**3.1 Cholinergic agonists and antagonists**

Cholinergic agonists including parasympathomimetics, especially muscarinic agonists and cholinergic antagonists encompassing parasympatholytics or muscarinic antagonists are the two categories of medications influencing the parasympathetic nervous system.

#### **3.2 Acetylcholinesterase inhibitors**

They are also referred to as anti-cholinesterases that prevent the cholinesterase to disintegrate Ach, hence boosting the amount and intensity of synaptic activity. AChE inhibitors are classified into two types based on their method of action: irreversible and reversible (**Figure 2**). Reversible inhibitors, whether competitive or noncompetitive, have largely medical benefits, whereas irreversible AChE activity modulators have harmful consequences. Reversible AChE inhibitors are crucial in the pharmacological modulation of enzyme activity. These inhibitors include compounds with various functional groups (carbamate, quaternary, or tertiary ammonium group) and have been used in the diagnosis and/or treatment of diseases such as myasthenia gravis, Alzheimer's disease, post-operative ileus, bladder distention, glaucoma, and as an antidote to anticholinergic overdose. Irreversible anticholinesterases comprise organo-phosphates which produce a phosphorylated enzyme that does not regenerate appreciably when hydrolyzed. They have little therapeutic efficacy but are extremely toxicologically significant. Isoflurophate, the most known and investigated chemical

#### **Figure 2.**

*Agonists and antagonists of acetylcholine.*


#### **Table 1.**

*Commonly used reversible and irreversible AChE inhibitors.*

in this family; malathion, an extensively employed insecticide; echothiophate, among the first substances in this family possesing a therapeutical efficacy; and tabun, the deadly and poisonous nerve gases, are four examples. Some of the commonly used reversible and irreversible inhibitors of Acetylcholinesterase are listed in **Table 1**.
