sinoatrial node (SA node) (sy-noh-ay-tri-ăl) n. the pacemaker of the heart: a microscopic area of specialized cardiac muscle located in the upper wall of the right atrium near the entry of the vena cava. Fibres of the SA node contract at around 70 times per minute. Following each contraction, the impulse spreads along connecting fibres to the atrioventricular node. Impulses arriving at the SA node accelerate or decrease the heart rate.

sinoatrial node See pacemaker.