Neuro 8 - Physiology and pharmacology of the ANS Flashcards
From where do parasympathetic nerves emerge from?
From the base of the brain
End of the spinal cord (sacral)
Give 4 parasympathetic cranial nerves
Oculomotor (3)
Facial nerve (7)
Glossopharyngeal (9)
Vagus (10)
What is the name of the nerve which emerges from the sacral region?
Splachnic nerve
Describe the parasympathetic outflow
Usually 2 neurons are set up:
one is pre ganglionic, the other post ganglionic, synapse at the ganglion.
The ganglion is situated close to the targeted tissue
What is the important neurotransmitter in the parasympathetic nervous system?
Acetylcholine
From where do neurons emerge from in the sympathetic nervous system? where do they synapse?
Thoracolumbar (T1 - L2)
They all synapse in the sympathetic trunk (=alignment of ganglia)
Short pre ganglionic fiber, long post ganglionic
What neurotransmitters are involved in the sympathetic nervous system?
ACh for preganglionic synapse.
NA for postganglionic (to produce effect on organ)
Except adrenal medulla, skin/sweat glands/ peripheral blood vessels
What is particular about the sympathetic innervation of the adrenal medulla?
Adrenal medulla is innervated by sympathetic pre-ganglionic nerve fibres (so doesn’t synapse in sympathetic trunk), which release acetyl choline that binds to the nicotinic acetyl choline receptors and causes release of catecholamines (A or NA) from the chromaffine cells which will act via bloodstream on target organ.
What is special about the sympathetic innervation of the sweat glands?
The sympathetic nerves to the sweat glands (postganglionic) release acetylcholine instead of noradrenaline.
What is different between the innervation of the sympathetic and parasympathetic systems?
In parasympathetic, one nerve leads to one nerve. However, in sympathetic one neuron can branch off and synapse elsewhere, divide after ganglion to have different postganglionic fibers.
Describe how the sympathetic nervous system exerts control over the trachea and bronchi.
The trachea and bronchi have no sympathetic nerve fibres. They are affected by the release of catecholamines from the adrenal medulla.
Describe the response of blood vessels to skeletal muscle during a period of increased sympathetic discharge. What causes this response?
Blood vessels supplying the skeletal muscle dilate. This is caused by circulating catecholamines.
Describe the effect of the SNS/PNS on the heart.
The SNS has an inotropic (increase force of contraction) and chronotropic (increase heart rate) effect.
The parasympathetic decreases HR. When baroreceptors sense a high BP, it activates parasympathetic pathway to slow down HR. ALSO stimulates inhibitory neuron that acts on sympathetic
BOTH systems are on at any time, just one dominates.
Intrinsic resting HR is 110 bpm however normal is 70 so parasympathetic dominates at rest.
Describe the effect of increased sympathetic firing on the blood vessels.
Increased sympathetic firing causes constriction of most blood vessels (except those supplying the skeletal muscle, which dilate)
How does the SNS affect arterial BP?
SNS increase cardiac output via the chronotropic and inotropic effects and it increases TPR by causing vasoconstriction so it increases blood pressure.
Describe the effects of sympathetic and parasympathetic activity on the eye muscles.
Sympathetic - causes dilation of the pupil (contraction of radial muscles)
Parasympathetic - constriction of the pupil and ciliary muscle contracts (lens bulges allowing near vision)
So pupillary constriction is a parasympathetic effect.
What can the vagus nerve do in the stomach?
Controls secretions like HCL/pepsin and motility
Parasympathetic action
Describe how the SNS affects the GI tract.
SNS decreases gut motility and tone
SNS stimulates contraction of sphincter
SNS generally inhibits secretory activity –> tends to say “don’t need gut atm”
–> PARAsympathetic = rest and digest so logic
How does the sympathetic nervous system increase oxygen delivery to the lungs?
Via NA/A from the adrenals. There is no sympathetic innervation of the lungs.
Describe the nervous control of the bladder.
Has both parasympathetic and sympathetic nervous controls. Parasympathetic is dominant - exerts control over detrussor muscle (muscle that makes bladder contract to eject urine). Sympathetic has control over internal spincter. Motor nerves (ie voluntary control) is exerted over the external sphincter.
Nervous control is a reflex; pressure builds up in the bladder, this sends signal -> reflex activation of the parasympathetic activity. Detrussor muscle contracts, internal sphincter relaxes because inhibition caused.
What is acetylcholine made from?
Acetic acid + choline
What are catecholamines derived from?
Dopamine
What structure is common to all catecholamines?
They contain a cathecole ring, ie 6 carbon ring with double hydroxyl group
If you block acetylcholine action in autonomic ganglia, how would it affect heart rate?
Depends if you are at rest or not
If at rest - HR increases as parasympathetic is dominant
If exercising, sympathetic dominant so heart will slow down
What 2 receptors can recognise ACh?
Muscarinic and Nicotinic
What are the differences between nicotinic and muscarinic receptors?
Nicotinic receptors are situated at autonomic ganglia (both symp and parasymp)
Nicotinic is an ion channel linked receptor; binding of ACh causes influx of sodium and increases cell depolarisation - very fast
Muscarinic receptor is a Gprotein coupled receptor so the effect is slower.
Situated at all effector organs innervated by post ganglionic parasympathetic fibers as well as sympathetic post ganglionic fibers that involve ACh (not NA like usual so in sweat glands for example) so wherever ACh is released that is not autonomic ganglia.
Which receptor recognise NA at the effector organ in sympathetic NS?
Adrenoreceptors
G protein coupled
4 types:
alpha 1 - predominant receptor - constriction - responds well to NA
alpha 2
beta 1
beta 2 - important receptor - doesn’t respond well to NA - dilates. However, responds well to adrenaline. so wherever lots of receptors you will see vasodilation - prevalent in skeletal muscle.
Describe the synthesis and breakdown of acetyl choline.
Precursors - Acetyl Co A and choline transformed into NT acetylcholine + CoA by choline acetyl transferase.
ACh put into vesicle, AP, Ca2+ influx, vesicle exocytosis.
ACh binds to receptor. Rest is transformed into Choline + Acetate by Acetylcholine esterase and taken back up into cell. Choline used to synthesise more ACh.
What would happen if you inhibit acetylcholinesterase?
This would inhibit reuptake of acetylcholine into cell therefore prolong and increase effect of ACh.
Describe the synthesis of noradrenaline
Multiple steps
Tyrosine taken up into cell -> converted to DOPA by tyrosine hydroxylase -> converted into dopamine by DOPA decarboxylase -> into vesicle -> DA convertes to NA by dopamine beta hydroxylase
What can happen to the noradrenaline after it has had its effect?
Can be reuptaked in 2 different ways:
1 - taken into presynaptic cell and degraded in mitochondria by monoamine oxidase A (MAO-A) which forms metabolites
2 - taken into postsynaptic cell and degraded by COMT
Which cells in the adrenal medulla produce catecholamines?
Chromaffin cells
What extra step and enzyme is involved in the synthesis of adrenaline?
(Adrenaline is secreted into the blood not at synapse)
Same synthesis steps as NA - gives NA but extra step is converting NA to A by Phenylethanolamine Methyl transferase (PNMT)
Describe the effect of cortisol on adrenaline secretion
Cortisol increases PNMT effect so increases A secretion
