Lecture 25 Flashcards
What are the two types of contractions of the GI tract?
Tonic contractions
phasic contraction
What are phasic contractions?
This is when there is contractions and relaxations in seconds
What are tonic contractions?
These are sustained contractions that last minutes to hours
Where do phasic contractions occur?
oesophagus, gastric antrum (stomach), small intestine
Where do tonic contractions occur?
lower oesophageal sphinter, orad stomach (near mouth), ileocecal and internal anal sphincters
What are the two main stages in the motility patterns in the GI tract?
- fasting state
- fed state
What is the purpose of the fasting motility pattern?
- clearing undigested material and secretions
- regulating intestinal microflora
What is the purpose of the fed motility pattern?
- Storage
- Movement at a controlled rate
- Mixing
- Exposure to absorptive surfaces
What is the fasting motility complex called?
The migrating motor complex
Describe the migrating motor complex
This starts 4-5 hours post meal absorption and it is 2 hours in duration from the stomach to the end of the large intestine
What are the three phases of the MMC?
- intense
- inactive
- intermittent
What is the function of the MMC?
- clears undigested material and secretions
- regulates intestinal microflora
- epithelial cell turnover
Describe the hormonal regulation of the MMC?
hormonal - motilin released by intestinal M-cells (M-cells trigger sensory neurons which feed into the ENS and have an effect on the motor neurons)
neuronal - motilin stimulates both the ENS and autonomic NS
What are the three base motility patterns in the fed state?
- storage function
- peristalsis
- segmentation
Describe the storage function the fed state
This is mainly in the stomach and colon. It involves the relaxation of smooth muscle which allows the volume of luminal contents to increase without changes in pressure
Describe peristalsis function of the fed state
This is mainly in the lower oesophagus and stomach, small and large intestine. It is propulsive but also mixing as part of retropulsion in the stomach
Describe the segmentation function of the fed state
This is in the small and large intestine which is for mixing and exposure to absorptive surfaces
What are the two types of storage in the stomach?
- receptive relaxation
- accommodation
Describe receptive relaxation storage in the stomach
An event, (e.g. swallowing in the case of the stomach) triggers reduced muscle tone as content is moved along the tract (less resistance as content arrives). The relaxation of the stomach means that the pressure in the stomach is lower that in the oesophagus. This allows food to enter and not be pushed back.
Describe accommodation for storage in the stomach
This is progessive relaxation in response to a volume change
Describe peristalsis
This is when the ascending circular muscle contracts behind the bolus and the ascending circular muscle relaxes ahead of the bolus. The longitudinal muscle is shortening (contracting). It can also be combines with other motility patterns to produce complex patterns
Describe segmentation
This is when the circular muscle contracts in alternating segments. This is for mixing.
There is pinching and then a wave of peristalsis. The one inner circular layer is contracting rhythmically.
What is the major motility pattern in the small intestine?
Segmentation
The GI tract smooth muscles act as a what? What does this mean?
syncytium
This means that the cells contract together
What are the smooth muscles of the GI system interconnected with?
The are interconnected via gap junctions and adherens junctions
The muscle cells are arranged in ___________ with a ________ ________ in between them. Describe this
layers
neural plexus
There is the circular (inner) and longitudinal (outer) intestinal smooth muscle sandwiching neural components of the myenteric plexus
What is the dimension of the intestinal smooth muscle cells?
5 – 20 μM diameter and ~ 500 μM long
Individual muscle cells interact with how many other cells? How do they do this?
Around 10 surrounding cells via gap junctions and adherens junctions