Structure of the GI tract

Question 1

What is the role of the mouth and oropharynx?

Answer 1

chops and lubricates food
starts carbohydrate digestion
propels food to oesophagus

Question 2

What is the role of the oesophagus?

Answer 2

muscular tube: propels food to the stomach by peristalsis

Question 3

What is the role of the stomach?

Answer 3

stores/churns food
continues carbohydrate and initiates protein digestion
regulates delivery of chyme to duodenum

Question 4

What is the role of the small intestine?

Answer 4

principal site of digestion and absorption of nutrients

Question 5

What are the 3 parts of the small intestine?

Answer 5

duodenum
jejunum
ileum

Question 6

What is the role of the large intestines?

Answer 6

Colon reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum
Colon condenses and dries out the matter that it receives

Question 7

What are the 3 parts of the large intestine?

Answer 7

caecum
appendix
colon

Question 8

What is the role of the rectum and anus?

Answer 8

storage and regulated expulsion of faeces

Question 9

Name the 4 accessory structures of the GI tract?

Answer 9

salivary glands
the pancreas
the liver
gall bladder

Question 10

What are the 4 activities of teh Ailmentary Canal?

Answer 10

Motility
Secretion
Digestion
Absorption

Question 11

Describe motility within the Ailmentary Canal?

Answer 11

Mechanical activity mostly involving smooth muscle - skeletal at mouth, pharynx, upper oesophagus and external anal sphincter

Question 12

Describe Secretion within the Ailmentary Canal?

Answer 12

Into the lumen of the digestive tract occurs from itself and accessory structures in response to the presence of food, hormonal and neural signals. 
Required for: 
- digestion
- protection
- lubrication (with mucin)

Question 13

Describe Digestion within the Ailmentary Canal?

Answer 13

Chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbable, units (physical digestion in the mouth, stomach and small intestine contributes)

Question 14

Describe Absorption in the Ailmentary Canal?

Answer 14

Transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood, or lymph – largely mediated by numerous transport mechanisms

Question 15

What occurs due to circular muscle contraction?

Answer 15

lumen becomes narrower and longer

Question 16

What occurs due to longitudinal muscle contraction?

Answer 16

intestine becomes shorter and fatter

Question 17

What occurs due to muscularis mucosae contraction?

Answer 17

change in absorptive and secretory area of mucosa (folding), mixing activity

Question 18

Name in order (outer to inner) the layers of the digestive tract wall?

Answer 18

Serosa
Muscularis Externa
Submucosa
Mucosa

Question 19

What are adjacent smooth muscle cells coupled by in the GI tract?

Answer 19

gap junctions

Question 20

What do gap junctions allow?

Answer 20

spread of electrical currents from cell to cell forming a functional syncytium (continuality)

Question 21

What is spontaneous activity across the syncytium driven and modulated by?

Answer 21

Driven: specialised pacemaker cells
Modulated:
- intrinsic (enteric) and extrinsic (autonomic) nerves
- numerous hormones

Question 22

In the stomach, small intestine and large intestine how does spontaneous electrical activity occur?

Answer 22

slow waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions

Question 23

What is slow wave electrical activity?

Answer 23

• Determines max. frequency, direction and velocity of rhythmic contractions
• Is driven by the interstitial cells of Cajal (ICCs) – specialised pacemaker cells interspersed between the far more numerous smooth muscle cells (SMCs)
• Contraction only occurs if the slow wave amplitude is sufficient to trigger SMC action potentials (upstroke mediated by voltage-activated Ca2+ channels, downstroke by voltage-activated K+ channels).
• Force is related to number of action potentials discharged.

Question 24

Where are Interstitial cells of Cajal located?

Answer 24

between longitudinal and circular muscle layers and in the submucosa

Question 25

What do slow waves determine?

Answer 25

Basic electrical rhythm (BER)

Question 26

What affects a slow waves aplitude being able to reach threshold?

Answer 26

Neuronal stimuli Hormonal stimuli Mechanical stimuli (i.e. food) Slow waves all have roughly the same amplitude, but their starting point determines their ability to produce an action potential

Question 27

What is the BER frequency in the stomach?

Answer 27

3 slow waves per minute, rhythmic contractions occur after food intake

Question 28

What is the BER frequency in the small intestine?

Answer 28

approximately 12 (proximal) and 8 (distal) waves per minute in the duodenum and terminal ileum, respectively – tends to drive luminal contents in the aboral (anus) direction

Question 29

What is the BER frequency in the large intestine?

Answer 29

approximately 8 (proximal) and 16 (distal) waves per minute in the proximal and distal (sigmoid) colon, respectively – favours retention of luminal contents facilitating reabsorption of water and electrolytes (especially done by distal part)

Question 30

Describe the action of Parasympathetic innveration?

Answer 30

Preganglionic fibres (releasing ACh) synapse with ganglion cells within the ENS – classed in the enteric nervous system Excitatory influences: • Increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction Inhibitory influences: • Relaxation of some sphincters, receptive relaxation of stomach

Question 31

Describe the action of Sympathetic innervation?

Answer 31

Preganglionic fibres (releasing ACh) synapse in the prevertebral ganglia. Postganglionic fibres (releasing NA) innervate mainly enteric neurones, but also other structures Excitatory influences •Increased sphincter tone Inhibitory influences •Decreased motility, secretion and blood flow

Question 32

Which 2 plexuses is the Enteric Nervous System found?

Answer 32

Myenteric (Auerbach’s) plexus – mainly regulates motility and sphincters Submucous (Meissner’s) plexus – mainly modulates epithelia and blood vessels

Question 33

What is the role of the Enteric Nervous System?

Answer 33

Co-ordinates muscular, secretive and absorptive activities via: - Sensory neurones: mechanoreceptors, chemoreceptors, thermoreceptors - Interneurones: the majority, co-ordinating reflexes and motor programs - Effector neurones: excitatory and inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels

Question 34

What are the local, short and long nerve reflexes that control motor and secretory activity?

Answer 34

Local reflex - peristalsis Short reflex - intestino-intestinal inhibitory reflex (local distension activates sensory neurones exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas) Long reflex – gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)

Question 35

What are the methods of motility in the GI tract?

Answer 35

``` Peristalsis Segmentation Colonic Mass Movement Migrating Motor Complex Tonic Contractions ```

Question 36

Describe Peristalsis?

Answer 36

a wave of relaxation, followed by contraction, that normally proceeds along the gut in an aboral direction – triggered by distension of the gut wall

Question 37

Describe Segmentation?

Answer 37

mixing, or churning, movements – rhythmic contractions of the circular muscle layer that mix and divide luminal contents Occurs in the small intestine (in the fed state) and in the large intestine (where it is called haustration)

Question 38

Describe Colonic Mass Movement?

Answer 38

powerful sweeping contraction that forces faeces into the rectum – occurs a few times a day

Question 39

Describe Migrating Motor Complex?

Answer 39

powerful sweeping contraction from stomach (small intestine) to terminal ileum and into the colon

Question 40

Describe Tonic Contractions in high and low pressures?

Answer 40

low pressure - organs with a major storage function (e.g. stomach), put a degree of tone on the stomach high pressure – occurs at sphincters along the GI tract

Question 41

What are Sphincters generally made up of? | What are the 2 exceptions?

Answer 41

Circular smooth muscle | - upper oesophageal sphincter and external anal sphincter - skeletal muscle

Question 42

What is the role of Sphincters?

Answer 42

Act as essentially as one way valves by maintaining a positive resting pressure relative to two adjacent structures In general, stimuli (pressure) proximal and distal to a sphincter cause opening and closing, respectively Unless swallowing, the pressure in the stomach is greater than the oesophagus

Question 43

Name the 5 different types of Sphincters?

Answer 43

- Upper oesophageal sphincter (UOS) - Lower oesophageal sphincter (LOS) - Pyloric sphincter - Ileocaecal valve - Internal and external anal sphincters

Question 44

Describe the Upper Oesophageal Sphincter?

Answer 44

``` skeletal muscle - relaxes to allow swallowing - closes during inspiration If this does not work then there becomes a build up of gas in the stomach. Most powerful sphincter. ```

Question 45

Describe the Lower Oesophageal Sphincter?

Answer 45

- relaxes to permit entry of food to the stomach | - closes to prevent reflux of gastric contents to the oesophagus

Question 46

Describe the Pyloric Sphincter?

Answer 46

- regulates gastric emptying, - usually prevents duodenal gastric reflux

Question 47

Describe the Ileocaecal Valve?

Answer 47

- regulates flow from ileum to caecum - distension of ileum opens - distension of proximal colon closes

Question 48

What are the Internal and external anal sphincters regulated by?

Answer 48

defaecation reflex