Most glandular cells, posed with a massive influx of diacylglycerol and IP3 will begin to secrete stuff. Similarly, smooth muscle will contract when there is a calcium influx.
Broadly speaking, these receptors are membrane stabilizers.
Excitable tissues hyperpolarize, inward rectifying potassium currents start flowing, voltage-gated calcium channels are inhibited.
There are several varieties; structurally they are sufficiently different to bind different drugs.
The receptor species at the neuromuscular junction is distinct from the species you will find at the autonomic ganglia.
Yes, Nicotine is an agonist for both neuronal and neuromuscular subtypes of nicotinic acetylcholine receptors.
These receptors cause a RAPID increase in membrane permeability to sodium and calcium. The door is open for less than a millisecond.
During that time, some 50,000 sodium ions force their way into the cell, causing immediate depolarization of the membrane.
The channel is not particularly selective; while it is open, potassium is able to escape from the cell. This is the channel feature which causes suxamethonium-induced hyperkalemia.
The structure of these receptors is pentameric.
All of them have 5 subunits.
For the neuromuscular junction receptors, the five subunits are beta, delta epsilon and two alpha-1
The two alpha-1 subunits act as the binding sites of acetylcholine as well as the whole lot of neuromuscular blocking agents.
For the autonomic ganglia receptors, the five subunits are three beta and two alpha-3
The two alpha-3 subunits also act as binding sites for acetylcholine, but they will not bind neuromuscular blocking agents.
For this sort of really basic stuff, no matter where you look you will find essentially the same information.
I used chapters from "Goodman & Gilman's The Pharmacological Basis of Therapeutics" 11th ed by Brunton et al, and "Basic & Clinical Pharmacology" 11th ed. By Katzung et al.
I also perused Peck and Hill "Pharmacology for Anaesthesia and Intensive care" as well as the notoriously error-prone "Handbook of Pharmacology and Physiology in Anaesthetic Practice" by Stoelting and Hillier. Neither covered this subject in a depth I found satisfying.
Goodman and Gilman's remains a canonical text.