alimentary mechanisms Flashcards
enteric nervous system: explain the major features of the enteric nervous system and how it interacts with the autonomic nervous system
number of neurones in GIT wall
10-100 million (100 billion in brain)
what nervous system do neurones in GIT communicate with
autonomic
how are GIT neurones arranged
rich plexuses of nerves and supporting cells and ganglia
what do ganglia consist of
nerve cells to carry signals, and glial cells which provide insulation, protection and nutritional and structural support
what interconnect ganglia
tracts of fine, unmyelinated nerve fibres
main feature of enteric nervous system
integrating centre to specific stimuli independent of CNS for coordinating function
4 things which cause enteric nervous system disruption
inflammation (ulcerative colitis, Crohn’s disease), following invasive abdominal surgery, irritable bowel syndrome, ageing (constipation)
2 plexi in gut wall which communicate directly with each other to make “local” decisions based on “local” signals
myenteric (Auerbach’s), submucosal (Meissner’s)
where is myenteric plexus located
between circular and longitudinal smooth muscle layers
innovation of myenteric plexus
efferent of both layers of smooth muscle
what does myenteric plexus control and what does this allow
controls entire activity of muscularis externa, allowing for coordinated control of motor function and motility
afferent function of submucosal plexus
senses environment within lumen using mechanoreceptors, chemoreceptors and osmoreceptors
efferent function of submucosal plexus
fine tune local blood flow, epithelial transport and secretory, paracrine (diffusion) or endocrine cell function (circulation)
4 key functions of enteric nervous system (SPAM)
secretion, perfusion, absorption, motility
what areas of gut are heavily perfused
VSMCs, submucosal glands, absorbing enterocytes
what is absorbed and how is this controlled
nutrients, vitamins, minerals and ions by adjusting expression of luminal transport proteins
what causes effective gut transit
SMCs in circular and longitudinal muscle layers contract and relax at different rates
what type of neurones are most present
multipolar (one axon, multiple dendrites)
what do sensory neurones respond to
mechanical, thermal, osmotic and chemical stimuli
where do motor cells terminate
SMCs, secretory cells of GI blood vessels
function of interneurones
integrate sensory input and effector output
where are SNS preganglionic neuronal cell bodies located
in spinal cord in lower thoracic and upper lumbar spinal cord
where are SNS postganglionic neuronal cell bodies located
in pre- and paraverterbal ganglia
where are PNS preganglionic neuronal cell bodies located
in brainstem and sacral spinal cord
where are PNS postganglionic neuronal cell bodies located
very close to target organs
length of SNS preganglionic and postganglionic nerves
preganglionic short, postganglionic long from sympathetic chain
length of SNS preganglionic and postganglionic nerves
preganglionic long, postganglionic short
SNS preganglionic innervation
preganglionic neurones are in splanchnic nerves from thoracic and lumbar regions
what do thoracic branches of SNS innervate
foregut and associated organs
what do lumbar branches of SNS innervate
hindgut
PNS innervation
most GIT innervation in branch of ANS arises from vagus nerve; structures from descending colon onwards from pelvic splanchnic nerves (S2-S4)
SNS neurotransmitters between SNS and enteric nervous system
norepinephrine
SNS neurotransmitters in sympathetic chain
acetylcholine
PNS neurotransmitter
acetylcholine
effect on gut function of SNS and PNS
SNS reduces activity (innovation of VSMCs allows rapid shut down of gut perfusion in high stress situations), PNS increases activity
examples of minor plexuses
deep muscular plexus (inside circular muscle), ganglia supplying biliary system and pancreas
what does sympathetic control directly
blood vessels for vasoconstriction (coeliac, superior and inferior mesenteric)
