GI physiology- motility

Question 1

Describe smooth muscle and GI motility

Answer 1

GI motility depends on smooth muscle contractions.
 GI motility functions to mix and propel food down the GI tract.
 Motility determines the transit time in different GI regions:
o Oesophagus: solid food takes 7-10 seconds; liquid takes 1-2 seconds due
to peristalsis and gravity causing a quick movement of food through the
oesophagus.
o Stomach: liquid is only retained for 40 minutes in the stomach; food is
retained for 2-4 hours.
o Small intestine: 3-5 hours transit time is faster despite it being longer
due to muscle contractions.
o Large intestine: 12-24 hours.

Question 2

What is the structure of the GI tract wall

Answer 2

Muscularis mucosa
 Muscularis externa: made up of circular smooth muscle and
longitudinal smooth muscle.
o The circular smooth muscle is thicker than the
longitudinal smooth muscle.
o Circular smooth muscle is thicker at certain regions,
forming a sphincter.

Question 3

How is GI motility controlled (by both nerves and hormones)

Answer 3

GI motility is dependent on the interactions between:
o Smooth muscle
o Autonomic nervous system/enteric nervous system
 Parasympathetic:
 Dominant during the digestive phase.
 Excitatory enteric fibres (acetylcholine).
 Inhibitory enteric fibres (VIP (vaso-inhibitory peptide), NO).
 Sympathetic:
 Inhibitory effects (noradrenaline).
o Hormones:
 Gastrin, CCK, motilin, glucagon like peptides.

Question 4

What are the characteristics of GI motility

Answer 4

Spindle like cells connected by tight junctions.
 Activation of a cell causes propagation to neighbouring cells.
 Smooth muscle cells act as a functional syncytium, allowing information
to be passed along, allowing for syncytial conduction.

Question 5

What is the basal electrical rhythm/slow wave rhythm in the GI tract?

Answer 5

Also termed basal electrical rhythm.
 Generated by cells of Cajal.
 Frequency depends on the region of the GI tract:
o Stomach: 3 contractions per minute.
o Duodenum: 12 contractions per minute.
o Ileum: 8 contractions per minute.
o Colon: 3 contractions per minute.
 Propagation of slow waves generates segmental and peristaltic contractions.

Question 6

Describe the swallowing reflex

Answer 6

Food is pushed using the tongue onto the
hard palate. This has lots of receptors which
initiates the involuntary response.
 The receptors are activated, causing the
larynx to life up, closing the epiglottis,
causing food to move down the oesophagus
and not into the trachea.
 The food then moves down the oesophagus
and is propelled by peristaltic waves and
gravity.

Question 7

Describe the ‘normal swallow’

Answer 7

Deglutination initiated by swallowing
centres in the medulla.
 Peristaltic waves pass down the oesophagus
and last 7-10 seconds.
 Lower oesophageal sphincter allows food to pass into the stomach.

Question 8

What is Achalasia

Answer 8

Failure of the lower oesophageal
sphincter to relax.
 Food sticks on its way down, and
it is more difficult to swallow
food in comparison to liquid.
 There is also damage to the
muscle of the oesophagus,
preventing the oesophagus from
being able to produce peristalsis.
 Unknown cause, although it could be hereditary or caused by
inflammation.
 Treatment: physical distension of the LES or surgical removal of the
LES.
 Diagnosed using a barium swallow showing distension (bird’s beak formation) of the oesophagus.

Question 9

What are the motor functions of the stomach

Answer 9

Receives food
 Stores food (volume empty 50 ml, full 2 l (extreme uncomfortable feeling)).
 Breaks up food and mixes it with gastric juice to form chyme.
 Delivers chyme at a controlled rate into the duodenum

Question 10

What forms the proximal motor unit

Answer 10

The stomach is made of three regions: fundus, body, antrum.
 This can be further divided by function: the fundus and body
forms the proximal motor unit. This acts as a temporary reservoir
for food. It is aided by the shape of the stomach, with rugae,
allowing to stomach to expand. Receptive relaxation is where the
muscle fibres can stretch without increasing their tone. This is
initiated by vago-vagal inhibitory reflex. Stretching of muscle
without increasing tone, means that there can be a minimal
increase in intra-gastric pressure. When empty, the pressure is 5
mmHg. Ingesting a meal of up to 1 L is allowed before this will
change, and stretching causes the uncomfortable feeling (increase in intra-gastric pressure).

Question 11

What is the distal motor unit

Answer 11

Comprised of the antrum and pyloric sphincter.
 Functions to propel chyme, mix chyme with gastric secretions and regulate
gastric emptying.
 Thicker muscular walls. About 3 contractions per unit.
 Retropulsion is where the food contents are moved back into the more
proximal regions of the stomach, further mixing and churning the food.

Question 12

How do hunger pangs occur

Answer 12

Occurs after 8 hours of the stomach being empty. The stomach can hold food
for up to 4 hours, so hunger pangs occur 12 hours after ingestion.
 Rhythmic peristaltic contractions
 Contractions may merge and last 2-3 minutes.
 Maximum intensity after fasting for 3-4 days

Question 13

How does gastric emptying depend on the composition of the meal

Answer 13

 Liquid meals: quickly released form the stomach. Hypertonic liquid released
much slower than an isotonic liquid. This is because of osmoreceptors in the duodenum. A hypertonic liquid in the
duodenum would cause water release (osmotic diarrhoea) and potential hypovolaemia.
 Lag phase: breaking up of food particles in the distal motor part of the stomach (solid and semi-solid) meals.
 Components of solid meals (mainly fats) are detected by enteroendocrine cells which release CCK, which feeds back to slow
gastric emptying

Question 14

How is gastric emptying regulated

Answer 14

Distension of the stomach and presence of partially digested food, alcohol or caffeine increases parasympathetic
innervation, and increases secretion of gastrin.
 Contraction of the lower oesophageal sphincter increases stomach motility. Relaxation of the pyloric sphincter allows for
gastric emptying.
 Gastrin stimulates contraction of the atrium which promotes mixing of chyme, however, it also inhibits gastric emptying by
relaxing the pyloric sphincter, causing it to constrict.

1.2. Intestinal phase (inhibition)
 Distension of the duodenum, presence of fatty acids, glucose and partially digested proteins or hypotonic solution initiates
feedback mechanisms: increased secretion of CCK and initiation of enterogastric reflex.
 This decreases stomach motility and inhibits gastric emptying.

Question 15

How does the vomiting reflex occur

Answer 15

1. Stimulus to vomiting centre in the midbrain.
2. Hypersalivation (protect epithelial lining), pallor,
   sweating, tachycardia and retching.
3. Glottis closes. Soft palate rises to close off airway
   (prevents swallowing of contents into trachea).
4. Stomach and pyloric sphincter relax.
5. Reverse peristalsis
6. Diaphragm and abdominal wall contracts.
7. Increased pressure forces chyme upwards from stomach out of mouth.

Question 16

What are the components of intestinal motility

Answer 16

1. Small intestinal motility aids digestion and absorption
    The muscularis externa generates movements for segmentation and
   peristalsis.
    Contraction of the muscularis mucosa serves to agitate fluid around the villi.
2. Segmentation
    Principally in the small intestine:
   o Duodenum: 12 contractions per minute (strongest).
   o Jejunum and ileum: 8 contractions per minute.
    Contraction of circular smooth muscle facilitates mixing.
    Last 5-6 s and occur throughout the digestive period.
    No net forward movement.
3. Peristalsis
    Propulsive movements throughout the intestine:
   o Duodenum: 11-12 contractions per minute.
   o Colon: 3-4 contractions per minute.
    Waves of contraction and relaxation of circular and longitudinal smooth
   muscle facilitates propulsion.
    Net forward movement of chyme at a rate of 1 cm per minute.
4. Control of segmentation and peristalsis
    Both movements are:
   o Initiated by distention.
   o Require enteric nervous pathways.
   o Influenced by the autonomic nervous system: increased by parasympathetic, reduced by sympathetic.
   o Influenced by some GI and other hormones.

Question 17

What is the migrating motor complex (non digestive role of intestinal motility)

Answer 17

Occurs during the interdigestive phase. Triggers peristaltic waves which facilitate transportation of indigestible substances
(bone, fibre, foreign bodies) from the stomach, through the small intestine, past the ileocecal sphincter and into the colon.
 Initiated every 70-90 minutes.
 Takes 80-120 minutes for one activity.
 It is also a method for transporting bacteria from the small intestine, and prevents overgrowth of microorganisms within the
small intestine.
 Motilin ↑ MMC.
 CCK and gastrin ↓ MMC. This dampens movement when there is food to digest.

Question 18

What is the ileal brake reflex

Answer 18

Prevents unabsorbed nutrients from entering the colon, otherwise it would change the microbiome structure.
 Unabsorbed nutrients in the lower ileum delays: gastric emptying, jejunal
transit and suppresses hunger.
 Involves the hormone GLP-1 (glucagon like peptide 1) and peptide YY (PYY).

Question 19

Describe the movement of chyme into the caecum

Answer 19

Controlled by the ileocecal sphincter and is under feedback control (like
gastric emptying).
 As food is delivered into the lower ileum, distension and chemical irritation
enhances peristalsis and relaxes the sphincter, allowing emptying of food
from the small intestine to the large intestine.
 Fluidity of chyme promotes emptying.

Question 20

Describe the movements of the colon

Answer 20

Normally sluggish (5-10 cm per hour).
 Long transit time (12-24 hours).
 Two types of movement:
o Mixing (haustrations)
o Propulsive (peristalsis and mass movements)
 Mass movements: usually initiated by food intake, gastrocolic reflex and moves contents from the sigmoid colon to the
rectum.

Question 21

What are colonic haustrations

Answer 21

Similar to segmentation, but more marked.
 Bulging of large intestine into bag like sacs as a result of circular and longitudinal muscle
contractions.
 Dependent on interaction between the ENS and smooth muscle

Question 22

Describe the defecation reflex

Answer 22

Reflex occurs in response to sudden distension of the rectal wall.
 Involves voluntary and involuntary process.
 Mass movements forces chyme into the sigmoid colon which initiates stretch receptors.
 These signal to the central nervous system
to initiate the defecation reflex.
 Both parasympathetic and sympathetic
nerve activity:
o Parasympathetic: causes
contraction of the rectum.
o Sympathetic: causes relaxation of
the internal sphincter.
 Final part of the defecation reflex is
voluntary control which originates in the
cerebral cortex. This allows relaxation of
external anal sphincter and contraction of
abdominal muscles, allowing the passing of
faeces through the anal canal.