Digestive system Mouth

Describe the PHYSIOLOGICAL STRUCTURE OF 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

16. 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

17. Dentition

18. 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

19. 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

20. 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

21. 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

22. 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)

23. 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

24. Movement of food through oesophagus

25. 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

26. 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