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Neuroscience 15-19 (Digestion (Model of neural circuit (contraction on…
Neuroscience 15-19
Autonomic NS
Roles: control bodily functions, regulate organs/tissues, essential for homeostasis/allostasis, reflexes, coordinated movement, complex behaviour
Targets: smooth/cardiac muscle, epithelial transport of ions (absorption/secretion), hormone/mucous secretion, metabolism, immune cells
Functional anatomy
in symp, all pre ganglionic in thoracolumbar spinal cord (arranged topographically) -in parasymp, pre ganglionic in craniosacral (in brainstem or sacral spinal cord)
post ganglionic in symp are closer to CNS, in parasymp post ganglionic closer to target organs
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Boundaries: motor pathways but also includes info from periphery (eg visceral sensory nerves), CNS circuits (interpret signals, decide on outcome, activate nerves)
Sympathetic NS
from T1-L3, vast majority of pre ganglionic neurons in intermediolateral nucleus
two types of ganglia: paravertebral (symp chain) and prevertebral ganglia (mesenteric ganglia) and symp pre ganglionic innervate adrenal medulla - secretes hormones
mass activation of symp pathways at once -achieved by turning on adrenal system and CNS has ability to switch on synapses to symp in spinal cord - synapse with lots of organs
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essential for normal functions - adjustment of blood pressure, small temp changes
Parasympathetic NS
brainstem nuclei axons project via cranial nerves - number of nuclei in separate locations in brainstem - connect to diff pathways
Sacral: intermediolateral nucleus - side column of neurons that go to organs - sacral post ganglionic lie in pelvic plexus -pelvic ganglia also contain symp neurons (mixed ganglia, long axons, vulnerable in surgery)
no mass activation system, less axon collaterals to simultaneoulsy activate numerous post ganglionic
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Autonomic dysfunction
Spinal cord injury: partial or complete loss of ascending and descending connection w the brain - cant sense/respond properly to changes in environment -unregulated organs can undergo changes (altered trophic signalling to peripheral nerves)- problems depend on spinal level and extent of damage
Examples (SCI) - Autonomic dysreflexia - occurs when injury above T6- risk varies between individuals - acute hypertensive episode triggered by peripheral signals (eg bladder distension) - rapid extreme hypertension can lead to stroke/seizure/death - sensory activation triggers vasoconstrictor reflex(eg large vascular bed in gut), brain detects rise in BP but cant control it
Example: bladder dysfunction: bladder function requires integration of visceral and somatic NS, both disrupted by SCI - get incontinence, uncoordinated voiding, bladder/renal damage, hypertrophy of bladder muscle
Visceral pain: pain originating from internal organs, perceived as diffuse, imprecisely localised or reffered (visceral sends one axon to large field) - visceral nociceptors have extensive branching of peripheral and cnetral processes - in somatic tissues, nociceptors initiate pain but in organs, they respond to more diverse stimuli and send different messages --can be involved in normal autonomic reflexes - challenge to develop therapies that reduce visceral pain w/out disrupting normal reflexes
Electroceuticals: using devices instead of drugs, to activate/reduce activity of neural pathways -problem: in many peripheral nerves, sensory and autonomic motor axons are intermingled, need to be able to control only relevant nerves in bundle
Sex, steroids, NS
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males have early rise in testosterone which causes masculinisation of genetalia, levels rise at puberty - females get low estrogens until puberty - cyclical patterns during adulthood
release of steroids tightly controlled - secretion determined by hypothalamus -(controlled by circulating hormones)
Gynandromorphs - in birds, mixture of female/male tissues, both sides exposed to same hormones, so any differences in tissues must be due to genes
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Steroid effects on pain
pain can be sexually dimorphic eg migraines more prevelant in females, males (cluster headaches) - exposure to certain hormones can increase sensitivity of nociceptors
many potential sites of actions, steroid receptors expressed in many but not all parts of pain neural circuitry
Digestion
somatic (chewing, swallowing, peristalsis in oesoophagus) and visceral (salivation, primary peristalsis) neurons involved - visceral includes symp/parasymp, many visceral afferents and the ENS
cephalic phase: initiated when we see/smell food- salivation, gastric relaxation, gastric acid secretion (via parasymp vagus nerve) - no food needs to be ingested for response
Mouth - chewing depends on motor pattern generator in brainstem (produces rhythmic movement)- under voluntary contorl, tongue, swallowing (motor pattern generator complex coordination)
Oesophagus - primary peristalsis controlled by CNS via vagus (mixed control - some somatic skeletal, driven by vagus) - ENS also has role (secondary peristalsis)- upper & lower sphincters important to regulate reflux
Stomach- central regulation via vagus -also controlled by pacemaker cells (interstitial cells of Cajal ICC)
small/large intestine- digestion absorption and propulsion , mixing /propulsion under neurla control - control largely peripheral (ENS)
Fed state- (in vitro) - activity only when food in lumen, activity blocked by tetrodotoxin (blocks nerve AP but not smooth muscle AP) - segmenting and propulsive contractions - only ENS present in intestinal wall
ENS - all neurons needed for complex behaviours (intrinsic sensory, excitatory/inhibitory, motor, interneurons), secretomotor (secretion/absorption of water/salt)
Model of neural circuit
contraction on oral/relaxation on anal side of stimulus, movement towards anal end
recurrent excitatory circuit of intrinsic sensory neurons (Requires intrinsic inhibition to prevent spasm) - in gut wall, intrinsic sensory neurons take info from terminals -have outputs to interneurons/local motor to produce behaviours
also make synaptic connections w other sensory neurons - work as cohort (can work as interneurons - lead to propagation of activity ) -drives how motor pathway works -neurons change state/function according to input
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