Molecules in the cell membrane that relay .hormonal and neurotransmitter signals bind to receptors on the .cell surface (M2, M3) that initiate either activation of a potassium .channel (M2), or release of intracellular .calcium (M3). M2; Muscarinic type 2 acetylcholine (ACh) receptor: M3; Muscarinic .type 3 ACh receptor: G; G protein: GIRK; G protein .gated inward rectifier channel subunit; PLC; ...