Week 4 The digestive system II Flashcards
What does digestion depend on?
Secretions from:
- Multicellular glands
- Single secretory cells scattered throughout GIT
Where do digestive secretions take place?
Mouth
Stomach
Pancreas
Liver
What glands exist in the oral cavity?
• Salivary glands
– typical mammal has multiple pairs of extrinsic salivary glands
What glands does a dog have in its oral cavity?
Dog – Parotid glands (anterior to ear) – Orbital glands (near eye) – Mandibular glands (near lower jaw) – Sublingual glands (beneath tongue) x2 cell types • mucus secreting • serous secreting
What is saliva made of?
Saliva – 99.5% H2O, 0.5% electrolytes & protein
– important salivary proteins = amylase, mucus, lysozyme
How much is salivary secretion in humans?
1-2L in humans
How is salivary secretion controlled?
Parasympathetic system:
- simple reflex
- conditioned reflex
What is the simple reflex in the parasympathetic system?
– food in mouth
– chemoreceptors & pressure receptors activated (buccal cavity)
– stimulus → impulses → salivary centre → impulses (extrinsic
autonomic nerves) → salivary glands → secretion
What is the conditioned reflex in the parasympathetic system?
Conditioned reflex – learned response (Ivan Pavlov)
– salivation occurs without oral stimulation
– thinking/seeing/smelling food → salivation
– mediated via cerebral cortex
Who coined the term conditioned reflex?
Ivan Pavlov (learned response-conditioned reflex)
Under what conditions is salivary secretion inhibited?
Dehydrated animals – Initiate sympathetic response – Blood flow to salivary glands restricted = Prevents secretion • Humans – stressful situations – Dry mouth – Same sympathetic response
Describe the inside of the stomach
Sphincters x2 • Surface = columnar epithelium with tight junctions • Luminal surface – gastric pits
What are gastric pits?
Gastric pits are cavities in the mucosa that include:
-mucousneck cells
-chief cells
-parietal cells
-enteroendocrine cells
and are responsible for the production of gastric fluid
What are the types of secretory cells?
• x4 cell types
1. Mucous neck cells (secrete acid mucous)
– protective
2. Parietal cells (secrete acid)
– primarily HCl
1. Chief cells (secrete digestive enzymes)
– Pepsinogen (protease) - digests protein
2. Enteroendocrine cells (secrete hormones)
– E.g. gastrin, excitatory – promotes digestive activity (secretion)
Which vertebrates do NOT have acidic stomachs?
• Platypus – Reasons unclear • Gastric brooding frog – Swallows fertilised eggs – Young produce prostaglandin E2 – Inhibits acid secretion – Frogs hop up oesophasgus & out through mouth
How is gastric secretion regulated?
Complex regulation – neural and hormonal control
• Presence of food
• Anticipation of food
• Gastrin = key
– Stimulates Enterochromaffin-like cells
» secrete histamine – stimulates HCl secretion
– Stimulates Chief cells
What is the pancreas?
Organ with exocrine &endocrine roles,
- duct cells & acinar cells = exocrine portion of pancreas
- islets of lagerhans = endocrine portion of pancreas
What does the exocrine portion of the pancreas do?
secrete through ducts
(exo = external, crine = secretion)
secretory product = pancreatic juice – function relates to GIT
What does the endocrine portion of the pancreas do?
lack ducts (endo = internal) release secretory products into blood key secretory products = insulin & glucagon – function relates to GIT
What do duct cells and acinar cells secrete?
Duct cells: -secrete aqueous NaHCO(3)- solution Acinar cells: -secrete digestive enzymes (enzymes leave through zymogen granules)
What is the secretory product of the exocrine portion of the pancreas?
-pancreatic juice – function relates to GIT-
Pancreatic juice:
Bicarbonate, Pancreatic amylase, Pancreatic lipases, Proteases,Nucleases
Acinar cells
• produce small volume of primary secretion & digestive enzymes
• digests chyme
Duct cells
• produce bicarbonate-rich fluid
• neutralises chyme
How is pancreatic juice secretion regulated?
-acid in duodenal lumen leads to
-the release of secretin and VIP (vasoactive intestinal peptide) from duodenal mucosa which in turn
-stimulates pancreatic duct cells and
-increases pancreatic HCO(3)- secretion which neutralizes the acid
AND
-digestive products in duodenal lumen lead to
-the release of CCK (cholecystokinin) from duodenal mucosa which
-stimulates pancreatic acinar cells and
-increase pancreatic enzyme secretion that digest the products in the lumen.
How is glucose regulated?
In the Endocrine portion of pancreas = key secretory products = insulin & glucagon – function relates to GIT and to glucose regulation
• Islets of Langerhans contain pancreatic β cells & pancreatic α cells
Alpha cells secrete Glucagon (when Low blood glucose)
Beta cells secrete Insulin (when High blood glucose)
Act as a team to maintain blood glucose between set points (5 mmol)
What happens after digestion?
Molecules are transported across plasma membrane in different ways
How are polar molecules transported?
Polar molecules e.g. monosaccharides, amino acids
‒ Cannot penetrate plasma membrane
‒ Require specific protein transporter
‒ Type of transporter depends on transmembrane gradient
1. Favorable concentration gradient (high – low)
‒ Facilitated diffusion e.g. glucose transported by GLUT proteins
2. Against concentration gradient (low – high)
‒ Active transport e.g. amino acids use secondary transport
How are other molecules transported?
Vesicular
• Cells engulf regions of plasma membrane to form vesicles
1. Nutrients in solution = pinocytosis
2. Nutrients are particulate = phagocytosis
1 +2= endocytosis
• Cells can also expel nutrients by exocytosis
• E.g. pathways are important for complex lipids
How are carbohydrates digested?
dietary carbohydrates → monosaccharides
– digestion begins in the mouth with salivary amylase
-then in the small intestine pancreatic amylase as well as sucrase lactase and maltase break dissacharides down into monosacharides
How is glucose and galactose absorbed?
Through: Na+ glucose-transporter • aka SGLT-1 (symporter) • secondary active transport GLUT-2 • facilitated diffusion (passive)
How is fructose absorbed?
Through: GLUT-5 • facilitated diffusion (passive) GLUT-2 • facilitated diffusion (passive)
How are proteins digested?
Digestion proteins → AA • digestion begins in the stomach Gastric pepsin and in the small intestine: Pancreatic proteolytic enzymes Aminopeptidases Intracellular peptidases
How are proteins absorbed?
• Amino acids (transport similar to glucose) – apical membrane • Na+ co-transporters (symporter) • secondary active transport – basolateral membrane • facilitated diffusion (passive)
How are lipids digested?
lipids are hydrophobic > challenge chemical & physical transformations before digested & absorbed Bile is key • synthesized in liver • stored in gall bladder • secreted into small intestine -detergent action • aids digestion by pancreatic lipase • lipid → monoglycerides & free fatty acids • participates in formation of micelles • aids absorption
What is bile and bile salts?
• BILE
-bile salts
-cholesterol
-lecithin
-bilirubin
= aqueous alkaline fluid
Bile salts
– lipid-soluble part & water-soluble part
– adsorb on surface of fat droplets
– mixing movement of SI > break up fat droplets
– water soluble surface prevents re-coalescing
What do bile salts and lecithin do?
formation of micelles – water soluble
– core = hydrophobic (monoglycerides & free fatty acids)
– shell = hydrophilic
micelles transport H20 insoluble substances → enterocytes
How are lipids absorbed?
-apical membrane • passive diffusion enterocyte • TGs re-synthesised • chylomicrons -basolateral membrane • exocytosis • enter lymphatic vessels
Hows the GIT controlled?
Neural & hormonal control
• Overlapping regulatory pathways > very complex
What are the layers of GIT wall?
Muscularis externa: -Smooth muscle layer • inner circular layer (↓ diameter) • outer longitudinal layer (↓ length) • nerve plexus - myenteric plexus Then: -submucosa Layer of connective tissue • blood & lymph vessels • nerve plexus - submucosal plexus • plexus = network of nerves
How is GIT motility determined?
Determined by contractile activity of circular smooth muscle
• Tonic contraction controls diameter of lumen
• Regulated by intrinsic/local pathways
• muscle cells (myogenic)
• neurotransmitters released from motor nerves (neurogenic)
-Myenteric plexus – motility & enzyme secretion
-Submucosal plexus – GIT blood flow & ion/water transport
-Enteric Nervous System (ENS) – works to regulate gut function
How do suites of neurotransmitters & hormones control gut motility?
-Neural control (CNS and ENS)
receptors (chemoreceptors & mechanoreceptors)
→ hypothalamus & spinal cord
→ parasympathetic / sympathetic neurons send impulses back
-Parasympathetic
Acetylcholine
↑ motility
-Sympathetic
Norepinephrine, somatostatin & neuropeptide Y
↓ motility
What does the myenteric plexus do?
myenteric plexus integrates information → smooth muscle
What role does timing play in GIT motility?
• Timing is key
– Optimal time for digestion & assimilation
↑ motility > ↑ rate of passage through GIT > ↓ absorption
BUT
↓ motility > animal carries around indigestible material
e.g. fruit-eating birds vs nectar eating birds
Do Fruit-eating birds have a faster passage rate of contents through the GIT
compared to nectar-eating birds?
False , nectar eating birds have a faster passage rate of contents as they do not carry around indigestible material.
