Explain the mechanisms involved in chewing and salivation Describe the composition and functions of saliva Describe the control of salivary secretion and xerostomia Explain the oral, pharyngeal and oesophageal phases of swallowing Describe the control of the lower oesophageal sphincter and achalasia Describe the causes of vomiting and the reflexes involved 2 Outline of Lecture 1) Introduction 2) Saliva a. Regulation of saliva production b. composition of saliva c. chewing 3) Swallowing a. phases of swallowing b. control of swallowing 4) Xerostomia, Ptyalism and Vomiting a. hypo- and hypersalivation b. factors contributing to vomiting 3  The GI Tract Today GIT: Gastric Function GIT: GIT Hormones GIT: Colon Function 4 Anatomy of the Autonomic NS The GI tract is innervated by the autonomic nervous system (ANS). The ANS can be divided into the extrinsic nervous system (sympathetic and parasympathetic) intrinsic, or enteric, nervous system. Fig. 2.1 Extrinsic branches of the autonomic nervous system. (A)Parasympathetic. Dashed lines indicate the cholinergic innervation of striated muscle in the esophagus and external anal sphincter. Solid lines indicate the afferent and preganglionic efferent innervation of the rest of the gastrointestinal tract. (B) Sympathetic. Solid lines denote the afferent and preganglionic efferent connections between the spinal cord and the prevertebral ganglia. Dashed lines indicate the afferent and postganglionic efferent innervation. CG , Celiac ganglion; IMG , inferior mesenteric 5 Anatomy of the Autonomic NS Fig. 2.2 The integration of the extrinsic (parasympathetic and sympathetic) nervous system with the enteric (myenteric and submucosal plexuses) nervous system. Preganglionic fibers of the parasympathetic synapse with ganglion cells located in the enteric nervous system. Their cell bodies, in turn, send signals to smooth muscle, secretory, and endocrine cells. They also receive information from receptors located in the mucosa and in the smooth muscle that is relayed to higher centers via vagal afferents. This may result in vagovagal (long) reflexes. Postganglionic efferent fibers from the sympathetic ganglia innervate the elements of the enteric system, but they also innervate smooth muscle, blood vessels, and secretory cells directly. The enteric nervous system relays information 6up Salivary Glands Parotid glands (~20-25% saliva) – serous watery secretions containing amylase enzyme – located in subcutaneous tissues of the face, anterior to ear – largest (15 – 30 g) Submandibular glands (~70-75% saliva) – major salivary glands – mixed serous and mucus (1:6) – viscous mucinous* secretions Sublingual glands (~3-5% saliva) – mostly viscous mucinous secretions (high mucin content, low amylase) – located inferior to tongue on floor of mouth – *Mucussmallest (2 – 4 g) of water, ions, mucin glycoproteins, and a variety of other is comprised macromolecules. Mucins are heavily glycosylated linear glycoproteins that protect 7 and lubricate epithelial cell surfaces. Mucins comprise the major protein component  Salivary Glands Parotid Gland and Duct Sublingual Gland and Duct Submandibular Gland and Duct 8 Composition of Saliva Saliva flow rate is typically ~1-2 L/day – varies throughout the day and with stimulus (see later) – spontaneous basal rate ~0.5 ml/min with a max flow of ~5 ml/min Composition is ~99.5% H2O and ~0.5% electrolytes and protein/enzymes – salivary α-amylase (initiates digestion of starches) – lingual lipase (initiates digestion of dietary lipids) – mucins (glycoproteins) (lubricant) – IgA and lysozyme (immune defence) Hypo-osmolal compared to plasma – saliva ~130 mosmol/k g vs. plasma 280 mosmol/k g – lower [Na+] and [Cl-], higher [K+] and [HCO3-] – pH 6.0 - 8.0 (pH optimum for amylase = 7.0) 9  Composition of Saliva Fig. 7.4 Saliva Plasma Movements of 150 Plasma ions and water (H 2 O) in the acinus 125 and duct of the Concentration (mmol/L) salivon. From: 100 Gastrointestinal Na+ Physiology 75 Johnson, L.R. HCO3- 50 Cl- 25 K+ 0 1.0 Na+ HCO3- Cl- K+ 2.0 3.0 4.0 Flow Rate (ml/min) Concentrations of major ions in saliva as a function of the rate of salivary secretion. Values in plasma are shown for comparison. Cl − , Chloride; HCO3–, bicarbonate; K + , potassium; Na + , sodium. 10 Regulation of Salivary Secretion Controlled by autonomic nervous system (-no hormonal control!) – parasympathetic branch (ACh) increases flow Chemoreceptors and pressure receptors respond to food in the mouth – simple (unconditioned) salivary reflex Action potentials transmitted along afferent fibres to salivary centre in the brainstem and stimulate PNS efferents that innervate the salivary glands – cranial nerves VII, IX and also V – ACh increases salivation Also an acquired (conditioned) salivary reflex – salivation occurs without direct stimulation but as learned response based on experience (thinking, smelling) 11 Regulation of Secretion Simple reflex Conditioned reflex Salivary Centre ­ thoughts/smell (brain stem)  Food in mouth of food  PNS  SNS ACh Noradrenaline M3 muscarinic Receptor α- and β-adrenoceptor  salivation 12 Autonomic Control of Secretion Parasympathetic (ACh) stimulation is dominant – hence dry mouth with anti-muscarinics such as atropine Increases salivary flow rate – increased flow rate and decreased electrolyte reabsorption along the duct so salivary electrolyte concentrations increase The higher the flow rate of saliva, the more closely the final saliva resembles plasma in its ionic content. – Also increases blood flow to the salivary gland via vasodilatation (provides more oxygen and glucose) Sympathetic more complex, but limited physiological role – α-AR activation seems to increased flow (may be related to resistance in duct) – β-AR activation increases amylase content 13  Function of Saliva Begins digestion of dietary starches (amylase) and lipids (lipase) – digestion in mouth is minimal, largely due to time – no absorption of nutrients Lubrication facilitates swallowing – moistens food and provides lubrication (mucus) for creation of bolus – solubilises food molecules for tasting – facilitates speech: movement of lips and tongue Oral Hygiene – buffering: rich in bicarbonate buffers, neutralise acid and maintain pH (6-8) – antibacterial action of lysozyme: lyses and destroys certain bacteria – antibacterial action IgA: respond to bacteria and maintain homeostasis of oral microbiota 14 Mastication (Chewing) Mechanically breaks up food* – grinds and breaks up food into smaller pieces – increases surface area for salivary enzymatic digestion Lubricates food (tongue) – mixes food with saliva (salivary amylase) starts enzymatic digestion – facilitates swallowing softens and lubricates food (mucus) Stimulates taste buds – reflex increase in salivary, gastric, pancreatic and bile secretion to prepare for arrival of food *The absence of teeth and neuromuscular defects in the elderly after strokes can 15 severely interfere with mastication Outline of Lecture 1) Introduction 2) Saliva a. Regulation of saliva production b. composition of saliva c. chewing 3) Swallowing a. phases of swallowing b. control of swallowing 4) Xerostomia, Ptyalism and Vomiting a. hypo- and hypersalivation b. factors contributing to vomiting 16 The Pharynx: Nasopharynx The pharynx is the common pathway for respiratory and GI tract – can be split into three regions The Nasopharynx lies behind nasal cavity Continuation of nasal cavity: conditioning inspired air and propagation to larynx* Lymphoid tissue on posterior wall – pharyngeal or ‘adenoid’ tonsils Openings to left and right auditory (eustachian) tubes *The primary function of the larynx is to protect the lower respiratory tract from aspirating food into the trachea while breathing. It also contains the vocal cords and functions as a voice17  The Pharynx: Oropharynx Oropharynx (middle region behind mouth) – between soft palate (uvula) and epiglottis Directs food from mouth to oesophagus Involved in voluntary and involuntary phases of swallowing Lymphoid tissue on lateral wall – palatine tonsils 18 The Pharynx: Laryngopharynx Laryngopharynx – inferior region attached to larynx Extends from hyoid bone to entrance of larynx, becoming continuous with the oesophagus – corresponding to C6 vertebrae 19 Swallowing or Deglutition Swallowing is initiated voluntarily and then under reflex control A coordinated and ordered sequence of motor events that moves food from the mouth to the stomach There are three sequential phases to swallowing – oral/buccal (voluntary): bolus is rolled from mouth to pharynx – pharyngeal (reflex) bolus moves from pharynx to oesophagus – oesophageal (reflex) bolus moves from oesophagus to stomach 20  Swallowing Oral (voluntary) Pharyngeal (reflex) Oesophageal (reflex) 21  Oral (voluntary) Pharyngeal Phase Control As bolus enters pharynx it triggers pressure receptors (involuntary) – afferent signals to brain deglutition centre (medulla oblongata, pons) – efferent signals initiate muscular contractions Pharyngeal (reflex) Contraction of the palatal muscle pull soft palate over the nasopharynx – don’t really want substances coming out of your nose Pharyngeal muscles pull the epiglottis and close off the trachea while moving the bolus downward. – These muscles also raise the larynx to direct the bolus into the oesophagus and enlarge the opening of the oesophagus Oesophageal (reflex) Lasts approximately 1 sec during which time breathing is inhibited – deglutition apnoena 22 Swallowing in Order tongue up & back 1 nasopharynx closed 2 larynx elevated 3 airway closed 4 UES opens 5 pharynx contracts oral pharyngeal oesophageal 0.5 1.0 1.5 Time (s) 23 Oesophageal Phase Control Upper oesophageal sphincter (UES) contracts peristalsis Primary peristaltic wave (4cm/sec) forces bolus through oesophagus – from UES to the lower oesophageal sphincter (LES) – May take bolus 10 seconds to reach the LES Secondary peristaltic waves initiated by mechanoreceptors in oesophagus to clear large lodged bolus failing to reach stomach with primary peristalsis – local reflex Increased salivary secretion » increased lubrication Controlled by vagal (X) and intrinsic reflexes – 1° peristaltic contraction initiated by swallowing – 2° peristaltic contraction initiated by stimulation of receptors within oesophagus – Mechanism not fully understood 24 GI Tract Muscle Layers: Peristalsis Fig. 3.4. Efferent innervation of the body of the 25 esophagus. From: Gastrointestinal Physiology. Lower Oesophageal Sphincter (LES) The smooth muscle of the LES is tonically active – due to vagal cholinergic activity: prevents reflux, heartburn Relaxes shortly before peristaltic contraction reaches LES, – due to VIP and NO released by vagal nerves that inhibit the smooth muscle LES contracts once bolus has passed into stomach – more on this in the Gastric Function session Secretion from oesophagus is entirely mucus – lubrication – protect from acid and enzymes in gastric juice 26 Swallowing Complications Due to the amount of striated muscle involved, swallowing can be impaired in conditions which affect striated muscle – e.g. stroke, myasthenia gravis, Parkinson’s disease, MS, etc. Swallowing can also be a concern in the elderly – dysphagia In achalasia, swallowing is impaired due to excess LES tone, weak oesophageal peristalsis and failure of LES relaxation – food remains in the oesophagus 27 Outline of Lecture 1) Introduction 2) Saliva a. Regulation of saliva production b. composition of saliva c. chewing 3) Swallowing a. phases of swallowing b. control of swallowing 4) Xerostomia, Ptyalism and Vomiting a. hypo- and hypersalivation b. factors contributing to vomiting 28 Xerostomia (Dry Mouth) Dry mouth resulting from reduced or absent saliva flow – various medical conditions – decreased salivary gland function (hyposalivation) – physiological causes: dehydration, anxiety – medications e.g. anticholinergic medication (atropine, scopolamine) Problems associated with dry mouth are – difficulty with speech, changed taste perception – dental caries (decay), gum disease, mouth infections (due to increases acidity) – opportunistic infection with Candida (loss of antimicrobial actions of saliva) – dysphagia (difficulty swallowing and chewing) Treatment is via saliva stimulants and substitutes (water, chewing gum, parasympathomimetic drugs) – prevention of caries and oral infection (candidiasis), 29 Ptyalism (hypersalivation) The opposite of dry mouth is hypersalivation Can be due to a number of causes such as oral infections (tonsillitis), gastro-oesophageal reflux and Sjögren Syndrome (pronounced Show-grin’s) – SS is an autoimmune disease of lacrimal and salivary glands It is also common in early pregnancy And often it accompanies the nausea, sweating, pallor and irregular heart rate that precedes emesis (=vomiting) 30 Vomiting (Emesis) Defence mechanism of GI tract that eliminates harmful substances Stimulation of vomiting (emetic) centre in medulla, integrates input and modulates response to noxious stimuli, multiple pathways Common causes include – tactile sensation in throat/irritation of pharynx activates the pharyngeal (gag) reflex via vagus nerve – irritation or distension in stomach/intestine (mechano-/chemoreceptors) – motion sickness – emetics in blood & CSF (drugs, toxins stimulating chemoreceptor trigger zone) – elevated intracranial pressure (cerebral haemorrhage) – psychogenic factors (pain, fear, smell, sight) 31  Vomiting (Emesis) Drugs, toxins Chemoreceptor Salivary Centre Chemotherapy Trigger Zone (CTZ) Distension/Obstruction Afferent Peripheral Drugs/Chemotherapy Nerves Emetic Centre Emesis (Medulla) (vomiting reflex) Motion Vestibular apparatus Respiratory Centre Sensory (pain, smell) Pyschological (fear, Higher memory Anticipatation) Cortical Centres 32 Vomiting Reflex Deep inspiration and closure of the glottis – elevation of soft palate and uvula Relaxation of stomach, LES, oesophagus and UES Contraction of abdominal muscles and diaphragm Increases intra-abdominal pressure forcing gastric contents upwards – against gravity 33  Summary The parotid, submandibular, and sublingual glands produce saliva, contributing to digestion, lubrication, and oral hygiene. Saliva is composed mainly of water, electrolytes, enzymes (like amylase and lipase), and mucins. Salivary secretion is controlled by the autonomic nervous system, primarily the parasympathetic branch, which increases saliva production. Salivation can be triggered by both unconditioned (food presence) and conditioned (learned responses) reflexes. Mastication (chewing) mechanically breaks down food, increases surface area for enzymatic digestion, and stimulates saliva production. Swallowing involves three phases (oral, pharyngeal, and oesophageal) and is controlled by reflexes to ensure food moves safely from the mouth to the stomach. The lower oesophageal sphincter (LES) prevents acid reflux into the oesophagus by maintaining a tonic contraction. It relaxes to allow the passage of food into the stomach and contracts again once the food has passed. Vomiting is a defense mechanism triggered by various stimuli, including irritation in the GI tract or motion sickness. Conditions like xerostomia (dry mouth) and achalasia can impair swallowing. Hypersalivation can occur due to infections or conditions like Sjögren Syndrome. Vomiting is a defense mechanism of the gastrointestinal tract that eliminates harmful substances by stimulating the vomiting center in the medulla. It integrates various inputs from the throat, stomach, intestines, and other areas, with common triggers including throat irritation, stomach distension, motion sickness, toxins in the blood, and psychological factors. 34 Additional Information A YouTube video of swallowing – http://www.youtube.com/watch?v=wqMCzuIiPaM Gastrointestinal Email: [email protected] Physiology Johnson, Leonard R., PhD Copyright © 2019 Elsevier Inc. All Rights Quiziology How does food move down the oesophagus? 1. Entirely gravity 2. Entirely peristalsis 3. Some gravity, some peristalsis 4. Some fourth option, I dunno - Let’s say magic 36 Self study MCQs A 30-year-old man visits the clinic complaining of frequent dry mouth and difficulty in swallowing. He mentions that he feels anxious most of the time and often forgets to drink water throughout the day. On examination, his oral mucosa appears dry, and there are no signs of infection or inflammation. Which of the following is the most likely cause of his symptoms? A) Sjögren's syndrome B) Dehydration C) Anticholinergic medication D) Diabetes mellitus E) Gastroesophageal reflux disease (GERD) Correct Answer: B) Dehydration Explanation: Dehydration is a common physiological cause of xerostomia (dry mouth). It is often related to insufficient fluid intake and can be exacerbated by anxiety, which may reduce saliva production. This patient's symptoms align with the common causes of dry mouth without other underlying systemic conditions​. Self study MCQs A 45-year-old man experiences recurrent vomiting triggered by motion sickness during travel. He also reports nausea, sweating, and dizziness before vomiting. Which of the following pathways is primarily involved in his vomiting reflex? A) Pharyngeal (gag) reflex via vagus nerve B) Chemoreceptor trigger zone stimulation by emetics C) Mechanoreceptors in the gastrointestinal tract D) Vestibular system activation E) Increased intracranial pressure Correct Answer: D) Vestibular system activation Explanation: Motion sickness-induced vomiting is primarily mediated by the vestibular system, which detects motion and sends signals to the vomiting center in the medulla. This response involves pathways that are distinct from those triggered by direct gastrointestinal irritation or toxins​