Final answer:
ACh decreases blood pressure when injected intravenously by stimulating muscarinic receptors on endothelial cells which induces vasodilation.
Step-by-step explanation:
The question at the core is why does intravenous injection of acetylcholine (ACh) decrease blood pressure (BP) even though there is no direct parasympathetic innervation to the blood vessels? The decrease in BP is not due to activation of sympathetic nerves, direct stimulation of alpha receptors, or release of norepinephrine, as these would all typically increase BP. Moreover, it is not due to blockade of beta receptors either. Instead, the injection of ACh stimulates the muscarinic receptors located on the endothelial cells of blood vessels.
Referring to the provided reference information, it is stated that all ganglionic neurons of the autonomic system, in both sympathetic and parasympathetic ganglia, are activated by ACh. This is through nicotinic receptors, but the effect on BP pertains to muscarinic receptors on endothelial cells, not the nicotinic ones on ganglionic neurons. An incorrect pairing from the question is 'acetylcholine decreases digestion' (option c) because acetylcholine typically increases digestion by stimulating digestive activities. Regarding drugs affecting the autonomic system, those that affect both divisions would interact with nicotinic receptors, as indicated in option a of another question.