Physiology- Motility of the GI tract

Question 1

Describe the role of sphincters and what can happen during their dysfunction

Answer 1

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.

Question 2

The GI tract is innervated via the intrinsic pathway and extrinsic pathway. Describe the intrinsic pathway.

Answer 2

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.

Question 3

The GI tract is innervated via the intrinsic pathway and extrinsic pathway. Describe the extrinsic pathway.

Answer 3

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

Question 4

Which nerve plexuses arise from preganglionic vagus?

Which nerve plexuses arise from the pelvic nerve?

Answer 4

Oesophageal, stomach, liver, pancreas, small intestine, caecum, ascending colon, appendix, transverse colon

The rest of colon (descending colon, sigmoid) via hypogastric plexus

Question 5

Where do the sympathetic preganglionic fibres arise from?

Which postganglionic bodies arise from it?

Answer 5

T8-L2

Inferior and superior mesenteric ganglia

Question 6

Describe Hirschsprings disease?

Answer 6

Congenital lack of mesenteric plexus

Lack of peristalsis, undergoes continuous spasms -> functional obstruction and severe constipation

Question 7

Outline the behaviour of smooth muscle lining the GI tract. Include how they are depoarised.

Answer 7

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.

Question 8

Describe, with examples, excitation transmitters and inhibitory transmitters.

Answer 8

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

Question 9

State the side effects of anticholinergic drugs

Answer 9

e.g. amitriptyline

Dry mouth (Xerostoma)
Constipation
Nausea/vomiting
Ileus

Question 10

Describe the three types of GI movement

Answer 10

1. SEGMENTATION- for mixing food with enzymes (mainly in SI). Closely spaced contractions of circular smooth muscle followed by relaxation
2. 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.

Question 11

Describe the migrating motor complex (the sweeper)

Answer 11

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.

Question 12

What is paralytic ileus?

Answer 12

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

Question 13

Consider movement along the GI tract. Outline what happens during swallowing.

Answer 13

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

Question 14

Consider movement along the GI tract. Outline what happens to food in the oesophagus.

What dysfunction can occur here?

Answer 14

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.

Question 15

Consider movement along the GI tract. Outline what happens to food in the stomach.

Answer 15

Pacemaker cells regulate gastric peristalsis (located in mid region of greater curvature)

1. Storage of food as rate of ingestion>digestion
2. MIXING
3. 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.

Question 16

How is motility in the stomach regulated?

Answer 16

Activation of mechanoreceptors due to stomach distenstion

Gastin release in response to food in stomach

Question 17

Consider movement along the GI tract. Outline what happens during stomach emptying

State two dysfunctions

Answer 17

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

Question 18

Which factors decrease gastric emptying

Answer 18

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

Question 19

Consider movement along the GI tract. Outline what happens in the SI

Answer 19

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

Question 20

Describe dysfunction of the small intestines.

.

Answer 20

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.

Question 21

Consider movement along the GI tract. Outline what happens in the large intestine.

Answer 21

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

Question 22

Consider movement along the GI tract. Outline what happens in the rectum

Answer 22

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.