What are the names and locations of pacemaker cells in the heart?

Cardiac Pacemaker Cells: Names and Locations

The primary pacemaker cells in the heart include the sinoatrial node (SAN) cells, atrioventricular node (AV node) cells, and specialized conduction system cells including those in the His-Purkinje system, each with distinct locations and functions in cardiac electrical conduction.

Sinoatrial Node (SAN)

• Primary pacemaker of the heart
• Located in the right atrium at the junction of the superior vena cava
• Composed of a heterogeneous population of specialized cardiac myocytes 1
• Contains distinct cell types:
  • Central pacemaker cells - contain the dominant pacemaking function with longer action potentials and faster rates of phase 4 diastolic depolarization 1
  • Transitional cells - connect pacemaker cells to atrial myocardium
  • Endothelial cells and fibroblasts - provide structural support 1

Atrioventricular Node (AV Node)

• Secondary pacemaker (takes over if SAN fails)
• Located in the inferior-posterior region of the interatrial septum
• Contains distinct populations of cells with different electrophysiological properties 1
• Firing rate: 40-60 beats per minute (slower than SAN)

His-Purkinje System

• Tertiary pacemakers
• Components include:
  • Bundle of His - located at the top of the interventricular septum
  • Bundle branches - extend down the interventricular septum
  • Purkinje fibers - extend throughout the ventricular myocardium
• Firing rate: 30-40 beats per minute (slowest intrinsic rate)

Functional Characteristics of Pacemaker Cells

Sinoatrial Node Characteristics

• Surrounded by strands of connective tissue that electrically insulate pacemaker cells from atrial myocardial tissue 1
• Contains a unique ion channel and connexin expression profile that results in chronotropic automaticity 1
• Features enhanced diastolic phase 4 depolarization, leading to spontaneous action potential generation 1
• Contains multiple functionally defined intranodal pacemaker clusters responsible for generating heart rhythm at different rates 2

Pacemaker Hierarchy

• The origin of cardiac action potentials is not static but dynamically shifts according to prevailing conditions 2
• Pacemaker shifts occur in response to:
  • Neurohormonal modulation
  • Pharmacological agents
  • Mechanical stretch
  • Temperature changes
  • Extracellular electrolyte concentration changes 2

Clinical Significance

• Dysfunction of the SAN leads to sinus node dysfunction (SND) or sick sinus syndrome 2, 3
• SND manifestations include severe bradycardia, tachy-brady arrhythmias, and conduction abnormalities 3
• Fibrosis of the SAN is often associated with fibrosis in the AV node, potentially causing broader conduction issues 1
• Asymptomatic sinus bradycardia has not been associated with adverse outcomes, but symptomatic SND carries high risk of cardiovascular events 1

Understanding the complex structure and function of cardiac pacemaker cells is essential for diagnosing and treating arrhythmias and other cardiac conduction disorders.