6. GI tract motility- Motility of the stomach, small and large intestine

Question 1

What is are the major functions of gastric motility? (3)

Answer 1

1. Allows stomach to act as a reservoir for the large volume of food ingested at a single meal
2. Breaks food into smaller particles and mixes with gastric secretions
3. Empties gastric contents into duodenum at controlled rate

Question 2

Smooth muscle of the stomach:
3 layers?
Change in thickness?

Answer 2

3 layers:
Outer= Longitudinal
Middle= Circular
Inner= Oblique

Muscle wall thickness increases from proximal to distal

Question 3

Innervation of the stomach:
Innervation from extrinsic nerves?
Enteric nervous system?
Sensory afferent fibres?

Answer 3

Rich innervation from extrinsic nerves
– Parasympathetic: Stimulate gastric smooth muscle motility and secretions
– Sympathetic: Inhibit motility and secretions

Enteric nervous system
–Myenteric plexus:
• Parasympathetic innervation via the vagus
• Sympathetic innervation via the coeliac ganglion

Sensory afferent fibres
–Between sensory receptors and the ENS (pressure, distension, pH, pain) and centrally via the vagal and splanchnic nerves

Question 4

Discuss the receptive relaxation of the stomach and functions

Answer 4

The oral region has a thin muscular wall
Distension of the lower oesophagus induces relaxation of the lower oesophageal sphincter and the oral region of the stomach
Reduces pressure and increases volume of the stomach

Question 5

What is the structure of the afferent and efferent information in the vasovagal reflex?

Answer 5

Afferent and efferent nerve fibres in the vagus
Afferent information: Mechanoreceptors associated with chewing, oesophageal and stomach distension relay information to CNS via sensory neurons
Efferent information from the CNS causes oral relaxation. The neurotransmitter, VIP (vasoactive intestinal peptide), released from postganglionic peptidergic vagal neurones is responsible for oral relaxation

Question 6

Which region of the stomach is responsible for mixing?

Answer 6

Thick muscular wall of the CAUDAD REGION is responsible for mixing.
Contraction waves begin in the middle of the body, move distally with increasing strength towards the pylorus

Fundus and body muscle layers are thin.

Question 7

What is the function of retropulsion in the stomach?

Answer 7

Propels gastric contents back for further mixing in the stomach

Question 8

Control of slow wave frequency in the stomach

Answer 8

3-5 per minute
Neural and hormonal input DO NOT AFFECT slow wave frequency but do affect action potential frequency

Parasympathetic stimulation, gastrin and motilin INCREASE action potential frequency (and force of contraction)

Sympathetic stimulation and secretin DECREASE action potential frequency

Question 9

What is the activity of the stomach during fasting?

Answer 9

Periodic gastric contractions (MMC’s)
MMC= Migrating Myoelectric Complexes
These are mediated by motion released from endocrine cells in the upper GI tract at 90 min intervals.

Function: Clears stomach of residue remaining from previous meal

Question 10

Why is gastric emptying rate regulated?

Answer 10

To ensure that gastric H+ is neutralised in the duodenum and there is adequate time for digestion and absorption of nutrients

Question 11

Physical factors affecting gastric emptying? 3

Answer 11

1. Liquids empty more rapidly than solids
2. Isotonic fluids empty more rapidly than hypo- or hypertonic fluids
3. Solids must be reduced to particles < 1 mm3 or less. Retropulsion continues until this is achieved

Question 12

Chemical factors inhibiting gastric emptying? 2

Answer 12

Presence of fat and H+ ions in the duodenum
Effect of fat: Mediated by cholecystokinin, secreted when fat reaches the duodenum
Effect of H+ ions: Mediated by reflexes in the enteric nervous system. H+ receptors in the duodenum detect low pH and relay information to the gastric smooth muscle via interneurons in the myenteric plexus

Question 13

3 functions of the motility of the small intestine

Answer 13

1. Mixes chyme with digestive enzymes and pancreatic secretions
2. Exposes nutrients to the intestinal mucosa for absorption
3. Propels unabsorbed chyme into large intestine

Question 14

Parasympathetic innervation of SI:
Nerve?
Action?
Neurotransmitter?

Answer 14

Nerve: Vagus
Action: Increases contraction
NT: ACh and motilin

Question 15

Sympathetic innervation of SI:
Nerve?
Action?
Neurotransmitter?

Answer 15

Nerve: Coeliac and superior mesenteric ganglia
Action: Decreases contraction
NT: Noradrenaline

Question 16

Slow waves in the small intestine:
Duodenum/ileum __ per min?
How? Why?

Answer 16

Duodenum: 12 per min
Ileum: 9 per min

MMC’s occur every 90 minutes to clear the SI of residual chyme

Question 17

What two forms contraction are coordinated by the ENS?

Answer 17

Segmentation for mixing

Peristalsis for forward movement:

• Orad contraction by ACh and substance P
• Caudad relaxation by VIP and NO

Question 18

Structure of muscles and innervation of the colon?

Answer 18

Longitudinal muscle concentrated in 3 bands – Taeniae(tenia)coli

PARASYMPATHETIC
Vagus nerve:
- Caecum, ascending colon and transverse colon
- Stimulation causes segmental contractions of the proximal colon
Pelvic nerves:
- Descending and sigmoid colon, rectum and anal canal
- Stimulation causes expulsive contractions of the distal colon

SYMPATHETIC
• Stimulation stops colonic movements

Question 19

Segmental contractions in caecum and proximal colon?

Answer 19

Contractions mix contents

Reverse peristalsis and segmental propulsion towards the caecum can occur.
[This retention favours Na+ and water absorption]

Question 20

Large intestinal motility

Answer 20

• Material not absorbed in the small intestine enters the large intestine - faeces destined for excretion
• After contents of of the small intestine enter the caecum and proximal colon, the ileoceacal sphincter contracts. Faecal material moves from the caecum, through the colon to the rectum and on to the anal canal
• Colon receives 500 - 1500 ml of chyme per day but most of the salt and water are absorbed (100 ml per day lost in faeces)
• Contractions associated with sac-like segments called haustra (haustrations)

Question 21

Mass movements in the colon due to..

Answer 21

Gastrocolin and duodenocolic reflexes

Question 22

Two nerves in inguinal canal

Answer 22

Ilio-inguinal nerve

Genito-femoral nerve

Question 23

Deep inguinal ring at…

Answer 23

Mid point of inguinal ligament

Question 24

Femoral pulse felt at….

Answer 24

mid inguinal point

Question 25

Gastrocolic reflex?

Answer 25

Distension of the stomach by food increases the motility of the colon and the frequency of mass movements in the large Intestine

Afferent limb in the stomach mediated by the parasympathetic nervous system.
The efferent limb of the reflex increasing colon motility is mediated by CCK and gastrin.

Question 26

Rectosphincteric reflex?

Answer 26

As rectum fills with faeces, the smooth muscle of the rectum contracts and the internal anal sphincter relaxes

Question 27

Muscle relaxation and contraction process of defecation?

Answer 27

External anal sphincter ( composed of striated muscle and under voluntary control) remains tonically contracted.

External anal sphincter is relaxed voluntarily, the smooth muscle of the rectum contracts and the pressure forces faeces through the anal canal.

Question 28

Vomiting:
Centre where?
Afferent information?
Efferent response?

Answer 28

Vomiting centre in the medulla

Afferent information:
• Vestibular system
• Back of throat
• GI tact
• Chemoreceptor trigger zone in the 4th ventricle

Efferent response:
• Reverse peristalsis in small intestine
• Relaxation of the stomach and pylorus
• Forced inspiration to increase abdominal pressure
• Relaxation of the lower oesophageal sphincter
• Forceful expulsion of gastric and duodenal contents