Gut motility Flashcards
Why do we need motility within the GI tract
- To propel ingesta along the tract
- To retain ingesta at a given site for digestion/ absorption/ storage
- To physically break up food material and mix it with digestive secretions
- To circulate ingesta so that it comes into contact with absorptive substances
How is motility achieved in the GI tract (3 ways)
1) Peristalsis
2) Mechanical digestion
3) Mixing
What does mechanical digestion mean?
Involuntary contractions and relaxations of muscles in the tract to help break down food.
What are the 3 regulations of motility in the GI tract?
1) Neural
2) Hormonal
3) Myogenic
What parts of the nervous system are responsible for motility (in a neural sense)
- ANS (inc sympathetic and parasympathetic)
- ENS (this uses the myenteric plexus and the submucosal plexus)
What 3 types of hormone are responsible for motility in the GI tract?
- Classical hormones (endocrine)
- Local hormones (paracrine)
- Neurohormone (neurocrine)
How are endocrine substances transported?
Classical hormones are carried by the blood from endocrine cells to target cells
How are paracrine substances transported?
Local hormones diffuse through interstitial fluid and act on target cells in the immediate vicinity of the secretory cell
How are neurocrine substances transported?
Neurohormones are released from neurones and transported by blood to target cells.
What does myogenic control of motility refer to?
The intrinsic/ natural rhythm of the GI muscularate
How is the parasympathetic nervous system involved in the motility of the GI tract?
- Relies on the vagus nerve down to the level of the transverse colon and then pelvic nerves to the rest of the tract
- Activation of parasympathetic nervous system usually stimulates activity of the GI tract
- Contributes to rest and digest response
How is the sympathetic nervous system involved in the motility of the GI tract?
- Relies on pathway via postganglionic adrenergic fibres which terminate in the submucosal and myenteric plexus
- Activation of the sympathetic nervous system inhibits activity of the the GI tract
- Contributes to fight/ flight response
What are interneurones?
Interneurones are nerves that connect the myenteric plexus and the submucosal plexus
What is the role of interneurones?
Interneurones within the plexus connect affect neurones with efferent neurones to smooth muscle (forming reflex arcs within the wall of the GI tract).
What do axons from neurones in the plexus innervate?
- Smooth muscle cells in the muscularis externa and muscularis mucosae
- Secretory cells in the mucosa and submucosa
- Intramural endocrine cells
- Blood vessels
What happens during the short reflex arc of the GI tract?
- Chemoreceptors and mechanoreceptors in the wall of the GI tract send signals to the myenteric plexus and submucosal plexus via local afferents
- The myenteric plexus and submucosal plexus send signals to the muscularis externa, the muscularis mucosae, endocrine cells, secretory cells and blood vessels via local efferents.
What happens during the long reflex arc of the GI tract?
- Taste and smell senses send signals to the CNS
- Chemoreceptors and mechanoreceptors send signals to CNS via splanchnic and vagal afferents
- CNS interacts with submucosal and myenteric plexus via sympathetic and parasympathetic pathways (efferent nerves).
- These efferents also send signals to the muscularis external, muscularis mucosae, endocrine cells, secretory cells and blood vessels.
How are endocrine cells stimulated within the GI tract?
- Stimulated by long + short reflex arcs
- Stimulated by composition of luminal contents
- Stimulated by stretch
What are the 8 different hormones that act in the GI tract?
1) Gastrin
2) Cholecystokinin
3) Secretin
4) Gastric Inhibitory Peptide
5) Vasoactive Intestinal peptide
6) Gastrin releasing peptide
7) Histamine
8) Serotonin
Some Say Good Girls Can Go Very Hormonal
1-4 = Endocrine. 5-6 = Neurocrine. 7-8 = Paracrine
Where are the interstitial cells of Cajal located?
Within the smooth muscle of the GI tract
What is the function of the interstitial cells of cajal?
They exhibit slow oscillations in membrane potentials to make the basic electrical rhythm of the GI tract
How does the oesophagus move food from the pharynx to the stomach?
Via peristalsis (specifically the primary peristaltic wave)
What is the secondary peristaltic wave?
A wave of peristalsis further down the oesophagus that is mediated by local reflexes originating in the enteric nervous system at the site of distention
What is the interdigestive motility complex?
Alternating periods of quiescence and peristaltic contractions
What is receptive relaxation and how is it controlled?
- Receptive relaxation = when smooth muscle in the proximal part of the stomach relaxes to allow food to enter
- Controlled/ regulated by the swallow centre as the vagus nerve carries information from the swallow centre to the stomach
How does muscle contraction occur in the stomach?
- Contraction begins in the fundus, narrowing the lumen and forcing food down into the pyloric region.
- Pyloric region contracts, some food goes down into the duodenum but most of it is circulated back up to the fundus region
What are the 2 types of muscle contraction in the small intestine?
1) Segmentation = mixing movements
2) Peristalsis = propulsive movements
What is the migrating motility complex?
Myoelectric waves that sweep through the intestine in a regular cycle during the interdigestive state
What are the 4 types of contraction possible in the large intestine?
1) Haustration (sacs which store and propel contents)
2) Peristalsis
3) Antiperistalsis (in opposite direction)
4) Mass movements (peristaltic contraction) moving from transverse colon to rectum
