Final answer:
The SA node is supplied by sympathetic and parasympathetic nerves. The sympathetic nerves release norepinephrine to speed up the heart rate, while the parasympathetic nerves release acetylcholine to slow down the heart rate.
Step-by-step explanation:
The SA node, also known as the sinoatrial node, is supplied by two nerves: the sympathetic nerves and the parasympathetic nerves. The sympathetic nerves originate from the cardioaccelerator center and form the cardiac nerves that innervate the SA node. These nerves release the neurotransmitter norepinephrine (NE) which speeds up the heart rate. On the other hand, the parasympathetic nerves originate from the cardioinhibitory region and travel via the vagus nerve to innervate the SA node. These nerves release the neurotransmitter acetylcholine (ACh) which slows down the heart rate.