Lecture 24: Introduction and Motility Flashcards
What are two functions of the GI system?
- obtain nutrients required for growth and energy needs
2. replace fluids and salt lost in sweating, breathing, urination
What is chemical digestion?
extracting nutrients to be made available for absorption
What is absorption?
selecting material we want and leaving behind waste
The purpose of regulation of the gastrointestinal function is to
regulate the conditions in the intestinal lumen to change composition of luminal components to create optimal conditions for absorption and digestion
The regulation of conditions in the intestinal lumen is carried out by
receptors in the GI tract
What two things do receptors in the walls of the GI tract respond to?
stretch (due to food in the lumen) and change in composition
What things could change when the composition changes?
pH, osmolarity, amino acids, sugars, fats
The receptors in the GI tact respond to changes (such as stretch or changes in composition) and innervate effectors. What are the effectors and what does innervation of them involve?
The effectors of them are smooth muscle and glands and innervation of them can cause smooth muscle contraction and gland secretion
How are the effectors (smooth muscle and glands) regulated?
through nervous and hormonal regulation
The central nervous system coordinates activity over ______ distances
long
The central nervous system consist of the parasympathetic nervous system and the sympathetic nervous system. Which of these stimulates motility and secretion and which one inhibits it?
The parasympathetic nervous system stimulates motility and secretion and the sympathetic nervous system inhibits motility and secretion
Does the central nervous system directly innervate the GI system?
No
instead, it modulates activity of the ENS which directly innervates the smooth muscle and glands
Give an example showing the importance of nervous regulation of the GI system
What does it come as a result of?
When food arrives in the stomach, we change motility in the small intestine so the food in the SI is moved along to the LI to make room for new food. This comes as a result of PNS activation and SNS inhibition
What two parts make up the ENS and what are these parts used for?
- submucosal plexus for regulation of secretion
- myenteric plexus for regulation of motility
The ENS is involved in _____ _______ over ______ distances
local reflexes
short
Give two examples of local reflexes controlled by the ENS
peristalsis and segmentation
The hormonal regulation of the GI system includes cells that have both ______ and ______ function
endocrine
paracrine
What is the role of endocrine cells and how does this differ to paracrine cells?
Endocrine cells secrete hormones into the blood whereas paracrine cells secrete hormones into the interstitial fluid
What are four critical hormones in the GI tract?
gastrin
gastric inhibitory peptide (GIP)
secretin
cholecystokinin (CCK)
What is the role of gastrin?
control of secretion in the stomach
Motility in the GI system involves
moving it through the system and mixing it up
What are the four functions of the motility in the GI tract?
- movement at a controlled rate
- mechanical digestion
- mixing
- exposure to absorptive surfaces
Why is it important that contents are moved through the system at an appropriate rate?
to aid chemical digestion and absorption
What two things contribute to motility and moving contents at an appropriate rate?
propulsion and storage
What is the importance of the mechanical digestion function of motility?
it increases surface area which is essential for chemical digestion
What is the importance of the mixing function of motility?
mixing with digestive enzymes to come in contact with a low pH and chemical composition so that the composition is optimal for digestion and absorption to occur
What is the importance of exposing contents to the absorptive surfaces function of motility?
it aids absorption
Motility is driven by _____ muscle (ie. ______ and ________ muscle)
smooth
circular and longitudinal
The smooth muscle involved in GI motility is _________ active
spontaneously
The smooth muscle involved in the GI motility is spontaneously active which means that it contract without an
external input
The frequency of contraction is a property of
each region
The strength of contraction of the smooth muscle for motility is regulated by
nervous and hormonal input
What is the generalised motility pattern that occurs whilst fasting?
migrating motor complex
Describe the migrating motor complex
- it occurs 4 hours after a meal and repeats every two hours until you eat again
- in the first hour, nothing happens
- in the next 50 mins, there is uncoordinated activity
- in the next 10 mins, there is coordinated activity
What is the housekeeping activity that occurs during the migrating motor complex?
the sweeping of undigested food and secretions down the tract from the SI to the LI
What are the three generalised motility patterns that occur during feeding?
- storage
- propulsion
- mixing
Storage occurs in which organs?
stomach and colon
What does storage involve?
relaxation of smooth muscle which allows the volume to increase without there being a change in pressure
Propulsion occurs in which organs?
oesophagus, stomach, small and large intestine
Propulsion occurs due to which action?
Peristalsis
Mixing occurs in which organs?
Stomach and small and large intestines
Mixing in the stomach occurs due to which action?
retropulsion
Mixing in the small and large intestine occurs due to which action?
segmentation
What is peristalsis?
a motility pattern described by a wave of contractions behind the food to propel it forwards
What is segmentation?
Alternative regions of relaxation and contraction to move the food forwards and backwards to mix it
What is the advantage of segmentation causing mixing?
it allows the food to collide and mix with excretions so that once digestion occurs, the products are exposed to absorptive surfaces
What are two specific motility patterns in the mouth/oesophagus?
Chewing
Swallowing
What is the purpose of chewing and what are the advantages of this?
To reduce the size of the food
which allows for ingestion and allows it to mix with saliva
Is chewing voluntary or involuntary? Because of this, what sort of muscle is involved?
Voluntary so skeletal muscle is used
What is swallowing?
the rapid transfer of material from mouth to stomach
Is swallowing voluntary?
It is initiated at will but it proceeds reflexly
What are the four functions of gastric motility in the stomach?
- storage
- Mechanical digestion
- mixing
- controlled delivery to duodenum
Where is food stored in the stomach and what is the purpose of this?
In the fundus and body of the stomach because the small intestine takes longer to digest the food so it is held in the stomach
Which part of the stomach is involved in mechanical digestion?
the antrum
Which part of the stomach is involved in mixing?
antrum
Which part of the stomach is involved in controlling the delivery to the duodenum?
the pyloric sphinctor
Why do we need to control the delivery of food to the duodenum?
To give the small intestine enough time to digest and absorb all the nutrients.
During feeding, there is gastric motility in the stomach. What are the two motility patterns of this process?
Storage and peristalsis
What are the two reflexes which are involved with storage in the stomach?
- receptive regulation
2. gastric accomodation
Describe the process of receptive regulation in the stomach
Receptors monitor the stretch in the oesophagus and this information is sent via the vagus nerve to the CNS, and then efferent information is sent to the smooth muscle to relax the stomach
Describe the process of gastric accomodation in the stomach
Receptors monitor the stretch of the fundus in the stomach and this information is sent via the vagus nerve to the CNS. Efferent information is then sent down to inhibit the activity of the ENS to relax the smooth muscle without directly inhibiting the smooth muscle cells
In the stomach, peristalsis is initiated on the _____ _____ and then it spreads to the _______
greater curvature
antrum
Peristalsis in the stomach is involved in
retropulsion
Describe the process of retropulsion
the pyloric sphincter is closed so when peristalsis forces the food against the closed sphincter, it forces it to go back through the constriction which breaks the food up into smaller pieces
forcing the food back into the stomach also collides with the new food to encourage mixing
What is controlling the rate of gastric emptying into the small intestine from the stomach?
The duodenum
Describe the reflex of gastric emptying
As food enters the small intestine, the receptors in the duodenum respond to the stretch and the presence of fatty, hypertonic, acidic chyme. They respond by stimulating duodenal endocrine cells which secrete hormones which reduce contractile forces of the stomach and the rate of stomach emptying. They also respond by activating chemoreceptors and stretch receptors which trigger a short and long reflex.
Describe the short and long reflex which controls gastric emptying
the short reflex occurs via enteric neurons and the long reflex occurs via the CNS which increases sympathetic activity and decreases parasympathetic activity
both of these together decrease contractile force of stomach and decrease the rate of stomach emptying
What are the functions of motility in the small intestine?
- mixing food with secretion from the pancreas, biliary system and intestine
- controlled movement
- exposure of products of digestion to absorptive surfaces
What are the motility patterns of the small intestine whilst fasting and after a meal?
Fasting:
migrating motor complex
After a meal:
segmentation and limited peristalsis
What is the purpose of the segmentation in the small intestine?
for mixing and exposure to absorptive surfaces
What are the motility patterns of the large intestine?
- segmentation
- storage of faeces
- peristalisis
What is the purpose of segmentation in the large intestine?
- exposure to absorptive surfaces
- absorption of fluid and salt from faeces
What is the purpose of peristalsis in the large intestine?
it drives the faeces into the rectum and initiates defecation
Motility is a property of ____ _________
smooth muscle
What are the four functions of motility?
- movement at a controlled rate
- mechanical digestion
- mixing
- exposure of absorptive surfaces
What are the two type of motility patterns?
peristalsis
segmentation
