Autonomic Nervous System Flashcards
What are the sympathetic effects on the heart?
Post-ganglionic fibres of sympathetic trunk innervate SAN, AVN, & myocardium
Noradrenaline acts on beta-1 adrenoceptors
Increased heart rate (+ve chronotropic effect)
Increased force of contraction (+ve inotropic effect)
Gs = increased adenylate cyclase = increased c.AMP = increased pacemaker potential
What are the parasympathetic effects on the heart?
Vagus nerve synapses with post-ganglionic neurone on epicardial surface/within walls of heart at SAN & AVN
ACh acts on M2 receptors
Decreased heart rate (-ve chronotropic effect)
Decreased AVN conduction velocity
Gi = reduced adenylate cyclase = reduced c.AMP = reduced pacemaker potenial By = increases K+ conductance
How does noradrenaline increase the force of contraction of the heart?
Noradrenaline -> beta-1 adrenoceptors in myocardium -> increased c.AMP -> phosphorylation of calcium channels
Therefore increased calcium entry during AP -> increased uptake of calcium in SR -> increased sensitivity of contractile machinery to calcium
What receptors affect the vasculature of the heart and circulatory system?
Most arteries and veins have alpha-1 adrenoceptors -> IP3 -> increased [Ca2+]i -> vasoconstriction
Coronary and skeletal muscle & liver have beta-2 adrenoceptors -> c.AMP -> opens K+ channels -> vasodilatation
note: circulating adrenaline has a higher affinity for beta-2 adrenoceptors
What happens to the resting heart rate if you give a beta-receptor antagonist to someone? What about during exercise?
At rest - little effect unless there is excessive sympathetic activation for some reason (as the resting heart rate is controlled by the vagal nerve - parasympathetic)
Exercise - bradycardia prevents the cardiac output from increasing sufficiently
What is the origin of the sympathetic fibres?
Thoracolumbar
Paravertebral chain/prevertebral ganglia
What is the origin of the parasympathetic fibres?
Cervicosacral
What are the neurotransmitters present in the ANS?
Noradrenaline
Acetylcholine
Non-adrenergic, non-cholinergic transmitters (NANC) (co-released with NA/ACh)
e.g. 5HT, ATP, NO, neuropeptides (VIP, substance P)
Give some examples of autonomic disorders.
Pheochromocytoma - cancer resembles chromaffin cells causing increased adrenaline secretion (note: cancer does not necessarily have to be in the adrenal medulla)
Orthostatic intolerance syndrome - abnormal response to changes in posture (exaggerated response; often caused by treating other things)
Guillain-Barré syndrome - numbness & weakness in limbs after an infection (antigens destroy myelin)
How do post-ganglionic sympathetic fibres synapse with smooth muscle?
Varicosities - specialised sites for calcium-dependent noradrenaline release
