Physiology - Upper GI Tract Structure and Function Flashcards
Why do we chew our food?
Prolonged taste experience
Defence against respiratory failure
How is chewing controlled?
- Voluntary - somatic nerves in the skeletal muscles of the jaw/mouth (allows us to control)
- Reflex - You contract jaw, pressure of food against gums, hard palate and tongue is sensed mechanoreceptors are activated, jaw muscles are inhibited, pressure reduces, contraction then stimulated
This allows chewing when you don’t think about it
Saliva is secreted by which 3 glands?
Parotid
Submandibular
Sublingual
What 5 things are in saliva and why?
- water- 99% of the secreted fluid in saliva. Softens moistens, dilutes. Also acts as a solvent so that soluble food reacts easier with enzymes.
- Mucins- Major protein component. Mucins + water = mucous. This is a viscous solution that has a lubricant function
- a-amylase - catalyses breakdown of polysaccharide (starch and glycogen) into disaccharide (maltose) + glucose
- Electrolytes- keeps tonicity to prevent massive fluid movements. Also helps to maintain the pH of the oral cavity around 7
- Lysozyme- Bactericidal by cleaving polysaccharide component of bacterial cell wall. this is basically the first line of defence.
What do serous cells do inside the salivary ducts?
Produce a fluid isotonic with blood plasma and also alpha-amylase. Both of which are secreted into the serous alveolus and then into the salivary duct.
How is salivary secretion controlled?
Controlled by both the parasympathetic and sympathetic NS which are both stimulatory just have different effects.
Also controlled by reflex control
What is the effect of the parasympathetic system on salivary secretion?
Cranial nerves VII (facial) and IX (glossopharyngeal)
Stimulates salivary production to produce profuse watery salivary secretion.
What is the effect of the sympathetic system on salivary secretion?
Stimulates a small volume that is very viscous.
Has a high mucous content (a1 adrenoreceptors) and a high amylase content (B2 adrenoreceptors)
Why you get a sticky mouth in stressful situations
how does the reflex control of salivary secretion work?
Presence of food in mouth activates chemoreceptors and pressure receptors on the walls of the mouth and tongue.
Chewing itself will cause the production of saliva
What is the role of the oesophagus?
Simply the oesophagus is just a conduit between the pharynx and stomach.
It is approximately 25cm in length
What is the structure of the oesophagus?
It has 4 layers: Mucosa, submucosa, muscular external and adventitia
Mucosa lined by stratified squamous epithelium which is non keratinised)
Submucosal layer has mucous glands to provide lubrication.
The upper 1/3rd (superior) of the muscularis externa is controlled by skeletal muscle so that you can control swallowing
The lower 2/3rd (inferior) is smooth muscle. No longer needed so body takes over
The upper and lower oesophageal sphincters regulate movement of material into and out of the oesophagus.
Describe the oral phase of swallowing
This phase is voluntary.
The food bolus is pushed to the back of the mouth by the tongue.
Describe the pharyngeal phase of swallowing
Presence of the bolus in the pharynx causes a sequence of reflex contractions of pharyngeal muscles.
This is co-ordinated by swallowing centre located in the medulla
The soft palate is reflected backward and upward to close off the nasopharynx.
A cleft palate compromises this nasopharyngeal seal and so if you swallow or vomit your have a very high chance of stuff ending up in your nose.
Describe the phase of swallowing as the food approaches the oesophagus
As the bolus approaches the oesophagus the upper oesophageal sphincter relaxes and epiglottis covers the opening to the larynx preventing food entering the trachea.
Once food has entered the oesophagus the upper oesophageal sphincter contracts preventing a food reflux
Describe the oesophageal phase of swallowing.
The bolus is propelled to the stomach.
A peristaltic wave sweeps along the entire oesophaus.
the bolus is propelled to the stomach in around 10 seconds
Describe swallowing as the bolus nears the stomach and enters
The lower oesophageal sphincter relaxes and the bolus enters the stomach.
Receptive relaxation of the stomach is initiated following relaxation of lower oesophageal sphincter and entry of the bolus into the stomach.
Vagal reflexes cause the relaxation of thin elastic smooth muscle of the gastric fundus (tip) and body os stomach)
The stomach volume varies between 50ml and 1500ml with no change in pressure.
What structure of the stomach allows the size of the stomach to change so much without a change in pressure?
The stomach has Rugae (pleates) which allow it to expand just like drawing and closing the curtains.
What are the 5 functions of the stomach?
- Temporary store of ingested material (this is its primary function.
- Dissolve food particles and initiate digestive process
- Control delivery of contents to small intestine
- Sterilise ingested material
- Produce intrinsic factor for Vitamin B12 absorption.
Lifestyle factor can compensate for all of the stomachs functions bar intrinsic factor. After 3 years will need injections every 3 months.
Which sphincter controls the feed of food into the duodenum?
Pyloric sphincter.
Prevents dumping syndrome
In addition to the circular muscle and longitudinal muscle of the rest of the GI tract the stomach has an extra layer to the muscularis externa. What is it called and what is its function?
The oblique muscle layer.
In addition to shortening the diameter and length of the stomach it allows an action much like wringing out a cloth. This really helps grind your food.
Describe the overall histology of the stomach
Serosa = connective tissue layer
Muscularis externae = three layers: longitudinal, circular and then oblique layers
Submucosa and mucosa are folded into rugae. these are packed tightly together to give an inner surface which is mostly flat however these stretch out as the stomach fills.
Lumenal surface= almost entire surface covered by mucous cells. Inside the gastric pits are gastric glands.
