The M muscarinic receptors are located in the heart and lungs.  In the heart, they act to slow the heart rate down below the normal baseline sinus rhythm, by slowing the speed of depolarization. In humans, under resting conditions, vagal activity dominates over sympathetic activity. Hence, inhibition of M receptors (e.g. by atropine) will cause a raise in heart rate. They also moderately reduce contractile forces of the atrial cardiac muscle, and reduce conduction velocity of the atrioventricular node (AV node). It also serves to slightly decrease the contractile forces of the ventricular muscle.
M muscarinic receptors act via a G type receptor, which causes a decrease in cAMP in the cell, inhibition of voltage-gated Ca channels, and increasing efflux of K, in general, leading to inhibitory-type effects.