Final answer:
Indirect-acting cholinergic drugs inhibit acetylcholinesterase, leading to increased levels of ACh and parasympathetic stimulation. They are used therapeutically in diseases like Alzheimer's and myasthenia gravis, and can be found in certain insecticides. Anticholinergic drugs, such as atropine, block the overstimulation caused by excessive ACh.
Step-by-step explanation:
Indirect-acting cholinergic drugs increase the levels of acetylcholine (ACh) by inhibiting the enzyme acetylcholinesterase. This enzyme is responsible for breaking down ACh in the synaptic cleft, and its inhibition leads to an accumulation of ACh, which in turn has enhanced cholinergic effects in the parasympathetic nervous system. These effects are also observed in conditions like Alzheimer's disease and myasthenia gravis, where similar drugs are used to improve nerve transmission by conserving available ACh.
Organophosphorus compounds and carbamates, often found in insecticides, and drugs like physostigmine, neostigmine, and pyridostigmine, affect acetylcholinesterase activity. By doing so, they prevent ACh degradation, increasing cholinergic stimulation, which can result in continuous muscle contractions or severe parasympathetic effects like salivation and bradycardia. Hence, individuals exposed to these compounds can experience symptoms like muscle twitching and, in severe cases, paralysis or cardiac arrest due to overstimulation of the nervous system.
Anticholinergic drugs, such as atropine, work in the opposite way by blocking muscarinic receptors to reduce parasympathetic activity, which is useful in cases of poisoning by nerve agents that cause excessive ACh accumulation.