Final answer:
Hexamethonium blocks cholinergic nicotinic receptors at the autonomic ganglia, thereby affecting the transmission in both the sympathetic and parasympathetic systems without impacting the neuromuscular junction.
Step-by-step explanation:
The autonomic receptor blocked by hexamethonium at the ganglia, but not at the neuromuscular junction, is the cholinergic nicotinic receptor. Hexamethonium acts as a nicotinic antagonist at autonomic ganglia where it blocks the action of acetylcholine (ACh) on nicotinic receptors. This results in a suppressed response in both the sympathetic and parasympathetic systems, as the transmission of impulses through the ganglia is inhibited. However, hexamethonium does not affect the muscarinic receptors, which are found at the neuromuscular junctions in the parasympathetic nervous system, or the adrenergic receptors, which are associated with sympathetic nervous system target organs.