Physiology of Mouth, Pharynx and Oesophagus

Physiology of Mouth, Pharynx and Oesophagus Dr Helen Strachan-Jones: [email protected] Learning Outcomes Signpos ts t o other T...

Physiology of Mouth, Pharynx and Oesophagus Dr Helen Strachan-Jones: [email protected] Learning Outcomes Signpos ts t o other T heme a n d B lo State the functions of saliva teachin ck g State the composition of saliva Describe the mechanism and control of salivary secretion Describe the process of taste Describe the structures and processes involved in mastication Describe the process of swallowing State the functions of saliva Saliva function Lubricates and wets food – helps create bolus for swallowing Helps with taste Begins digestion of starch (via α-amylase) and lipids (via lingual lipase) Protects oral environment – Washes away bacteria and food particles – Keeps mucosa moist – Cools hot foods – Contents destroy bacteria – Maintains alkaline environment State the composition of saliva Key features of saliva 800ml-1500ml produced each day pH 6.2-8.0 Hypotonic solution Osmosis B1 CTB Cell Membra : ne and ‘The movement of water across a semi-permeable Transpo rt membrane from an area of low-solute concentration to high-solute concentration’ Concentration of solute exerts an osmotic pressure creating an osmotic pressure difference across the membrane The pressure that is required to stop osmosis is equal to the osmotic pressure of the solution containing the non- diffusible solute Solution 1: Solution 2: High solute Low solute concentrati concentrati Q. Is osmosis an example on on of passive or active transport? Explain why. Osmolarity and osmolality More later in Block 1 The concentration of osmotically active particles (particles which exert an osmotic pressure) in a solution – Osmolarity - per litre of solution (mOsm/L) – Osmolality - per kg of water(mOsm/kg) Lower osmolarity/osmolality – lower concentration Higher osmolarity/osmolality – higher concentration Tonicity How one solution affects the movement of water by osmosis into or out of another solution separated from it by a semi-permeable membrane, determined by the relative osmolarities of the two solutions If two solutions have the same osmolarity they have the same osmotic pressure – no net movement of water by osmosis – The solutions are isotonic If two solutions have a different osmolarity, there will be an osmotic pressure difference, and there will be a net movement of water by osmosis Tonicity Whiteboard Animation: Tonicity See notes section for text Tonicity in context Tonicity often described relative to normal plasma Saliva is hypotonic – it has a lower osmolarity relative to normal plasma Composition of saliva Water High concentration of some electrolytes: – Potassium (K+) – Bicarbonate (HCO3-) – maintains alkaline environment Low concentration of some electrolytes: – Sodium (Na+) – Chloride (Cl-) Mucous – lubrication Digestive enzymes – salivary α-amylase, lingual lipase Antibacterial agents – proteolytic enzymes (e.g., lysozyme), antibodies (IgA) Tip: link composition of substances to their function – it will make it much easier to Describe the mechanism and control of salivary secretion Salivary glands Block 3 CA I Exocrine glands Parotid gland – serous saliva, watery and rich in α-amylase Sublingual gland – mostly mucous saliva Submandibular gland – mixed serous and mucous saliva Many tiny buccal glands Von Ebner’s glands of tongue – lingual lipase Q. What is an exocrine gland? Histological structure of B1 CTB Epithel : iu major salivary glands m Histolo y g Compound (branched) tubuloacinar glands Secretory portion: acinar cells – Serous cells – α- amylase and immune components – Mucous cells – mucin (becomes mucus) Myoepithelial cells – contract to compress acinus -> saliva forced into ducts Histological structure of B1 CTB Epithel : iu major salivary glands m Histolo y g Branching duct system – Small intercalated ducts – lined by simple cuboidal epithelium and myoepithelial cells – Striated ducts lined by simple cuboidal to simple columnar epithelium – Terminal (principle) duct -> oral cavity Structure of salivary glands Mucous Serous acinus acinus Duct Saliva production – primary secretion Acinar cells secrete initial saliva Initial saliva is isotonic and has a similar electrolyte concentration to plasma Saliva production – ductal modification Transporters on luminal and basolateral membranes of ductal cells enable modification of initial saliva Absorption of sodium chloride is greater than secretion of potassium and bicarbonate → net absorption of solute Ductal cells are relatively impermeable to water → hypotonic solution Effect of flow rate Degree of modification is dependent on flow rate Bicarbonate [HCO3-] secretion is selectively stimulated so its concentration increases with increasing flow rate Resting saliva Stimulated saliva Volume Low High Degree of Highly modified Less modification modification Tonicity Very hypotonic Less hypotonic pH Neutral or slightly Alkaline acidic Enzymes? Few enzymes Lots of enzymes Effect of flow rate Control of saliva secretion The dominant neural input to the salivary glands is the parasympathetic nervous system (PNS) –“rest and digest” Stimulation results in: – Increased saliva production – Increased bicarbonate and enzyme secretions – Contraction of myoepithelial cells – Increased blood flow (PNS) Hormones (aldosterone and antidiuretic hormone) result in increased sodium and water reabsorption when blood volume is low Xerostomia Xerostomia – dry mouth from reduced/absent salivary secretion or change in the composition of saliva Many potential causes, including: – Dehydration – Anxiety – Damage to salivary glands or their innervation, e.g., trauma, surgery, radiotherapy – Medication side effect – Sjögren’s syndrome Q. Thinking about the function and composition of saliva, what signs and symptoms might patient with reduce saliva production have? Signs and symptoms of reduced saliva production Dry and painful throat Dry and rough tongue Dry and cracked lips Problems with swallowing and speaking Difficulty keeping dentures in place Altered taste Halitosis (bad breath) Dental caries and periodontal disease Signs of oral infections, e.g., candidiasis End of Part 1 Questions 1. Define osmolarity 2. What does it mean when a solution is described as hypertonic relative to plasma? 3. Which cells in the salivary glands are responsible for primary secretion of saliva? 4. Describe the events that occur during ductal modification and how this relates to the final composition of saliva 5. What effect will stimulation of the parasympathetic nervous system during eating have on the composition of secreted saliva? Describe the process of taste Taste (gustation) Block 3 CA I 5 taste classifications: sweet, sour, bitter, salty, and umami Taste buds are found on the tongue, palate, larynx and pharynx Taste buds in the tongue located in taste papillae Taste buds: – Taste receptor cells – Supporting cells – Basal cells Taste receptor cells Taste receptor cells are chemoreceptors – detect chemical signals Microvilli provide a large surface area Tastant molecules bind to receptors or enter taste receptor cells → depolarisation Appreciation of flavour involves olfaction (smell) Describe the structures and processes involved in mastication Mastication Block 3 CA I Physical digestion – Breaks down food into small pieces – Increase surface area for enzyme action – Mix food with saliva – Create bolus for swallowing Structures involved: – Teeth – Tongue – Mandible – Temporomandibular joint – Muscles of mastication Describe the process of swallowing The pharynx Block 2 CA I Muscular tube that connects the nasal cavity, oral cavity, larynx, and oesophagus 3 parts: – Nasopharynx – posterior to nasal cavity – Oropharynx – posterior to the oral cavity – Laryngopharynx – posterior to the larynx Muscles of the pharynx Block 2 CA I Inner longitudinal layer – shortens, elevates and widens the pharynx during swallowing External circular layer (pharyngeal constrictors) – Contract sequentially to force bolus through pharynx and into oesophagus – Cricopharyngeus – upper oesophageal sphincter Lower oesophageal sphincter (LOS) Physiological sphincter at the gastro-oesophageal junction Prevents reflux of gastric contents into the oesophagus Lower oesophageal sphincter (LOS) Intrinsic component – Smooth muscle Extrinsic component – The right crus of the diaphragm (“pinch-cock”) Other components – Acute angle at which the oesophagus enters the stomach – Mucosal folds present at the gastro-oesophageal junction (“cork in a bottle”) Gastro-oesophageal reflux disease (GORD) Reflux of stomach contents through the LOS into the oesophagus Occurs due to impairment of normal anti-reflux mechanisms, e.g., – Increased frequency of transient lower oesophageal sphincter relaxations – Increased intra-abdominal pressure, e.g., pregnancy, obesity – Low LOS pressure, e.g., due to smoking – Hiatus hernia Can cause inflammation of the oesophageal mucosa (oesophagitis) Symptoms include: – Heartburn – Acid brash Complications of GORD GI Tract Histolo g y; Cellular Oesophageal stricture A d ap ta tions – Scarring and narrowing of the oesophagus and Cel l Death Barrett’s oesophagus – Metaplasia of squamous epithelium of oesophagus to gastric mucosa (columnar epithelium) – Associated with an increased risk of oesophageal cancer Swallowing Oral phase (1 second) – Voluntary Pharyngeal phase (1 second) – Involuntary Oesophageal phase (10 seconds) – Involuntary Oral phase Tongue moves bolus back towards oropharynx Somatosensory receptors, including mechanoreceptors, send afferent information to the swallowing centre in the medulla (in the brainstem) – Via vagus (CN X) and glossopharyngeal (CN IX) nerves Involuntary swallowing reflex initiated – Motor information sent to muscles of pharynx and upper oesophagus  Pharyngeal phase Respiratory tract Upper oesophageal Peristaltic protected sphincter relaxes wave of o Soft palate elevates contraction o Glottis closes and propels bolus larynx elevates into o Respiration inhibited oesophagus o Epiglottis tilts to cover Oesophageal phase Upper oesophageal sphincter closes Larynx falls, glottis opens, and respiration recommences Primary peristatic wave – Mediated by the swallowing reflex Lower oesophageal sphincter relaxes Secondary peristatic wave – Stimulated by mechanoreceptors in the wall of the oesophagus – Mediated by the enteric nervous system Dysphagia Difficulty with swallowing Q. Note down potential causes of dysphagia using the mind map as a guide Mouth Neurologic Pharyn al Dysphagia x causes Stomach Oesophagus  Dysphagia – potential causes Clift lip or palate Mouth cancer Mouth Parkinson’s Pharyngeal Stroke disease cancer Neurologic Pharyn Tonsillitis al Dysphagia x Head injury Dement Pharyngeal ia causes pouch Mediastinal tumour Oesophagus Oesophageal Stomach cancer Gastric Gastro- Achalasia cancer oesophageal reflux disease -> oesophageal stricture Assessment of swallowing History and examination Speech and Language Therapy (SLT) – Clinical assessment – Bedside swallow test – Instrumental assessment Investigations: – Endoscopy – Barium swallow – Manometry End of Part 2 Questions 1. What type of receptor are taste receptor cells? 2. How does mastication help with the digestion of food? 3. What mechanisms protect the respiratory tract during the pharyngeal phase of swallowing? 4. During swallowing, how does the lower oesophageal sphincter open to allow food to enter the stomach? 5. What additional features prevent the reflux of gastric contents into the oesophagus through the lower oesophageal sphincter? Key Points The composition of saliva reflects its function Saliva is secreted by acini and undergoes ductal modification in salivary glands to create a hypotonic alkaline solution The composition of resting saliva is different from stimulated saliva due to the degree of ductal modification Taste receptor cells are chemoreceptors found in taste buds The organisation of muscles in the pharynx and oesophagus reflects their role in swallowing The lower oesophageal sphincter consists of several components and is important in preventing reflux of gastric contents into the oesophagus There are a 3 phases to swallowing, both voluntary and involuntary The swallowing reflex is an important brainstem function Resources Available on Clinical Key Student (https://www.clinicalkey.com/student): Physiology. Seventh Edition. Costanzo, L. S. 2022. Elsevier. – Osmosis: https://www.clinicalkey.com/student/content/book/3-s2.0-B978032379333 9000109#hl0001914 – Chewing and swallowing: https://www.clinicalkey.com/student/content/book/3-s2.0-B978032379333 9000171#hl0001542 – Salivary secretion: https://www.clinicalkey.com/student/content/book/3-s2.0-B978032379333 9000171#hl0001738 Other: Osmosis and Tonicity: https://www.khanacademy.org/science/biology/membranes-and-transport/diff usion-and-osmosis/a/osmosis Part 1 Slide Questions: Answers Q. Is osmosis an example of passive or active transport? Explain why. Osmosis is an example of passive transport as water moves spontaneously from the solution with low solute concentration to the solution with high solute concentration. This does not require energy and so is described as passive. Osmosis is facilitated by the presence of aquaporins channels. The movement of water is therefore classed as facilitated diffusion. Q. What is an exocrine gland? A gland which secretes its contents via a duct onto an epithelial surface. This is in comparison to an endocrine gland which secretes hormones directly into the blood stream, e.g., thyroid gland. Q. Thinking about the function and composition of saliva, what signs and symptoms might patient with reduce saliva production have? See slides. End of Part 1 Questions: Answers 1. Define osmolarity. Osmolarity is the concentration of osmotically active particles per litre of solution. 2. What does it mean when a solution is described as hypertonic relative to plasma? It means the solution has a higher concentration of osmotically active particles (higher osmolarity) relative to plasma. If the solution is separated from plasma by a semi-permeable membrane, there will be an osmotic pressure different across the membrane, and water will move out of the plasma into the solution via osmosis from the area of low solute concentration to the are of high solute concentration. 3. Which cells in the salivary glands are responsible for primary secretion of saliva? Acinar cells. End of Part 1 Questions: Answers 4. Describe the events that occur during ductal modification and how this relates to the final composition of saliva. As saliva passes through the ducts it undergoes ductal modification. On the luminal membrane sodium-hydrogen exchangers remove sodium from the saliva, and potassium-hydrogen exchangers secrete potassium into the saliva (both are driven by sodium-potassium ATPase on the basolateral membrane. Chloride-bicarbonate exchanger secretes bicarbonate into the saliva and is selective stimulated. The absorption of sodium chloride is greater than the secretion of potassium and bicarbonate, and so there is a net absorption of solute. The ductal cells are impermeable to water, and it is not absorbed along with the solute. The result is a hypotonic solution with a high concentration of potassium and bicarbonate (relative to plasma) and a low concentration of sodium and chloride (relative to plasma). End of Part 1 Questions: Answers 5. What effect will stimulation of the parasympathetic nervous system during eating have on the composition of secreted saliva? Parasympathetic nervous system stimulation results in an increase in blood flow to the salivary glands and increased saliva production. Therefore, the flow rate of the saliva will be high and there will be less ductal modification, resulting in a saliva that is less hypotonic compared to resting saliva. The saliva will also be more alkaline (due to selective stimulation of bicarbonate secretion) and contain lots of enzymes (due to stimulation of enzyme secretion). Part 2 Slide Questions: Answers Q. Note down potential causes of dysphagia using the mind map as a guide. See slides. End of Part 2 Questions: Answers 1. What type of receptor are taste receptor cells? Chemoreceptors as they detected chemical signals. 2. How does mastication help with the digestion of food? Mastication breaks up food into smaller pieces (physical digestion). It mixes the food with the saliva and increases the surface area for enzymes in the mouth (⍺-amylase and lingual lipase) to start chemical digestion. It helps create a bolus that can be swallowed to undergo further digestion in the rest of the GI tract. 3. What mechanisms protect the respiratory tract during the pharyngeal phase of swallowing? Soft palate elevates, glottis closes, larynx elevates, respiration is inhibited, and epiglottis tilts to cover the larynx. 4. During swallowing, how does the lower oesophageal sphincter open to allow food to enter the stomach? Smooth muscle of the lower oesophageal sphincter relaxes. End of Part 2 Questions: Answers 5. What additional features prevent the reflux of gastric contents into the oesophagus through the lower oesophageal sphincter? The right crus of the diaphragm (acts like a “pinch-cock”), the angle at which the oesophagus enters the stomach, and the mucosal folds present at the gastro-oesophageal junction (act like a “cork in a bottle”).