introduction to structure, functions and control of the alimentary canal

Question 1

function of mouth and pharynx

Answer 1

chops and lubricates food, starts carbohydrate digestion and delivers food to the oesophagus

Question 2

oesophagus

Answer 2

propels food to stomach

Question 3

stomach

Answer 3

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

Question 4

small intestine

Answer 4

(duodenum, jejunum, ileum) principal site of digestion and absorption of nutrients

Question 5

large intestine

Answer 5

(caecum, appendix, colon)

colon reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum

Question 6

rectum anal canal and anus

Answer 6

regulated expulsion of faeces

Question 7

accessory structures are …

Answer 7

salivary glands
the pancreas
the liver and gall bladder
(hepatobilliary system)

Question 8

what consists of the generalised structure of the digestive wall

Answer 8

mucosa, submucosa, muscular externa, serosa

Question 9

mucosa

Answer 9

> mucous membrane (epithelial, exocrine gland and endocrine gland cells)
lamina propria (neuroendocrine cells)
muscular mucosae (gut motility)

Question 10

submucosa

Answer 10

connective tissue
larger blood and lymph vessels
glands
submucous plexus (neurone network)

Question 11

muscular externa

Answer 11

circular, longitudinal muscle layer 
myenteric plexus (neurone network)

Question 12

serosa

Answer 12

connective tissue

Question 13

main functions of the alimentary canal

Answer 13

motility
secretion
digestion
absorption

Question 14

motility

Answer 14

mechanical activity mostly involving smooth muscle - skeletal muscle at mouth, oesophagus and others

Question 15

secretin

Answer 15

into the lumen of the digestive tract occurs from itself and accessory structure in response to the presence of food,
hormonal and neural signals
this is required for 1) digestion 2) protection 3) lubrication

Question 16

digestion

Answer 16

chemical breakdown of enzymatic hydrolysis of complex foodstuffs to smaller, absorbable units

Question 17

absorption

Answer 17

transfer of the absorbable products of digestion - with water electrolytes and vitamins - from the digestive tract to the blood or lymph

Question 18

how does GI motility work

Answer 18

> circular muscle contraction ie the lumen becomes narrower and longer
longitudinal muscle contraction ie the intestine becomes shorter and fatter
muscular mucosae contraction ie change in absorptive and secretory area of mucosa (folding), mixing activity
*mostly due to the activity of smooth muscle but skeletal muscle is important in features such as the mouth pharynx external anal sphincter and upper oesophagus

Question 19

explain electrically coupled smooth muscle in GI tract

Answer 19

> adjacent smooth muscle cells are couples by gap junctions - electrical currents flow from cell to cell
hundreds of cells are depolarised and contract at the same time as a synchronous wave (ie a single unit of smooth muscle as opposed to multiunit smooth muscle)
spontaneous activity across the coupled cells is driven by specialised pacemaker cells and is modulated by: intrinsic (enteric) and extrinsic (autonomic) nerves and
numerous hormones
in the stomach, small, and large intestine spontaneous electrical activity occurs as slow waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions

Question 20

what is the purpose of slow wave electrical activity

Answer 20

determines: frequency, direction and velocity of rhythmic contractions

Question 21

how does slow wave electrical activity work?

Answer 21

it is driven by the interstitial cells of Cajal - pacemaker cells located largely between the circular and longitudinal muscle layers
>ICCs from gap junctions with each other and smooth muscle cells electrically coupling them
>slow waves in ICCs drive slow waves in the smooth muscle cells couple to them
>some ICCs form a bridge between nerve endings and smooth muscle cells
»depolarising slow waves do not necessarily result in smooth muscle contraction
»contraction in the intestine occurs only if the slow wave amplitude is sufficient to reach a threshold to trigger smooth muscle cell calcium action potentials (spikes)
»force is related to number of action potentials discharged ie driven in turn by the duration of the slow wave that is above threshold

Question 22

what are slow waves aka ?

Answer 22

basic electrical rhythm
>BER
»varies in frequency along the length of the GI tract ie stomach is 3 mins while small intestine is 8-12mins

Question 23

whether or not the slow wave amplitude reaches threshold depends on:

Answer 23

neuronal stimuli, hormonal stimuli, mechanical stimuli

> these generally act to depolarise smooth muscle cells rather than influence slow waves directly - depolarisation shifts slow wave peak to threshold

Question 24

what does the parasympathetic autonomic innervation of the GI tract consist of ? and how do they work

Answer 24

> Preganglionic fibres (releasing ACh) synapse with ganglion cells (in essence post-ganglionic neurones) within the enteric nervous system (ENS)
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 25

what consists of the sympathetic autonomic innervation of the GI tract ? and how do they work

Answer 25

*functionally less important than the parasympathetic NS >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 26

what is the ENS

Answer 26

is is about 100 million neurones, cells mostly located in ganglia connected by fibre tracts within - the myenteric plexus -submucous plexus >is intrinsic to GI tissue -reflex circuits can operate independently but hormones and extrinsic nerves exert a strong regulatory influence >co-ordinates muscular, secretive and absorptive activities via: sensory neurones, interneurones, effector neurones

Question 27

myenteric plexus

Answer 27

"Auerbach's" | >mainly regulates motility and sphincters

Question 28

submucous plexus

Answer 28

"Meissner's" | >mainly modulates epithelial and blood vessels

Question 29

sensory neurones

Answer 29

mechanoreceptors, chemoreceptors, thermoreceptors

Question 30

interneurones

Answer 30

the majority, co-ordinating reflexes and motor programmes

Question 31

effector neurones

Answer 31

>excitatory and inhibitory motor neurones ``` >they supply: both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels ```

Question 32

what do nerve reflexes do ? and give examples

Answer 32

control motor and secretory activity >a local reflex = peristalsis >short reflex = intestino-intestinal inhibitory reflex >long reflex = gastroileal reflex (a vaso-vagal reflex)

Question 33

the major motility patterns

Answer 33

peristalsis and | segmentation

Question 34

peristalsis

Answer 34

>Peristalsis – a wave of relaxation, followed by contraction, that normally proceeds a short distance along the gut in an aboral direction – triggered by distension of the gut wall * *aboral - altered activity of interneurones > altered activity of motor neurones > circular muscle relaxes (release of VIP and NO from inhibitory motor neurone) OR > longitudinal muscle contract (release of ACh and substance P from excitatory motorneurone) * *oral - altered activity of interneurones >altered activity of motor neurones >circular muscle contracts (release of ACh and substance P from excitatory motor neurone) OR longitudinal muscle relaxes (release of VIP and NO from inhibitory motor neurone)

Question 35

segmentation

Answer 35

>Segmentation (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) (greater detail in small intestine lecture) >colonic mass movement = powerful sweeping contraction from stomach to terminal ileum >tonic contractions (sustained contractions) = low pressure ie stomach = high pressure ie sphincter

Question 36

functions of the sphincters of the tract

Answer 36

*there are 6 >act as one way valves by maintaining a positive resting pressure relative to two adjacent structures >stimuli (pressure) proximal and distal to a spitter cause opening and closing respectively

Question 37

upper oesophagus sphincter UOS

Answer 37

skeletal muscle relaxes to allow swallowing and closes during inspiration

Question 38

lower oesophageal sphincter LOS

Answer 38

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

Question 39

pyloric sphincter

Answer 39

regulates gastric emptying and usually prevents duodenal gastric reflux

Question 40

ileocaecal sphincter

Answer 40

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

Question 41

internal and external anal sphincters

Answer 41

are regulated by defaecation reflex