Inside the brain acetylcholine functions as a neuromodulator —a chemical that alters the way other brain structures process information rather than a chemical used to transmit information from point to point. The brain contains a number of cholinergic areas, each with distinct functions.4) Acetylcholine binds to postsynaptic receptors. (5) This binding causes ion channels to open and allows sodium ions to flow into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential which induces muscle contraction.