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A ganglionic blocker (or ganglioplegic) is a type of medication that inhibits transmission between preganglionic and postganglionic neurons in the autonomic nervous system, often by acting as a nicotinic receptor antagonist.[1] Nicotinic acetylcholine receptors are found on skeletal muscle, but also within the route of transmission for the parasympathetic and sympathetic nervous system (which together comprise the autonomic nervous system). More specifically, nicotinic receptors are found within the ganglia of the autonomic nervous system, allowing outgoing signals to be transmitted from the presynaptic to the postsynaptic cells. Thus, for example, blocking nicotinic acetylcholine receptors blocks both sympathetic (excitatory) and parasympathetic (calming) stimulation of the heart. The nicotinic antagonist hexamethonium, for example, does this by blocking the transmission of outgoing signals across the autonomic ganglia at the postsynaptic nicotinic acetylcholine receptor.
The opposite of a ganglionic blocker is referred to as a ganglionic stimulant. Some substances can exhibit both stimulating and blocking effects on autonomic ganglia, depending on dosage and/or duration of action. An example for such a "dual" action is nicotine, which does this via depolarization block (see classification below).
Ganglionic blockers are used less frequently now than they were in the past, because antihypertensives with fewer side effects are now available. Hexamethonium has been described as the "first effective antihypertensive drug".[4] However, they are still used in some emergency situations, such as aortic dissection or autonomic dysreflexia.