General gut physiology Flashcards
what is the innermost gut layer and what is it composed of
Mucosa: the innermost layer, consisting of:
○ Epithelium
○ Lamina propria loose connective tissue containing glands, lymph nodules and capillaries
○ Muscularis mucosae a thin layer of smooth muscle which throws the mucosa into folds
Villi increase surface area
what is the layer following mucosa
submucosa containing the submucosal plexus
muscularis externa
including inner and outer layers of smooth muscle, there is the myenteric plexus located between the two layers
what is circular and what is the longitudinal layer of smooth muslce
circular = inner, longitudinal = outer
what is the serosa
outermost layer consisting of connective tissue and simple squamous epithelium
what is splanchnic circulation
blood supply to the pancreas, stomach, intestines, liver and spleen
75% of the blood passes through what vessel to the liver
hepatic portal vein
oxygenated blood reaches the liver via what vessel
hepatic artery
what is functional hyperaemia
increased splanchnic blood flow following a meal
how does parasympathetic stimulation work
increases blood flow locally e.g. in salivary gland
increase in blood flow else where following parasympathetic stimulation could be due to….
secondary effect following increased metabolic rate
what can sympathetic vasoconstriction do
reduce splanchnic blood flow
what are the vessels responsive to
circulating vasoconstrictors such as angiotensin II and ADH at high levels
great veins of the gut act as …… vessels
capacitance vessels at rest.
what does venoconstriction do
add blood from mesenterin veins and blood from the liver to the general circulation
what is the counter current arrangement of blood supply to the villi
arterial blood supply to villi ascends in from the base while the venous supply descends out.
what does the counter current arrangement allow for
monosaccharides and AA enter the descending vessels which drain into the hepatic portal vein, then transported to the liver
where do products of fat digestion enter the small intestine
they enter lacteals within the intestinal villi
how can lacteals be emptied
irregular contractions of smooth muscle within lamina propria, stimulated by an increase in interstitial fluid presure
the central lacteals are emptied via what motion/movement
squeezing, the lymph is moved in this way into the lymphatic system proper
how is backflow in lymph vessels prevented
valves in the submucosal lymph vessels
what is the layer of the gut epithelium
single layer of columnar epithelial cells
what is the role of the gut epithelium layer
preventing microbial invasion of the body
why must gut epithelium be renewed continuously with a high turnover rate of 2-6days
they are vulnerable to mechanical damage
in the SI where are old epithelial cells shed from
shed from the villus tips and are replaced by new ones moving up the sides of the villus
what are the crypts of lieberkuhn
blind ending tubules projecting into the gut lining between the villi
where do new cells arise from
a stem cell population in the crypts of lieberkuhn
what happens before the older cells are shed
new gap junctions are formed beneath them between neighbouring cells, ensures barrier function of gut is not compromised so microbes cannot invade the mucosa
what is the enteric nervous system
includes the submucosal and myenteric plexuses which extend from the middle of the oesophagus to the colon
diff in function between myenteric and submucosal plexus
SM = secretion vs myenteric = motility
how are the plexuses able to coordinate entirely intrinsic reflex activities e.g. peristalsis
sensory cells in the gut wall pass messages to the two plexus relating to e.g. stretch or chemical composition of the lumen
why are bowel transplants so successful
ENS can perform many functions independently
what does the autonomic nervous system ANS do
provide extrinsic innervation from the central nervous system
what do the ANS fibres normally form synapses with
ENS fibres, passing information about the conditions of the body
where is ANS input particularly important
proximal gut and rectum
where is intrinsic ENS and hormonal control more important
in between proximal gut and rectum
where do sympathetic fibres synapse
OUTSIDE of CNS.
where can the cholinergic synapse (sympathetic) be located
in one of the paravertebral ganglia of the sympathetic chain or in a separate pre vertebral ganglion within abdominal cavity (more common)
what are most of the postganglionic fibres
noradrenergic
what is the effect of sympathetic stimulation on gut motility and secretion
inhibitory, but sphincter contraction stimulated
where is parasympathetic supply to the gut carried
in the vagus
where do the cholinergic preganglionic fibres synpase
within the ENS, often with many cholinergic postganglionic fibres,
what is the effect of parasympathetic stimulation on gut motility and secretion
excitatory, but sphincters may be relaxed via inhibitory postganglionic fibres
what do the pelvic nerves supply and what fibres are they (PS or S)
the distal colon, rectum and anus. they are sympathetic fibres arising in the sacral spinal cord
what are the 3 categories of sensory neurons in the gut
IPANS, general visceral afferent fibres, and IFANS
what are the intrinsic primary afferent neurons
sensory fibres located entirely within the ENS. these fibres form the afferent limbs of local reflexes incl those of peristalsis/mixing/secretion
what are general visceral afferent fibres
cell bodies in dorsal root ganglia or a homologous ganglion of the vagus. their axons transmit signals from the gut to the spinal cord or brain stem and are involved in stomach reflexes, pain and defaecation reflexes.
what are vagovagal reflexes
reflexes where both afferent and efferent arms are carried by the vagus nerve
what are intestinofugal afferent neurons
sensory fibres with cell bodies in the enteric nervous system, sends axons with the sympathetic nerves to synapse in the prevertebral sympathetic ganglia. often forms the afferent limbs of long range inhibitory relfexes used to coordinate activity of diff parts of the gut.
ifans and prevertebral ganglionic connections do what
short circuit a long and mulstisynaptic pathway through the ENS
what are long range reflexes
coordinates activities of gastrointestinal tract, usually involve a synpase in the prevertebral gangla
what do gastrointestinal hormones contribute to
a neural response either directly or through stim of vagal afferent fibres
what do the feedback mechanisms of some reflexes do
reduce motility in proximal sections of the gut if contents are moving too quickly, there are feedforward reflexes enhancing motility of distal parts of the gut in order to make room for incoming contents
what is the ileal brake mechanism
effect of nutrients esp fat which have reached the ileum without being absorbed reducing the motility and secretion of more proximal parts of the digestive tract.
what gut peptide does the ileal brake mechanism involve
PYY and GLP-1 + nerve fibres
what is the gastrocolic reflex
food entering the stomach promotes motility of the colon = urge to defaecate
what does the gastrocolic reflex involve
nerves, gastrin and CCK
how can voluntary control exert over swallowing and defaecation
striated muscle is present at each end of digestive tract
what is neurocrine transmission
nerve terminals release a transmitter onto a target cell or into the blood. NT are a type of neurocrine secretion
examples of neurocrine transmitters
ach, nitric oxide, vasoactive intestinal peptide, noradrenaline
Ach
released onto muscarininc receptors, excites gut smooth muscle and stim secretion of alnds
NO and VIP
relax smooth muscle, VIP stim secretion
Nadr
released by sympathetic neurons rather than neurons of the ENS itself, typically inhibitory but promotes contraction of sphincters and vascular smooth muscle
paracrine transmission
involves a locally produced substance diffusing through the extracellular fluid to work on neighbouring cells of a different cell type
endocrine tranmission
this includes hromones travelling via the blood
what are gastrointestinal hormones
they are all peptides secreted by the enteroendocrine cells in the gut epithelium
describe the features of the enterendocrine cells
apical membrane exposed to the gut lumen, receptors on this membrane detect luminal conditions and stimulate hormone release in response to certain nutrient subtances
give an example of a particular enteroendocrine cell type releasing a particular hormone
s cells and secretin
what is secretin
secreted by s cells of the duodenum in response to the presence of acid.
what are the key roles of secretin (5 roles)
1 stimulates pancreatic growth, 2 bicarbonate and 3 water secretion, 4 inhibits gastric acid secretion and motility and 5 promotes constriction of the pyloric sphincter
what is gastrin
secreted by g cells of gastric antrum and duodenum in response to nervous stim + presence of peptides and aa.
roles of gastrin (2)
1 stim gastric acid secretion by parietal cells and 2 promote growth of oxyntic mucosa
what is cholecystokinin CCK
secreted by I cells in duodenum and jejunum in response to longchain FFA and monoglycerdies
role of CCK (5)
1 stim gall bladder contraction, 2 pancreatic secretion and 3 pancreatic growth. 4 inhibits gastric emptying and 5 appetite
what is meant by GIP
glucose dependent insulinotropic polpeptide
incretins are not gip true or false
false, they are GIP
what is meant by GLP-1
glucagon like peptide 1
is incretin a GLP-1
yes
GIP secretion
secreted from k cells in upper small instestine
GLP-1 secretion
secreted from L cells in both small and large intestine
what is motilin
secreted cyclically during fasting by M cells in upper small intestine under neural control
what is the role of motilin
initiate migtating myoelectric complex
what is ghrelin
secreted by endocrine cells of stomach by fasting.
role of ghrelin
works on hypothalamus to stim appetite, promotes growth horomone release from pituitary gland
most gut hormones …. appetite
inhibit
what is potentiation
response of such a cell to a combination of messengers exceeds the sum of the responses to each messenger delivered individually
what does potentiation reflect
activation of different intracellular pathways contributing to the same end.
what is the incretin effect
a form of potentiation. although plasma glucose alone stimuates insulin release from the pancreas, if the beta cells are stimulataneously exposed to the incretins insulin production is augmented
describe the distribution of gut smooth muscle
from middle to lower oesophagus onwards, the gut is lined with smooth muscle
how does smooth muscle in the sphincters contract
tonically for minutes to hours, but it relaxes when needed
what is phasic contraction
contraction in walls of stomach and intestines which is slow and rhythmical
what happens during phasic contraction
wave of depolarisation spreads through gap junctions and the cells are mechanically coupled, allowing coordinated contraction
what is single unit smooth muscle
smooth muscle in which cells are electrically coupled
how is smooth muscle contraction induced
calcium induced calcium release from sacroplasmic reticulum
what are caveolae
indentations of the plasma membrane that increase the surface area and act as calcium stores
what is the ratio of actin to mysoin
10:1
z lines are replaced by …
dense bodies which serve as attachment points for thin filaments and are themselves connected to the cytoskeleton
what does smooth muscle lack
TROPONIN
how does excitation contraction coupling occur
calcium binds to calmodulin which activated myosin light chain kinase. this phosphorylates a regulatory light chain on myosin allowing it to bind with actin and undergo the cross bridge cycle
what happens when calcium level falls
The myosin is dephosphorylated by myosin light chain phosphatase to prevent further cycling
what is peristalsis
general term referring to gut motility patterns which propel food in the anal direction. an example is peristaltic reflex
what is the peristaltic reflex
occurs when stretching of the gut wall elicits contraction of the inner and outer smooth muscle behind a bolus (small lump of substance) but relaxation of the muscle in front of the bolus
describe the control of the peristaltic reflex
contraction mediated by ach but relaxation controlled by nitric oxide. it is mediated entirely in the ENS so can occur without extrinsic innervation
detection of food
mechanical stretch receptors in the myenteric plexus or by mechanical/chemical stimuli to (5-HT) release from enterochromaffin cells: the 5-HT stimulates local sensory neurons.
stretch and chemosensitive neurons modulate activity of smooth muscle of the muscularis externa indirectly via….
myenteric plexus
peristalsis in the oesophagus
controlled by somatic motor neurons which cause sequential contractions of the striated muscle
reverse peristalsis can occur where…
colon
what is the basal electrical rhythm
slow waves of electrical activity are slow, undulating depolarizations of amplitude between 10-50mV. these slow waves represent a basal electrical rhythm to the gut - responsible for phasic contractions
what is tone
between slow waves, the smooth muscle retains a basal level of tension
what are the interstitial cells of cajal ICCs
ICCs act as pacemakers in the gut, initiating and propagating the slow waves. they are specialised smooth muscle cells containing few contractile elements located mainly between the longitudinal and circular muscle layers.
which nervous system innervates the ICCs rather than the smooth muscle directly
the enteric nervous system
what is the pacemaker activity of the ICCs
appears to be based on calcium uptake or release from intracellular stores, resulting in changes in the activity of nearby plasma-membrane ion channels.
how are gap junctions formed within the ICCs
fine processes of the ICCs form gap junctions within each other and with nearby smooth muscle cells in both the circular and longitudinal layers; the slow waves are propagated within the ICC network and spread from there to the smooth muscle cells
depolarization of smooth muscle cells by slow waves originating in the ICCs results in……
opening of L type voltage gated calcium channels in their plasma membrane.
if calcium conc exceeds the contraction threshold ….
smooth muscle will contract
if it exceeds electrical threshold….
action potentials may be generated based on calcium entry through more VG channels
where are spike potentials superimposed
onto slow wave in smooth muscle of many parts of the gut
does all smooth muscle require action potentials for contraction to occur
no
how can the amplitude of slow waves be increased
excitatory substances such as ach e.g. opening more cation channels will contribute to the depolarisation > more depol > more calcium entering cell > stronger contraction
how to decrease slow wave amplitude
by inhibitory substances e.g. the opening of hyperpolarizing potassium channels > resulting in weaker contraction or no contraction at all if the amplitude is under the threshold
what form of contraction of sphincters does not depend on slow wave activity
tonic contractions - it may be caused by a continuous sequence of action potentials, partial depolarisation of the smooth muscle cell membrane without APs or other mechanisms resulting in sustained levels of intracellular ca
what is segmentation
where different regions of the circular muscle of the gut tube wall contract to aid mixing. slow waves initiated by the ICCs drive segmental contractions but are modulated by nerves and hormones
describe neuronal control of segmentation
Parasympathetic
stimulation is excitatory, sympathetic stimulation is
inhibitory; segmentation contractions become very weak in the
absence of myenteric stimulation.
