7.6 GI Motility Flashcards
Describe what surrounds the gastrointestinal tract lumen
The gastrointestinal tract lumen is surrounded by the mucosa which is the innermost layer, responsible for absorption and secretion. Followed by the submucosa containing blood vessels, nerves and lymphatics. Then coming the muscularis externa with 2 layers, the inner circular muscle that constricts the lumen and the outer longitudinal muscle that shortens the GI tract. Then comes the serosa which is the outermost protective layer. Additionally there is the mesentery which is a fold of membrane that attaches the intestine to the abdominal wall
What are the 4 motility actions of the smooth muscle in the GI wall
Peristalsis, segmentation, migrating motor complex and reservoir
Describe peristalsis
Peristalsis acts to propel the bolus (food in the GI) in an aboral direction (away from the mouth). It does this via contraction of circular muscle behind the bolus and relaxation of the circular muscle ahead of the bolus whilst relaxing the longitudinal muscle behind the bolus and contracting ahead of it. This peristalsis can also help to mix the food. The peristalsis can be initiated by distension, chemical irritation of the epithelium or parasympathetic stimulation and it is coordinated by the enteric nervous system
Describe segmentation
This is the alternating contractions of circular smooth muscle that occurs after a meal has been ingested, to create segments in the GI tract. This helps to mix the content of the lumen so that mechanical and chemical digestion can occur. It does not cause net forward movement of the lumen content
Describe the migrating motor complex
It is a specialised type of peristalsis that occurs in a fasted state. Undigested material, bacteria, sloughed cells and residual secretions are swept towards the colon. The sphincters are generally open. It is regulated via motilin and the autonomic nervous system
Describe reservoir
This is the relaxation of muscles with the contraction of sphincters to provide storage in the stomach and large intestine. There are 3 types of relaxation it can be. Receptive relaxation which is triggered by swallowing and controlled by the vasovagal reflex. Adaptive relaxation which is triggered by stretch and controlled by the vasovagal reflex. Feedback relaxation which is triggered by the presence of nutrients in the small intestine and controlled by hormones.
What are the neuronal systems controlling gastrointestinal motility
The parasympathetic nervous system and the enteric nervous system
What are some hormonal mechanisms controlling gastrointestinal motility
Motility can be promoted by motilin and ghrelin and can be inhibited by GIP and GLP-1
Describe the regional motility patterns in the stomach
In the proximal region of stomach, there is the reservoir function which is mediated by the autonomic nervous system.
In the distal region of the stomach the muscle is thicker and there is segmentation.
In the pyloric sphincter, gastric emptying is controlled and varied based on the composition of the meal, there is feedback from the small intestine via CCK and the vasovagal reflex.
Describe the regional motility patterns in the small intestine
Mostly segmentation and peristalsis which are regulated by nervous system. When lipids reach the ileum, peptide YY is released to slow down motility and allow for complete digestion.
Describe the regional motility patterns in the large intestine
In the proximal section of the large intestine there are segmentation. Mass peristalsis occurs around 3 times a day, it is stimulated by stomach distension and colonic irritation, this fills the rectum and creates the urge to defecate. Large intestine motility is mostly controlled by the enteric nervous system.
Describe defecation in relation to the GI
Defecation can vary from 3 times a day to 3 times a week. It can be caused by a spinal reflex where a stretch in the rectum causes relaxation of the internal anal sphincter causing faeces to pass into the anus, this is involuntary. The brainstem’s supraspinal defecation center can either speed up or slow down this reflex, this is voluntary. The final passage of the faeces involves contracting the rectal muscle which is helped by the valsalva maneuver
Describe vomiting in relation to the GI
Vomiting is caused by antiperistalsis in the oesophagus, stomach and intestines, this is stimulated by excessive irritation, distension or excitation of the GI tract. Vagal and sympathetic afferents are sent to the vomiting centre, autonomic afferents are sent to the GI muscle and somatic afferents are sent to the diaphragm and abdominal muscles.
What are some additional factors that influence GI motility
diet- e.g the effects of dietary fibre
microbiome- microbial composition can affect transit time
drugs- e.g prokinetics and smooth muscle relaxants
age- constipation more likely in old age
physical activity- increased physical activity increases colonic transit speed
