Overview of GI Physiology

Question 1

name the layers of the go tract from inside out

Answer 1

Mucosa
muscular mucosa 
internal submucosal plexus (meissner plexus)
submucosa 
external submucosal plexus
circular muscle layer
myenteric plexus 
longitudinal muscle 
serosa

Question 2

where does blood from the gut, spleen and pancreas flow to the liver via

Answer 2

the portal vein

Question 3

blood filtered by the liver sinuses leaves via the

Answer 3

hepatic vein and drains into the vena cava

Question 4

what does the blood filtering process by the liver sinuses allow

Answer 4

the removal of bacteria/particulate matter that might enter the blood from the GI tract and prevents direct transport of potentially harmful agents into the body

Question 5

what are villi

Answer 5

small arteriole and venule that interconnect with a system of multiple looping capillaries (countercurrent system)

Question 6

increase in blood flow to the villus is most likely cause by

Answer 6

• vasodilators released from GI mucosa (CCK, VIP, Gastrin)
• vasodilators released from GI glands (bradykinin)
• decreased o2 concentration in the gut wall due to increased metabolism
• subsequent adenosine release

Question 7

what differs in the oesophagus and distal rectum anatomy in the composition of the GI tract

Answer 7

they lack serosa and mesentery

Question 8

where is the submucosal plexus found

Answer 8

small and large intestine - between the circular smooth muscle layer and the mucosa (innervating glandular epithelium, endocrine glands and blood vessels - controls GI secretion

Question 9

where is the myenteric plexus found

Answer 9

throughout the GIT - between the circular and longitudinal muscle (innervating both layer)

Question 10

what are the four functions of motility

Answer 10

1. propel ingested food along the entire length of the GIT
2. mixing and grinding of the contents of the GIT
3. Aid absorption of nutrients and water
4. Clear the stomach and intestines of luminal contents

Question 11

fed state

Answer 11

• peristalsis and mixing movements

- functions 1-3

Question 12

inter digestive state - between meals

Answer 12

• also known as the migrating motor complexes (MMCs)

- responsible for clearing the stomach and intestines of luminal contents

Question 13

role of the parasympathetic neurones in the ENS

Answer 13

branches of the vagus, pelvic and splanchic nerves

• innervate neurones in the myenteric plexus
• main function is to stimulate GI motility and secretion

Question 14

role of the sympathetic neurones in the ENS

Answer 14

these originate from the celiac, superior and inferior mesenteric and hypogastric plexuses

• terminate on the enteric nerves, the smooth muscles and the mucosa
• main function is to inhibit GIT motility and secretion

Question 15

tell me about smooth muscle

Answer 15

• muscle fibres connected bu gap junctions and focal adhesions (adherens junctions)
• actin and myosin filaments aligned diagonally along long axis of the cells
• filaments anchored at dense bodies (alpha - actinin - rich) scattered throughout sarcoplasm
• contractile arrays anchored to sarcolemma by dense plaques

Question 16

electrical activity in the GI smooth muscle is initiated where

Answer 16

interstitial cells of cajal

Question 17

what are the two types of electrical activity

Answer 17

• slow waves

- spike potentials

Question 18

role of slow waves

Answer 18

to excite the appearance of spike potentials

Question 19

the higher the slow wave potential

Answer 19

greater frequency of spike potentials, therefore the stronger the contraction of the smooth muscle

Question 20

how can baseline membrane potential vary

Answer 20

• depolarisation leads to increased excitability

- hyper-polarisation leads to decreased excitability

Question 21

what ions are involved in electrical activity in smooth muscle

Answer 21

due to the opening of calcium ion channels leading to the influx of ca2+ into the cells, and is then terminated by K+ efflux

Question 22

peristalsis

Answer 22

the stretch initiates the circular muscles behind (oral end) it to contract (ascending excitatory reflex) and the circular muscles in front (anal end) of it to reflex (descending inhibitory reflex)

Question 23

segmentation - large intestine

Answer 23

• contractions of circular muscle layer at short intervals along GIT
• contractions start as n 1 opposite
• contractions then move forward to now contract in original relaxed area (2 opposite)

Question 24

migrating motor complexes

Answer 24

• between meals or when fasting
• function to clear stomach and small intestine debris and bacteria
  happens in 4 stages

Question 25

what are the phases of MMC

Answer 25

• Phase I (45-60 mins) - quiescent period
  with slow waves
  – Phase II (30-35 mins) - action potential activity on slow waves but sporadic contractions of circular muscle
  – Phase III (2-12 mins) - action potential activity on slow waves but regular contractions of circular muscle
  – Phase IV – period in which action potential activity and contraction reduce and merge into phase I
  Peristalsis