Lecture 24: Intro GI physiology Flashcards
What does mechanical digestion do and where does it take place?
breaks the food into smaller and smaller pieces
Occurs in:
Mouth = chewing
Stomach = retropulsion
What does chemical digestion do? How is this achieved?
breaks the large nutrient molecules into smaller and smaller pieces which can be absorbed
How?
By acid and enzymes
What are the two main types of receptors in the GI and what do they respond to?
Mechanoreceptors are activated by stretch:
◼ Food/chyme in the lumen
Chemoreceptors are activated by changes in composition:
◼ △pH, △osmolarity, amino acids, sugars, fats
How do regulatory systems change the activity of the GIT effectors?
◼ Smooth muscle: change in motility
◼ Glands/secretory cells: change in amount or type of secretion
What two plexuses make up the Enteric nervous system (ENS) What are their roles and can the ENS function independently?
Submucosal plexus
▪ Regulation of secretion
Myenteric plexus
▪ Regulation of motility
YES it can function independently
▪ Totally self-contained
▪ Can function independently of the CNS
How does the parasympathetic and sympathetic branches of the Central Nervous Sytstem (CNS) differ in regards to GIT?
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
What are the two reflex pathways for neural regulation?
▪ Local/short reflex pathway: ENS only
▪ CNS & Long reflex pathway: CNS → ENS & ENS → CNS → ENS
How does nervous system regulation differ from hormonal regulation?
Nervous system regulation:
▪ Generates fast responses
Hormonal regulation:
▪ Slower, but longer lasting & can affect multiple parts of the GIT at once
What is the name for endocrine cells for the GIT and what do they have/do?
enteroendocrine cells:
▪ Epithelial cells with receptors
▪ Release hormones
Can we combine neural (nervous system) and hormonal regulation for digestion and absorption?
YES definitely
Combination of neural and hormonal regulation:
▪ Responses that start quickly but last longer
▪ Can coordinate responses across different parts of the GIT
What is the local/short reflex pathway? Include (function, stimulus, receptors, response)
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.
What are the two functions of the CNS/long reflex pathway?
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)
What are some stimulus for the CNS/long reflex pathway?
- Sight, smell or tasting food
- Emotional responses
- Change in GI lumen contents
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.
1
What are tonic contractions?
▪Tonic contractions:
sustained contractions for minutes to hours e.g. sphincters
What are phasic contractions?
▪Phasic contractions: waves of contraction and relaxation, each wave lasting seconds
▪ e.g. peristalsis, segmentation
What is a motility pattern?
▪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
What is movement/propulsion in regards to the GIT?
▪Movement / Propulsion: motility patterns that specifically move contents along the GIT
What type of muscle undergoes phasic contractions and what are they controlled by?
▪Smooth muscle
▪Controlled by pacemaker cells - Spontaneously contacts and relaxes without external input
Can the frequency of phasic contractions change?
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
How can we modify phasic contractions?
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
What are the five ‘fed’ motility patterns?
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
What is the main ‘fasting’ motility pattern and what is its function?
▪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
When does the Migrating motor complex begin and what are the periods of activity?
▪ 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
