<Ul> <Li> The M1 muscarinic receptors (CHRM1) are located in the neural system . </Li> <Li> The M2 muscarinic receptors (CHRM2) are located in the heart, and act to bring the heart back to normal after the actions of the sympathetic nervous system: slowing down the heart rate, reducing contractile forces of the atrial cardiac muscle, and reducing conduction velocity of the sinoatrial node and atrioventricular node . They have a minimal effect on the contractile forces of the ventricular muscle due to sparse innervation of the ventricles from the parasympathetic nervous system . </Li> <Li> The M3 muscarinic receptors (CHRM3) are located at many places in the body, such as the endothelial cells of blood vessels, as well as the lungs causing bronchoconstriction . The net effect of innervated M3 receptors on blood vessels is vasodilation, as acetylcholine causes endothelial cells to produce nitric oxide, which diffuses to smooth muscle and results in vasodilation . They are also in the smooth muscles of the gastrointestinal tract, which help in increasing intestinal motility and dilating sphincters . The M3 receptors are also located in many glands that help to stimulate secretion in salivary glands and other glands of the body . They are also located on the detrusor muscle and urothelium of the bladder, causing contraction . </Li> <Li> The M4 muscarinic receptors: Postganglionic cholinergic nerves, possible CNS effects </Li> <Li> The M5 muscarinic receptors: Possible effects on the CNS </Li> </Ul> <Li> The M1 muscarinic receptors (CHRM1) are located in the neural system . </Li> <Li> The M2 muscarinic receptors (CHRM2) are located in the heart, and act to bring the heart back to normal after the actions of the sympathetic nervous system: slowing down the heart rate, reducing contractile forces of the atrial cardiac muscle, and reducing conduction velocity of the sinoatrial node and atrioventricular node . They have a minimal effect on the contractile forces of the ventricular muscle due to sparse innervation of the ventricles from the parasympathetic nervous system . </Li> <Li> The M3 muscarinic receptors (CHRM3) are located at many places in the body, such as the endothelial cells of blood vessels, as well as the lungs causing bronchoconstriction . The net effect of innervated M3 receptors on blood vessels is vasodilation, as acetylcholine causes endothelial cells to produce nitric oxide, which diffuses to smooth muscle and results in vasodilation . They are also in the smooth muscles of the gastrointestinal tract, which help in increasing intestinal motility and dilating sphincters . The M3 receptors are also located in many glands that help to stimulate secretion in salivary glands and other glands of the body . They are also located on the detrusor muscle and urothelium of the bladder, causing contraction . </Li>

The parasympathetic nervous system releases what at ganglionic synapses