structure and function of the alimentary canal

Question 1

What does the GI tract consist of?

Answer 1

series of hollow organs between mouth and anus (alimentary canal, separated by sphincters.
also several accessory structures

Question 2

what is the function of the mouth and oropharynx?

Answer 2

1) chops up and lubricates food
2) starts carbohydrate digestion (amylase)
3) propels food to oesophagus

Question 3

what is the function of the oesophagus?

Answer 3

actively propels food from the oropharynx to the stomach via peristalsis

Question 4

what is the function of the stomach?

Answer 4

1) storage organ
2) churns food
3) continues carbohydrate digestion and begins protein digestion
4) regulates delivery of chyme to duodenum (via enzyme/churning activity and secretions)

Question 5

what 3 structures does the small intestine consist of?

Answer 5

duodenum (short)
jejunum (longer)
ileum

Question 6

what is the function of the small intestine?

Answer 6

principal site of digestion and absorption via secretions from small intestine and pancreas
begins fat digestion

Question 7

what parts of the small intestine are mainly involved in absorption?

Answer 7

duodenum and jejunum

Question 8

what are the 3 component structures of the large intestine?

Answer 8

caecum, appendix and colon

Question 9

what are the functions of the large intestine?

Answer 9

1) contains left over substances
2) colon reabsorbs water and electrolytes
3) delivers fairly solid stool to the rectum

Question 10

what is the function of the rectum?

Answer 10

storage and expulsion of faeces

Question 11

what are the 3 accessory structures?

Answer 11

pancreas
salivary glands
liver and gall bladder (hepatobiliary system)

Question 12

describe the motility activity of the alimentary canal

Answer 12

mechanical activity mainly involving smooth muscle

Question 13

describe the secretion activity action of the alimentary canal

Answer 13

secretion into the digestive tract from itself and accessory structures in response to food.
required for digestion, protection and lubrication

Question 14

describe the digestion activity of the alimentary canal

Answer 14

chemical breakdown by enzymatic hydrolysis of complex foodstuffs into smaller absorbable units

Question 15

describe the absorptive activity of the alimentary canal

Answer 15

transfer of absorbable products of digestion from the digestive tract to the blood or lymph
(involves specialised transport mechanisms)

Question 16

name the 4 layers of the GI wall from the lumen inwards (7-10 cm)

Answer 16

mucosa
submucosa
muscularis externa
serosa

Question 17

what is the mucosa?

Answer 17

epithelial/endocrine/exocrine cells, lamina propria, muscularis mucosae (thin, circular smooth muscle that changes shape of mucosa so involved in mixing of contents of lumen)

Question 18

what is the submucosa?

Answer 18

elastic connective tissue, larger blood and lymph vessels, glands, submucous plexus

Question 19

what is the muscularis externa?

Answer 19

circular muscle layer and longitudinal muscle layer with nerve network in between (myenteric plexus)

Question 20

what is the serosa?

Answer 20

connective tissue, squamous cell layer covering it allowing smooth movement of lumen contents over it

Question 21

what special adaptations exist in the GI wall in the stomach?

Answer 21

another oblique smooth muscle layer in muscularis externa

Question 22

what drives GI motility?

Answer 22

smooth muscle activity (circular and longitudinal)

Question 23

what parts of the GI tract are skeletal muscle?

under autonomic control

Answer 23

mouth
pharynx
upper oesophagus
external anal sphincter
(rest is smooth)

Question 24

GI smooth muscle acts as a single unit, how does this happen?

Answer 24

cells linked via gap junctions allowing electrical continuity between cells and depolarization spreads through all of them as a synchronous wave and they contract as one

Question 25

what drives spontaneous activity across the syncytium (group of SM cells) and how is this modulated?

Answer 25

driven by specialised pacemaker cells | modulated by intrinsic (enteric) and extrinsic (autonomic) nerves and numerous hormones

Question 26

what are "slow waves"?

Answer 26

rhythmic patterns of depolarization spreading via gap junctions (occurs in stomach and both intestines)

Question 27

how so slow waves determine smooth muscle activity?

Answer 27

contraction only occurs if small wave peak reaches threshold and action potential fires Number/direction/velocity proportional to number of AP's

Question 28

what drives slow waves?

Answer 28

interstitial cells of cajal (ICCs) - pacemaker cells interspersed between muscle layers

Question 29

why does basic electrical rhythm vary along the length of the GI tract?

Answer 29

not all slow waves reach threshold

Question 30

what determines whether slow wave amplitude reaches threshold?

Answer 30

neuronal stimuli hormonal stimuli mechanical stimuli - eg. food (don't affect amplitude of wave, just shift starting point of wave)

Question 31

BER of stomach?

Answer 31

3 slow waves per min | 3 contraction per min when food present

Question 32

BER of small intestine?

Answer 32

12 per min in duodenum 8 per min in terminal ileum propels matter downwards

Question 33

BER of large intestine?

Answer 33

8 per min proximal colon 16 per min distal colon pushes matter back up so more time to reabsorb

Question 34

describe parasympathetic innervation of smooth muscle cells

Answer 34

preganglionic fibres synapse with post ganglionic within the enteric NS excitory = increased gastric/pancreatic/small intestine secretion, blood flow and muscle contraction Inhibitory = relaxes smooth muscle and sphincters

Question 35

describe sympathetic innervation of smooth muscle cells

Answer 35

preganglionic fibres synapse in the prevertebral ganglia (celiac, superior and inferior mesenteric) Excitory = increased sphincter tone Inhibitory = decreased motility, secretion and blood flow

Question 36

what 2 plexuses is the enteric nervous system contained within?

Answer 36

myenteric (auerbach's) plexus (motility and sphincters) Submucous (Meissner's) plexus (epithelia and blood vessels) All within the GI tissue

Question 37

what 3 types of neurones of the enteric nervous system co-ordinate muscular, secretive and absorptive activities?

Answer 37

``` sensory neurons interneurons (majority) effector neurons (excitory and inhibitory motor neurones supplying the 2 smooth muscle layers, secretory epithelium, endocrine cells and blood vessels) ```

Question 38

describe local nerve reflex in the GI tract

Answer 38

occurs completely in the walls of the GI tract

Question 39

describe the short nerve reflex of the GI tract

Answer 39

intestine-intestinal inhibitory reflex (neurone leaves GI wall, activating sensory neurones exciting sympathetic fibres that inhibit muscle activity in adjacent areas)

Question 40

describe the long nerve reflex in the GI tract

Answer 40

neurone goes all the way to CNS (nucleus of vagus in medulla) gastroileal reflex - brain tells muscles to deficate

Question 41

how does peristalsis happen?

Answer 41

before bolus: circular muscle contracts, longitudinal relaxes after bolus: circulator relaxes, longitudinal contracts

Question 42

What is segmentation?

Answer 42

rhythmic contractions of adjacent portions of circular smooth muscle that mix and divide lumen contents (called haustration in large intestine)

Question 43

what is colonic mass movement?

Answer 43

powerful sweeping contraction wave that forces faeces into rectum

Question 44

what is migrating motor complex?

Answer 44

powerful sweeping contraction wave from the stomach t the terminal ileum (cleans out small intestine)

Question 45

describe tonic contraction in the sphincters and storage organs

Answer 45

high pressure in sphincters | low pressure in storage organs

Question 46

how many sphincters are there in the GI tract?

Answer 46

6 (excluding sphincter of Oddi)

Question 47

what are sphincters made of?

Answer 47

circular smooth muscle | external anal sphincter and .... are skeletal

Question 48

how do sphincters work?

Answer 48

act as one way valves by maintaining a positive resting pressure (tone) relative to two adjacent structures (eg. oesophagus and stomach)

Question 49

describe the function of each sphincter

Answer 49

upper oesophageal = entry to oesophagus, open in swallowing, closed on inspiration lower oesophageal = permits entry of food into stomach then closes to prevent reflux Pyloric = regulates gastric emptying, prevents reflux of duodenal contents) ileocaeccal = one way movement of ileum contents to caecum internal/external anal = control exit of faeces

Question 50

what is gastric outlet obstruction?

Answer 50

overactive pyloric sphincter so stomach doesn't empty into duodenum so backs up causing vomiting