motility in the GI tract

Question 1

what does segmentation allow

Answer 1

rhythmic contraction and relaxation of the intestine.

mixes chyme thoroughly in both directions (unlike peristalsis)

Question 2

what does segmentation allow

Answer 2

the contents of intestine to mix with intestinal juices for digestion and touch the intestinal wall for absorption.

Question 3

where is the skeletal muscle found

Answer 3

in only the upper third of the oesophagus

Question 4

is contraction of the skeletal muscle voluntary

Answer 4

yes

Question 5

where is smooth muscle found

Answer 5

all of GI tract except upper third of the oesophagus

Question 6

when does contraction of smooth muscle occr

Answer 6

in response to neuronal and hormonal or paracrine input

Question 7

what stimulates skeletal muscle to contract

Answer 7

neurons

Question 8

what do motor neurons do

Answer 8

innervate skeletal muscle via axons branching and forming junctions with the muscle.

Question 9

what do axon terminals contain

Answer 9

vesicles which contain neurotransmitter.

Question 10

what happens at the neuromuscular junction

Answer 10

Action potential in motor
neurone leads to ACh
release at cleft

ACh binds to receptors on
motor end plate (muscle
plasma membrane under axon
terminal), opening ion channels
and resulting in depolarisation

Propagation of action potential in muscle cell
plasma membrane

Question 11

excitation-contraction coupling

Answer 11

AP in t-tubule triggers
Ca2+ entry and increased
[Ca2+]i.

Ca2+ binding to troponin
allows force generation and
contraction.

Removal of Ca2+ from the
cytosol is required for
relaxation.

Question 12

what is the type of muscle in the small intestine and large intestine

Answer 12

smooth muscle

Question 13

how are smooth muscle cells connected

Answer 13

by gap junctions so are electrically coupled

Question 14

why do smooth muscles appear to be smooth

Answer 14

they lack striations/banding pattern seen in skeletal muscle

Question 15

what allows coordinated control of smooth muscle

Answer 15

depolarisation spreads through adjacent sections

Question 16

what initiates segmentation

Answer 16

the electrical activity of ‘interstitial cells of cajal’ (pacemaker cells) in circular smooth muscle.

Question 17

what regulates contraction of smooth muscle

Answer 17

neurotransmitters released by autonomic nerve endings. No specialised motor end plate but swollen regions of axon that contain NT.

can also be regulated by hormonal or mechanical input via receptor activation.

Question 18

what can a neurotransmitter be

Answer 18

either excitatory or inhibitory to smooth muscle contraction

Question 19

what type of input does the skeletal muscle receive

Answer 19

only excitatory input

Question 20

how does E-C coupling work in smooth muscle

Answer 20

no troponin involved

instead, contractile proteins are regulated by Ca2+/CaM activation of Myosin Light Chain Kinase resulting in phosphorylation of myosin

Question 21

example of excitatory neurotransmitter which leads to contraction

Answer 21

acetylcholine and 5-HT

Question 22

example of inhibitory neurotransmitter which leads to relaxation

Answer 22

VIP and NO

Question 23

how long does small intestine transit last

Answer 23

its relatively constant and little affected by diet or exercise

Question 24

how quick is transit in duodenum last

Answer 24

its rapid, slowing down as chyme processes through small intestine

Question 25

where does most digestion and absorption occur

Answer 25

in the duodenum and jejunum

Question 26

how long is transit time and rate of absorption in the ileum

Answer 26

both are slower, allows absorption of more slowly digested substances

Question 27

what happens in the fasted state

Answer 27

interdigestive peristaltic activity occurs.

This is known as the migrating myoelectic complex (MMC) and begins in the stomach

Question 28

what happens as the MMC reaches the ileum

Answer 28

one starts at the stomach again which allows undigested material to move towards to large intestine

Question 29

what initiates the MMC

Answer 29

an increase in motilin (hormone) which increases neuronal activity

Question 30

what happens in a fed state

Answer 30

segmentation

Question 31

what affects the number of contractions

Answer 31

physical nature of the food

Question 32

does solid food induce more contractions than equicalorific liquid

Answer 32

yes, 2x as many

Question 33

conditions that increase small intestine transit time

Answer 33

• Diarrhoea
• Thyrotoxicosis (hyperthyroidism)
• Irritable bowel syndrome
• Chronic Pancreatitis

Question 34

conditions that decrease small intestine transit time

Answer 34

• Constipation
• Myxoedema (hypothyroidism)
• Pseudo-obstruction
• Ileal Resection
• Partial gastrectomy
• Jejunal bypass
• Diabetes (constipation affects blood sugar control & high blood sugar causes constipation)

Question 35

what do anti-spasmodics do

Answer 35

reduce smooth muscle contraction
e.g. anti-muscarinic antagonists (hyoscine butylbromide – Buscopan, mebeverine)

Question 36

what do anti-motility drugs do

Answer 36

reduce transit through the GI tract by decreasing activity of the myenteric plexus e.g. loperamide (opioid agonist)

Question 37

what do pro kinetics do

Answer 37

stimulate GI tract activity by increasing contraction
e.g. drugs that act on 5-HT receptors e.g. Tegaserod (5-HT4 agonist), metoclopramide (acts at 5-HT receptors and dopamine receptors)

Question 38

what do laxatives do

Answer 38

stimulate intestinal movement by increasing bulk, adding lubrication or acting as a local irritant to the mucosal layer.