 Muscarinic acetylcholine receptors are found in both the central nervous system and the peripheral nervous system of the heart, lungs, upper gastrointestinal tract, and sweat glands.Acetylcholine is the substance the nervous system uses to activate skeletal muscles, a kind of striated muscle.These are the muscles used for all types of voluntary movement, in contrast to smooth muscle tissue, which is involved in a range of involuntary activities such as movement of food through the gastrointestinal tract and constriction of blood vessels.