Flashcards in Physiology of mastication, swallowing and GI tract motility Deck (52):
What are the 4 basic digestive processes?
Motility, secretion, digestion and absorption
Describe the motility of the GI tract.
Mechanical activity mostly involving smooth muscle (although the muscle is skeletal at the mouth, pharynx, upper oesophagus and external anal sphincter).
It consists of Propulsive movement, mixing movements and tonic contractions.
Describe the secretion process in the GI tract.
What do digestive secretions contain?
Secretion occurs into the lumen of the digestive tract from the digestive tract and accessory structures in response to hormonal and neuronal signals. It is required for digestion and protection.
Digestive secretions contain water (a large volume extracted from plasma), electrolytes and organic compounds (e.g bile salts, enzymes, mucous etc).
What is digestion?
The biochemical breakdown (or enzymatic hydrolysis) of complex foodstuffs into smaller, absorbable units.
What is absorption?
The transfer of absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood or lymph. (products of digestion are initially absorbed into enteric cells, and then either the bloodstream or the lymph in the case of some fats).
Describe the structure of the digestive tract wall.
Mucosa: Epithelial cells for absorption
Exocrine cells which secrete digestive juices
Endocrine gland cells which secrete hormones
Submucosa: Connective tissue
Blood and lymph vessels
Muscularis externa: Circular muscle
Serosa: connective tissue
Which reflexes mediate chewing?
Masseteric and diagastric
What is the function of chewing?
1. It breaks down the food and mixes it with the saliva
2. Stimulates taste buds
3. Stimulation of taste buds triggers reflexes which increases salivary, gastric, pancreatic and bile secretion.
What is the function of the palate?
It separates the mouth from the nasal passages and allows chewing and breathing to occur simultaneously.
What is the function of the uvula?
It helps seal of nasal passages during swallowing.
Name the 3 major pairs of salivary glands, and state what proportion of saliva comes from each.
Sublingual - 5%
What are the functions of saliva?
Solvent: important for taste
Antibacterial: Contains lysozyme, lactoferrin (starves bacteria of iron) and immunoglobulins. This is important in preventing dental caries.
Lubricant: Important for swallowing and speech
Digestion of complex carbohydrates: contains amylase
Neutralisation of acid: contains bicarbonate
Facilitates sucking by infants : creates a fluid seal
Name a condition in there is limited saliva production and the patient develops dental caries.
What are the two stages of saliva formation and where do they occur?
Primary secretion - by the acinus
Secondary modification - by duct cells
What happens in primary secretion by the acinus?
Cells produce a primary secretion with Na, K, Cl and HCO3 content similar to plasma.
What happens in secondary modification of saliva by the duct cells?
Why is there a diluting effect?
Cells modify the secretion by removing Na and Cl and to a lesser extent adding K and HCO3.
The duct cells are impermeable to water, and more ions are removed than are added, so there is a diluting effect.
What compound is required for saliva production?
What is the rate of saliva production when we are a) sleeping, b) awake by resting and c) actively salivating?
a) 0.05 ml /min
b) 0.5 ml/min
Describe the composition of saliva, and how it changes with flow rate.
The NaCl content of saliva is much lower than that of plasma, which perhaps explains why we can detect a salty taste.
The bicarbonate content increases as flow rate increases, and decreases as flow rate decreases.
There is no glucose in saliva, which may explain why we can detect a sweet taste.
Give an overview of the reflex regulation of saliva production.
This is the neuronal control of saliva production.
There is the simple, unconditioned reflex and the acquired, conditioned reflex.
Describe the simple, unconditioned reflex which regulates saliva production.
chemoreceptors and pressure receptors in the mouth are activated by the presence of food or another stimulus, impulses are then sent via afferent nerves to the salivary centre in the medulla. This sends out impulses via external autonomic nerves (both sympathetic and parasympathetic) to increase the production of saliva in the salivary glands.
Describe the acquired, conditioned reflex which regulates saliva production.
Thinking about, smelling or seeing an appetising dish causes the cerebral cortex to send impulses to the salivary centre in the medulla. This then sends impulses via external autonomic (both sympathetic and parasympathetic) nerves, to increase the production of saliva.
Describe the role of the autonomic nerves in the production of saliva.
Parasympathetic stimulation has the dominant role in normal saliva production.
Impulses travel via glossopharyngeal and facial nerves to produce a large volume of watery, enzyme rich saliva. This is mediated by M3 muscarinic receptors.
Sympathetic stimulation is dominant at stressful times (e.g. we have a dry mouth when nervous).
Impulses travel via postganglionic nerves from superior cervical ganglia to produce a small volume of thick, mucous rich saliva. This is mediated by Beta 1 adrenoceptors.
What is another word for swallowing?
What are the 2 stages of deglutition?
The oropharyngeal stage and the oesophageal stage.
Describe the oropharyngeal stage of swallowing.
Voluntary: Food bolus is formed in the mouth by the action of teeth and the tongue.
The tongue forces the bolus into the pharynx at the rear of the mouth.
Involuntary: Pressure stimulates pharyngeal pressure receptors.
Afferent impulse sent to the swallowing centre in the medulla.
Efferent signals initiate an all or nothing reflex sequence of muscle movements.
The upper oesophageal sphincter opens.
Food passes through the pharynx into the oesophagus.
What happens to the larynx and epiglottis during swallowing?
The larynx elevates and the epiglottis tilts, to prevent the food from entering the trachea. The vocal cords also close across the glottis under the influence of laryngeal muscles to prevent food from entering the trachea.
What does the swallowing centre in the brain do to prevent food from entering the trachea?
It inhibits the respiratory centre in the brain.
How long does the oropharyngeal stage of swallowing take?
About 1 second.
Describe the stages in the oesophageal stage of deglutition.
The swallowing stage in the medulla oblongata triggers a primary peristaltic wave and closure of the upper oesophageal sphincter.
The wave is mediated by skeletal muscle in the upper oesophagus and smooth muscle in more distal regions.
In primary peristalsis, there is orderly contraction of circular smooth muscle around the food bolus and at the same time, the longitudinal muscle in front of the food bolus shortens, shortening the distance the bolus has to travel.
The lower oesophageal sphincter opens 2-3 seconds after the initiation of a swallow (and closes after the bolus has passed into the stomach to prevent reflux).
Secondary peristalsis occurs when a bit of food gets stuck. Local pressure receptors trigger the secondary peristaltic wave (mediated by the enteric nervous system) and also trigger more production of saliva which is swallowed and helps dislodge the bolus.
How long does the oesophageal stage of deglutition take?
What sort of epithelium lines the oesophagus?
Stratified squamous epithelium, as this is best suited to protect against mechanical attack.
It also has a rapid turnover.
What is secreted in the oesophagus, and what is its function?
The secretion is entirely mucous
This lubricates for the passage of food, and protects the epithelium from attack by acid and enzymes in gastric juice.
Why does no absorption take place in the oesophagus?
The transit time is too rapid to permit absorption to take place.
Stimulation from which nerve causes the stomach to relax to accommodate food from the oesophagus?
The vagus nerve
What is the capacity of the stomach?
What type of compound can begin to be broken down in the stomach?
Which contents of stomach secretions can enable this to happen?
HCl and pepsin.
What is produced when the stomach mixes the food with gastric secretions?
Where is gastric juice secreted from?
How much is secreted a day?
It is secreted from gastric pits in the gastric mucosa.
About 2 litres a day is secreted.
Describe the fundus.
It is next to the oesophagus and has a thin smooth muscle layer.
It receives food but there is little mixing in the fundus.
There is little food stored in the fundus- it is usually a pocket of gas.
Describe the body of the stomach.
This is the middle section of the stomach.
It also has a thin smooth muscle layer.
There is little mixing in the body, but food is stored here.
Describe the antrum of the stomach.
This is next to the duodenum. It has a thicker smooth muscle layer and is highly contractile.
There is a lot of mixing in the antrum.
Is there any absorption through the stomach wall?
There is very little absorption, with the exception of highly lipophilic substances.
Where are the pacemaker cells in the stomach located and what are they called?
They are located in the fundus.
They are called interstitial cells of Cajal.
What do the pacemaker cells in the stomach do?
They establish a basal electrical rhythm (BER) that spreads over the stomach from the fundus to the pyloric sphincter. The BER occurs continuously (3 per minute) and may or may not generate smooth muscle contraction.
What is required to get the basal electrical rhythm to threshold?
An external stimulus, such as the presence of food, which stimulates pressure receptors which sense the distension.
How does the BER spread across the stomach?
The muscle cells are joined by gap junctions, which allows the electrical signal to travel from cell to cell.
Why does all of the chyme not leave the stomach with each contraction?
Because the pyloric sphincter has constant tone and is semi-closed, so it does not allow all of the chyme to leave with each contraction. Some is therefore pushed back and further mixed with the gastric acid.
What determines the escape of chyme through the pyloric sphincter?
The strength of the antral wave. This itself depends on several factors which are either gastric or duodenal.
What are the gastric factors controlling stomach emptying?
The volume of chyme in the stomach: the rate of emptying is proportional to this, as the distension increases motility due to stretch of smooth muscle, stimulation of intrinsic nerve plexuses, increased vagus nerve activity and gastrin increase.
The consistency of chyme: emptying is facilitated by finely divided, thick liquid chyme. This affects emptying as the consistency of chyme affects how easily it can pass through the pyloric sphincter.
What are the duodenal factors affecting gastric emptying?
Fat- is particularly potent and a delay in gastric emptying is required for digestion and absorption in the small intestine.
Acid - time is required for neutralisation of the acid by bicarbonate secreted from the pancreas. This is important for optimal function of pancreatic digestive enzymes.
Hypertonicity- products of carbohydrate and protein digestion are osmotically active and draw water into the small intestine. There is therefore a danger of reduced plasma volume and circulatory disturbances (known as dumping syndrome).