Regulation of the GI tract - Motility Flashcards
Why do we need motility within the GI tract?
- to propel ingesta along the tract
- retain ingesta at a given site for digestion and absorption or storage
- physically break up food material and mis it with digestive secretions
- circulate ingesta so that it comes into contact with absorptive surfaces
How is motility achieved in the GI tract?
- peristalsis
- mechanical digestion
- mixing
What is peristalsis?
- Series of wave-like muscle contractions that move food through the tract
What is mechanical digestion?
- involuntary contractions and relaxations of muscles in the tract helps to break down material
What are the 3 regulations of motility in the GI tract?
- neural
- hormonal
- myogenic
What parts of the nervous system are responsible for motility?
- autonomic nervous system ( sympathetic and parasympathetic)
- enteric nervous system (this uses the myenteric and submucosal plexus)
What 3 types of hormone are responsible for motility in the GI tract?
- classical hormones (endocrine)
- local hormones (paracrine)
- neurohormone (neurocrine)
What is myogenic?
- intrinsic rhythm of GI musculature
How are endocrine substances transported?
- Classical hormones are carried by the blood from the endocrine cells to target cells
How are paracrine substances transported?
- Local hormones diffuse through interstitial fluid and act on target cells in the intermediate vicinity of the secretory cell
How are nuerocrine substances transported?
- Neurohormones are released from neurons and transported by blood to target cells
How is the parasympathetic nervous system involved in the motility of the GI tract?
- provided by the vagus nerve down to the level of the transverse colon, and pelvic nerves to remainder of the tract
- activation usually stimulates motor and sensory activity of GIT
- rest and digest response
How is the sympathetic nervous system involved in the motility of the GI tract?
- via postganglionic adrenergic fibres which terminate in the myenteric and submucosal plexuses
- activation usually inhibits motor and secretory activity of the GIT
- fight or flight response
What are interneurons? (enteric NS)
- interneurons are nerves that connect the myenteric plexus and the submucosal plexus
What is the role of interneurons? (enteric NS)
- interneurons within the plexus connect afferent neurons with efferent neurons to smooth muscle (to form reflex arcs within the wall of the GI tract)
What do axons from neurons in the plexus innervate? (enteric NS)
- smooth muscle cells in the muscularis external 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 gastrointestinal 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 CNS
- chemoreceptors and mechanoreceptors send signals to CNS via splanchnic and vagal afferents
- CNS interacts with submucosal and myenteric plexus vis sympathetic and parasympathetic pathways (efferent nerves)
- these efferent also send signals to the muscularis externa, 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?
*(some say good girls can go very hormonal)
- Gastrin
- Cholecystokinin
- secretin
- Gastric inhibitory peptide
(all endocrine action) - Vasoactive intestinal peptide
- gastrin releasing peptide
(both neuroncrine action) - histamine
- serotonin
(both paracrine action)
Where are the interstitial cells of cajal located? (intrinsic myogenic rhythm)
- within the smooth muscle of the GI tract
What is the function of the interstitial cells of Cajal? (intrinsic myogenic rhythm)
- they exhibit repetitive, spontaneous, slow oscillations in membrane potentials to make the basic electrical rhythm
Why are well coupled electrically, smooth muscle cells important in intrinsic myogenic rhythm?
- it means waves of depolarisation will spread throughout a particular gut segment
For intrinsic myogenic rhythm when the amplitude of the slow wave reaches a threshold what happens?
- when a slow waves reaches a threshold level an action potential occurs leading to contraction of the smooth muscle cells
