The chemical reactions and pathways involving acetylcholine, the acetic acid ester of the organic base choline. Acetylcholine is a major neurotransmitter and neuromodulator both in the central and peripheral nervous systems. It also acts as a paracrine signal in various non-neural tissues. [ http://openisbn.com/search.php?q=0192800752&isbn=1/ GOC:nln GOC:jl ]