Physiology Flashcards
From inner to outer, what are the 4 layers of a wall of the GI tract?
Mucosa, submucosa, muscularis externa, serosa
From inner to outer, what are the 5 layers of the mucosa?
Epithelial cells, exocrine cells, endocrine gland cells, lamina propria, muscularis mucosa
What is contained in the lamina propria?
Connective tissue, immune cells and capillaries
From inner to outer, what are the 4 layers of the submucosa?
Connective tissue, larger blood and lymph vessels, glands, submucus plexus
From inner to outer, what are the 3 layers of the muscularis externa?
Circular muscle layer, myenteric plexus, longitudinal muscle layer
What forms the serosa?
Connective tissue
Where is skeletal muscle found in the GI tract?
Mouth, pharynx, upper oesophageal and external anal sphincters
What happens upon circular muscle contraction?
Lumen becomes longer and narrower
What happens upon longitudinal muscle contraction?
Lumen becomes shorter and fatter
What happens upon contraction of the muscularis mucosae?
Changes in absorptive and secretory area of mucosae
What are adjacent smooth muscle cells coupled by?
Gap junctions
Spontaneous activity is generated by specialised pacemaker cells. What modifies these?
Enteric and autonomic nerves
What does electrical activity occur as in the stomach and intestines?
Slow waves
What are slow waves?
Rhythmic patterns of depolarisation and repolarisation which spread from cell to cell via gap junctions
What do slow waves determine?
Frequency, duration and velocity of rhythmic contractions
What are interstitial cells of Cajal?
Pacemaker cells interspersed between smooth muscle cells
More specifically, where are interstitial cells of Cajal found?
Between circular muscle and submucosa
What has to happen in order for contractions to occur?
Slow wave amplitude has to be sufficient
What in terms of action potentials, is force of contraction related to?
Number of action potentials produced
What factors determine whether slow waves reach potential or not?
Neuronal, hormonal or mechanical stimuli which act to depolarise smooth muscle cells
What does the myenteric plexus do?
Regulates motility and sphincters
What does the submucous plexus do?
Modulates epithelia and blood vessels
What are found in the myenteric and submucous plexus’?
Enteric nerves
What is the main autonomic influence of the GI tract?
Parasympathetic
What are parasympathetic excitatory influences?
Increased secretions, blood flow and smooth muscle contractions
What are parasympathetic inhibitory influences?
Relaxation of some sphincters and receptive relaxation of the stomach
What are sympathetic excitatory influences?
Increased sphincter tone
What are sympathetic inhibitory influences?
Decreased motility, secretion and blood flow
What is a local reflex in the GI tract?
Peristalsis
What is a short reflex in the GI tract?
Intestine-intestine inhibitory reflex
What is a long reflex in the GI tract?
Gastroilial reflex
What is a process which is known as haustration in the intestines, and involves mixing/churning ovements of the circular muscle layer?
Segmentation
What is colonic mass movement?
Powerful sweeping contraction which forces faeces into the rectum
What is a sweeping contraction from the stomach to terminal ileum?
Migrating motor complex
What does the upper oesophageal sphincter do?
Relaxes to allow swallowing and closes during inspiration
What does the lower oesophageal sphincter do?
Relaxes to permit food to enter the stomach and contracts to avoid reflux
What does the pyloric sphincter do?
Regulates gastric emptying
What opens and closes the ileocaecal valve?
Distension of ileum opens and distension of proximal colon closes
What are the anal sphincters regulated by?
Defaecation reflex
What afferent nerve impulses are sent to the pons and medulla at the start of the swallowing reflex?
CN IX and X
What efferent nerve impulses are sent to the skeletal muscles of the pharynx and larynx at the start of the swallowing reflex?
CN VII, IX, X, XI
What do the laryngeal muscles do during swallowing?
Close the glottis and raise larynx to inhibit ventilation
Where is the swallowing centre?
Pons and medulla
What nerve is involved in generated a primary peristaltic wave?
Vagus nerve
Where are the parotid glands?
Anterior to ear
Where does the parotid gland enter the mouth?
Above the 2nd maxillary molar
Where is the submandibular gland?
Medial to the body of the mandible
Where do the submandibular glands enter the mouth?
Under the tongue
Where are the sublingual glands?
Medial to the submandibular glands
Where do the sublingual glands enter the mouth?
Connect with the submandibular glands at the sublingual caruncula and enter under the tongue
What type of cells do the parotid glands have and what do they produce?
Serous cells producing a watery alpha amylase rich secretion
What type of cells do the submandibular glands have and what do they produce?
Mixed serous and mucous cells which produce a more viscous solution
What type of cells do sublingual glands have and what do they produce?
Mainly mucus cells producing a thick mucus
Are salivary glands endo or exocrine?
Exocrine
What is the functional unit of a salivary gland?
Salivon
What does a salivon consist of?
Secretory acinus, intercalated duct and striated duct
What do striated ducts combine to form?
Interlobular and excretory ducts
What type of cells surround the acinus of a salivary gland?
Contractile myoepithelial cells
What type of epithelium are intercalated ducts?
Cuboidal
What type of epithelium are striated ducts?
Columnar
What give striated ducts their appearance?
Infoldings between cells containing mitochondria
What are some functions of saliva?
Lubrication, protection, digestion, protective secretion prior to vomiting, facilitates suckling
What electrolytes are found in saliva?
Na+, K+, Ca++, Cl-, I-, PO4–, HCO3-
Which electrolytes are found at higher concentrations in saliva than in plasma?
K+ and HCO3-
What is the relationship between bicarbonate and rate?
Increases with rate
What is the relationship between K+ and rate?
Decreases with rate
What parasympathetic nerve acts on the sublingual and submandibular glands?
CN VII
What parasympathetic nerve acts on the parotid glands?
IX
What receptors are there a large volume of in the saliva?
Watery, enzyme rich M1/M3 acetylcholine receptors
What acts on sympathetic fibres in times of stress and where does this come from?
Adrenaline from adrenal medulla
What type of receptors are there a small volume of in saliva?
Mucus rich alpha and beta 1 adrenoceptors
What nerve relaxes the stomach?
Vagus
What is produced when the stomach mixes food with gastric acid?
Chime
How can the duodenum delay gastric emptying?
Neuronal response- decrease peristalsis
Hormonal response- inhibit stomach contraction
What factors cause delayed gastric emptying?
Fat, acid, hypertonicity and distension
What is secreted from the oxyntic mucosa?
HCl, pepsinogen, intrinsic factor, histamine, mucus
What is secreted from the pyloric gland area?
Gastrin, somatostatin, mucus
What 3 things does HCl do?
Activates pepsinogen to pepsin, kills bacteria ingested with food, denatures proteins
What is pepsinogen?
Inactive precursor of pepsin
What does intrinsic factor do?
Binds vitamin B12 to allow absorption in the terminal ileum
What does histamine do?
Stimulates HCl production
What does mucus do?
Protective
What does gastrin do?
Stimulates HCl secretion
What does somatostatin do?
Inhibits HCl secretion
When does the cephalic stage of gastric secretion occur?
Before food reaches the stomach
What happens in terms of production of molecules during the cephalic and gastric stage of gastric secretion?
Decreased somatostatin, increased ACh, histamine and gastrin
When does the gastric stage of gastric secretion occur?
When food reaches the stomach
When does the intestinal stage of gastric secretion occur?
After food has left the stomach
What happens to somatostatin secretion as the stomach is emptied?
It is increased
What are peptic ulcers?
Any ulcers where the mucosa is exposed to HCl and pepsin
What protects the mucosa from attack of HCl and pesin?
Locally produced prostaglandins PGE2 and PGI2
What do PGE2 and PGI2 do?
Reduce acid secretion, increase mucus and bicarbonate secretion and mucosal blood flow
What reduces prostaglandin formation?
NSAIDs
What does H. pylori secrete?
Agents causing inflammation which weakens the mucosal barrier and leaves the submucosa open to attack
What are drugs that are used to reduce gastric acid secretion used for?
Peptic ulcers, GORD, acid hypersecretion
What are sucralfate and bismuth chealate examples of?
Mucosal strengtheners
What are the 3 main components of food?
Carbohydrates, lipids and proteins
What are examples of carbohydrates?
Starch, glycogen, disaccharides, cellulose
Which carbohydrate is indigestible?
Cellulose
What are examples of lipids?
Triacylglycerols, phospholipids, cholesterol and fatty acids
What is luminal digestion?
Digestion through pancreatic enzymes secreted into the duodenum
What is membrane digestion?
Mediated by enzymes situated at the brush border of epithelial cells
What happens to the absorbable products of digestion during absorption?
They are transferred across both the apical and basolateral borders of the enterocytes
What are enterocytes?
Absorptive cells of intestinal epithelium
What is assimilation?
Combined digestion and absorption
What are digestible carbohydrates?
Polysaccharides, oligosaccharides and monosaccharides
What are polysaccharides?
Glucose polymers joined by alpha 1,4 linkages
What is an example of a straight glucose chain?
Amylase
What is an example of a branched glucose chain?
Amylopectin
Explain the structure of glycogen compared to amylase and amylopectin?
Similar but with a greater molecular weight and more branches
What must all dietary carbohydrates be converted to for absorption?
Monosaccharides
Which enzyme breaks down internal alpha 1,4 linkages but not terminal ones?
Alpha amylase
Because of the way it works (not breaking terminal bonds) what can alpha amylase not produce?
Glucose
What are the products of alpha amylase breakdown?
Glucose oligomers e.g. maltose
What are integral membrane proteins with a catalytic domain which faces the lumen of the GI tract?
Oligosaccharides
What does lactase do?
Breaks down lactose to glucose and galactose
What are enzymes which convert oligosaccharides to monosaccharides known as?
Oligosaccharisases
Apart from lactase, what do all other oligosaccharidases yield?
Glucose
What can degrade alpha 1,4 linkages in straight chain oligomers up to 9 monomers in length?
Maltase
What does sucrase do?
Hydrolyses sucrose to glucose and fructose
What is the only enzyme which can split the branching alpha 1,6 linkages of alpha limit dextrins?
Isomaltase
What are monosaccharides which can be absorbed?
Glucose, fructose, galactose
What is primary lactose intolerance?
A genetic cause
What is secondary lactose intolerance?
Caused by damage to/infection of the proximal small intestine
What is congenital lactose intolerance?
Rare autosomal recessive disease
What can it cause if lactose is eaten in someone who is lactose intolerant?
Bloating, abdominal pain and flatulence
What can undigested lactose cause?
Acidification of the colon and increased osmotic load
Where does absorption of final monosaccharides take place?
Duodenum and jejunum
How are glucose and galactose absorbed?
Secondary active transport mediated by SGLT1
How is fructose absorbed?
Facilitated diffusion mediated by GLUT5
What is the exit out of enterocytes for all monosaccharides?
Facilitated diffusion by GLUT2
What other transporter is expressed in enterocytes?
GLUT1
What must proteins be broken down into for efficient absorption?
Oligopeptides and amino acids
How many different pathways can be used for protein breakdown?
4
What does pepsin do to proteins?
Cleaves them into peptides
What are the endopeptidases secreted as proenzymes from the exocrine pancreas to be converted to the active form in the duodenum?
Trypsin, chymotrypsin, elastase
What does digestion by endopeptidases produce?
Oligopeptides 2-6 amino acids long
What are the exopeptidases secreted as proenzymes from the exocrine pancreas to be converted to the active form in the duodenum?
Procarboxypeptidase A and B
What does digestion by exopeptidases produce?
Single amino acids
Peptidases found where are numerous and have affinity for larger oligopeptides?
Brush border
Peptidases found where are less numerous and primarily hydrolyse di and tripeptides?
Cytoplasm
Where are amino acids absorbed?
Brush border and basolateral membrane
What transporters for moving protein breakdown products out of the brush border are present?
7 different mechanisms- 5 Na+ dependent co-transporters (secondary active transport) and 2 Na+ independent
In the basolateral membrane there are 5 different mechanisms of moving protein breakdown products, what are these?
2 Na+ dependent and mediate influx of amino acids, 3 Na+ independent and mediate efflux of amino acids
What are fats?
Long chain fatty acyl esters of glycerol
What is formed when heat and movements mix foods with gastric lipase?
An emulsion
What does pancreatic lipase do in the small intestine?
Hydrolyses triacylglycerols to 2 x monoglycerides and 2 x free fatty acids
What aids the action of pancreatic lipase?
Bile salts from the gallbladder
What is formed when a triglyceride is broken down by gastric lipase?
Diglyceride and free fatty acid
What does the free fatty acid produced by breakdown with gastric lipase stimulate release of?
CCK from duodenum and pancreatic lipase
How are short and medium chain fatty acids absorbed?
Diffusion through the enterocyte to enter the villus capillaries
How are long chain fatty acids and monoglycerides absorbed?
Resynthesised to triglycerides in the ER and incorporated into chylomicrons
What are chylomicrons?
Small fat globules which transport lipids from the intestines to other body sites
Triglycerides in chylomicrons are released by exocytosis into where?
Lymphatic system to enter the systemic system at the subclavian vein
Chylomicron triglycerides are metabolised in capillaries by what?
Lipoprotein lipase present on endothelial cells
What do free fatty acids and glycerol bind to to be taken up by tissues?
Albumin
Where is cholesterol from chylomicrons released?
Secreted unaltered in bile to be stored or oxidised to bile salts
What is albumin?
The dominant carrier of lipids in blood
What is ezetimibe?
A drug which binds to a transporter and prevents cholesterol absorption and is sometime used alongside statins
What type of transport is generally used in absorption of Ca++ in chime?
Active
What mediates Ca++ absorption from chime?
Dihydroxyvitamin D3 and parathyroid hormone
What is iron an important constituent of?
Haemoglobin, myoglobin and any haem containing enzymes
What does transport of iron take place through?
Divalent metal transporter 1 and haem carrier protein 1
What is a hormone released from the liver which controls iron absorption?
Hepcidin
What are examples of fat soluble vitamins?
A, D, E, K
What do fat soluble vitamins require to be absorbed?
Adequate bile secretion and intact intestinal mucosa
How are fat soluble vitamins transported into enterocytes?
Passively in mixed micelles
How are fat soluble vitamins distributed?
By intestinal lymphatics
What are water soluble vitamins?
B, C and H
What transporters do water soluble vitamins use to be absorbed?
Na+ dependent or independent transporters
Why does vitamin B12 require specially effective absorption?
It is present in minute amounts in the diet
What is energy homeostasis?
Energy intake is matched to energy expenditure over time
What has a mismatch in energy homeostasis over time led to?
Obesity
What other things can obesity cause?
Metabolic syndrome, dyslipidaemia, type 2 diabetes and insulin resistance, cardiovascular disease
What are the main factors influencing obesity?
Genetics and environment
What do we need fat for?
Energy storage, energy buffer during prolonged illness and prevention of starvation
How does the CNS influence energy balance and body weight?
Behaviour (exercise/feeding), ANS activity, neuroendocrine secretions
Where is the site of integration of CNS mechanisms controlling energy balance and body weight and what neural centre is responsible?
Site of integration in the brain, neural centre is hypothalamus
What is satiation?
The feeling of fullness generated during a meal
What is satiety?
The time between the termination of one meal and initiation of the next
What is a satiation signal which is secreted from enteroendocrine cells in the duodenum and jejunum and signals via sensory nerves to the hindbrain?
Cholecystokinin (CCK)
What satiation signal is secreted from the endocrine mucosal L cells of the GI tract?
Peptide YY
What does peptide YY do?
Slows gastric emptying, inhibits gastric motility and reduces food intake
What are satiation signals which are products of the pro-glucagon gene?
Glucagon like peptide and oxyntomodulin
What does oxyntomodulin do?
Suppresses appetite
What is ghrelin?
A hunger signal
Where is ghrelin produced?
Oxyntic cells in the stomach
What raises levels of ghrelin?
Fasting and hypoglycaemia
What are the two main adiposity signals in humans?
Insulin and leptin
Where is leptin made and released?
Fat cells
Where is insulin made and released?
Pancreatic cells
What does reduced leptin cause?
It mimics starvation and causes unrestrained appetite
What are some roles of leptin?
Food intake and fat deposition, glucose homeostasis and insulin sensitivity, maintenance of reproductive/immune systems, angiogenesis, tumourigenesis
What is food reward?
The pleasure derived from eating
What pathways are involved in food reward?
Dopamine
What limits the use of leptin as an obesity therapy?
Resistance
What causes leptin resistance?
Defective transport to the brain or altered signal transduction following leptin binding to its receptor
What is the present obesity drug?
Orlistat
What does orlistat do?
Inhibits pancreatic lipase and decreases triglyceride absorption
What are side effects of orlistat?
Cramping and diarrhoea
What is liraglutide?
Glucagon like receptor antagonist used in treatment of type 2 diabetes
What is the only way to produce sustainable weight loss medically?
Bariatric surgery
What does brown adipose tissue do?
Produces heat after eating to get rid of energy
