Mechanisms of GI tract motility and its control Flashcards
What type of muscle is involved in gut motility?
Mainly smooth muscle, but skeletal muscle is present at a few critical points (e.g., upper esophagus, external anal sphincter).
What part of the nervous system controls gut motility?
The enteric nervous system (ENS), with some extrinsic control by the autonomic nervous system (ANS).
How do endocrine influences affect gut motility?
Hormones link the presence of food in the gut to increased motility.
What is the role of sphincters and valves in gut motility?
They regulate the timing of movement and ensure one-way traffic of food through the gastrointestinal tract.
What is another name for the myenteric plexus?
Auerbach’s plexus.
What is the main function of the myenteric (Auerbach’s) plexus?
Coordination of muscle contraction in the gastrointestinal tract.
How is the myenteric plexus influenced?
It is influenced by autonomic nervous system (ANS) supply.
What is another name for the submucosal plexus?
Meissner’s plexus.
What is the primary role of the submucosal (Meissner’s) plexus?
Sensory functions and local responses to stimulation of sensory nerve endings in the mucosa.
What is the enteric nervous system (ENS)?
A largely independent nervous system that controls the gastrointestinal tract.
What are the 3 types of neurons in the ENS?
Motor neurons, interneurons, and sensory neurons.
What are the functions of motor neurons in the ENS?
What is the role of interneurons in the ENS?
They coordinate reflexes within the gastrointestinal system.
What are the functions of sensory neurons in the ENS?
How is the ENS regulated?
It operates independently but is influenced by overall regulation by the autonomic nervous system (ANS).
Does the ENS have contributions from somatic motor neurons?
Yes, in a few specific regions like the upper esophagus.
What is required to ensure one-way traffic in the GI tract?
Valves or sphincters between adjacent segments of the GI tract.
Name the 5 sphincters involved in one-way traffic in the GI tract.
Upper oesophageal sphincter
Lower oesophageal sphincter
Pyloric sphincter
Ileo-caecal sphincter
Anal sphincters (internal and external)
What happens in the oral phase of swallowing?
What happens in the pharyngeal phase of swallowing?
What happens in the oesophageal phase of swallowing?
What type of muscle is found in the upper third of the oesophagus?
Striated muscle.
What type of muscle is found in the lower third of the oesophagus?
Smooth muscle.
What is the overlap zone in the oesophagus?
A transition area where both striated and smooth muscle are present.
What controls the striated muscle in the oesophagus?
Somatic motor neurons from the brainstem, which generate activity patterns for contraction.
What controls the smooth muscle in the oesophagus?
Autonomic nervous system (ANS, parasympathetic) and neurons of the enteric nervous system (ENS).
Which nerve is involved in oesophageal muscle control?
The vagus nerve.
What is peristalsis?
A series of wave-like muscle contractions that move food (bolus) through the digestive tract.
What happens at time zero during peristalsis?
- The bolus is in place.
- The muscle behind the bolus contracts.
- The receiving segment relaxes, allowing movement forward.
What happens seconds later during peristalsis?
- The contracting region moves forward, pushing the bolus ahead.
- The next receiving segment relaxes, continuing the motion.
What is the main function of peristalsis?
To propel food forward through the gastrointestinal tract.
What forms the lower oesophageal sphincter?
A functional sphincter formed by tonic contraction of circular smooth muscle in the wall of the abdominal oesophagus.
When does the lower oesophageal sphincter relax?
It relaxes only during swallowing and vomiting.
How does the angle at which the oesophagus enters the stomach help prevent reflux?
It allows intragastric pressure to close the end of the oesophagus by squashing one wall.
How does the diaphragm contribute to reflux prevention?
The surrounding diaphragm restricts the orifice diameter and contracts during inspiration and when intra-abdominal pressure rises.
How much can the stomach’s capacity increase when food arrives?
It increases from 50 mL to 1.5 L.
What happens to the internal rugae when the stomach expands?
The internal rugae flatten to accommodate food.
How does the stomach wall respond to food arrival?
The muscle wall relaxes in a process called receptive relaxation, which occurs by reflex action (brainstem, PNS).
What is the function of stomach contractions?
They mix the stomach contents and propel food toward the pyloric region.
What initiates gastric contractile waves?
A pacemaker in the stomach wall, which sets off contractile waves at a rate of 3 per minute.
How do contractile waves change as they move toward the pyloric zone?
The waves get stronger as they approach the pyloric zone.
How does pressure in the pyloric antrum affect gastric emptying?
Increased pressure in the pyloric antrum squirts liquid chyme through the narrow sphincter.
What happens to larger food lumps that do not pass through the pyloric sphincter?
Due to the narrow sphincter and incomplete occlusion of the pyloric antrum, larger lumps are regurgitated back into the antrum for further mixing.
What are the 5 major factors in the duodenum that inhibit gastric emptying?
Increased acidity
Increased fat content
Increased amino acids
Hypertonicity
Distension
What is the main type of motility in the small intestine?
Segmentation, which is responsible for mixing.
How does segmentation occur in the small intestine?
Simultaneous contractions of different segments result in mixing without net forward movement.
What happens once absorption is complete?
The Migrating Myoelectric Complex (MMC) takes over to move remaining contents forward.
What is the overall transit rate in the small intestine?
Slow, approximately 2 hours.
Why is peristalsis less frequent in the small intestine?
To allow time for digestion and absorption.
What role does the small pressure gradient play?
The pressure is higher in the proximal small intestine than in the distal part.
This helps move chyme forward.
What structure controls the passage of contents from the small to large intestine?
The ileocaecal valve and sphincter.
What is the function of the ileocaecal valve?
It prevents reflux of contents back into the ileum.
How does the smooth muscle sphincter behave in response to pressure?
Relaxes in response to upstream pressure (from the ileum).
Constricts in response to downstream pressure (from the colon) and sympathetic stimulation.
What additional factor helps maintain one-way movement?
A small pressure gradient assists the forward flow of contents.
What are haustra and what is their function?
Haustrations are short segmental contractions of the circular and longitudinal muscle in the large intestine, responsible for mixing contents.
What are mass movements and their role in large intestine motility?
Mass movements are bulk contractions of longer segments of circular muscle, occurring approximately once every 30 minutes, to move contents forward.
How does the consistency of gut contents change through the large intestine?
Due to progressive reabsorption of water, the contents transition from:
Fluid (ascending colon)
Semi-fluid
Mush (transverse colon)
Semi-mush
Soft solid (descending colon)
Solid (rectum)
How is the exit of gut contents controlled?
By two sphincters:
Internal anal sphincter
External anal sphincter
What does defecation involve?
Co-ordination of contraction and relaxation of both:
Skeletal muscle (under voluntary control).
Smooth muscle (under parasympathetic control).
What are the two main types of muscle in the rectum?
Rectal circular muscle
Longitudinal muscle
What are the two sphincters that control defecation?
Internal anal sphincter
External anal sphincter
What is the role of the levator ani muscle in defecation?
It supports the pelvic floor and helps maintain continence.
What is the function of the anal canal?
It acts as the final passage for feces before expulsion.
What triggers the desire to defecate?
Filling of the rectum is detected by afferent nerves, leading to the urge to defecate.
What happens to the muscles in the rectum and anal canal when the urge to defecate occurs?
Rectal circular muscle contracts.
Internal anal sphincter relaxes (both under parasympathetic control).
External anal sphincter remains contracted for voluntary control.
What happens when defecation is socially acceptable?
External anal sphincter relaxes.
Levator ani contracts.
Rectal smooth muscle contracts.
Feces are expelled.
Which muscles are under voluntary control during defecation?
External anal sphincter
Levator ani
Which muscles are under involuntary (parasympathetic) control during defecation?
Rectal circular muscle
Internal anal sphincter
How does the autonomic nervous system (ANS) regulate gut motility?
Parasympathetic (PS) stimulation → Increases motility
Sympathetic (S) stimulation → Decreases motility
Are there exceptions to the general ANS regulation of gut motility?
Yes, a few specific exceptions exist, mainly affecting sphincter muscles.
What hormone regulates motility in the inter-digestive period (fasting period between meals)?
Motilin, which regulates background motility via the migrating contractile activity that starts in the stomach and propagates along the small intestine (SI).
What hormone initially increases motility in the post-prandial period (when digesting a meal)?
Gastrin, which increases gastric motility.
What hormones decrease gastric motility as digestion proceeds?
CCK (Cholecystokinin)
GIP (Gastric Inhibitory Peptide) & GLP-1 (Glucagon-Like Peptide-1)
What are the three extrinsic factors that can affect gastrointestinal motility?
Diet composition
Medication
Age
How does diet composition affect gastrointestinal motility?
It influences water retention in gut contents, affecting the consistency of stools (hard/soft).
It influences microbiota in the gut lumen, which may directly affect local enteric nervous system (ENS) neurons.
How can medication impact gut motility?
Prescribed, over-the-counter, and other medications can alter gut motility.
How does age affect gastrointestinal motility?
ENS neuron degeneration in the elderly can lead to reduced gut motility.
What are the 2 consequences of regurgitation of acidic gut contents?
Erosion of teeth
Damage to the oesophagus and larynx
What are some causes of regurgitation of acidic gut contents?
Diaphragmatic hernia
Obesity
Bulimia
Stroke (affects swallowing and upper oesophageal motility)
Side effects of some medications
How can gut motility issues affect nutrient absorption?
Malabsorption of foodstuffs, including vitamins and minerals.
How can gut motility issues affect the gut microbiota?
Bacterial overgrowth within the intestine, leading to a change in gut microbiota balance.
What condition can result in dehydration and acid-base problems?
Diarrhoea.
What condition can cause discomfort and dangerous straining?
Constipation.
What is the Valsalva manoeuvre?
A forced expiration against a closed glottis, leading to large changes in blood pressure (BP) and heart rate (HR).
