Regulation of the GI tract - Motility

Question 1

Why do we need motility within the GI tract?

Answer 1

• to propel ingesta along the tract
• retain ingesta at a given site for digestion and absorption or storage
• physically break up food material and mis it with digestive secretions
• circulate ingesta so that it comes into contact with absorptive surfaces

Question 2

How is motility achieved in the GI tract?

Answer 2

• peristalsis
• mechanical digestion
• mixing

Question 3

What is peristalsis?

Answer 3

• Series of wave-like muscle contractions that move food through the tract

Question 4

What is mechanical digestion?

Answer 4

• involuntary contractions and relaxations of muscles in the tract helps to break down material

Question 5

What are the 3 regulations of motility in the GI tract?

Answer 5

• neural
• hormonal
• myogenic

Question 6

What parts of the nervous system are responsible for motility?

Answer 6

• autonomic nervous system ( sympathetic and parasympathetic)
• enteric nervous system (this uses the myenteric and submucosal plexus)

Question 7

What 3 types of hormone are responsible for motility in the GI tract?

Answer 7

• classical hormones (endocrine)
• local hormones (paracrine)
• neurohormone (neurocrine)

Question 8

What is myogenic?

Answer 8

• intrinsic rhythm of GI musculature

Question 9

How are endocrine substances transported?

Answer 9

• Classical hormones are carried by the blood from the endocrine cells to target cells

Question 10

How are paracrine substances transported?

Answer 10

• Local hormones diffuse through interstitial fluid and act on target cells in the intermediate vicinity of the secretory cell

Question 11

How are nuerocrine substances transported?

Answer 11

• Neurohormones are released from neurons and transported by blood to target cells

Question 12

How is the parasympathetic nervous system involved in the motility of the GI tract?

Answer 12

• provided by the vagus nerve down to the level of the transverse colon, and pelvic nerves to remainder of the tract
• activation usually stimulates motor and sensory activity of GIT
• rest and digest response

Question 13

How is the sympathetic nervous system involved in the motility of the GI tract?

Answer 13

• via postganglionic adrenergic fibres which terminate in the myenteric and submucosal plexuses
• activation usually inhibits motor and secretory activity of the GIT
• fight or flight response

Question 14

What are interneurons? (enteric NS)

Answer 14

• interneurons are nerves that connect the myenteric plexus and the submucosal plexus

Question 15

What is the role of interneurons? (enteric NS)

Answer 15

• interneurons within the plexus connect afferent neurons with efferent neurons to smooth muscle (to form reflex arcs within the wall of the GI tract)

Question 16

What do axons from neurons in the plexus innervate? (enteric NS)

Answer 16

• smooth muscle cells in the muscularis external and muscularis mucosae
• secretory cells in the mucosa and submucosa
• intramural endocrine cells
• blood vessels

Question 17

What happens during the short reflex arc of the GI tract?

Answer 17

• chemoreceptors and mechanoreceptors in the wall of the gastrointestinal tract send signals to the myenteric plexus and submucosal plexus via local afferents
• the myenteric plexus and submucosal plexus send signals to the muscularis externa, the muscularis mucosae, endocrine cells, secretory cells and blood vessels via local efferents

Question 18

What happens during the long reflex arc of the GI tract?

Answer 18

• Taste and smell senses send signals to CNS
• chemoreceptors and mechanoreceptors send signals to CNS via splanchnic and vagal afferents
• CNS interacts with submucosal and myenteric plexus vis sympathetic and parasympathetic pathways (efferent nerves)
• these efferent also send signals to the muscularis externa, muscularis mucosae, endocrine cells, secretory cells and blood vessels

Question 19

How are endocrine cells stimulated within the GI tract?

Answer 19

• stimulated by long + short reflex arcs
• stimulated by composition of luminal contents
• stimulated by stretch

Question 20

What are the 8 different hormones that act in the GI tract?

Answer 20

*(some say good girls can go very hormonal)

• Gastrin
• Cholecystokinin
• secretin
• Gastric inhibitory peptide
  (all endocrine action)
• Vasoactive intestinal peptide
• gastrin releasing peptide
  (both neuroncrine action)
• histamine
• serotonin
  (both paracrine action)

Question 21

Where are the interstitial cells of cajal located? (intrinsic myogenic rhythm)

Answer 21

• within the smooth muscle of the GI tract

Question 22

What is the function of the interstitial cells of Cajal? (intrinsic myogenic rhythm)

Answer 22

• they exhibit repetitive, spontaneous, slow oscillations in membrane potentials to make the basic electrical rhythm

Question 23

Why are well coupled electrically, smooth muscle cells important in intrinsic myogenic rhythm?

Answer 23

• it means waves of depolarisation will spread throughout a particular gut segment

Question 24

For intrinsic myogenic rhythm when the amplitude of the slow wave reaches a threshold what happens?

Answer 24

• when a slow waves reaches a threshold level an action potential occurs leading to contraction of the smooth muscle cells

Question 25

Describe swallowing

Answer 25

• initial voluntary phase followed by an involuntary phase

Question 26

How does the oesophagus move food from the pharynx to the stomach?

Answer 26

• by peristalsis - a wave of contraction in the circular muscle layer which travels down the length of the oesophagus

Question 27

What is the primary peristaltic wave?

Answer 27

• peristaltic wave initiated in the pharynx and runs along whole length of oesophagus

Question 28

What is the secondary peristaltic wave?

Answer 28

• A wave of peristalsis further down the oesophagus that is mediated by local reflexes originating in the enteric nervous system at the site of distention

Question 29

How is receptive relaxation regulated?

Answer 29

• regulated by the swallow centre as the vagus nerve carries information from the swallow centre to the stomach

Question 30

What is the interdigestive motility complex?

Answer 30

• Alternating periods of quiescence and peristaltic contractions

Question 31

What is receptive relaxation?

Answer 31

• when smooth muscle on the proximal part of the stomach relax to allow food to enter

Question 32

How does muscle contraction occur in the stomach?

Answer 32

1. Contraction begins in the fundus, narrowing the lumen and forcing food down into the pyloric region
2. Pyloric region contracts, some food goes down into the duodenum but most of it is circulated back up into the fundus region

Question 33

What are the two types of muscle contraction in the small intestine?

Answer 33

• segmentation (mixing movements)
• peristalsis (propulsive movements)

Question 34

What is the migrating motility complex?

Answer 34

• myoelectric waves that sweep through the intestine in a regular cycle during the interdigestive state

Question 35

What are the 4 types of contraction in the large intestine?

Answer 35

• Haustration - similar to segmentation but with formation of sacs which store and propel contents
• peristalsis
• antiperistalsis
• mass movements - uniform peristaltic contraction of smooth muscle originating at the transverse colon and rapidly moves formed faces into the rectum