Cholinergic signal transduction the nicotinic

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Cholinergic Signal Transduction   The nicotinic response of skeletal muscle has been discussed in detail. ACh causes  depolarization by a sudden increase in Na+ conductance. Repolarization depends upon  the outward flow of K+.  The transduction of muscarinic responses depends upon the tissue. For example ACh  causes hyperpolarization of cardiac conducting tissue by increasing K+ conductance  (M2). In G.I. smooth muscle it causes partial depolarization by increasing Na+ and Ca2+  conductance (M3).  Adrenergic Neurotransmission 
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Neurotransmission from almost all sympathetic postganglionic nerves is adrenergic, that  is, it involves noradrenaline (NE, norepinephrine, levarterenol). Adrenergic receptors  were first divided into  α and  β by Ahlquist in 1948. "Tachyphylaxis" Certain drugs like ephedrine act by releasing intraneuronal stores  of preformed norepinephrine. After repeated dosing, the supply of  neurotransmitter may be temporarily exhausted.  Receptor Down Regulation This may be a decrease in the number of available  receptors or a decrease in ligand affinity for the receptor. Ex: Decrease in  β1 receptors in the myocardium of patients with congestive failure.  Desensitization Reversible uncoupling of receptor occupancy and cellular  response by receptor phosphorylation and internalization. Ex:  β adrenergic  receptor and nicely 
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