The figure below shows the two main families of acetylcholine receptors: muscarinic and nicotinic. In structural terms, muscarinic receptors are G-coupled protein receptors, while nicotinic receptors are ligand-gated ion channels. They can be found on both sides of the synaptic cleft (presynaptic and postsynaptic).
After the pressure drops below 40 psi, the electrical switch turns on the pump and the pressure in the tank increases. Any failure in any component or loss of electricity can cause a well to suddenly stop producing water when a faucet is turned on.
There are three causes of green pool water. The first is a lack of chlorine. Without chlorine the pool water will quickly start to grow algae and turn green. Water without chlorine, especially if it has an algae bloom, is unsafe to swim in, and can be a breeding ground for mosquitoes and other insects. The second thing that causes pool water to look green (or yellow) is pollen.