A G-protein coupled acetylcholine receptor signaling pathway that contributes to an decrease in frequency or rate of heart contraction through inhibition of adenylate cyclase (AC) activity. Binding of acetylcholine to a G-protein coupled (muscarinic) receptor on the surface of the signal-receiving cell results in the activation of an intracellular Gi/o protein. Gi/o inhibits adenylate cyclase to decrease cyclic-AMP (cAMP) levels. Since cAMP binds directly to F-channels to allow an inward flow of sodium (funny current, If current), a reduction in cAMP reduces the funny current to bring about membrane hyperpolarization and a decrease in heart rate.