introduction to structure, functions and control of the alimentary canal Flashcards
function of mouth and pharynx
chops and lubricates food, starts carbohydrate digestion and delivers food to the oesophagus
oesophagus
propels food to stomach
stomach
stores/churns food, continues carbohydrate digestion, initiates protein digestion, regulates delivery of chyme to duodenum
small intestine
(duodenum, jejunum, ileum) principal site of digestion and absorption of nutrients
large intestine
(caecum, appendix, colon)
colon reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum
rectum anal canal and anus
regulated expulsion of faeces
accessory structures are …
salivary glands
the pancreas
the liver and gall bladder
(hepatobilliary system)
what consists of the generalised structure of the digestive wall
mucosa, submucosa, muscular externa, serosa
mucosa
> mucous membrane (epithelial, exocrine gland and endocrine gland cells)
lamina propria (neuroendocrine cells)
muscular mucosae (gut motility)
submucosa
connective tissue
larger blood and lymph vessels
glands
submucous plexus (neurone network)
muscular externa
circular, longitudinal muscle layer myenteric plexus (neurone network)
serosa
connective tissue
main functions of the alimentary canal
motility
secretion
digestion
absorption
motility
mechanical activity mostly involving smooth muscle - skeletal muscle at mouth, oesophagus and others
secretin
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
digestion
chemical breakdown of enzymatic hydrolysis of complex foodstuffs to smaller, absorbable units
absorption
transfer of the absorbable products of digestion - with water electrolytes and vitamins - from the digestive tract to the blood or lymph
how does GI motility work
> 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
explain electrically coupled smooth muscle in GI tract
> 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
what is the purpose of slow wave electrical activity
determines: frequency, direction and velocity of rhythmic contractions
how does slow wave electrical activity work?
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
what are slow waves aka ?
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
whether or not the slow wave amplitude reaches threshold depends on:
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
what does the parasympathetic autonomic innervation of the GI tract consist of ? and how do they work
> 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
what consists of the sympathetic autonomic innervation of the GI tract ? and how do they work
*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
what is the ENS
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
myenteric plexus
“Auerbach’s”
>mainly regulates motility and sphincters
submucous plexus
“Meissner’s”
>mainly modulates epithelial and blood vessels
sensory neurones
mechanoreceptors, chemoreceptors, thermoreceptors
interneurones
the majority, co-ordinating reflexes and motor programmes
effector neurones
> excitatory and inhibitory motor neurones
>they supply: both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels
what do nerve reflexes do ? and give examples
control motor and secretory activity
> a local reflex = peristalsis
short reflex = intestino-intestinal inhibitory reflex
long reflex = gastroileal reflex (a vaso-vagal reflex)
the major motility patterns
peristalsis and
segmentation
peristalsis
> 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)
segmentation
> 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
functions of the sphincters of the tract
*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
upper oesophagus sphincter UOS
skeletal muscle relaxes to allow swallowing and closes during inspiration
lower oesophageal sphincter LOS
relaxes to permit entry of food to the stomach and closes to prevent reflux of gastric contents to the oesophagus
pyloric sphincter
regulates gastric emptying and usually prevents duodenal gastric reflux
ileocaecal sphincter
regulates flow from ileum to caecum, distension of ileum opens and distension of proximal colon closes
internal and external anal sphincters
are regulated by defaecation reflex
