Digestive system Mouth
- 1. Gastro intestinal tract –part I Dr. Radhakrishna G Pillai Department of Life Sciences University of Calicut
- 2. Introduction • The gastrointestinal (GI) tract, also called the gut • Muscular tube approximately 9m (30 feet) in length • Running from the mouth to the anus • Mechanically and chemically break down foods • Into simple components that can be absorbed by the body
- 3. GI Tract • It produces various enzymes that help in the chemical digestion of food • It produces various hormones that play a role in regulating digestion, appetite and satiety • Different parts with different functions • Home to myriad micro-organisms (mostly bacteria): many are essential to health • These form a large part of the human microbiome
- 4. Mouth • Opening of the GI tract : receives food, tastes it and prepares it for swallowing • The average volume of the adult mouth is 72 ml in men and …. ml in women • 55 • The mouth is lined by mucous membranes and consists of two major regions: – Vestibule – the space between the inner surface of the cheeks/lips and the teeth – Oral cavity proper – the space inside the teeth, where food is chewed and mixed with saliva before being swallowed.
- 5. Tongue and sense of taste • The tongue is a muscular structure • Approx 10cm long and weighs 60-70g • It is anchored posteriorly to the hyoid bone via small muscles and • Attached to the floor of the mouth by a thin flap of tissue, the lingual frenulum
- 6. Tongue and sense of taste • Tiny extensions called papillae make its surface rough textured • Papillae help to grip food as it is moved around the mouth and mixed with saliva • The papillae contain taste buds Taste buds contain gustatory cells that act as taste receptors
- 7. Taste buds • Food molecules dissolve in saliva and enter taste buds • Taste receptors are activated Nerve impulses are relayed to the gustatory cortex of the brain, where they are perceived as distinct tastes
- 8. Sensing tastes • In humans, there are five primary tastes • Each taste has only one corresponding type of receptor • Each receptor is specific to its stimulus • The five tastes are sensed through different mechanisms based on the molecular composition of the tastant • A salty substance (containing NaCl) provides the sodium ions (Na+) that enter the taste neurons and excite them directly
- 9. Sensing tastes • Map of the tongue showing the distribution of different taste receptor types are misleading – distributed across • Sour molecules are acids • Binding of an acid or other sour-tasting molecule triggers a change in the ion channel and these increase hydrogen ion (H+) concentrations in the taste neurons, thus depolarizing them • Sweet, bitter, and umami tastants require a G- protein coupled receptor
- 10. • These molecules bind to their respective receptors, thereby exciting the specialized neurons associated with them
- 11. Taste and age • Tasting abilities change with age • In humans, the senses decline dramatically by age 50 and continue to decline • A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing
- 12. Olfaction and taste • The sense of taste is intimately linked to olfaction (sense of smell) • Food cannot be tasted properly if the nose is blocked • As soon as the olfactory apparatus detects the smell of food, the parasympathetic nervous system is activated • Salivary glands stimulated - release extra saliva, preparing the mouth to receive the food
- 13. Retronasal and ortho nasal olfaction • Retronasal olfaction -perception of odours emanating from the oral cavity during eating and drinking • The retronasal olfactory pathway, which contributes to the flavor of foods or drinks, is commonly associated with the sense of taste • Also named as “gustatory olfaction” • Orthonasal olfaction- during sniffing- through olfactory epithelium of nose
- 14. Retronasal and ortho nasal olfaction
- 15. Gustatory pathway • Taste neurons project from taste cells in the tongue, esophagus, and palate to the medulla in the brainstem • From the medulla, taste signals travel to the thalamus • Then to the primary gustatory cortex • Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex
- 17. Dentition • Adults normally have 32 teeth, which are evenly distributed between the upper and lower jaws • The teeth comprise: • Eight incisors – flat, chisel-like teeth that bite into food and break it into small manageable pieces • Four canines – sharp, fang-like teeth just outside the incisors that grip and tear food • Eight premolars – small teeth between the canines and molars that grind up and cut into food • Eight molars – square teeth in the rear of the mouth that grind food to increase its surface area
- 18. Dentition
- 19. General structure of teeth • Their central portion, the pulp cavity, is composed of living tissue – mostly blood vessels and sensory nerve endings • Teeth are susceptible to erosion as a result of wear and tear as well as exposure to acidic foods Teeth are mainly composed of a bony material (dentine) covered by a layer of white enamel
- 20. Mastication • Carried out by the teeth • The tongue moves food towards the premolars and molars • If teeth are missing, the gums can also chew food- requires much more effort • Chewing makes food easier to swallow • Increases the surface area that is available to the digestive enzymes-enhance chemical digestion
- 21. Salivary glands • The mouth has three pairs of salivary glands • Sublingual glands – located below the tongue • Submandibular glands – located below the mandible • Parotid glands – located to the side of the earlobes • SGs innervated by the parasympathetic branch of the autonomic NS
- 22. Saliva • The salivary glands continuously produce small amounts of saliva to keep the mouth moist (basal secretion) • In a typical 24-hour period 0.5-1.5L of saliva is secreted • Eating induces a significant increase in saliva production (induced secretion) • The submandibular glands do most of the basal secretion • The parotid glands are the major contributors to secretion induced by the presence of food • The sublingual glands only produce around 5% of both basal and induced saliva secretion • The volume of saliva that is produced can be increased or decreased according to the nature of the food
- 23. Swallowing • Towards the end of mastication • The tongue and roof of the mouth gather and shape the chewed food into a pellet called bolus • Moisture and mucus bind and coat the bolus • Pushed towards the back of the mouth and into the oropharynx • Bolus in the posterior pharyngeal wall trigger the swallowing reflex (deglutition)
- 24. Muscular contractions in swallowing • Involves precisely coordinated muscular contractions • The pharynx walls contract around the bolus • The epiglottis closes over the airway, preventing food from entering the trachea and bronchial tree • The bolus is then funnelled, through the upper oesophageal sphincter, into the oesophagus
- 25. Movement of food through oesophagus
- 26. • Voluntary nervous system help to swallow • Involuntary nervous system takes over swallowing once the liquid or food enters the top of your esophagus • Stretching from food stimulates the smooth muscles on the walls of esophagus • This stimulates network of nerves inside muscles known as intrinsic nerves, start a coordinated muscle action called peristalsis Peristalsis
- 27. Peristalsis • Peristalsis squeezes esophageal muscles from top to bottom • This pushes food and liquid along • Sphincters act like one-way valves • They relax to let food and liquid pass into the stomach and then tightens to prevent backflow, called reflux • Food enters stomach
Editor's Notes
- #5: Source: https://www.nursingtimes.net/clinical-archive/gastroenterology/gastrointestinal-tract-1-the-mouth-and-oesophagus-28-05-2019/
- #15: http://www.stinciclab.com/olfaction-in-taste
- #22: Fig source: https://www.wjgnet.com/2218-6263/full/v4/i2/56.htm
- #26: Source: Peter Lamb