The opposite type of drug, something that binds to the receptors and does not allow them to be activated is called an antagonist. A class of chemicals in snake and other poisonous venoms, neurotoxins, do exactly that. If you are bitten by a krait or a cobra, for example, and enough venom gets into the blood, there will be enough of their neurotoxin in your body to shut down the diaphragm muscle expands your lungs. Without that muscle functioning, the person ceases to breathe and dies of asphyxiation. Plants use them to prevent being eaten by herbivores.
Animals use similar substances to paralyze their prey.
So, as you can see, nAChRs are important to life. In the nervous system, the actions of nAChRs are not nearly as well characterized. We also know that nicotine's effects are diverse and at least somewhat dependent on its actions within the nervous system. One complication is that several types of nicotinic receptors are expressed in the nervous system. What we do know is that each of these classes is just a little different.