Function and neurochemistry of the autonomic nervous system (ANS)- Theme 6

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What are the two rapid responses to changes in the environment?
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1) Voluntary: Somatic nervous system acts on skeletal muscles. 2) Involuntary: Autonomic nervous system acts on cardiovascular system and other organs.
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What is the slow response to changes in the environment?
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Hormones, neurotransmitters, growth factors, cytokines and other substances cause changes in gene expression leading to long term adaptation.
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What is the autonomic nervous system?
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The neuronal groups and fibre connections that control the activity of the heart, visceral organs, blood vessels and glands.
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What is the function of the autonomic nervous system?
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Maintains homeostasis by directly or indirectly facilitating the response of every system to external and internal changes.
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What are the four components of the peripheral nervous system?
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1) Somatic motor nerve fibres 2) visceral afferents 3) Autonomic efferent nerve fibres 4) Other afferents
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Define afferent
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Conducting towards the CNS in nerves (sensory neurones) or towards the organ supplied in blood vessels.
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Define efferent
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Conducting away from the CNS in nerves (motor neurones) or away from the organ supplied in blood vessels.
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What are the three subdivisions of autonomic efferent nerve fibres?
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1) Enteric nervous system 2) Sympathetic nervous system 3) Parasympathetic nervous system
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What is the parasympathetic branch of the autonomic nervous system?
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Mainly important for digestion, excretion and visual accommodation. Promotes the ‘couch potato’ state. Less widespread innervation and effects than the sympathetic branch.
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What is the sympathetic branch of the autonomic nervous system?
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Mainly important for ongoing control of the cardiovascular system and reflex responses to stressful situations. Controls fight or flight response. More widespread innervation and effects.
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What are preganglionic nerve fibres?
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Fibres from the central nervous system to the ganglion.
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Are all preganglionic nerve fibres cholinergic?
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Yes, regardless of whether they are in the sympathetic/ parasympathetic nervous system.
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Hence what do all preganglionic fibres release at the ganglion (in the sympathetic/ parasympathetic nervous system)?
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Acetylcholine
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What are cholinergic fibres?
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Nerve fibres which use acetylcholine as their neurotransmitter.
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Why are sympathetic preganglionic fibres shorter than parasympathetic postganglionic fibres?
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Sympathetic ganglia are often closer to the spinal cord than the parasympathetic ganglia, so the parasympathetic fibres have further to travel.
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What is an autonomic ganglion?
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A cluster of nerve cell bodies in the autonomic nervous system. There are two types- sympathetic ganglion and parasympathetic ganglion.
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What are 5 types of efferent nerve fibres (away from the CNS in nerves/ organs in blood vessels)?
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1) Salivary gland 2) Blood vessel 3) Sweat gland 4) Alpha and beta receptors 5) Muscle fibre
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What are the effects of the parasympathetic nervous system on heart rate?
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Decreased heart rate
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What effect does the PNS have on the GI tract?
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Increased gastrointestinal and other secretions Increased peristalsis (GI contractions which mix and propel GI contents)
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What effect does the PNS have on visual accommodation?
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Pupil constriction (allows for focussing)
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What effect does the PNS have on micturition (urination)?
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Detrusor muscle contracts (a smooth muscle found in the wall of the bladder, which relaxes to store urine and contracts to release urine).
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What other effect does it have on the digestive system?
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Defaecation
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What effect does the sympathetic nervous system have on heart rate?
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Increased heart rate and force of contraction
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What effect does the sympathetic nervous system have on blood vessels?
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Vasoconstriction and increased renin release (which raises BP)
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What effect does the sympathetic nervous system have on the lungs?
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Bronchodilation
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What effect does the SNS have on blood glucose?
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Increased blood glucose from free fatty acids (beta oxidation)
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What effect does the SNS have on pupils?
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Pupil dilation
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What effect does it have on the sweat glands?
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Increased activity
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What effect does it have on the bladder?
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Bladder relaxation (stores urine)
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What effect does the parasympathetic nervous system have on arteries?
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Usually no effect
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What effect does the sympathetic nervous system have on arteries?
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Constriction
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What effect does the sympathetic nervous system have on the exocrine glands?
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No effect
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What effect does the parasympathetic NS have on the exocrine glands?
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Increases secretion
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What effect does the sympathetic and parasympathetic NS have on metabolism?
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Sympathetic= increase in plasma glucose Parasympathetic= no effect
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What effect does the sympathetic and parasympathetic NS have on renin?
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Sympathetic= increases secretion from the kidney Parasympathetic= no effect.
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Does the parasympathetic nervous system innervate blood vessels in humans?
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No, it is not involved in the dilation/ constriction of blood vessels.
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What is generally the only nervous system involved in secretion?
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Parasympathetic nervous system
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Under resting conditions, which nervous system dominates heart rate?
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The parasympathetic nervous system- hence why if the ANS is blocked there is a slight increase in heart rate.
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What does it mean when the sympathetic and parasympathetic NS drive to an organ is normally reciprocal?
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As one drive increases the other decreases.
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Where is the pre- ganglionic cell body located in the sympathetic and parasympathetic nervous system?
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SNS= thoracic or upper lumbar PNS= brain stem or sacral
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Where length is the preganglionic axon in the sympathetic and parasympathetic nervous system?
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SNS= short (as ganglia are located closer to the spinal cord) PNS= long
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Where are the ganglia located in the sympathetic and parasympathetic nervous system?
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SNS= paravertebral (beside or near the vertebral column) PNS= in or near organ wall
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What is the ganglionic transmitter in both the sympathetic and parasympathetic nervous system?
answer

Acetylcholine
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What is the ganglionic receptor in both the sympathetic and parasympathetic nervous system?
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Nicotinic.
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What is the length of the post- ganglionic axon in the PNS and SNS?
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SNS= long, with varicosities (release points for neurotransmitter along the effector). PNS= short
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What is the transmitter at the neuro effector junction in the SNS and PNS?
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Sympathetic= noradrenaline (rarely acetylcholine) Parasympathetic= acetylcholine
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What is the receptor at the neuro-effector junction in the SNS and PNS?
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Sympathetic= alpha or beta adrenergic receptors (rarely muscarinic) Parasympathetic= muscarinic receptors
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What are the three effects of the parasympathetic nervous system on the heart?
answer

1) Inhibition of cardiac pacemaker (SA node). This decreases heart rate, CO and BP. 2) Decreased conduction velocity in AV node, as when HR slows down this causes more of a delay which enables the heart to fill properly when it is beating slower. 3) Little direct effect on ventricular contraction
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What vein mediates this response of the PNS on the heart?
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The vagus nerve
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What effect does the parasympathetic nervous system have on blood vessels?
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Parasympathetic nervous system does not usually innervate blood vessels (the main exception being the arteries to the penis). Hence there is little effect on total peripheral resistance.
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However would artificial injection of acetylcholine result in any parasympathetic response in blood vessels?
answer

Yes, as there are cholinergic muscarinic receptors in blood vessels, so artificial injection of acetylcholine would cause vasodilation (which would cause BP to fall)
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What nerves mediate this induced parasympathetic response in blood vessels (vasodilation)?
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Vagus and sacral nerves
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Is there normally Ach in the blood?
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No
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What are the parasympathetic effects on other smooth muscle?
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1) Bronchioles constrict 2) GI tract: spontaneous contractions of gut wall enhanced, intestinal sphincters relaxed, which aids propulsion of GI contents. 3) Detrusor muscle of the bladder contracts (external spincter relaxed= urination).
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What nerves mediate bronchoconstriction and contractions of the internal sphincter in the gut wall and external sphincter in the detrusor muscle?
answer

Vagus and sacral nerves
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In what 4 parts of the body does the parasympathetic nervous system stimulate secretions?
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1) bronchosecretion (mucus) 2) gastrointestinal (gastric acid, pancreatic enzymes) 3) Salivary glands (watery saliva) 4) Lacrimal glands (tears)
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What are the 2 parasympathetic effects on the eye?
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1) Accommodation for near vision (focuses) 2) Pupil constriction (miosis)
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How does the PNS cause focusing of the eye?
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Contraction of ciliary muscle relaxes tension on the lens, allowing it to thicken and shortening focal distance.
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How does the PNS cause pupil constriction?
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Contraction of sphincter pupillae muscle in the iris.
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What nerve mediates this parasympathetic response in the eye?
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The oculomotor nerve
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What is the effect of blocking the ganglionic nicotinic receptors?
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Loss of all autonomic function
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What is an antagonist of the ganglionic nicotinic receptor, which blocks the receptor?
answer

Hexamethonium
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What are 2 effects of blocking the ganglionic nicotinic receptor on the sympathetic nervous system?
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1) Loss of vasomotor tone (as SNS stimulates vasoconstriction), so blood vessels dilate. 2) Dry skin (as sweating controlled by the CNS)
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What are 5 effects of blocking the ganglionic nicotinic receptor on the parasympathetic nervous system?
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1) Heart rate increases (as PNS usually decreases HR) 2) Reduction of visual accommodation (focus, as pupils constrict normally) 3) Reduction of various secretions 4) Constipation 5) Difficulty in micturition (urination)
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What are the effects of blocking muscarinic receptors in the ANS?
answer

Loss of all parasympathetic functions and sweating. No effect on the sympathetic nervous system, only on the sweat gland, as this is the only sympathetic pathway which has muscarinic receptors at its neuro effector junction.
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Hence what is the effect of blocking muscarinic receptors on the sympathetic nervous system?
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Dry skin (due to no sweating)
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What are 5 effects of blocking muscarinic receptors on the parasympathetic nervous system?
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1) Heart rate increases 2) Reduction of visual accommodation 3) Reduction of various secretions 4) Constipation 5) Difficulty in micturition (same effects as blocking ganglionic nicotinic receptors).
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What are the effects of blocking all adrenergic receptors?
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Loss of all sympathetic function except sweating (as the sweat glands have muscarinic receptors at its neuro effector junction).
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In particular what is the effect of blocking the alpha adrenergic receptors?
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Loss of vasomotor tone, so blood vessels dilate (as the SNS usually results in vasoconstriction)
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What is the effect of blocking the beta 1 adrenergic receptor?
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Heart rate falls
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What are 2 effects of blocking the beta 2 adrenergic receptor?
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1) Blood sugar and free fatty acids fall 2) Airways narrow
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When do the effects of blocking the SNS become more important and widespread?
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During stress, excitement and increased physical activity.
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What condition can be caused by stimulating the sympathetic nervous system?
answer

Phaeochromocytoma
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What is Phaeochromocytoma?
answer

A tumour of chromaffin cells which causes secretion of large amounts of noradrenaline and adrenaline.
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What are the 5 signs and symptoms of phaeochromocytoma?
answer

1) Increased blood pressure 2) Pallor (as redness of the skin is affected by blood flow and constriction of vessels limits blood supply to the skin). 3) Increased and irregular heart rate 4) Hyperglycaemia (increased plasma glucose) 5) Increased metabolic rate
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What is the enteric nervous system?
answer

Complex networks (plexuses) of sensory, motor and interneurons forming 2 layers within the walls of the GI tract.
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What are two plexuses inside the enteric nervous system?
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Myenteric and submucosal plexuses (together containing approximately 100 million neurones).
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How many neurotransmitters does the enteric nervous system have approximately?
answer

30 different neurotransmitters, including non- adrenergic, non- cholinergic, acetylcholine, vasoactive intestinal polypeptide and nitric oxide.
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What does the ENS coordinate and control?
answer

Peristalsis Fluid transport Glandular secretion Blood flow within the GI tract.
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Are the effects of the PNS and SNS on the GI tract exerted mainly directly or indirectly?
answer

Indirectly, via modulation of enteric nervous system activity.
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What system can be manipulated to treat MI, angina, arrhythmias, heart failure and hypertension?
answer

Block of sympathetic stimulation of the heart by blocking the beta 1 adrenergic receptor via an antagonist for this receptor at the neuro effector junction.
question

What system can be manipulated to treat asthma?
answer

B2 receptor agonists can be used to dilate the airways (activates sympathetic bronchodilation) Muscarinic receptor antagonists can also be used to dilate the airways (inhibits parasympathetic bronchoconstriction)
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How can drugs be used to treat overactive bladder?
answer

Blocking M3 receptors (inhibits the PNS stimulating detrusor contraction) Activating B3 receptors (stimulates the SNS to stimulate detrusor relaxation) Blocking alpha1 adrenergic receptors
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What is a treatment of hypertension achieved by blocking the SNS?
answer

Alpha 1 receptor antagonists block sympathetically- mediated vasoconstriction.
question

How can premature labour be delayed? (not very successfully)
answer

B2 receptor antagonists (inhibits sympathetic nervous system)
question

Why can Botulinum toxin (Botox) be used to treat incontinence?
answer

Reduces acetylcholine release, so relaxes muscle spasms and wrinkles.
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Why can M antagonists be used to speed up the heart?
answer

It blocks the parasympathetic nervous system from decreasing heart rate, so HR speeds up.

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