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HOM > General gut physiology > Flashcards

General gut physiology Flashcards

1
Q

what is the innermost gut layer and what is it composed of

A

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

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2
Q

what is the layer following mucosa

A

submucosa containing the submucosal plexus

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3
Q

muscularis externa

A

including inner and outer layers of smooth muscle, there is the myenteric plexus located between the two layers

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4
Q

what is circular and what is the longitudinal layer of smooth muslce

A

circular = inner, longitudinal = outer

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5
Q

what is the serosa

A

outermost layer consisting of connective tissue and simple squamous epithelium

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6
Q

what is splanchnic circulation

A

blood supply to the pancreas, stomach, intestines, liver and spleen

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7
Q

75% of the blood passes through what vessel to the liver

A

hepatic portal vein

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8
Q

oxygenated blood reaches the liver via what vessel

A

hepatic artery

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9
Q

what is functional hyperaemia

A

increased splanchnic blood flow following a meal

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10
Q

how does parasympathetic stimulation work

A

increases blood flow locally e.g. in salivary gland

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11
Q

increase in blood flow else where following parasympathetic stimulation could be due to….

A

secondary effect following increased metabolic rate

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12
Q

what can sympathetic vasoconstriction do

A

reduce splanchnic blood flow

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13
Q

what are the vessels responsive to

A

circulating vasoconstrictors such as angiotensin II and ADH at high levels

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14
Q

great veins of the gut act as …… vessels

A

capacitance vessels at rest.

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15
Q

what does venoconstriction do

A

add blood from mesenterin veins and blood from the liver to the general circulation

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16
Q

what is the counter current arrangement of blood supply to the villi

A

arterial blood supply to villi ascends in from the base while the venous supply descends out.

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17
Q

what does the counter current arrangement allow for

A

monosaccharides and AA enter the descending vessels which drain into the hepatic portal vein, then transported to the liver

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18
Q

where do products of fat digestion enter the small intestine

A

they enter lacteals within the intestinal villi

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19
Q

how can lacteals be emptied

A

irregular contractions of smooth muscle within lamina propria, stimulated by an increase in interstitial fluid presure

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20
Q

the central lacteals are emptied via what motion/movement

A

squeezing, the lymph is moved in this way into the lymphatic system proper

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21
Q

how is backflow in lymph vessels prevented

A

valves in the submucosal lymph vessels

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22
Q

what is the layer of the gut epithelium

A

single layer of columnar epithelial cells

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23
Q

what is the role of the gut epithelium layer

A

preventing microbial invasion of the body

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24
Q

why must gut epithelium be renewed continuously with a high turnover rate of 2-6days

A

they are vulnerable to mechanical damage

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25
Q

in the SI where are old epithelial cells shed from

A

shed from the villus tips and are replaced by new ones moving up the sides of the villus

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26
Q

what are the crypts of lieberkuhn

A

blind ending tubules projecting into the gut lining between the villi

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27
Q

where do new cells arise from

A

a stem cell population in the crypts of lieberkuhn

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28
Q

what happens before the older cells are shed

A

new gap junctions are formed beneath them between neighbouring cells, ensures barrier function of gut is not compromised so microbes cannot invade the mucosa

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29
Q

what is the enteric nervous system

A

includes the submucosal and myenteric plexuses which extend from the middle of the oesophagus to the colon

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30
Q

diff in function between myenteric and submucosal plexus

A

SM = secretion vs myenteric = motility

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31
Q

how are the plexuses able to coordinate entirely intrinsic reflex activities e.g. peristalsis

A

sensory cells in the gut wall pass messages to the two plexus relating to e.g. stretch or chemical composition of the lumen

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32
Q

why are bowel transplants so successful

A

ENS can perform many functions independently

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33
Q

what does the autonomic nervous system ANS do

A

provide extrinsic innervation from the central nervous system

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34
Q

what do the ANS fibres normally form synapses with

A

ENS fibres, passing information about the conditions of the body

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35
Q

where is ANS input particularly important

A

proximal gut and rectum

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36
Q

where is intrinsic ENS and hormonal control more important

A

in between proximal gut and rectum

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37
Q

where do sympathetic fibres synapse

A

OUTSIDE of CNS.

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38
Q

where can the cholinergic synapse (sympathetic) be located

A

in one of the paravertebral ganglia of the sympathetic chain or in a separate pre vertebral ganglion within abdominal cavity (more common)

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39
Q

what are most of the postganglionic fibres

A

noradrenergic

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40
Q

what is the effect of sympathetic stimulation on gut motility and secretion

A

inhibitory, but sphincter contraction stimulated

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41
Q

where is parasympathetic supply to the gut carried

A

in the vagus

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42
Q

where do the cholinergic preganglionic fibres synpase

A

within the ENS, often with many cholinergic postganglionic fibres,

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43
Q

what is the effect of parasympathetic stimulation on gut motility and secretion

A

excitatory, but sphincters may be relaxed via inhibitory postganglionic fibres

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44
Q

what do the pelvic nerves supply and what fibres are they (PS or S)

A

the distal colon, rectum and anus. they are sympathetic fibres arising in the sacral spinal cord

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45
Q

what are the 3 categories of sensory neurons in the gut

A

IPANS, general visceral afferent fibres, and IFANS

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46
Q

what are the intrinsic primary afferent neurons

A

sensory fibres located entirely within the ENS. these fibres form the afferent limbs of local reflexes incl those of peristalsis/mixing/secretion

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47
Q

what are general visceral afferent fibres

A

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.

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48
Q

what are vagovagal reflexes

A

reflexes where both afferent and efferent arms are carried by the vagus nerve

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49
Q

what are intestinofugal afferent neurons

A

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.

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50
Q

ifans and prevertebral ganglionic connections do what

A

short circuit a long and mulstisynaptic pathway through the ENS

51
Q

what are long range reflexes

A

coordinates activities of gastrointestinal tract, usually involve a synpase in the prevertebral gangla

52
Q

what do gastrointestinal hormones contribute to

A

a neural response either directly or through stim of vagal afferent fibres

53
Q

what do the feedback mechanisms of some reflexes do

A

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

54
Q

what is the ileal brake mechanism

A

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.

55
Q

what gut peptide does the ileal brake mechanism involve

A

PYY and GLP-1 + nerve fibres

56
Q

what is the gastrocolic reflex

A

food entering the stomach promotes motility of the colon = urge to defaecate

57
Q

what does the gastrocolic reflex involve

A

nerves, gastrin and CCK

58
Q

how can voluntary control exert over swallowing and defaecation

A

striated muscle is present at each end of digestive tract

59
Q

what is neurocrine transmission

A

nerve terminals release a transmitter onto a target cell or into the blood. NT are a type of neurocrine secretion

60
Q

examples of neurocrine transmitters

A

ach, nitric oxide, vasoactive intestinal peptide, noradrenaline

61
Q

Ach

A

released onto muscarininc receptors, excites gut smooth muscle and stim secretion of alnds

62
Q

NO and VIP

A

relax smooth muscle, VIP stim secretion

63
Q

Nadr

A

released by sympathetic neurons rather than neurons of the ENS itself, typically inhibitory but promotes contraction of sphincters and vascular smooth muscle

64
Q

paracrine transmission

A

involves a locally produced substance diffusing through the extracellular fluid to work on neighbouring cells of a different cell type

65
Q

endocrine tranmission

A

this includes hromones travelling via the blood

66
Q

what are gastrointestinal hormones

A

they are all peptides secreted by the enteroendocrine cells in the gut epithelium

67
Q

describe the features of the enterendocrine cells

A

apical membrane exposed to the gut lumen, receptors on this membrane detect luminal conditions and stimulate hormone release in response to certain nutrient subtances

68
Q

give an example of a particular enteroendocrine cell type releasing a particular hormone

A

s cells and secretin

69
Q

what is secretin

A

secreted by s cells of the duodenum in response to the presence of acid.

70
Q

what are the key roles of secretin (5 roles)

A

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

71
Q

what is gastrin

A

secreted by g cells of gastric antrum and duodenum in response to nervous stim + presence of peptides and aa.

72
Q

roles of gastrin (2)

A

1 stim gastric acid secretion by parietal cells and 2 promote growth of oxyntic mucosa

73
Q

what is cholecystokinin CCK

A

secreted by I cells in duodenum and jejunum in response to longchain FFA and monoglycerdies

74
Q

role of CCK (5)

A

1 stim gall bladder contraction, 2 pancreatic secretion and 3 pancreatic growth. 4 inhibits gastric emptying and 5 appetite

75
Q

what is meant by GIP

A

glucose dependent insulinotropic polpeptide

76
Q

incretins are not gip true or false

A

false, they are GIP

77
Q

what is meant by GLP-1

A

glucagon like peptide 1

78
Q

is incretin a GLP-1

A

yes

79
Q

GIP secretion

A

secreted from k cells in upper small instestine

80
Q

GLP-1 secretion

A

secreted from L cells in both small and large intestine

81
Q

what is motilin

A

secreted cyclically during fasting by M cells in upper small intestine under neural control

82
Q

what is the role of motilin

A

initiate migtating myoelectric complex

83
Q

what is ghrelin

A

secreted by endocrine cells of stomach by fasting.

84
Q

role of ghrelin

A

works on hypothalamus to stim appetite, promotes growth horomone release from pituitary gland

85
Q

most gut hormones …. appetite

A

inhibit

86
Q

what is potentiation

A

response of such a cell to a combination of messengers exceeds the sum of the responses to each messenger delivered individually

87
Q

what does potentiation reflect

A

activation of different intracellular pathways contributing to the same end.

88
Q

what is the incretin effect

A

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

89
Q

describe the distribution of gut smooth muscle

A

from middle to lower oesophagus onwards, the gut is lined with smooth muscle

90
Q

how does smooth muscle in the sphincters contract

A

tonically for minutes to hours, but it relaxes when needed

91
Q

what is phasic contraction

A

contraction in walls of stomach and intestines which is slow and rhythmical

92
Q

what happens during phasic contraction

A

wave of depolarisation spreads through gap junctions and the cells are mechanically coupled, allowing coordinated contraction

93
Q

what is single unit smooth muscle

A

smooth muscle in which cells are electrically coupled

94
Q

how is smooth muscle contraction induced

A

calcium induced calcium release from sacroplasmic reticulum

95
Q

what are caveolae

A

indentations of the plasma membrane that increase the surface area and act as calcium stores

96
Q

what is the ratio of actin to mysoin

A

10:1

97
Q

z lines are replaced by …

A

dense bodies which serve as attachment points for thin filaments and are themselves connected to the cytoskeleton

98
Q

what does smooth muscle lack

A

TROPONIN

99
Q

how does excitation contraction coupling occur

A

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

100
Q

what happens when calcium level falls

A

The myosin is dephosphorylated by myosin light chain phosphatase to prevent further cycling

101
Q

what is peristalsis

A

general term referring to gut motility patterns which propel food in the anal direction. an example is peristaltic reflex

102
Q

what is the peristaltic reflex

A

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

103
Q

describe the control of the peristaltic reflex

A

contraction mediated by ach but relaxation controlled by nitric oxide. it is mediated entirely in the ENS so can occur without extrinsic innervation

104
Q

detection of food

A

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.

105
Q

stretch and chemosensitive neurons modulate activity of smooth muscle of the muscularis externa indirectly via….

A

myenteric plexus

106
Q

peristalsis in the oesophagus

A

controlled by somatic motor neurons which cause sequential contractions of the striated muscle

107
Q

reverse peristalsis can occur where…

A

colon

108
Q

what is the basal electrical rhythm

A

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

109
Q

what is tone

A

between slow waves, the smooth muscle retains a basal level of tension

110
Q

what are the interstitial cells of cajal ICCs

A

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.

111
Q

which nervous system innervates the ICCs rather than the smooth muscle directly

A

the enteric nervous system

112
Q

what is the pacemaker activity of the ICCs

A

appears to be based on calcium uptake or release from intracellular stores, resulting in changes in the activity of nearby plasma-membrane ion channels.

113
Q

how are gap junctions formed within the ICCs

A

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

114
Q

depolarization of smooth muscle cells by slow waves originating in the ICCs results in……

A

opening of L type voltage gated calcium channels in their plasma membrane.

115
Q

if calcium conc exceeds the contraction threshold ….

A

smooth muscle will contract

116
Q

if it exceeds electrical threshold….

A

action potentials may be generated based on calcium entry through more VG channels

117
Q

where are spike potentials superimposed

A

onto slow wave in smooth muscle of many parts of the gut

118
Q

does all smooth muscle require action potentials for contraction to occur

A

no

119
Q

how can the amplitude of slow waves be increased

A

excitatory substances such as ach e.g. opening more cation channels will contribute to the depolarisation > more depol > more calcium entering cell > stronger contraction

120
Q

how to decrease slow wave amplitude

A

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

121
Q

what form of contraction of sphincters does not depend on slow wave activity

A

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

122
Q

what is segmentation

A

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

123
Q

describe neuronal control of segmentation

A

Parasympathetic
stimulation is excitatory, sympathetic stimulation is
inhibitory; segmentation contractions become very weak in the
absence of myenteric stimulation.

HOM (8 decks)

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