• Location: Upper right atrium, near the opening of the superior vena cava.
• Function: It spontaneously produces electrical impulses that start each heartbeat.
• Pathway: SA node → atrial muscle → AV node → His bundle → bundle branches → Purkinje fibers → ventricular contraction.
• Normal intrinsic firing rate: About 60-100 beats/minute in adults at rest.
• Autonomic control:
  • Sympathetic stimulation increases SA-node firing and heart rate.
  • Parasympathetic (vagal) stimulation decreases firing and heart rate.
• If the SA node fails, a slower backup pacemaker, often near the AV junction, may take over.

Functions of the SA node

1. Acts as the normal pacemaker of the heart
   It produces spontaneous electrical impulses automatically.
2. Initiates each heartbeat
   The impulse spreads through both atria, causing atrial contraction.
3. Sets the heart rate and rhythm
   Because it normally fires faster than other potential pacemakers, it controls the normal sinus rhythm.
4. Conducts the impulse toward the AV node
   The signal travels across the atria to the AV node, then onward to activate the ventricles.
5. Responds to autonomic nerves
   • Sympathetic stimulation increases SA-node firing, causing tachycardia.
   • Vagal stimulation decreases firing, causing bradycardia.