Flashcards in CVS - autonomic system Deck (63)
On what basis is the autonomic system divided into parasympathetic and sympathetic?
On anatomical grounds - where the spinal nerves exit.
Some textbooks also divide the autonomic nervous system into a third division: sympathetic, parasympathetic and ...
Enteric nervous system - a network of neurones surrounding the GIT. It is normally controlled by a mixture of parasympathetic and sympathetic fibres.
Describe the general set up of the neurones in the parasympathetic nervous system e.g. start, middle and finish.
Two neurones arranged in parallel.
1. Pre-ganglionic neurone (cell body is in the CNS)
2. Post-ganglionic neurone
3. Target cell
Where do the preganglionic neurones arise from in the sympathetic nervous system?
Thoracolumbar origin ( T1 to L2 or L3)
Where do the pre-ganglionic and post-ganglionic neurones synapse in the sympathetic nervous system?
Most synapse in the paravertebral chain of ganglia but some synapse in a number of prevertebral ganglia (between paravertebral chain and target organ).
Where do the preganglionic neurones arise from in the parasympathetic nervous system?
Craniosacral origin ( cranial nerves 3,7,9 and 10; S2-S4)
Where do the cranial nerves arise from?
They are twelve pairs of nerves which arise directly from the brain, not from the spinal cord, and pass through separate apertures in the skull.
Where do the pre-ganglionic and post-ganglionic neurones synapse in the parasympathetic nervous system?
They synapse with neurones in ganglia close to the target tissues. Then have short post-ganglionic neurones (these can actually be embedded within the wall of the target organ)/
What neurotransmitters are released by the pre-ganglionic neurones and post-ganglionic neurones of the sympathetic system? What receptors are present for these neurotransmitters?
Pre-ganglionic: ACh (bind to nAChR)
Post-ganglionic NA (bind to adrenergic receptors)
The sympathetic input to sweat glands is an exception - what type of neurotransmitter does this use?
Cholinergic (though some like hands are triggered by adrenaline too).
Describe the specialisations of chromaffin cells of the adrenal medulla:
They are specialised postganglionic sympathetic neurones, with no axons. In response to ACh binding to their nAChRs they release adrenaline which circulates in the blood stream.
What type of receptors does adrenaline and noradrenalin e work upon?
Adrenoreceptors = G protein-coupled receptors e.g. alpha1,2; beta1,2 (and others)
Why do different tissues have different sub-types of adrenoreceptors? How is this useful to practitioners?
This allows them diversity in their actions/response to the same neurotransmitters. This can be used to selectively target drugs to specific tissues.
What co-transmitters can also be released with NA or adrenaline at the synapse of post-ganglionic neurones with effector cells?
Neuropeptide Y (NPY) and ATP
What neurotransmitters are released by the pre-ganglionic neurones and post-ganglionic neurones of the parasympathetic system? What receptors are present for these neurotransmitters?
Preganglionic - ACh (nAChR)
Post-ganglionic - ACh (mAChR)
What type of receptors are muscarinic receptors?
G-protein coupled receptors - unlike nAChR they do not have an integral ion channel
When is the sympathetic system dominant over the parasympathetic system and vice versa?
Sympathetic activity is increased under stress.
Parasympathetic system is more dominant under basal conditions.
What is the sympathetic and parasympathetic actions on the pupil of the eye? Which receptors mediate these actions?
Sympathetic = pupil dilation (radial muscle contraction), mediated by alpha-1 receptors
Parasympathetic = pupil contraction (sphincter muscle contraction), mediated by M3 receptors.
What is the sympathetic and parasympathetic actions on the airways of the lungs? Which receptors mediate these actions?
Sympathetic = relaxation of bronichole smooth muscle -> vasodilation of bronchioles, mediated by Beta-2 receptors
Parasympathetic = vasoconstriction, mediated by M3 receptors.
What is the sympathetic and parasympathetic actions on the heart? Which receptors mediate these actions?
Sympathetic = increased chronotropy and ionotropy, mediated by beta-1 receptors
Parasympathetic = decreased chronotropy, mediated by M2 receptors.
What is the sympathetic and parasympathetic actions on the sweat glands? Which receptors mediate these actions?
1. localised secretion (e.g. palms), mediated by alpha-1 receptors
2. Generalised secretion -> temperature control, mediated by M3 receptors
Parasympathetic = NO EFFECT!
Sympathetic drive to different tissues is independently regulated. When is this not the case?
On some occasion (fight and flight) there can be a more co-ordinated sympathetic response.
What does the ANS control in the cardiovascular system?
1. Heart rate
2. Force of contraction of heart
3. Peripheral resistance of blood vessels
What is the affect of the ANS on electrical activity in the heart?
It does not initiate electrical activity - the heart has its own automaticity (a denervated heart still beats but at a faster rate (~100bpm)). Therefore the parasympathetic NS dominates (vagal influence) at rest -> a decrease in this natural heart rate set by SA cells.
Describe the pre-ganglionic neurones of the parasympathetic input to the heart and where they synapse. What neurotransmitters and receptors do they use?
Preganglionic fibres = 10th cranial nerve (vagus). They synapse with the post-ganglionic cells on the epicardial surface or within the walls of the heart at SA and AV node.
Post-ganglionic cells release ACh which acts on M2 receptors.
What is the effect of parasympathetic input at the SA and AV node?
Decreased chronotropy (heart rate)
Decreased AV node conduction velocity (it enhances the natural slowing down of conduction that occurs anyway at the AV node).
Describe the pre-ganglionic neurones of the sympathetic input to the heart and where they synapse. What neurotransmitters and receptors do they use?
Pre-ganglionic fibres synapse with post-ganglionic at the sympathetic trunk/chain. The post-ganglionic fibres then innervate the SA node, AV node and myocardium. They release NA and act mainly on B1 receptors (B2 and B3 are also present but the main effect is mediated by B1).
What is the effect of parasympathetic input at the SA, AV node and myocardium?
Positive ionotropy (force of contraction).
Why does the parasympathetic nervous system not cause a change in ionotropy?
The parasympathetic NS, unlike the sympathetic NS, does not innervate much myocardium.