What does sinoatrial node release when it fires?

When the sinoatrial (SA) node fires, it generates electrical impulses that trigger the heart's contractions. This process leads to the release of neurotransmitters, primarily norepinephrine, which enhances heart rate by stimulating the surrounding cardiac muscle cells. Additionally, the electrical impulses promote the release of calcium ions from the sarcoplasmic reticulum in cardiac cells, facilitating muscle contraction. Overall, the SA node plays a crucial role in regulating the heartbeat and maintaining normal heart rhythm.