Lecture #25: Digestive System--Stomach

Question 1

What muscle does the stomach have?

Answer 1

Smooth muscle

Question 2

What does the smooth muscle do in the stomach?

Answer 2

Smooth muscle modulates luminal pressure and tension

Primarily under control of ANS-‘involuntary’

Question 3

What does the smooth muscle look like?

Answer 3

Is present in sheets, bundles, or sheaths around tissues in the cardiovascular, respiratory, digestive, urinary, and reproductive system

Question 4

When smooth muscles diffuse into nerve fibers, what transmitters are they inducing?

Answer 4

ACh and NE

Membran potential ~-50 to -60mV

Question 5

What types of categories do smooth muscles have when they get excited?

Answer 5

1. Single/unitary (visceral

2. Multi-unit

Question 6

Where is the unitary smooth muscle cells found?

Answer 6

• Walls of the digestive tract, gallbladder, urinary bladder

Question 7

What is the function of the unitary smooth muscle cells?

Answer 7

• Autorhythmic. Adjacent cells
connect via gap junctions →
transmission of APs from one
fiber to another.

• Cell membranes adhere to one
another → transmission of force

• Slow synchronized “graded”
contraction.

• Entire sheet of muscle contracts - “functional syncytium”

Question 8

The structure of smooth muscle cells are composed of what?

Answer 8

• Sarcoplasmic reticulum is
poorly developed relative to
skeletal muscle.

• Thick filaments of SM have
actin-gripping heads along
their entire length.

• Thick and thin filaments are
arranged diagonally within
the cell contraction results
in twisting motion

Question 9

What are some of the special features of smooth muscle?

Answer 9

• Capable of sustained contraction without fatigue and at very little energy cost.

• Maintains a low level of tension or tone even in the
absence of action potentials.

Question 10

What response does smooth muscle give?

Answer 10

Stress-relaxation response

– Stretch causes initial increase in tension –> tension decreases within 1-2 minutes -> allows SM to change length but maintain ability to contract (important for storage organs)

Question 11

What is the length-tension relationship?

Answer 11

• smooth muscles operate over a wider range of resting
lengths

• generally broader length-tension relationship in smooth
muscle compared to skeletal muscle

Question 12

Name the major part of the stomach.

Answer 12

Cardia 
Fundus 
Body 
Pylorus 
Pyloric Sphincter

Question 13

What happens when the stomach is being filled?

Answer 13

Triggers secretions and motility

Question 14

Where does the stomach empty?

Answer 14

The stomach empties
slowly into the duodenum as small squirts of chyme
leave through the pyloric sphincter

Question 15

What cell types are in the stomach?

Answer 15

Surface mucous cell 
Muscous neck cell 
Parietal cell 
Chief cell 
G cell

Question 16

What does the surface and muscous neck cell secrete?

Answer 16

Surface mucous: secrets mucous, forms protective barriers that prevents digestion of stomach wall

Muscous neck cell are used for absorption, small quantity of water, ions, short-chain fatty acids, and some drugs enter bloodstream

Question 17

What do parital cell secrete?

Answer 17

Secretes hydrochloric acid (kills microbes of food) and intrinsic factor (needed for absorption of vitamin B12)

Question 18

What do chief cells secrete?

Answer 18

Secretes Pepsinogen (pepsin activated form-breaks down proteins into peptides) and gastic lipase (splits triglycerides into fatty acids and monoglycerides)

Question 19

What do G cells secrete?

Answer 19

Secretes the hormone gastrin (stimulates HCl to be absorb by parietal cells and chief cells to secrete pepsinogen, relaxes pyloric sphincter)

Question 20

What happens in the mechanical digestion of the stomach?

Answer 20

• Gentle mixing waves occur in the stomach to mix
the bolus of food with gastric juice (enzymes, acid,
water) and turn it into chyme (a thin liquid)

• More vigorous waves traveling from the body of stomach to the pyloric move the chyme along
• Intense waves near the pylorus lead to opening of the pyloric sphincter - squirting 1-2 teaspoons into the duodenum with each wave

Question 21

What happens in the chemical digestion of the stomach?

Answer 21

• Protein digestion begins in the stomach
– HCl denatures (unfolds) protein molecules
– HCl activates pepsinogen into pepsin - an enzyme
that breaks peptide bonds between certain amino
acids

• Fat digestion continues
– gastric lipase splits the triglycerides in milk fat,
although this is most effective at pH 5-6 (infant
stomach)

• HCl kills microbes in food
• Mucous cells secrete mucus to protect the stomach walls from being digested

Question 22

What nutrients does the stomach absorb?

Answer 22

• Water
• Electrolytes
• Some drugs (aspirin) & alcohol

• Fat content in the stomach
slows the passage of alcohol to
the intestine, where absorption
is more rapid

• Alcohol is absorbed more
slowly if taken with a meal

Question 23

What are phases of digestion?

Answer 23

1. Cephalic Phase
2. Gastric Phase
3. Intestinal Phase

Question 24

What is the role of the cephalic phase?

Answer 24

• Prepare the mouth and
stomach for food

• Cerebral cortex - sight,
smell, taste & thoughts of
food stimulate the
parasympathetic nervous
system to stimulate
--> Salivation (Facial and
Glossopharyngeal)
-->Gastric glands to secrete
gastric juice (Vagus)

Question 25

What is happening in the gastric phase?

Answer 25

• Neural influences over stomach activity – stretch receptors & chemoreceptors (pH) signal bolus entry – vigorous peristalsis and gastric gland secretions – chyme periodically released into the duodenum • Endocrine influences over stomach activity – distention and presence of food in stomach cause G cells to secrete gastrin into the bloodstream; gastrin increases gastric gland secretions and motility, and causes pyloric sphincter relaxation

Question 26

Once the gastrin is emptying, and as the bolus of food enters the stomach, what happens to the gastrin hormones?

Answer 26

``` distention of the stomach and presence of undigested contents increase the secretion of gastrin hormone & vagal nerve impulses ```

Question 27

What happens when the gastrin hormone is secreted?

Answer 27

``` stimulates contraction of the lower esophageal sphincter and the stomach, as well as relaxation of the pyloric sphincter for emptying ```

Question 28

What happens in the intestinal phase?

Answer 28

• Entry of chyme into duodenum slows gastric activity and increases intestinal activity • Neural influences: distension of the duodenum and chemical contents of the chyme activate sympathetic nerves, which slow gastric activity (enterogastric reflex) • Endocrine influences: distension of duodenum and contents of chyme trigger hormonal release from enteroendocrine cells in the duodenum – secretin hormone decreases stomach secretions – cholecystokinin (CCK) decreases stomach emptying

Question 29

What reflex regulates the amount of chyme released into the duodenum?

Answer 29

Enterogastric reflux

Question 30

How is the enterogastric reflux initiated?

Answer 30

• initiated by distension of duodenum & contents of the chyme they also cause cholecystokinin and secretin release from the duodenum and stimulate sympathetic impulses, both of which inhibit gastric emptying

Question 31

So in short terms what does the cephalic phase do?

Answer 31

Anticipation

Question 32

In short terms, what does the gastric phase do?

Answer 32

Breakdown

Question 33

In short terms, what does the intestinal phase do?

Answer 33

Release