AUTONOMIC NERVOUS SYSTEM-PART-V-Continued...

Ganglionic Blockers-Continued...

These agents are antagonizing or blocking acetylcholine to act on its neuronal nicotinic (Nn,)receptors at the ganglia.

As we all know the Nn receptor action of acetylcholine is necessary to release the respective transmitters of cholinergic and adrenergic postganglionic end to act on the respective target cells.If these ganglionic Nn receptors are blocked both the systems will be affected not at the same degree but in different degrees in different effector sites as follows:-

Heart-In heart normally parasympathetic (cholinergic) nervous system is prominent to keep the heartbeat in a proper peaceful rhythm. But by blocking the ganglia, the system would lose its prominence and the unopposed sympathetic(adrenergic)system would raise the heart rate leads to tachycardia, and arrhythmias and blood pressure would rise.

Blood vessels-The The arteries and veins are dilated to deliver more blood to the periphery as the predominant sympathetic(adrenergic)system loses its prominence.

Eye-The normally predominant parasympathetic system loses its hold and the unopposed sympathetic system produces cycloplegia and mydriasis (dilation of the pupil)

Digestive system-The The normally predominant parasympathetic nervous system loses its hold leads to unopposed sympathetic effects such as constipation, dry mouth, reduced gastric and pancreatic secretions.

The urinary system-The usual predominant parasympathetic system loses its hold leads to urinary retention 

Sweat glands-The usually predominant sympathetic (cholinergic) effect loses its hold to sweat block(hot flashes)

Classifications of Ganglionic Blockers

1. Depolarizing GBs-These are the agents such as nicotine first stimulating the Nn-receptor in the ganglia and depolarize it similar to acetylcholine. But with persisting prolonged depolarization the action potential is nullified and the receptor is further desensitized to acetylcholine followed by the ganglionic blockade.

That is why in the tobacco(nicotine) poisoning at first the heart rate is increased due to the stimulation of the sympathetic ganglia and adrenal medulla to release more catecholamines followed by the ganglionic blockade to counteract these effects

(e.g.) Nicotine and Labeling

These drugs have no therapeutical values.

2.Non-depolarizing GBs -These drugs directly block the receptors without prior stimulations or depolarizing effects.

(e.g) hexamethonium, mecamylamine, and trimetaphane.

Because of the severe toxicities and complications these drugs used very rarely in treatments.