Also, a lot of drugs bind to these receptors. Obiously, nicotine and muscarine bind to these respective receptors.
As well, at the neuromuscular junction, antagonistic drugs like neostigmine and eserine bind to nicotinic receptors to alleviate symptoms of Myasthenia Gravis (by allowing ACh to stay longer in the synaptic cleft).
I'm not too sure, but I also think atropine acts as an antagonist at muscarinic receptors and is known to dilate the pupils of the eye. Hmm... (goes on a mission to find old Neurobio text) 😳