Autonomic ganglia nicotinic acetylcholine receptors

Atropine, an alkaloid from Atropa belladonna, is the classical parasympatholytic compound. It competes with acetylcholine for the binding at the muscarinic receptor. Its affinity towards nicotinic receptors is very low, so that it does not interfere with the ganglionic transmission or the neuromotor transmission, at least in therapeutic dosages. However, in the central nervous system muscarinic receptor do play an important role and while atropine can penetrate the blood-brain barrier it exerts pronounced central effects. Atropine, like all other antagonists of the muscarinic acetylcholine receptor inhibit the stimulatory influence of the parasympathetic branch of the autonomous nervous system. All excretory glands (tear, sweat, salivary, gasto-intestinal, bronchi) are... 

Ganglionic transmission Acetylcholine transmits both sympathetic and parasympathetic impulses from the "preganglionic" neurons in the brain and spinal cord to nicotinic ganglionic (N ) receptors on "postganglionic" neurons of the autonomic nervous system. This occurs in sympathetic ganglia, which are located along the spinal cord, and in parasympa-... 

The basis for the antihypertensive activity of the ganglionic blockers lies in their ability to block transmission through autonomic ganglia (Fig. 20.2C). This action, which results in a decrease in the number of impulses passing down the postganglionic sympathetic (and parasympathetic) nerves, decreases vascular tone, cardiac output, and blood pressure. These drugs prevent the interaction of acetylcholine (the transmitter of the preganglionic autonomic nerves) with the nicotinic receptors on postsynaptic neuronal membranes of both the sympathetic and parasympathetic nervous systems. 

Schematic diagram comparing some anatomic and neurotransmitter features of autonomic and somatic motor nerves. Only the primary transmitter substances are shown. Parasympathetic ganglia are not shown because most are in or near the wall of the organ innervated. Cholinergic nerves are shown in blue noradrenergic in red and dopaminergic in green. Note that some sympathetic postganglionic fibers release acetylcholine or dopamine rather than norepinephrine. The adrenal medulla, a modified sympathetic ganglion, receives sympathetic preganglionic fibers and releases epinephrine and norepinephrine into the blood. ACh, acetylcholine D, dopamine Epi, epinephrine M, muscarinic receptors N, nicotinic receptors NE, norepinephrine.

Two distinct receptor groups have been identified for acetylcholine, the nicotinic and the muscarinic groups (Table 11.1). Furthermore, there are at least four subtypes of nicotinic and five subtypes of muscarinic receptors. Nicotinic receptors are ubiquitous and exist at the neuromuscular junctions of skeletal muscles and on ganglion cells in the autonomic nervous system. Nicotinic receptors located on cation-specific ion channels, when opened, evoke fast, transient depolarizations of the recipient cell. Muscarinic receptors are found in smooth muscle receiving parasympathetic innervation and elsewhere, and can be blocked by atropine. Muscarinic receptors are coupled indirectly to slow and fast ion channels via G proteins. 

Acetylcholine is stored in storage vesicles. 3). Upon nerve stimulation, an action potential travels down the neuron and causes calcium influx into the nerve terminal. Calcium influx causes the vesicles to fuse with plasma membrane and release acetylcholine. Acetylcholine diffuses through the synaptic cleft and may bind to (4) Nm receptors (nicotinic receptors on muscle cells, (5) Ng receptors (nicotinic receptors on ganglionic synapses of the autonomic nervous system), (6) Ml muscarinic receptors, (7) M2 muscarinic receptors or (8) M3 muscarinic receptors. At least six muscarinic receptors have now been identified, of which Ml, M2 and M3 receptors have been most carefully studied (Table 2.5). 

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