Physiology- Motility of the GI tract Flashcards
Describe the role of sphincters and what can happen during their dysfunction
They seperate the different structures of the GI tract by ensuring food is delivered at the right time, right composition and remain in compartments for the right duration.
Made from smooth muscle
Dysregulation results in achalsia, GERD, gastroparesis/dumping syndrome. Also peptic ulser and diarrhoea.
The GI tract is innervated via the intrinsic pathway and extrinsic pathway. Describe the intrinsic pathway.
Myenteric plexus controls gut motility- innervating the longitiudinal and circular muscle of the muscularis externa.
Submucosal plexus controls absorption and secretion via innervation of glandular epithelium, intestinal endocrine cells and submucosal blood vessels.
The GI tract is innervated via the intrinsic pathway and extrinsic pathway. Describe the extrinsic pathway.
Link via sensory and motor nerves oF:
PNS- Efferents induce intestinal motility and exocrine secretion, afferents determine pain, satiety, nausea and distention.
SNS- Intestinal motility inhibtion
Which nerve plexuses arise from preganglionic vagus?
Which nerve plexuses arise from the pelvic nerve?
Oesophageal, stomach, liver, pancreas, small intestine, caecum, ascending colon, appendix, transverse colon
The rest of colon (descending colon, sigmoid) via hypogastric plexus
Where do the sympathetic preganglionic fibres arise from?
Which postganglionic bodies arise from it?
T8-L2
Inferior and superior mesenteric ganglia
Describe Hirschsprings disease?
Congenital lack of mesenteric plexus
Lack of peristalsis, undergoes continuous spasms -> functional obstruction and severe constipation
Outline the behaviour of smooth muscle lining the GI tract. Include how they are depoarised.
Act as a functional syncythium
Exhibit pacemaker activity in slow waves (slow waves are spontaneous first arising in distal stomach) which can lead to spike potential if peak of slow wave >threshold. Upstroke due to Ca2+ influx.
Basic electrical rhythm determines when contractions can occur in a certain area of a bowel.
The number of spikes fired within each wave is determined by neural and hormonal input. This in turn determines the force of the contraction.
THE MAXIMUM FREQUENCY OF A RHYTHMIC CONTRACTION IN A GIVEN REGION IS SET BY THE LOCAL RATE OF SLOW WAVE GENERATION.
Describe, with examples, excitation transmitters and inhibitory transmitters.
Excitatory: Open non-selective cation channels in smooth muscle. Vm-> depolarised and more slow waves reach threshold-> spike potential. eg. Ach
Inhibitory: Open K+ channels which hyperpolarises the Vm so it becomes harder for slow waves to reach threshold. e.g. Anticholinergic drugs
State the side effects of anticholinergic drugs
e.g. amitriptyline
Dry mouth (Xerostoma)
Constipation
Nausea/vomiting
Ileus
Describe the three types of GI movement
- SEGMENTATION- for mixing food with enzymes (mainly in SI). Closely spaced contractions of circular smooth muscle followed by relaxation
- TONIC CONTRACTION- for seperation as in sphincter.
PERISTALSIS- longitudinal m. contraction followed half way through by circular m. contraction. Longitudinal m. relaxes half way through circular m. contraction. Distension of gut by food triggers it.NET FORWARD MOVEMENT.
Describe the migrating motor complex (the sweeper)
Intervals of strong propulsive contractions every 90 minnutes between meals that pass from distal stomach-> SI
Sweep stomach and SI of indigestible material (e.g. fibre)
Independent of external innervation
Property remains in pathology, only disturbed by radiotherapy. Loss causes overgrowth of gut microbiome.
What is paralytic ileus?
Temporary cessation of gut motility that is most common after abdominal surgery
Other causes include: infection, inflammation, hypokalaemia, narcotics.
Causes nausea/vomiting, distention, absent bowel sounds
Consider movement along the GI tract. Outline what happens during swallowing.
Food bolus formed by mastication
Propelled to oropharynx as tongue elevates and goes back against hard palate.
Involuntary reflux swallowing
Stimulation of mechanoreceptors in pharynx
Efferent impulses from vagus to pharyx, oesophagus and palate for coordinated muscle contraction.
Soft palate elevates and nasopharynx contracts
Respiration is inhibited
Larynx rises so epiglottis covers trachea
Upper eosophageal sphincter relaxes and bolus enters
Consider movement along the GI tract. Outline what happens to food in the oesophagus.
What dysfunction can occur here?
Upper oesophageal sphincter relaxes
PERISTALSIS
Lowe oesophageal sphincter and proximal stomach relax
Top 1/3 of oesophagus is skeletal muscle. Rest is smooth muscle. Lacks pacemakers
Achalasia (charactrised by dysphagia) as lower sphincter fails to relax
GERD- incompetent lower oesopaheal spincter. Distal oesopahgus becomes inflammed and ulcerated.
Consider movement along the GI tract. Outline what happens to food in the stomach.
Pacemaker cells regulate gastric peristalsis (located in mid region of greater curvature)
- Storage of food as rate of ingestion>digestion
- MIXING
- Deliver chyme to intestine at optimal rate
Increase in stomach pressure triggers reflux and dumping.
Receptive relaxation mediated by vagus as part of end of swallowing reflex.
As pyloric sphincter transiently opens, some chyme enters duodenum but most returns under pressure retropulsion.
How is motility in the stomach regulated?
Activation of mechanoreceptors due to stomach distenstion
Gastin release in response to food in stomach
Consider movement along the GI tract. Outline what happens during stomach emptying
State two dysfunctions
Pyloric antrum has thickened muscle wall (induced by increased chyme) and its sphincter controls exit.
Liquids leave first, lag, solids (increases time for mechanical digestion)
Small intestine can only accept small amounts of chyme/time. Therefore hormones secreted to inhibit gastric emptying.
Rate of emptying proportional to volume, pH, physical and chemical nature.
ENTEROGASTIC REFLUX inhibits emptying and consists of complex neuronal and hormonal signals.
Dumping syndrome- rapid dumping
Gastroparesis- insufficient dumping
Which factors decrease gastric emptying
Fatty acids and unemulsfiied monoglycerides in duodenum or low pH promote release of secretin, GIP and CCK leading to decreased gastric emptying
CNS decreases parasympathetic input and increases sympathetic input leading to decreased gastric emptying
Gastric chyme -> acid, fat,hyper/hypo-tonic solution -> receptors -> leading to decreased gastric emptying
Consider movement along the GI tract. Outline what happens in the SI
Function 1: mixing by segmentation contraction
Multiple short contractions along gradient from proximal to distal SI
Decrease in BER frequencty promotes the distal movements of intestinal chyme.
Function 2: peristalsis
Short range (approx 10cm), stimulated extrinsically and intrinsically
Vilus movements mix and drain lymphatics
Describe dysfunction of the small intestines.
.
Dysfunction occurs due to harsh environment causes by low pH, enzymes and antigens from food and bacteria. Defense by intestinal mucin, epithelial cell permeability barrier and peristalsis.
Impairment of peristalsis increases entero-bacteria which can cause diarrhoea and steattorhea. This is called intestinal blind loop syndrome.
Consider movement along the GI tract. Outline what happens in the large intestine.
Recieves 2.5L/day converting it to 100-200g of poo
Movements are slow and regular for the purpose of increasing contact with the absorbing surface.
Lacks longitudinal muscle- so no pacemaker (instead, 3 thick bands = taeniae coli)
Hausteration shuffling and segmental contractions cause bolus to squeeze and roll to expose fecal material for reabsorption and secretion of mucus for lubrication.
Also produce propulsive movements
Mass movement involves simultaneous segmental contraction of left and right colon causing content to be moved distally = GASTRIC COLIC REFLEX
This occurs all day at intervals
Consider movement along the GI tract. Outline what happens in the rectum
When stretch receptors become distended -> defecation reflex
Sphincter 1- internal anal (involuntary)
Sphincter 2- external anal (voluntary)- skeletal m.
Afferent stimulation leads to parasympathetic signal to relax internal sphincter.
If voluntary relaxation of 2 doesn’t occur via pudendal nerve, reverse peristalsis returns to colon.
