overview of the GIT Flashcards
Learning Outcomes Recall the basic structure of the Gastrointestinal Tract Recall the innervation of the Gastrointestinal Tract Compare and contrast the organisation and roles of the extrinsic and intrinsic (enteric) nervous system Describe the different types of Gastrointestinal tract motility and their functions Describe the electrophysiological properties of Gastrointestinal Tract smooth muscle cells Describe the neural and endocrine regulation of Gastrointestinal Tract motility
what does the gastrointestinal tract consist of?
- mouth
- oesophagus
- stomach
- small intestine
- large intestine
- appendix
- rectum
- anus
- accessory organs
what separates the gut into sections?
sphincters
overview of motility:
how does food get ingested?
- mastication (chewing food into smaller pieces)
- swallowing
what are the motor functions of the stomach?
- storage
- mixing/propulsion of food
- stomach emptying and its regulation
what are the movements of the small intestine?
- mixing contractions
- propulsive movements
- prevention of back flow
what are the movements of the large intestine?
- mixing/propulsive movements (the is called frustrations)
- defecation
describe the overview of splanchnic circulation:
(what happens to the blood flow once we have absorbed nutrients)
- blood from the gut, spleen and pancreas flow to the liver via the portal vein
- blood is filtered by liver sinuses before leaving via the hepatic vein into the vena cava
what does the filtration of blood before leaving hepatic system into the vena cava allow?
- it allows the removal of bacteria/particulate matter that might enter the blood from the GI tract
- it will prevent the direct transport of potentially harmful agents into the body
what type of cells will remove bacteria from the gut?
reticuloendothelial cells
describe the action of nervous control of the GIT blood flow?
parasympathetic stimulation:
- to the stomach and lower colon
- it will increase blood flow and glandular secretions
sympathetic stimulation:
- vasoconstriction of arterioles (switches off blood flow to the gut)
- happens during exercise
- circulatory shock
what are the 7 layers of the GIT?
(from outside to inside)
- serosa
- longitudinal muscle
- myenteric plexus
- circular muscle
- submucous muscle
- submucous plexus
- mucosa (gut epithelium with sub epithelial connective tissue)
what is the purpose of mucosa?
increases the surface area to allow for more time for absorption
what 2 things will the oesophagus and the rectum lack in terms of its general organisation?
serosa and mesentery
where is the submucosal plexus be found?
small and large intestine
when stimulates, what are the main effects of the myenteric plexus?
- increased tonic contraction (tone of the wall)
- increased intensity of rhythmical contractions
- increased rate of rhythm of contraction
- increased velocity of conduction od excitatory waves along the gut wall
how does the structure of the GIT smooth muscle allow the contraction of a syncytium?
- the muscle fibres are connected by gap junctions and focal adhesions (adherens junctions)
- actin and myosin filaments aligned diagonally along the axis of cells
- filaments anchored at dense bodies that are scattered throughout the sarcoplasm
- contractile arrays anchored to sarcolemma by dense plaques.
what are the 2 different neural controls of the GIT function and describe them?
EXTRINSIC- neuronal and hormonal
- Ach excites
- Noradrenaline and adrenaline inhibits
INTRINSIC (enteric) - myogenic, neural and chemical
- does not need sympathetic or parasympathetic (they will only modify the action.
- all GIT motilities do not require external nerve supply
- they are generated by enteric nervous system and extrinsic factors will only modify them.
Enteric nervous system:
1- what does it consist of?
2- where does it receive input from?
3- where does it lie between?
4- what is its main function?
1- consists of sensory, inter and motor neurons
2- receives inout from the ANS, but it can function independently from it
3- lies between the circular and longitudinal smooth muscle layers (it will innervate both layers)
4- main function is motor control of the GIT.
submucosal plexus:
1- where does it lie between?
2- what does it innervate?
1- it lies between the circular smooth muscle layer and the mucosa
2- it innervates the epithelium, endocrine glands and blood vessels (controls GIT secretion and local blood flow)
what are the 2 components of the extrinsic nervous system?
sympathetic and parasympathetic ns (branches of the autonomic nervous system)
parasympathetic neurons:
1- what are they branches of?
2- what do they innervate?
3- what is its main function?
1- they are branches of the vagus, pelvic and splanchnic nerves
2- they innervate neurons in the myenteric plexus
3- main function is to stimulate GIT motility and secretion.
sympathetic neurons:
1- where do they originate from?
2- where do they terminate?
3- what is their main function?
1- they originate from the celiac, superior and inferior mesenteric and hypogastric plexus
2- they terminate on the enteric nerves, smooth muscle and mucosa
3- main function is to inhibit GIT motility and secretion
what is the function of:
- intermediate filaments
- gap junctions
- focal adhesions
intermediate filaments = maintain cytoskeleton organisation
gap junctions = couples adjacent cells electrically and chemically in some smooth muscle types
focal adhesions = couples adjacent cells mechanically
what are the 2 types of waves of electrical activity in the GI tract? And what is the difference between them?
Slow waves - these are fluctuations that will not initiate smooth muscle contraction. (Basel electrical rhythm, pacesetter potentials)
Spikes = these are action potentials that will allow for enough calcium release to cause smooth muscle contraction.
1- what is GI smooth muscle stimulated by?
2- where and by what is it initiated by?
1- stimulated by continual slow, intrinsic electrical activity
2- initiated in the stomach by interstitial cells of Cajal (ICC)
describe what slow waves are?
describe what they do?
- they are not action potentials but slow undulating changes in resting membrane potential
- they do not themselves cause muscle contraction (they may only in the stomach)
- they will excite appearances of spike potentials
what is the intensity and frequency range of slow waves?
intensity = 5-15mV
frequency = 3-12 per min
what are spike potentials?
when do they occur?
how can they be effected by slow wave potential?
- true action potentials
- they occur when resting membrane exceeds 40mV
- the higher the slow wave potential, the greater the frequency of spike potentials
how long do spike potentials last?
10-50x as long in gastrointestinal muscle in large nerve fibres
- each GI spike lasts 10-20 ms
- baseline membrane potential can also vary.
what is the difference in excitability of GIT between depolarisation and hyperpolarisation?
- depolarisation leads to increased excitability
- hyperpolarisation leads to decreased excitability
what are 3 examples that can stimulate depolarisation?
what are 2 things that can stimulate hyperpolarisation?
1- stretch
2- ach
3- PSNS
1- Norepinephrine
2- SNS
the electrical activity of smooth muscle will spontaneously fluctuate between what 2 values?
what is this initiated by?
-65 and -45 mV
- this is initiated by the pacemaker cells known as interstitial cells of Cajal
describe how the electrical activity of smooth muscle works?
how does the activity start and stop?
- it is initiated b the pace maker cells (interstitial cells of Cajal)
- the activity is mainly due to the opening of the calcium ion channels leading to the influx of Ca into the cells.
- this is then terminated by Ca - dependant K efflux.
what are the 2 main types of GIT motility?
what does each consist of?
1- FED state:
- consists of peristalsis and mixing movement
2- INTERDIGESTIVE state
- also known as migrating motor complexes (gets rid of all the remaining remnants before we have another meal)
1- what is peristalsis?
2- when does it occur?
3- what direction does it move food (endless pathology) ?
1- peristalsis is an autonomic reflex in response to stretch of the GIT wall
2- it will occur during a fed state in all parts of the GIT from the oesophagus to the rectum.
3- it will always move food from oral to rectal direction at 2-25cm
If there is pathology this can be reversed leading to vomitting.
what controls peristalsis?
what happens to the different layers during peristalsis?
- peristalsis requires an active myenteric plexus
- the stretch initiates the circular muscles infront of the plexus to contract and the longitudinal muscles behind it to relax.
what is the function of segmentation?
What is it?
1- the function of segmentation is to mix digested food
It is contractions of circular muscle layer at short intervals along the GIT, these contractions start as 1 opposite and then they move forward and contract in a new original area. (2 opposite)
when does migrating motor complexes occur?
what is their function?
they occur during periods of fasting/between meals
their function is to clear the stomach and small intestine of debris and bacteria.
what are the 4 phases of MMC?
Phase 1= (45-60mins) - quiescent period with slow waves
Phase 2= (30-35mins) - action potential activity on slow waves but sporadic contractions of circular muscle
Phase 3= (2-12mins) action potential activity on slow waves but regular contractions of circular muscle
Phase 4= period in which action potential activity and contraction reduce and merge into phase 1
describe the spike potential between phases 1-3 of MMC?
phase 1= no spike potentials and no contractions
phase 2 = irregular spike potentials and contractions
phase 3= regular spike potential na contractions.
