GI physiology and motility

Question 1

What are the basic principles of regulation for GI function?

Answer 1

Conditions in intestinal lumen are regulated
Receptors in the wall of GI tract: stretch receptors, chemo receptors, osmoreceptors
Effectors are smooth muscle and glands- reflexes stimulated by receptors stimulate smooth muscle contraction and gland secretion
Nervous and hormonal regulation of GI function

Question 2

How is the nervous system regulated in GI function?

Answer 2

Sympathetic and parasympathetic nervous system act via the neurons in the enteric nervous system
The ENS is made of the submucosa plexus which regulates secretion and the myenteric plexus which regulates motility. It is involved in local reflexes that are self contained but also is modulated by CNS

Question 3

Regulation of GI function in the CNS

Answer 3

Function 1: integration of responses to external stimuli (senses, emotions)
Function 2: response to internal stimuli (GI lumen contents), long reflex path
Sympathetic: reflexes that generally inhibit GI function (noradrenaline)
Parasympathetic: reflexes that generally activate GI function (acetylcholine)

Question 4

Regulation of GI function - ENS

Answer 4

Function: response to internal stimuli (GI lumen contents)
Receptors: detect conditions in the GI tract (stretch and chemical composition)
ENS response: occurs via short local reflex pathways, acts on GI tract smooth muscle and epithelia/glands

Question 5

Hormonal regulation in the GI tract

Answer 5

Sensor cells are called enteroendocrine cells which have receptors for chemical, pH, stretch and osmolarity and also secrete hormones that can target cells at very far distances. They can also have a paracrine affect which means they affect neighbouring cells. Critical hormones found in the GI tract are gastrin, gastric inhibitory peptide, secretin and cholecystokinin

Question 6

Definition of tonic and phasic contractions

Answer 6

Tonic contractions: sustained contractions for minutes to hours (e.g. sphincters)
Phasic contractions: waves of contraction and relaxation, each wave lasting seconds (e.g. peristalsis)

Question 7

Definition of motility and movement/propulsion

Answer 7

Motility = any pattern of contraction or relaxation of GI tract smooth muscle, some are a combination of two or more simpler patterns
Movement/propulsion = motility patterns that specifically move/propel GI contents along the tract

Question 8

Basis of GI motility

Answer 8

Smooth muscle, spontaneously active meaning it contracts without any external input as it contains pacemaker cells
Frequency of contraction is a property of the GI region, stomach 3 per min, speeds up in duodenum 12 per min so it can mix with digestive enzymes and slows in Ileum 9 per min for absorption

Question 9

Fasting vs Feeding

Answer 9

Fasting:
When you haven’t eaten for four hours. Migrating motor complex which is a repeating pattern that helps to clear undigested material, repeats every 2hr until eat again
Feeding:
Storage in stomach and colon, relaxation of smooth muscles allows volume to increase without change in pressure
Propulsion/movement occurs in most of the tract, peristalsis
Mixing occurs in the stomach (retropulsion) and small and large intestine (segmentation)

Question 10

What is Peristalsis?

Answer 10

Bolus of food arrives in digestive system, circular muscles contract behind bolus, longitudinal muscles ahead of bolus contract and contraction in circular muscle layer forces bolus forwards

Question 11

What is Segmentation?

Answer 11

Circular muscles pinching and alternating segments to mix everything with digestive enzymes and expose everything with digestive surfaces

Question 12

What is chewing?

Answer 12

Chewing is a voluntary movement, skeletal muscle, functions to reduce size of food for swallowing and mixes food with salvia. Reflex element to chewing where how hard and how frequent our chewing is based on a reflex based on the consistency of our food

Question 13

What is swallowing?

Answer 13

Swallowing begins when tongue pushes bolus to back of mouth which induces stretch receptors to open oropharynx inducing a reflex. Rapid transfer of material from mouth to stomach once the epiglottis closes

Question 14

Functions of gastric motility

Answer 14

Storage happens in the fundus
Mechanical digestion and mixing occurs in the antrum
Controlled delivery to duodenum occurs in the pyloric sphincter, controlled so enzyme and food ratio is right for digestion

Question 15

Fasting in gastric motility

Answer 15

Stomach shrinks to ~50ml in volume
Migrating motor complex- inactive, uncoordinated activity and strong coordinated activity until eat something
Housekeeping: removes residual secretion and undigested material and promotes epithelial cell turn over

Question 16

Storage in feeding gastric motility

Answer 16

Stomach muscle walls relax, as more volume comes in, relaxing occurs hence, minimal change in pressure occurs to ensure chyme doesn’t reflux back into the oesophagus
Storage is allowed due to receptive relaxation and gastric accommodation

Question 17

Propulsion in feeding gastric motility

Answer 17

Propulsion and movement in the stomach is peristalsis. It is initiated at the greater curvature and spread to the antrum, there are 3 contractions per min , first 60 min following meal is gentle, 60-300 min after is more intense

Question 18

Mixing and mechanical breakdown in feeding gastric motility

Answer 18

When pyloric sphincter is shut, peristalsis contractions become stronger and retropulsion occurs causing mechanical and mixing breakdown

Question 19

Gastric emptying process

Answer 19

Regulated by feedback from duodenum. If lots of fatty, hypertonic, acidic chyme duodenal cells secrete enterogastrones. Chemoreceptors and stretch receptors will also trigger entergastric reflex that increase sympathetic activity and decrease parasympathetic activity. Both of these processes reduces the force of contraction and reduces rate of stomach emptying

Question 20

Small intestine motility

Answer 20

Between meals the small intestine has a migrating motor complex, after meals its main motility pattern is segmentation for mixing and exposure to absorptive surfaces with some peristalsis for propulsion

Question 21

Large intestine motility

Answer 21

Large intestine is where feces are stored, it has exposure to absorptive surfaces and is where absorption of fluid and salt from feces occurs. Mass movement/propulsion happens 1-2 times a day following meals which drives feces into rectum and initiates defecation (peristaltic wave)