Physiology Of The Gi Tract

Front 1

What is the muscular component of the GI tract called?

Back 1

Muscularis externa

Front 2

What is the neuronal component of the GI tract called?

Back 2

Myenteric plexus and submucosal plexus.

Front 3

How much liquid does the large intestine have the capacity to absorb?

Back 3

8,000-10,000ml per day

Front 4

How much does the large intestine usually absorb in a day?

Back 4

~400ml

Front 5

Does intestinal epithelial permeability increase or decrease in an aboral direction?

Back 5

Increase

Front 6

What is the potential difference between the lumen of the GI tract and the interstitial space?

Back 6

Negative

Front 7

Where is sodium mainly absorbed in the GI tract? And how?

Back 7

Jejunum. Exchanged for H+ ions.

Front 8

In the duodenum how is sodium absorbed in the GI tract?

Back 8

Coupled with glucose and amino acids - SGLT-1 transporter - 2Na+/Glucose.

Front 9

Where is potassium absorbed in the GI tract and how?

Back 9

Jejunum and paracellularly through solvent drag with water.

Front 10

Where is potassium excreted in the GI tract and how is this upregulated?

Back 10

Large intestine due toe the negative potential difference, regulated by aldosterone which increases apical potassium channels and basolateral sodium/potassium pumps.

Front 11

Where is calcium absorbed in the GI tract and how?

Back 11

Duodenum, passive paracellular. (Some transcellular in duodeumc with TRPV, regulated by calcitriol).

Front 12

Where is iron absorbed along the GI tract and how?

Back 12

Duodenum, H+ ions reduce ferric (Fe3+) ions to ferrous (Fe2+). When ferrous ions are absorbed, the are released as ferritin or processed by transferrin.

Front 13

What is the impact of excess spinach or citrate to iron absorption?

Back 13

Contain oxalic acid which binds to iron reducing its absorption.

Front 14

Is chloride absorbed or secreted and how?

Back 14

Absorbed (due to negative lumen). Bicarb is secreted as chloride is absorbed.

Front 15

What happens to bicarb stores during diarrhoea?

Back 15

You become bicarb depleted.

Front 16

Are electrolyte and water movements regulated through the coordination of secretion or absorption?

Back 16

Secretion

Front 17

VIP secreted by enteric neurones stimulates enterocytes how?

Back 17

Increasing the activity of Adenylate Cyclase which increases PKA/cAMP chloride secretion, pulling water in.

Front 18

Serotonin secreted from enterochromaffin cells does what?

Back 18

Stimulates Cl- secretion, activates PKC and calcium.

Front 19

Where is somatostatin secreted from and what does it do?

Back 19

D cells and stimulates NaCl absorption (drying stools).

Front 20

Where are enkephalins released from and what do they do?

Back 20

Enteric neurones and increase the absorption of solutes.

Front 21

What does serotonin stimulate in D cells

Back 21

Absorption via NaCl co-transport.

Front 22

How does cholera cause diarrhoea?

Back 22

Activates Gs --> massive PKA stimulation --> excessive chloride secretion --> excessive diarrhoea.

Front 23

How do mast cells enhance chloride secretion?

Back 23

They secrete histamine which increases cAMP through the Gs-alpha pathways, increasing chloride secretion.

Front 24

What are the four types of movements in the SI?

Back 24

Individual villi, pendular and segmentation mixing and peristaltic waves.

Front 25

How do the individual villi move?

Back 25

Fibres from muscularis mucosa cause shortening and elongation.

Front 26

How does segmentation work and what does it do?

Back 26

Causes pinching of the intestine by the contraction and relaxation of the circular muscles. Facilitates mixing of food with digestive enzymes.

Front 27

What is pendular mixing?

Back 27

The contraction of the londitudinal muscles

Front 28

How often does segmentation occur?

Back 28

Up to 12/min

Front 29

How often does pendular mixing occur?

Back 29

Up to 10/min

Front 30

What are peristaltic waves?

Back 30

Co-ordinated movements which move chyme.

Front 31

How fast can peristaltic waves move?

Back 31

~60cm/min

Front 32

What is migrating motility complex?

Back 32

During fasting housekeeping contractions occur to flush out the contents of the SI every ~1.5 hours. Takes nearly 2 hours to move along the length of the SI.

Front 33

What is the goal of the migrating motility complex?

Back 33

Prevents the stasis of material in the SI.

Front 34

Which are the excitatory neurotransmitters in the ENS?

Back 34

ACh and Substance P

Front 35

What are the inhibitory neurotransmitters in the ENS?

Back 35

VIP and NO

Front 36

Is the myenteric plexus primarily motor or sensory in function?

Back 36

Motor

Front 37

What is the function of the submucosal plexus?

Back 37

Primarily sensory but can affect the secretatory ability of cells.

Front 38

Does sympathetic activity increase or decrease motility in the GI tract?

Back 38

Decrease

Front 39

Does stretching the mechanoreceptors increase or decrease motor activity?

Back 39

Increase

Front 40

What are Interstitial Cells of Cajal (ICCs)

Back 40

Mesenchymal cells associated with smooth muscle cells which influence their contraction (part of the SIP syncytium)

Front 41

What does SIP stand for?

Back 41

Smooth muscle cells

Interstitial cells of Cajal

Platelet-derived growth facto receptor alpha-positive cells.

Front 42

What do ICCs do?

Back 42

Give rise to the myogenic nature of the contraction - they display pacemaker activity due to cyclic release of Ca++, causing slow de- and re-polarisations, generating slow waves.

These waves propagate in the antral direction, frequency decreases towards the anus.

Front 43

How is the amplitude and frequency of slow waves modulated?

Back 43

Intrinsic and extrinsic nerves, hormones and paracrine substances.

Front 44

What determines the magnitude of slow waves?

Back 44

Spike potentials.

Front 45

What happens when a bolus stimulates stretch receptors?

Back 45

The path ahead of it widens and the lumen behind narrows.

Front 46

What happens to the longitudinal (outer) muscles behind and ahead of a bolus?

Back 46

Behind: relax

Ahead: contract

Front 47

What happens to the circular (inner) muscles behind and ahead of a bolus?

Back 47

Behind: contract

Ahead: relax

Front 48

Is the ileocecal usually open or closed and what does it control?

Back 48

Closed and regulates the entry of the bolus into the colon.

Front 49

Which nerves control the tone of the ileocecal sphincter and what does it depend on?

Back 49

The vagus nerve, SNS and ENS. The presence of food proximal and distal to it. Ileal distension relaxes whist colon distention increases the tone.

Front 50

What stimulates the active transport of sodium out of the colon?

Back 50

Aldosterone

Front 51

What causes the final absorption of water?

Back 51

Osmosis

Front 52

What is the movement in the large intestine called?

Back 52

Segmentation

Front 53

What are haustra?

Back 53

Bulges in the colon

Front 54

How does segmentation occur?

Back 54

Circular smooth muscle contracts (2-3cm) and the 3 taenia coli also contract. This causes a rolling movement mixing the semi-solid matter making sure it makes contact with the wall of the colon to ensure maximum absorption.

Front 55

What causes the mass propulsive movement in the colon and how is this initiated? When does this occur?

Back 55

The contraction of the longitudinal muscles. Intrinsic reflex pathways, enhanced by vagal neurones. About 3 times a day, usually for about 15 minutes after eating.

Front 56

Which hormones increase colon excitability?

Back 56

Gastin & CCK

Front 57

What is Hirschsprung's disease?

Back 57

The loss of the ability to relax smooth muscle cells (ganglia are lost). The colon remains contracted, faecal matter builds up proximal to the area of disease.

Front 58

How big is the rectum?

Back 58

~500ml

Front 59

Describe the internal anal sphincter.

Back 59

Smooth muscle, not under voluntary control. Tonically contracted and has both para. (relax) and symp. (tonic contraction) neural supply.

Front 60

Describe the external anal sphincter.

Back 60

Skeletal muscle, attached to the coccyx and perineum. Innervated by the pudendal nerve. Under learned control from ~18 months.

Front 61

What has to happen for defection to occur?

Back 61

Anorectal angle has to elongate, puborectalis contracts and the pelvic floor has to lwoer.

Front 62

Describe the role of sensory afferents in defecation.

Back 62

They relay the information about the distension of the rectum (mechanoreceptors are stimulated by the mass propulsive movement in the sigmoid colon) to the sacral region of the spinal cord, via the pelvic plexus.

Front 63

Which area of the brain initiates the process of defecation?

Back 63

The cerebral cortex

Front 64

Describe the process of expulsion.

Back 64

The walls of the sigmoid colon and rectum contract (under para. control), internal sphincter relaxes, moving the faeces towards the anal canal. Anorectal angle increases and the external sphincter relaxes, allowing the bowel to be evacuated.

Front 65

Which muscles can contract to aid expulsion? Which relax?

Back 65

The diaphragm and glottis. The pelvic floor muscles (allow rectum to straighten).