AUTONOMIC NERVOUS SYSTEM-PART-III

NEUROMUSCULAR BLOCKING AGENTS

These drugs block completely the neuromuscular cholinergic transmission by blocking at the nicotinic receptors of the somatic system. The somatic nervous system (SNS) is as we have already seen as a part of the Peripheral Nervous System(PNS) apart from ANS. Even though it is to some extent limits to a voluntary nervous system but beyond the limits, it may become autonomous.

The drugs which block the cholinergic transmissions at the neuromuscular junction causes muscle relaxation and are used as adjuvant drugs to produce anesthesia.

Classifications:-

1.Nondepolarizing blockers

2.Depolarizing blockers

A.Nondepolarizing blockers:-

1.Tubocurarine

2.Pancuronium 

3.Atracurium

4.Vecuronium

Among them, pancuronium has a longer duration of action.

They are not affecting all the muscles together but in order of first, the eye muscles followed by the facial muscles and at last the respiratory muscles.

Because of the poor intestinal absorption, these medicines are recommended by intravenous routes.

Side effects

They produce severe allergic reactions as they stimulate histamine release causes severe anaphylactic reactions, shock, hypotension, tachycardia followed by respiratory failure.

The above side effects can be effectively counteracted by administering a cholinergic agonist such as edrophonium, or neostigmine.

B.Depolarizing Blockers:-

Membrane depolarization usually occurs by the action of acetylcholine at its nicotinic receptors and the sodium channels are opened and rapid exchange of positively charged sodium ions from outside to inside the neuronal axon(lumen) through its channels to depolarize the muscle and when the threshold reached the acetylcholine is rapidly inactivated by the acetylcholinesterase and action potential occur followed by the rapid repolarization to sensitize our muscle for its normal functioning.

The depolarizing blockers such as Succinylecholine(Suxamethonium) is acting in a similar manner to acetylcholine by binding to the nicotinic receptor and causes depolarization. But as succinylcholine is not inactivated by the enzyme the depolarization is prolonged and goes beyond the threshold and the receptor is continuously desensitized to acetylcholine so that there is no or very weak action potential occur and the muscle is relaxed or paralyzed.

The duration of action is very short (3 to 6 min)

Unlike acetylcholine which is metabolized by the enzyme acetylcholinesterase, succinylcholine is metabolized by plasma cholinesterase.

It is used as an adjuvant in general anesthesia.

Since the neuromuscular blockers have effect only on nicotinic Nm subtype receptors and hence they have no ganglionic blocking effect at Nn subtype.