Learning Objective
Understand the functional specializations of cardiac cells, including automaticity, conduction, and contraction, and identify the primary pacemaker and secondary pacemaking tissues in the heart.
Cardiac tissue consists of cells specialized for three major roles:
- Automaticity – the ability to spontaneously generate action potentials.
- Conduction – the rapid propagation of electrical signals through the heart.
- Contraction – the force generation that produces cardiac output.
All cardiac cells are electrically coupled via gap junctions, allowing coordinated depolarization across the heart. While all cardiac cells can spontaneously depolarize, certain regions are critical for maintaining rhythm.
Automaticity
Automaticity refers to the heart cells’ ability to initiate action potentials without external stimuli. Key sites of automaticity include:
Sinoatrial (SA) Node
- The primary pacemaker of the heart.
- Spontaneously depolarizes to threshold.
- Highest intrinsic rate: ~100 beats/min.
Atrioventricular (AV) Node
- Secondary pacemaker if the SA node fails.
- Intrinsic rate: 40–60 beats/min.
Purkinje Fibers
- Specialized for conduction but exhibits some automaticity.
- Intrinsic rate: ~35 beats/min.
Key Point: The SA node sets the pace under normal conditions due to its highest intrinsic rate. The AV node and Purkinje fibers act as backup pacemakers.
