Lecture 24: Intro GI physiology

Question 1

What does mechanical digestion do and where does it take place?

Answer 1

breaks the food into smaller and smaller pieces

Occurs in:
Mouth = chewing
Stomach = retropulsion

Question 2

What does chemical digestion do? How is this achieved?

Answer 2

breaks the large nutrient molecules into smaller and smaller pieces which can be absorbed

How?
By acid and enzymes

Question 3

What are the two main types of receptors in the GI and what do they respond to?

Answer 3

Mechanoreceptors are activated by stretch:
◼ Food/chyme in the lumen

Chemoreceptors are activated by changes in composition:
◼ △pH, △osmolarity, amino acids, sugars, fats

Question 4

How do regulatory systems change the activity of the GIT effectors?

Answer 4

◼ Smooth muscle: change in motility

◼ Glands/secretory cells: change in amount or type of secretion

Question 5

What two plexuses make up the Enteric nervous system (ENS) What are their roles and can the ENS function independently?

Answer 5

Submucosal plexus
▪ Regulation of secretion

Myenteric plexus
▪ Regulation of motility

YES it can function independently
▪ Totally self-contained
▪ Can function independently of the CNS

Question 6

How does the parasympathetic and sympathetic branches of the Central Nervous Sytstem (CNS) differ in regards to GIT?

Answer 6

Parasympathetic NS: “rest and digest”
▪ Increases the overall activity of the GIT by activating the ENS
▪ Stimulates motility and secretion

Sympathetic NS: “ fight, flight or freeze”
▪ Decreases the overall activity of GIT by inhibiting the ENS
▪ Inhibits motility and secretion

Question 7

What are the two reflex pathways for neural regulation?

Answer 7

▪ Local/short reflex pathway: ENS only

▪ CNS & Long reflex pathway: CNS → ENS & ENS → CNS → ENS

Question 8

How does nervous system regulation differ from hormonal regulation?

Answer 8

Nervous system regulation:
▪ Generates fast responses

Hormonal regulation:
▪ Slower, but longer lasting & can affect multiple parts of the GIT at once

Question 9

What is the name for endocrine cells for the GIT and what do they have/do?

Answer 9

enteroendocrine cells:
▪ Epithelial cells with receptors
▪ Release hormones

Question 10

Can we combine neural (nervous system) and hormonal regulation for digestion and absorption?

Answer 10

YES definitely

Combination of neural and hormonal regulation:
▪ Responses that start quickly but last longer
▪ Can coordinate responses across different parts of the GIT

Question 11

What is the local/short reflex pathway? Include (function, stimulus, receptors, response)

Answer 11

Function:
To return the lumen of the GIT back to its resting state.

Stimulus:
A change in the contents of the GIT lumen due to the arrival or departure of food/chyme.

Receptors:
Detect changes in local conditions, such as stretch and chemical composition, within the GIT.

ENS Response (Enteric Nervous System):
Occurs via short, local reflex pathways. Acts on GI tract smooth muscle and GI tract epithelia/glands to restore the lumen to its resting state.

Question 12

What are the two functions of the CNS/long reflex pathway?

Answer 12

Function 1:
to integrate the response of the GIT to external stimuli (smell) and/or changes in the rest of the body (stress/danger) eg. sympathetic vs parasympathetic responses.

Function 2:
to return the lumen of the GIT back to its resting state (long reflex pathway)

Question 13

What are some stimulus for the CNS/long reflex pathway?

Answer 13

• Sight, smell or tasting food
• Emotional responses
• Change in GI lumen contents

Question 14

In neural regulation of the GI tract by the Central Nervous System (CNS): (select the correct answer)
1. The long reflex pathway generates a response to internal stimuli (GI lumen contents).
2. The sympathetic NS usually stimulates GI function.
3. The parasympathetic NS usually inhibits GI function.
4. The Enteric nervous system is not involved in the response.

Answer 14

1

Question 15

What are tonic contractions?

Answer 15

▪Tonic contractions:
sustained contractions for minutes to hours e.g. sphincters

Question 16

What are phasic contractions?

Answer 16

▪Phasic contractions: waves of contraction and relaxation, each wave lasting seconds
▪ e.g. peristalsis, segmentation

Question 17

What is a motility pattern?

Answer 17

▪Motility pattern: any pattern of contraction or relaxation of GI tract smooth muscle.

Some motility patterns are a combination of two or more simpler patterns
▪ e.g. retropulsion: peristalsis and contraction of the pyloric sphincter

Relaxation is also a motility pattern
▪ e.g. storage in the stomach: receptive relaxation and accommodation

Question 18

What is movement/propulsion in regards to the GIT?

Answer 18

▪Movement / Propulsion: motility patterns that specifically move contents along the GIT

Question 19

What type of muscle undergoes phasic contractions and what are they controlled by?

Answer 19

▪Smooth muscle

▪Controlled by pacemaker cells - Spontaneously contacts and relaxes without external input

Question 20

Can the frequency of phasic contractions change?

Answer 20

NO

Frequency of contraction is a property of the region of the GIT
▪ Stomach 3 contractions per min
▪ Duodenum 12 contractions per min
▪ Ileum 9 contractions per min

DOES NOT CHANGE

Question 21

How can we modify phasic contractions?

Answer 21

We cannot change the frequency of phasic contractions HOWEVER we CAN change the strength of these contractions - this is especially important in the fed state

Question 22

What are the five ‘fed’ motility patterns?

Answer 22

CPRSC (Chewy Pork Really Sucks Cock)
▪ Chewing:
mechanical digestion of food

▪ Peristalsis:
movement of food/chyme along the GIT

▪ Relaxation:
storage of food/chyme by increasing the volume without an increase in pressure

▪ Segmentation:
mixing of chyme with secretions & exposure to absorptive surfaces

▪ Contraction or relaxation of sphincters:
prevents backwards movement of chyme/food & controls rate of forward movement of food/chyme

Question 23

What is the main ‘fasting’ motility pattern and what is its function?

Answer 23

▪Migrating motor complex (MMC):

Location:
Begins in the stomach and travels to the small intestine

Functions: “House keeping”
▪ Removes residual secretions
▪ Removes undigested material
▪ Promotes epithelial cell turnover

Question 24

When does the Migrating motor complex begin and what are the periods of activity?

Answer 24

▪ Occurs 4 h after a meal
▪ begins with 45-60 min of inactivity
▪ Then has 30 min intermittent uncoordinated activity
▪ Then has 5-15 min intense coordinated peristaltic contractions
▪ Repeats every 1.5-2 hours until more food is consumed

Question 25

What is the motility pattern in the mouth?

Answer 25

Process:
Chewing (Mastication) is the initial step of mechanical digestion.

Function:
Reduces the size of food particles to allow for ingestion and swallowing.

Control:
- Voluntary process involving skeletal muscle.
- Reflex control regulates the strength, frequency, and rhythm of chewing.

Mixes food with saliva, which aids in:
- Taste perception
- Lubrication of food for easier swallowing
- Protection of oral tissues

Question 26

What are the three motility patterns involving the esophagus?

Answer 26

Swallowing
▪ Rapid transfer of material from mouth to stomach
▪ Initiated under voluntary control
▪ Continues via reflexes

Relaxation of sphincters:
▪ Location: upper and lower oesophageal sphincters open
▪ Function: allows food to move freely down oesophagus
into the stomach

Peristalsis
▪ Location: oesophagus
▪ Function: Wave of peristalsis moves food down
oesophagus to the stomach

Question 27

What are the three main motility patterns in the stomach?

Answer 27

▪ Relaxation:
▪ Locations: fundus and body of stomach
▪ Function: storage
▪ Prevents reflux of food/chyme into the oesophagus

Retropulsion: peristalsis & contraction of pyloric sphincter
▪ Locations: body → pyloric antrum and pyloric sphincter
▪ Functions: mechanical digestion and mixing

Contraction & relaxation of pyloric sphincter
▪ Location: pyloric sphincter between stomach and SI
▪ Functions: Gastric emptying and controlled delivery of
chyme to the duodenum

Question 28

What are the two types of stomach relaxation?

Answer 28

Receptive relaxation:
▪ part of swallowing, the stomach is about to receive food
and relaxes

Accommodation:
▪ food is in the stomach (has arrived) and the stomach
need to find space/accommodation for it

Question 29

What is retropulsion in the stomach and what does it do to the food?

Answer 29

waves of peristaltic contractions grind the food particles against the closed pyloric sphincter

▪ the chyme is pushed backwards (retropulsion) from the pyloric antrum to the body of the stomach

Peristalsis
▪ Contractions begin in the body of the stomach and spread to antrum
▪ 3 contractions per min
▪ First 60 min following meal gentle, 60 - 300 min more intense activity

▪ Completion of mechanical digestion: physical break
down of food particles into smaller particles, resulting
in liquid “pureed” chyme

▪ Mixing: of food/chyme with stomach acid and enzymes
to help chemical digestion

Question 30

Is the pyloric sphincter closed in during gentle and or intense contractions?

Answer 30

▪ Closed during gentle contractions

▪ During intense contractions starts to partially relax, allowing small spurts of liquid chyme into the duodenum

Question 31

What is gastric emptying? Why is the rate of this process important?

Answer 31

controlled movement of fully mechanically digested chyme from the stomach into the duodenum

The rate of gastric emptying is adjusted to match the
digestive capacity of intestines - at the correct rate to match with release of bicarbonate, fluid, bile and digestive enzymes

Question 32

The rate of gastric emptying can be sped up or slowed
down, using feedback from the stomach and the
duodenum. What are some important factors that effect this rate?

Answer 32

Size of meal:
▪ the larger the meal, the greater the stretch/distension, and faster the emptying

Composition of meal:
▪ Fluids faster than solids:
▪ solids require reduction in the size of food particles to increase the surface area of the food - mechanical digestion (takes more time)

Fats, acid or a change in the osmolarity in the duodenum:
▪ Fats: because they take longer to digest & require bile
▪ Acid: need to neutralize by releasing the correct amount of bicarbonate (HCO3)

Question 33

What are the three main motility patterns of the small intestine?

Answer 33

Segmentation (main pattern):
▪ Location: throughout small intestine
▪ Functions:
- Mixing
- Exposure to absorptive surfaces
- Some movement

Peristalsis (propulsion):
▪ Location: throughout small intestine
▪ Function: movement of chyme
- (WEAK PERISTALSIS WHEN ABSORBING, mainly happens after most nutrients have been absorbed)

Contraction & relaxation of ileocecal sphincter
▪ Location: ileocecal sphincter between the small and large intestines
▪ Functions: delivery of chyme from the small intestine to the large intestine

Question 34

What are the three main motility patterns of the large intestine?

Answer 34

Large periods of inactivity

Segmentation:
▪ Location: throughout large intestine
▪ Function: Exposure to absorptive surfaces
▪ Absorption of water and Na+ from chyme turning liquid
chyme into semi-solid faeces

Relaxation:
▪ Location: descending and sigmoid colon
▪ Function: Storage of faeces

Peristalsis (propulsion):
▪ Peristaltic wave
▪ Mass movement of faeces 1-2 time a day following
meals
▪ Drives faeces into rectum and initiates defecation

Question 35

The motility pattern __________________ occurs in the
__________________
1. segmentation; stomach
2. relaxation; oesophagus
3. contraction of the pyloric sphincter; large intestine
4. retropulsion; stomach

Answer 35

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