Functions and control of the alimentary tract Flashcards
What are the digestive functions of the stomach?
→ Accommodation and storage
→ Mechanical and enzymatic breakdown
→ Slow delivery of chyme to the duodenum
What does the vagal reflex inhibit?
→ Smooth muscle tone
→ Mechanoreceptors
→ Fundic relaxation
Where is the antral region?
→ Lower region of the stomach
What are physical properties of the antral region?
→ Thick and muscular
What does the antral region do?
→ Mixes/grinds food with gastric secretions
What does the stomach act as?
→ A reservoir for unmixed food
What allows a large volume of food to be stored in the stomach?
→ Fundus and the body of the stomach relax
→ 1.5 L storage
What two nerves are involved in receptive relaxation?
→ parasympathetic nerves
→ Vagus inhibitory nerves
How much gastric juice does the stomach store?
→ 2-3 L
What does gastric juice contain and what is the function of it?
→ mucus → Pepsinogen → intrinsic factor →Lipase → helps in digestion + absorption of food
What does mucus do?
→ Acts as a lubricant by acting as a barrier that protects the stomach and colon especially from gastric acid
What does lipase do?
→ Converts triglycerides into fatty acids and glycerol
What does pepsin do?
→ Protein digestion
What is pepsin secreted by?
→ Secreted by chief cells or peptic cells
What is HCL secreted by?
→ Parietal cells
What is mucus secreted by?
→ Goblet cells or mucus cells
What are paracrine secretions?
→ Secreted from cells in the mucosa but the chemical acts locally on adjacent cells via interstitial fluid
What does somatostatin inhibit?
→ Gastric release in the stomach
What is somatostatin released by and what does it act on?
→ D cells and acts on G cells
Where are exocrine secretions secreted from?
→ Salivary glands
→ Gastric glands
→ Pancreas
→ Liver
What are endocrine secretions?
→ Hormones synthesized by ductless glands
→ Enter the bloodstream and travel to their target tissues where they bind to specific receptors to elicit their effects
What is gastrin secreted by?
→ G cells of the stomach
What are secretin and pancreozymin-cholecystokinin produced by?
→ Duodenal mucosa
Where is insulin secreted from?
→ Pancreas beta cells
Where does absorption of fluid occur?
→ Small intestine + colon
What does the colon absorb and what does it produce?
→ Colon absorbs 90% of water reducing volume to 200ml of semi solid fecal matter
What produces mechanical degradation?
→ The gastric antrum
What mixes the lumen contents?
→ The small intestine
How do drugs and normal products of metabolism leave the body?
→ saliva
→ bile
→ feces
→ vomit
How does indigestible material leave the body?
→ Via feces
What does the gut epithelium act as and what is it exposed to?
→ Interface with the outside world
→ Exposed to the external environment
What helps protect the gut from external pathogens?
→ Sight, smell and taste
→ Vomit reflex
→ Acid in the stomach kills most harmful bacteria
→ Mucus secretions
→ Natural bacteria flora prevent colonization by harmful bacteria
→ Aggregation of lymphoid tissue can mount a response against food-borne antigens
→ Peyer’s patches
What is the liver involved in?
→ Carbohydrate, nitrogen and lipoprotein metabolism
→ Production and excretion of bile + bilirubin
How can the gut initiate its own responses?
→ It has an enteric nervous system which can initiate its own responses to gut stimuli without higher functions of the brain
What are the contractions initiated by?
→ Excitatory reflexes and mediated by AcH or substance P
What are relaxation effects mediated by?
→ VIP or NO
What are splanchnic nerves?
→ Sympathetic
When does receptive relaxation occur?
→ Which happens by mechanical stimuli
→ Food is expected in the stomach
→ Sphincter relax to allow food to enter
When does adaptive relaxation occur and with what neurotransmitter?
→the food is within the stomach
→NANC
→Inhibitory AcH effect, preganglionic
The factors that induce relaxation is not AcH itself the post-ganglionic is mediated by NO and VIP
What is feedback relaxation initiated by?
→CCK
Why is CCK being released?
→ Due to lipids being in the food
What does CCK do?
→ Decreases feeding because relaxation is occurring
What does CCK promote?
→ Bile secretion
→ Gall bladder contracting
How does food move through the duodenum?
→ Ripples of contraction move the food towards the antrum (thicker muscle layer)
→Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave
→Repulsion of chyme causes the opening of pyloric sphincter
→Small partially digested material is squirted through the pyloric sphincter into duodenum
→Repulsion of antral contents backwards towards the body allows mixing/grinding
What is the sieving effect?
→ Viscous and solid matter are retained in the stomach
→pylorus can detect the size of food particles, 1-2mm are passed through
→anything larger goes back to the antrum for grinding.
When is somatostatin released?
→ When there is hypersecretion of acid
What does the vago-vagal reflex describe?
→ type of reflex in which both the afferent (“sensory”) and efferent (“motor”) axons are in the vagus nerve trunk
What kind of circuit is the vago-vagal reflex?
→ Reflex circuit within the GIT
What does the vago-vagal reflex control?
→control of responses to gut stimuli via the NTS and DMVN
When is the vago-vagal reflex active?
→ during the receptive relaxation of the stomach in response to swallowing
Where does the vago-vagal reflex go from and to?
→from stomach to brain and then back to stomach
What does the vago-vagal reflex promote?
→motility and acid secretion
What two nerve fibers are intrinsic to the gut and what are they for?
→Myenteric plexus - motor function
→ Submucosal plexus - Intestinal secretions
What do the two plexuses do?
→ Reflexely regulate GI functions entirely within the wall of the gut
→Mediated by enteric nervous system
How are the two plexuses connected to the CNS?
→ parasympathetic and sympathetic fibres
What are the neurotransmitters used in the plexuses?
→ ACh, NO, NA, 5-HT, GABA and ATP
Where are the two plexuses found?
→ The wall of the colon
What does parasympathetic activity do to the colon?
→ Increases the contraction of the proximal colon
What effects does the cholinergic innervation have on the auerbach plexus?
→ Increases gastric motility and secretion
What effects does adrenergic stimulation have on the Auerbach plexus?
→ Decrease in gastric motility and secretion
What does the rate of emptying depend on?
→ THe materials ability to be absorbed
How do carbohydrates, proteins and fats get emptied?
→ Carbohydrates are emptied quicker
→ Proteins have slow emptying
→ Fatty foods have even slower emptying
What is the result of having fatty foods in the duodenum?
→ Decrease in gastric emptying by increasing the contractility of the pyloric sphincter
What is peristalsis like in the large intestine compared to the small intestine?
→ Slower in the large intestine
What initiates contraction (peristalsis)?
→ Distension
How does the bolus get moved along?
→ Contractions are initiated by excitatory activity mediated by substance P and AcH
→ The circular muscles have to relax infront and contract behind
→Longitudinal muscles are contracted
Where are Peyer’s patches found?
→Lamina propria of the mucosa extend into the submucosa of the ileum
