To slow the heart rate, acetylcholine is released in response to nerve impulses, which inhibits discharge of the S-A node and transmission through the A-V node. Inhibiting the vagus nerve, which controls acetylcholine release, increases heart rate. To speed the heart rate, nerve impulses trigger the release of norepinephrine, which increases S-A discharge rate, A-V node conduction rate, and the force of contraction.