lecture 16 Flashcards
what is propulsive movement?
- fast
- propels things forwards
- eg. peristalsis
what is mixing/presentation?
- slow
- eg. segmentation
what is accommodation?
- preparing the gut for incoming material
- eg. receptive relaxation
what occurs in peristalsis?
- series of wave-like muscle contractions that move food through the digestive tract
- contraction and relaxation of food pipe moving food to stomach
where does peristalsis begin?
- oesophagus
what is peristalsis stimulated by?
- parasympathetic nervous system via the myenteric plexus
what is activated in segmentation?
- circular muscles in intestines that contract to move food back and forth
what does mixing allow?
- interaction with gastric juices
- helps with break down for digestion
what is receptive relaxation?
- smooth muscle of stomach relaxes when food enters
- enables large amount of food to pass with a minimal rise in intragastric pressure
what is the law of the intestine?
- moves material in oral to anal direction through lower oesophageal sphincter into the stomach
who discovered the law of the intestine?
- Bayliss and starling
what does the stimulus produce?
- excitation above the stimulation point (ascending contraction)
- inhibitor below the stimulatory point (descending relaxation)
what happens when the bolus reaches the bottom of the oesophagus?
- lower oesophageal sphincter relaxes
- food material enters the stomach
what phases occur in the stomach?
- accommodation
- mixing/churning
- propulsion
what occurs in accommodation in stomach?
- receptive relaxation
- allows stomach to store large amounts of food without a significant increase in pressure
what is receptive relaxation mediated by?
- vagal nerve
where does mixing in the stomach occur?
- antrum
- pylorus
- allows the mechanical breakdown of food
what occurs in propulsion?
- small volume of chyme moves through pyloric sphincter into duodenum
what is chyme?
- acidic fluid
- passes from stomach to small intestine
- consists of gastric juices
what is propulsion controlled by?
- basal electrical rhythm (BER)
what is basal electrical rhythm?
- regular wave of spontaneous depolarisations and repolarisations in smooth muscle of the GI tract
what is basal electrical rhythm initiated by?
- pacemaker areas high in the stomach
what parts of the stomach relax to allow material to enter?
- fundus
- body
what does this receptive relaxation allow?
- materials to enter
- allows reservior
what is receptive relaxation?
- reflex where the gastric fundus dilates when food passes down the pharynx and the esophagus
where does the mixing/churning/grinding occur in the stomach?
- antrum
- lower section of the pylorus
what does the mixing allow?
- exposure to acid and protease secretion in stomach
how many electrical waves occur per minute?
- 3 contractions and relaxations every minute to move food down
what are the pacemaker cells of the stomach?
- interstitial cells of cajal (ICC)
what is the function of the ICC?
- act as a conduction pathway for smooth muscle
where are the ICC located?
- greater curvature of the stomach
what are the ICC responsible for?
- normal persitaltic activity
what are the different types of interstitial cells of cajal?
- ICC-AP / ICC-MY (auerbachs plexus/myenteric plexus)
- ICC-IM (intramuscular)
where are the ICC-AP / ICC-MY located?
- between circular and longitudinal muscle layers of the gut
what is the shape and structure of ICC-AP / ICC-MY?
- triangular shaped
- irregular
- multiple processes forming highly branching networks between longitudinal and circular muscle layers
what are the functions of ICC-AP / ICC-MY?
- pacemaker generation of slow waves
- provide conduction pathway for regenerative propagation of slow waves
where are ICC-AP / ICC-MY located?
- colon
- small intestine
- stomach
what are the ICC-IM?
- intermediate step between nerve cell activity and smooth muscle cells
what is the shape and structure of ICC-IM?
- spindle shaped
- scattered throughout longitudinal and circular muscle layers
what are the ICC-IM closely associated with?
- excitatory fibres (Ach & substance P)
- inhibitory fibres (VIP & NOS)
what is the function of ICC-IM?
- mediate neurotransmission between nerve cells and smooth muscle cells
what is retropulsion?
- backwards movement of food from pylorus to the body of the stomach
- reverse peristalsis
when does retropulsion occur?
- when peristaltic contraction overshoots the movement of food
- causes wave to reflect back
what Is the vomiting reflex there for?
- protective reflex
- empties upper duodenum and stomach
- controlled by higher centres
what processes occur in the small intestine?
- segmentation
- propulsion (gastro-ileal reflux)
what occurs in segmentation in the small intestine?
- mixes chyme with enzymes
- exposes to the surface
- maximises absorption
what is the segmentation in small intestine controlled by?
- basal electrical rhythm (BER)
where is the highest segmentation activity in the small intestine?
- towards the top
what occurs in the gastro-ileal reflex (propulsion)?
- after another meal, presence of food in stomach causes peristaltic reflex to empty small intestine
- ensures empty stomach and small intestine before next meal
where does the chyme move?
- through ileocaecal sphincter into large intestine
what processes occur in the large intestine?
- haustration
- peristalsis / mass movement
- defaecation
what is the movement from the small to large intestine?
ileum -> ileocaecal junction -> large intestine
what are haustra?
- small pouches in colon that give segmented appearance
- represents a contraction at one end of the muscle
- enables chyme mixing with water for maximal absorption
what is haustration?
- slow ring like contractions of circular muscle that move food back and forth within colon
what does haustration allow?
- mixing with enzymes and mucus
- helps with absorption of fluid and electrolytes
what occurs in mass movement (peristalsis) of large intestine?
- several hausfrau contract to move material in oral to anal direction
what is different about the peristalsis in large intestine?
- uses mass action contractions to propel faeces towards the rectum
- triggered by the gastrocolic reflex
what occurs in defaecation?
- elimination of waste from digestive tract through anus
what are motor functions of GI tract performed by?
- smooth muscle cells (longitudinal and circular) from lower 2/3 of oesophagus to large intestine
where is skeletal muscle activity important?
- mouth
- top 1/3 of oesophagus
- external anal sphincter
what does pacemaker activity originate from?
- interstitial cells of cajal
what does pacemaker activity take the form of?
- slow waves of depolarisation and repolarisation that form BER
- alongside neural and hormonal reflexes
what are examples of movement under neuronal control?
- long reflex (receptive relaxation)
- short reflex (peristalsis) -> independent of CNS
what is activity of GI tract controlled by?
- sympathetic and parasympathetic nerves
- enteric nervous system
what does control by enteric nervous system allow?
- to be independent of the CNS
what neurotransmitters are used by the enteric nervous system?
- non adrenergic non cholinergic (NANC)
what are examples of NANC neurotransmitters?
- ATP
- dopamine
- GABA
how is gut motility modulated?
- bolus activity sensors are local
- activity of inhibitory neurones downstream of the stimulus
- enterochromaffin cells secrete serotonin
what is the intestinal peristaltic reflex stimulated by? (figure 1)
- chemical activation
- mechanical activation
what does chemical activation cause the release of?
- 5-HT (serotonin)
- irritates the intestines/certain chemicals activated
what does 5-HT bind to once released?
- 5-HT4 receptor on afferent neurones
where do the afferent neurones extend up to?
- myenteric plexus
what do the neurones contain?
- CGRP
what occurs once afferent neurones are activated?
- release CGRP into myenteric plexus
what is the mechanical activation?
- stretching of wall
- activates afferent neurones that also release CGRP
what does CGRP activate?
- interneurones in myenteric plexus which contain acetylcholine
- extends both downstream (caudad) and upstream (orad)
what occurs once Ach is released in myenteric plexus?
- binds to receptors on cell body of motor neurones
- then descend into the circular muscle
what receptor do the motor neurones have?
- cholinergic nicotinic receptors
what occurs downstream?
- VIP/NOS/PHI release allows descending relaxation
- interneurones go from orad to caudad direction
what occurs upstream?
- motor neurones in caudad to orad directions
- release Ach or substance P
- allow ascending contraction
