Introduction to the Control of the Alimentary Tract Flashcards
What are digestive functions of the stomach?
→Accommodation & storage
→Mechanical and enzymatic breakdown
→Slow delivery of chyme to duodenum
How long does food remain in the stomach unmixed?
1hr unmixed
How is a large volume of storage allowed?
Fundus and body of stomach (thinner muscle tone) relaxes
What chemicals aid in relaxation of the stomach?
Vasoactive intestinal peptide (VIP) and nitric oxide (NO)
Which region of the stomach mixes and grinds food?
Antral region mixes/grinds food with gastric secretions → Digestion
What is the role of the colon/rectum?
storage of indigestive residues and faecal matter
How many litres of gastric juices are is stored /24hr?
2-3 litres of gastric juice/24hr
What are the gastric juices and briefly what do they do?
→Mucus acts as a lubricant by acting as a barrier that protects the stomach and colon especially from gastric acid (prevents trauma)
→Lipase – converts triglycerides to fatty acids and glycerol
→Pepsin : protein digestion
→HCl – important in defence
→Intrinsic – for vitamin B12 absorption
Where are the gastric juices secreted from/by?
→mucus=secreted by goblet cells and mucus neck cells
→lipase= gastric chief cells in the fundic mucosa in the stomach.
→pepsin=secreted by chief cells or peptic cells as pepsinogen(inactive)
→HCL=secreted by parietal cells
→Intrinsic= secreted by parietal cells
What are paracrine secretions
→ Secreted from cells in the mucosa but the chemical acts locally on adjacent cells via interstitial fluid
Give an example of a paracrine secretion?
somatostatin
What does somatostatin do?
inhibits gastrin release in the stomach- reduced acid secretion
Where are exocrine secretions from?
→ Salivary glands
→ Gastric glands
→ Pancreas
→ Liver
What does the vagal reflex inhibit?
→ Smooth muscle tone
→ Mechanoreceptors
→ Fundic relaxation
What is somatostatin released by and what does it act on?
→ D cells and acts on G cells
What do the exocrine glands/organs release?
→Salivary glands: mucus (lubrication for mastication and speech); lipase
→Gastric glands: hydrochloric acid, pepsin, mucus
→Pancreas: bicarbonate ions, enzymes (e.g. amylase, lipase, carboxypeptidase)
→Liver: bile salts, bile acids. During feeding, all bile released goes to stomach
What are endocrine secretions?
→hormones synthesised by ductless glands
→enter the blood stream, travel to their target tissue(s) where they bind to specific receptors to elicit their effects
What are the endocrine secretions?
→gastrin: stomach (G-cells in antrum)
→secretin: duodenal mucosa- secretin acts on pancreas
→pancreozymin-cholecystokinin(CCK): duodenal mucosa- in response to fatty meal
→insulin:pancreas (-cells)
Where does fluid absorption occur?
the small intestine and colon
Where is 90% of water absorbed?
Colon absorbs 90% of water, reducing volume to 200ml of semi-solid faecal matter
What allows mechanical degradation?
. gastric antrum
What mixes lumen content?
small intestine
Estimated times for food in SI, and LI
90-180mins
16-48hrs
How do drugs and normal products of metabolism leave the body?
→Saliva
→Bile
→Faeces
→Vomit
What is the largest mucosal surface in the body?
intestine- probably exposed to the heaviest burden of environmental antigens
→largest lymphoepithelial organ
What helps protect the gut from external pathogens?
→Sight, smell and taste alerts us to harmful food substances
→Vomit reflex
→Acid in stomach (HCl) kills most harmful bacteria
→Mucus secretions- the duoderm is not as protected as the stomach with its acid
→Natural bacterial flora prevents colonisation of harmful bacteria
→Aggregation of lymphoid tissue (e.g. Peyer’s patches) able to mount a response to food-borne antigens - analyse and respond to pathogenic microbes
→Peyer’s patches: located in the lamina propria layer of the mucosa and extending into the submucosa of the ileum
What are Peyer’s patches and what do they do?
small masses of lymphatic tissue found throughout the ileum region of the small intestine.
→monitoring intestinal bacteria populations and preventing the growth of pathogenic bacteria
What is the lamina propia?
lies beneath the epithelium
How much does the liver weigh in an adult?
1.3kg
How is the liver involved in the metabolism of the gut?
involved in carbohydrate, nitrogen and lipoprotein metabolism
→ the production of bile and excretion of bilirubin stored in gall bladder
What are sphlanic nerves?
→contribute to the innervation of the internal organs
→paired visceral nerves- parasympathetic
How much ml does an empty stomach has?
→50ml
How is the relaxation of the fundus regulated?
→mediated by reflexes differentiated into 3 types and name one component mediator: name these:
→receptive
→adaptive (vagal innervation (NO/VIP), tension of stomach)
→feedback (nutrients, CCK).
What is vagotomy?
↓ accommodation and gastric compliance
How are the three regulations of relaxation mediated?
mediated by non-adrenergic, non-cholinergic (NANC) mechanisms
When does the pyloric sphincter close and open?
closes upon arrival of peristaltic wave
→Repulsion of chyme causes the opening of pyloric sphincter
How do digested material get to the duodenum?
squirted through the pyloric sphincter into duodenum
How is mixing and grinding allowed by the antral?
Repulsion of antral contents backwards towards the body
What is common to all hormones produced by the gut?
are peptides
Where does gastrin production occur?
→pyloric antrum
Role of fundus?
→Pacemaker zone
→Spontaneous →depolarisation
BER and peristalsis
Describe gastrin’s effects on cells
→Gastrin does not have to go through circulation to get to its target cell. It can go to ECL cells to produce histamine
→Histamines act on H2 receptors on parietal cells
What are ECL cells?
→synthesize and secrete histamine in response
What is vago-vagal reflex?
→a type of reflex in which both the afferent (“sensory”) and efferent (“motor”) axons are in the vagus nerve trunk
→Pathway is via the brain stem (medulla
When is the vago-vagal reflex active?
→during the receptive relaxation of the stomach in response to swallowing of food prior to it reaching the stomach.
→When food enters the stomach a “vagovagal” reflex goes from the stomach to the brain, and then back again to the stomach causing active relaxation of the smooth muscle in the stomach wall.
What are the NTS and DMVN?
→NTS= Nucleus tractus solitarius-main site of termination of vagal afferents →DMVN = Dorsal motor vagal nucleus-main site of origin of vagal efferents (motor) supplying the gut
What two nerve fibres are intrinsic to the gut?
→Myenteric plexus/ Auerbach’s plexus- motor function via NO
→Submucosal plexus Meissner’s plexus- intestinal secretions
What do the two plexus do?
→Reflexly regulate GI functions entirely within the wall of the gut
What is the difference between myenteric and submucosal plexus?
→Auerbach’s plexus has both parasympathetic and sympathetic input
→Meissner’s plexus has only parasympathetic fibres and provides secretomotor innervation to the mucosa nearest the lumen of the gut
Where does the Auerbach’s and Meissner’s plexus receive input from?
Parasympathetic and sympathetic nervous system
What do parasympathetic and sympathetic activity on the colon do?
→Parasympathetic activity → increase contraction of proximal colon
→Sympathetic activity → decrease colonic movement
What is the colo-colonic reflex?
Food/food products ↑ motility
→presence of food or food products/distension of the stomach increase motility of the colon in response to stretch
What do the cholinergic and adrenergic innervations do to the Auerbach’s plexus?
Most prominent plexus
Cholinergic innervation ↑ gastric motility and secretion
Adrenergic stimulation ↓ gastric motility and secretion
Carbs, Proteins, fatty foods, which one is quickly empting?
→Carbohydrates emptied quickly into duodenum
→Proteins – slow emptying
→Fatty foods – even slower due to emulsification
What is fatty acid’s effect on the duodenum?
↓ in gastric emptying by increasing the contractility of pyloric sphincter
Where is peristalsis slower?
Slower in large intestine compared to small intestine
How are contents moved from gut toward anus?
Wave of propulsive contractions moves contents of gut towards the anus
→Vagal inhibitory and excitatory fibres control movement
What initiates contraction?
distension
Describe the process of peristalsis
Activation of stretch in the mucosa induce the release of 5-HT andcalcitoningene-related peptide.
→ the proximal side of the bolus, the circular muscle shortens
→the longitudinal muscle relaxes, pushing the bolus forward.
Describe what happens when fed
→The bolus causes distension(stretches).
→There are specific types of receptors within the muscularis layer called stretch receptors
The descending fibres (toward the anal) of the myenteric plexus.
→The circular muscles act as sphincter- they constrict behind the bolus.
→The longitudinal contract and open up the lumen in front of the bolus. They stimulate longitudinal by releasing Ach and substance P.
The descending neurones release VIP and NO onto the circular muscles toward anal side.
→The ascending fibres release Ach and substance P on the circular smooth muscle to contract.
→They inhibit the longitudinal by releasing VIP and NO causing hyperpolarising of the cells and the longitudinal relax.
