Structure and Motility Flashcards
what is the alimentary canal
series of hollow organs running from mouth to anus (oral to aboral) that are separated by sphincters, controlling movement
function of mouth and oropharynx
chops and lubricates food
starts carbohydrate digestions via salivary glands
delivers food oesophagus
function of oesophagus
muscular tube that propels food to stomach
function of stomach
stores/churns food
continues carbohydrate, initiates protein digestion via gastric glands that contain digestive enzymes
regulates delivery of chyme to duodenum
what is chyme
thinly divided particles of food, small enough to leave stomach and enter the duodenum of the small intestine
function of small intestine
made up of duodenum, jejunum and ileum
principal site of digestion and absorption of nutrients - in particular the duodenum and jejunum
function of large intestine
made up of caecum, appendix and colon
colon - reabsorbs fluid and eelectrolytes (not absorbed in small intestine), stores faecal matter and further digestion by bacteria (vitamin K)
function of rectum and anus
regulated expulsion of faeces
accessoy structures of alimentary canal
salivary glands - parotid, sublingual and submandibular glands
liver and gallbladder - hepatobiliary system
exocrine pancreas
general structure of digestive tract wall - 4 layers
mucosa - inside
submucosa
muscularis externa
serosa - outside
general structure of digestive tract wall - mucosa
mucous membrane (epithelial, exocrine gland and endocrine gland cells) lamina propia (capillaries, enteric neurones, gut associated lymphoid tissue) muscularis mucosae
general structure of digestive tract wall - submucosa
connective tissue
larger blood and lymph vessels
glands
submucous plexus (enteric nervous system)
general structure of digestive tract wall - muscularis externa
circular muscle layer myenteric plexus (controls muscle) longitudinal muscle layer
general structure of digestive tract wall - serosa
connective tissue
4 major functions of alimentary canal
motility
secretion
digestion
absorption
describe motility of alimentary canal
mechanical activity mostly involving smooth muscle - circular, longitudinal layers and the muscularis mucosa
skeletal muscle at - mouth, pharynx, upper oesophagus and external anal sphincter
describe secretion of alimentary canal
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 and lubrciation
describe digestion of alimentary canal
chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbale, units
describe absorption of alimentary canal
transfer the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood, or lymph - mediated by numerous transport mechanisms
circular muscle contraction
lumen becomes narrower and longer
longitudinal muscle contraction
intestine becomes shorter and fatter
muscularis mucosae contraction
change in absorptive and secretory area of mucosa (folding), mixing activity
describe electrically coupled smooth muscle cells
adjacent smooth muscle cells are coupled by gap junctions, electrical currents flow from cell to cell
this allows for a slow, synchronous wave of depolarisation to spread across muscle sheets - single unit smooth muscle as opposed to multiunit smooth muscle (do not contract in isolation)
describe specialised pacemaker cells of the electrically coupled smooth muscle cells
specialised pacemaker cells drive spontaneous activity across the coupled cells
this is modulated by;
intrinsic (enteric) and extrinsic (autonomic) nerves
numerous hormones
describe electrical activity of the stomach, small and large intestine
spontaneous electrical activity occurs as slow waves - rhythmic patterns of membrane depolarisation and repolarisation that spread from cell to cell via gap junctions
describe slow wave electrical activity and how it is driven
determines the frequency, direction and velocity of rhythmic contractions
is driven by interstitial cells of Cajal (ICCs) - pacemaker cells (calcium dependent channels)
ICCs are located largely between the circular and longitudinal muscle layers
describe ICCs in slow wave electrical activity
ICCs form gap junctions with each other and smooth muscle cells electrically coupling them
slow waves in ICCs drive slow waves in the smooth muscle cells coupled to them - depolarising slow waves do not necessarily result in smooth muscle contraction
some ICCs form a bridge between nerve endings and smooth muscle cells
describe contraction in the intestine
occurs only if the slow wave (basic electrical rhythm) amplitude is sufficient to reach a threshold to trigger smooth muscle calcium action potentials
force is related to number of action potentials discharged (driven in turn by the duration of the slow wave that is above threshold)
slow waves vary in frequency along length of GI tract
stimuli dependent for slow wave amplitude to meet threshold
neuronal stimuli
hormonal stimuli
mechanical stimuli - e.g. presence of food
generally act to depolarise smooth muscle cells (by changing the starting point - membrane potential) rather than influence slow waves directly - depolarisation shifts slow wave peak to threshold
autonomic innervation of GI tract - parasympathetic nerves
vagal nerves (sensory and motor) from medulla oblongata
pelvic nerves from sacral spinal cord
pre-ganglionic fibres (neurotransmitter - ACh) synapse with ganglion cells within the enteric nervous system (ENS)
excitatory influences of parasympathetic nerves of GI tract
increased gastric, pancreatic and small intestinal secretion
increased blood flow
increased smooth muscle contraction
inhibitory influences of parasympathetic nerves of GI tract
relaxation of some sphincters
receptive relaxation of stomach
autonomic innervation of GI tract - sympathetic nerves
functionally less important than parasympathetic
superior cervical ganglion and pre-vertebral ganglia (celiac, superior mesenteric and inferior mesenteric) from thorco-lumbar region
pre-ganglionic fibres (neurotransmitter - ACh) synapse in the pre-vertebral ganglia
post-ganglionic fibres (neurotransmitter - NA) innervate mainly enteric neurones, but also other structures
excitatory influences of sympathetic nerves of GI tract
increased sphincter tone
inhibitory influences of sympathetic nerves of GI tract
decreased motility
decreased secretion
decreased blood flow
describe the enteric nervous system (ENS)
very complex
cell bodies are located in ganglia, connectived by fibre tracts wihtin;
myenteric (Aurebach’s) plexus
submucous (Meissner’s) plexus
intrinsic to GI tissue - reflex circuits operate independently but influenced by hormones and extrinsic nerves
co-ordinates muscular, secretive and absorptive activities
function of myenteric plexus
regulates motility and sphincters
function of submucous plexus
modulates epithelia and blood vessels
how does the ENS co-ordinate its activities
via (in order);
sensory neurones - mechanoreceptors etc.
interneurones
effector neurones
example of local reflex (major motility pattern)
peristalsis;
PP - 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
example of 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
example of long reflex
gastroileal reflex (a vago-vagal reflex) - increase in gastric activity causes increased propulsive activity in the terminal ileum
major motility patter - segmentation (mixing or churning movements)
rhythmic contractions of circular muscle layer that mix and divide luminal contents
occurs in small intestine (in fed state) and in large intestine (haustration)
major motility patter - colonic mass movement
powerful sweeping contraction that forces faeces into rectum
major motility patter - migrating motor complex
powerful sweeping contraction from stomach to terminal ileum
major motility patter - tonic contractions
sustained contractions;
low pressure - organs with major storage function
high pressure - sphincters
sphincters of GI tract
6 in total
circular, generally smooth, muscle
act as one way valves by maintaining positive resting pressure relative to 2 adjacent structures - e.g. oesophagus and stomach
stimuli (positive) proximal and distal to sphincter cause opening and closing respectively
upper oesophageal sphincter
skeletal muscle
relaxes to allow swallowing
closes during inspiration
lower oesophageal sphincter
relaxes to permit entry of food to the stomach
closes to prevent reflux of gastric contents to the oesophagus
pyloric sphincter
regulates gastric emptying
usually prevents duodenal gastric reflux
ileocaecal valve
regulates flow from ileum to caecum
distension of ileum opens, distension of proximal colon closes
internal (smooth muscle) and external (skeletal muscle) anal sphincters
regulated by defaecation reflex
