Week 4 The digestive system II

Question 1

What does digestion depend on?

Answer 1

Secretions from:

• Multicellular glands
• Single secretory cells scattered throughout GIT

Question 2

Where do digestive secretions take place?

Answer 2

Mouth
Stomach
Pancreas
Liver

Question 3

What glands exist in the oral cavity?

Answer 3

• Salivary glands

– typical mammal has multiple pairs of extrinsic salivary glands

Question 4

What glands does a dog have in its oral cavity?

Answer 4

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

Question 5

What is saliva made of?

Answer 5

Saliva – 99.5% H2O, 0.5% electrolytes & protein

– important salivary proteins = amylase, mucus, lysozyme

Question 6

How much is salivary secretion in humans?

Answer 6

1-2L in humans

Question 7

How is salivary secretion controlled?

Answer 7

Parasympathetic system:

• simple reflex
• conditioned reflex

Question 8

What is the simple reflex in the parasympathetic system?

Answer 8

– food in mouth
– chemoreceptors & pressure receptors activated (buccal cavity)
– stimulus → impulses → salivary centre → impulses (extrinsic
autonomic nerves) → salivary glands → secretion

Question 9

What is the conditioned reflex in the parasympathetic system?

Answer 9

Conditioned reflex – learned response (Ivan Pavlov)
– salivation occurs without oral stimulation
– thinking/seeing/smelling food → salivation
– mediated via cerebral cortex

Question 10

Who coined the term conditioned reflex?

Answer 10

Ivan Pavlov (learned response-conditioned reflex)

Question 11

Under what conditions is salivary secretion inhibited?

Answer 11

Dehydrated animals
– Initiate sympathetic response
– Blood flow to salivary glands restricted = Prevents secretion
• Humans – stressful situations
– Dry mouth
– Same sympathetic response

Question 12

Describe the inside of the stomach

Answer 12

Sphincters x2
• Surface = columnar
epithelium with tight
junctions
• Luminal surface – gastric pits

Question 13

What are gastric pits?

Answer 13

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

Question 14

What are the types of secretory cells?

Answer 14

• 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)

Question 15

Which vertebrates do NOT have acidic stomachs?

Answer 15

• Platypus
– Reasons unclear
• Gastric brooding frog
– Swallows fertilised eggs
– Young produce prostaglandin E2
– Inhibits acid secretion
– Frogs hop up oesophasgus & out through mouth

Question 16

How is gastric secretion regulated?

Answer 16

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

Question 17

What is the pancreas?

Answer 17

Organ with exocrine &endocrine roles,

• duct cells & acinar cells = exocrine portion of pancreas
• islets of lagerhans = endocrine portion of pancreas

Question 18

What does the exocrine portion of the pancreas do?

Answer 18

secrete through ducts
(exo = external, crine = secretion)
secretory product = pancreatic juice – function relates to GIT

Question 19

What does the endocrine portion of the pancreas do?

Answer 19

lack ducts (endo = internal)
release secretory products into blood
key secretory products = insulin & glucagon – function relates to GIT

Question 20

What do duct cells and acinar cells secrete?

Answer 20

Duct cells:
-secrete aqueous NaHCO(3)-  solution
Acinar cells:
-secrete digestive enzymes
(enzymes leave through zymogen granules)

Question 21

What is the secretory product of the exocrine portion of the pancreas?

Answer 21

-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

Question 22

How is pancreatic juice secretion regulated?

Answer 22

-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.

Question 23

How is glucose regulated?

Answer 23

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)

Question 24

What happens after digestion?

Answer 24

Molecules are transported across plasma membrane in different ways

Question 25

How are polar molecules transported?

Answer 25

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

Question 26

How are other molecules transported?

Answer 26

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

Question 27

How are carbohydrates digested?

Answer 27

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

Question 28

How is glucose and galactose absorbed?

Answer 28

``` Through: Na+ glucose-transporter • aka SGLT-1 (symporter) • secondary active transport GLUT-2 • facilitated diffusion (passive) ```

Question 29

How is fructose absorbed?

Answer 29

``` Through: GLUT-5 • facilitated diffusion (passive) GLUT-2 • facilitated diffusion (passive) ```

Question 30

How are proteins digested?

Answer 30

``` Digestion proteins → AA • digestion begins in the stomach Gastric pepsin and in the small intestine: Pancreatic proteolytic enzymes Aminopeptidases Intracellular peptidases ```

Question 31

How are proteins absorbed?

Answer 31

``` • Amino acids (transport similar to glucose) – apical membrane • Na+ co-transporters (symporter) • secondary active transport – basolateral membrane • facilitated diffusion (passive) ```

Question 32

How are lipids digested?

Answer 32

``` 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 ```

Question 33

What is bile and bile salts?

Answer 33

• 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

Question 34

What do bile salts and lecithin do?

Answer 34

formation of micelles – water soluble – core = hydrophobic (monoglycerides & free fatty acids) – shell = hydrophilic micelles transport H20 insoluble substances → enterocytes

Question 35

How are lipids absorbed?

Answer 35

``` -apical membrane • passive diffusion enterocyte • TGs re-synthesised • chylomicrons -basolateral membrane • exocytosis • enter lymphatic vessels ```

Question 36

Hows the GIT controlled?

Answer 36

Neural & hormonal control | • Overlapping regulatory pathways > very complex

Question 37

What are the layers of GIT wall?

Answer 37

``` 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 ```

Question 38

How is GIT motility determined?

Answer 38

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

Question 39

How do suites of neurotransmitters & hormones control gut motility?

Answer 39

-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

Question 40

What does the myenteric plexus do?

Answer 40

myenteric plexus integrates information → smooth muscle

Question 41

What role does timing play in GIT motility?

Answer 41

• 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

Question 42

Do Fruit-eating birds have a faster passage rate of contents through the GIT compared to nectar-eating birds?

Answer 42

False , nectar eating birds have a faster passage rate of contents as they do not carry around indigestible material.