RCSI Physiology of Mastication & Swallowing PDF 2024

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

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Physiology of Mastication & Swallowing Class DEM Year 2 Course Gastrointestinal & Hepatology Title Physiology of Mastication & Swallowing Lecturer Simon Furney Date 09.09.24 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 The GI Tract Outline of Lecture 1) Saliva a. composition of saliva b. regulation of saliva production c. chewing 2) Swallowing a. phases of swallowing b. control of swallowing 3) Xerostomia, Ptyalism and Vomiting a. hypo- and hypersalivation b. factors contributing to vomiting Salivary Glands Parotid glands (~20-25% saliva) – serous watery secretions containing amylase – 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 – smallest (2 – 4 g)  Salivary Glands Parotid Gland and Duct Sublingual Gland and Duct Submandibular Gland and Duct 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 ~0.5% electrolytes and protein – salivary α-amylase (initiates digestion of starches) – lingual lipase (initiates digestion of dietary lipids) – mucins (glycoproteins) (lubricant) – IgA and lysozyme (immune defence, anti-microbial) Compared to plasma – saliva ~130 mosmol/kg vs. plasma 280 mosmol/kg – lower [Na+] and [Cl-], higher [K+] and [HCO3-] – pH 6.0 - 8.0 (pH optimum for amylase = 7.0)  Composition of Saliva Saliva Plasma 150 Concentration (mmol/L) 125 100 Na+ 75 HCO3- 50 Cl- 25 K+ 0 1.0 Na+ HCO3- Cl- K+ 2.0 3.0 4.0 Flow Rate (ml/min) Regulation of Secretion Controlled by autonomic nervous system – parasympathetic branch (ACh) increases flow Chemoreceptors and pressure receptors respond to food in the mouth – simple (unconditioned) salivary reflex Afferent fibre carry this 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) 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 Autonomic Control of Secretion Parasympathetic (ACh) is dominant response – hence dry mouth with anti-muscarinics such as atropine Increases salivary flow rate – increased flow rate decreases electrolyte reabsorption along the duct so salivary electrolyte concentrations increase – and blood flow via vasodilatation Sympathetic more complex, but limited physiological role – α-AR activation seems to increase flow (may be related to resistance in duct) – β-AR activation increases amylase content Function of Saliva Begins digestion of dietary starches (amylase) – digestion in mouth is minimal, largely due to time – certainly 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 – antibacterial action of lysozyme: lyses and destroys certain bacteria – antibacterial action IgA: respond to bacteria and maintain homeostasis of oral microbiota 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 Outline of Lecture 1) Saliva a. composition of saliva b. Regulation of saliva production c. chewing 2) Swallowing a. phases of swallowing b. control of swallowing 3) Xerostomia, Ptyalism and Vomiting a. hypo- and hypersalivation b. factors contributing to vomiting 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 The Pharynx: Nasopharynx The pharynx is the common pathway for respiratory and GI tract – can be split into three regions Nasopharynx (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 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 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 vertebra  Swallowing Oral Pharyngeal Oesophageal 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 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 Lasts approximately 1 sec during which time breathing is inhibited – deglutition apnoena Oesophageal Phase Control Upper oesophageal sphincter (UES) contracts Primary peristaltic wave (4 cm/sec) forces bolus through oesophagus – from UES to 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 GI Tract Muscle Layers: Peristalsis The oesophagus, like the rest of the GI tract, has two muscle layers— circular and longitudinal—but the oesophagus is one of two places in the gut where striated muscle occurs, the other being the external anal sphincter The type of muscle (striated or smooth) in the oesophagus varies along its length The UES and LES are formed by thickening of striated or circular smooth muscle, respectively 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 Swallowing Complications Due to the amount of striated muscle involved in swallowing, 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 (difficulty in swallowing) In achalasia, swallowing is impaired due to excess LES tone, weak oesophageal peristalsis and failure of LES relaxation – food remains in the oesophagus Outline of Lecture 1) Saliva a. Regulation of saliva production b. composition of saliva c. chewing 2) Swallowing a. phases of swallowing b. control of swallowing 3) Xerostomia, Ptyalism and Vomiting a. hypo- and hypersalivation b. factors contributing to vomiting Xerostomia (Dry Mouth) Dry mouth resulting from reduced or absent saliva flow – various medical conditions Sjögren Syndrome - an autoimmune disease of lacrimal and salivary glands – 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) Ptyalism (hypersalivation) The opposite of dry mouth is hypersalivation Can be due to a number of causes such as oral infections (tonsillitis) and gastro-oesophageal reflux It is also common in early pregnancy And often it accompanies the nausea, sweating, pallor and irregular heart rate that precedes emesis 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)  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 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 Summary Saliva production is primarily PNS and it is hypotonic – low Na+ and Cl- but high K+ and HCO3- – concentration of everything increases at high flow Swallowing is a reflex involving the coordinated contraction and relaxation of the oesophagus and oesophageal sphincters Vomiting reflex involves the relaxation of GI tract and an increase in abdominal pressure Additional Information Your physiology texts Overview of the Gastrointestinal Tract, Netter's Essential Physiology Berne & Levy Physiology, from Chapter 27 A YouTube video of swallowing – http://www.youtube.com/watch?v=wqMCzuIiPaM