Myocardial Cell Types Overview

Introduction

The human heart is a highly specialized muscular organ whose efficiency depends not only on its chambers, valves, and great vessels but also on its cellular composition. The myocardium, the thick muscular layer of the heart wall, is primarily made up of cardiomyocytes, but these are not all identical. Two major cell populations exist:

Contractile myocytes – responsible for generating force and pumping blood
Specialized conduction system cells – responsible for initiating and propagating electrical impulses

Understanding these cell types, their histological features, and their distribution across atria, ventricles, and nodal tissue is fundamental to appreciating both normal cardiac physiology and the pathophysiology of rhythm disorders and heart failure.

1. Contractile Myocytes

1.1 Definition and General Features

Contractile myocytes are the workhorse cells of the heart. They:

Generate mechanical force
Contribute to the coordinated contraction of atria and ventricles
Account for >90% of myocardial mass

They are striated muscle cells, similar to skeletal muscle fibers but shorter, branched, and interconnected in a syncytium via intercalated discs.

1.2 Microscopic Structure

Feature	Description
Shape	Cylindrical, branched fibers (50–100 μm long, 10–20 μm wide)
Nucleus	Single, centrally located nucleus (sometimes binucleate)
Sarcomeres	Organized into repeating units, giving striated appearance
Mitochondria	Very abundant (30–40% of cell volume) → continuous ATP supply
Intercalated Discs	Contain desmosomes, fascia adherens, and gap junctions for mechanical & electrical coupling

1.3 Sarcomere Organization

The sarcomere is the fundamental contractile unit:

Thin filaments (actin) anchored to Z-discs
Thick filaments (myosin) in A-band
Regulatory proteins: troponin complex (T, I, C) and tropomyosin
Contraction is mediated by calcium binding to troponin C → conformational change → cross-bridge cycling

1.4 Functional Role

Responsible for atrial systole (“atrial kick”) and ventricular systole
Contractile force proportional to sarcomere length (Frank–Starling mechanism)
Excitation–contraction coupling dependent on calcium influx and calcium-induced calcium release (CICR)

2. Specialized Conduction System Cells

Specialized cardiomyocytes are not primarily contractile but are optimized for impulse generation and conduction. They include:

Pacemaker cells (SA node, AV node) – spontaneously generate action potentials
Conduction fibers (Bundle of His, bundle branches, Purkinje fibers) – rapidly propagate action potentials

2.1 SA Node Cells

Located in the sulcus terminalis at the junction of SVC and RA
Smaller than atrial myocytes, pale cytoplasm (fewer myofibrils)
Lack well-organized sarcomeres → poor contractility
Express HCN channels for “funny current” (If) → spontaneous diastolic depolarization
Primary pacemaker of the heart (60–100 bpm intrinsic firing)

2.2 AV Node Cells

Located in the interatrial septum, near the opening of the coronary sinus
Similar to SA nodal cells but slower intrinsic firing (40–60 bpm)
Provide physiological delay between atrial and ventricular activation (≈120 ms)
Critical for coordinated filling of ventricles before systole

2.3 Purkinje Fibers

Large diameter (≈50 μm), glycogen-rich, pale cytoplasm
Few contractile elements → weak contraction but extremely fast conduction (2–4 m/s)
Ensure synchronous activation of ventricular myocardium
Intrinsic pacemaker rate ≈20–40 bpm (backup in case of nodal failure)

2.4 Bundle of His and Bundle Branches

Located within interventricular septum
Intermediate conduction velocity between nodal tissue and Purkinje fibers
Divide into right and left bundle branches → further subdivide into Purkinje network

3. Electrophysiological Differences

Feature	Contractile Myocytes	Pacemaker Cells
Resting Membrane Potential	−85 to −90 mV	Unstable (−60 mV → spontaneous depolarization)
Action Potential Phases	0–4 (rapid depolarization, plateau)	0 (slow upstroke), 3 (repolarization), 4 (automatic depolarization)
Ion Channels	Fast Na⁺, L-type Ca²⁺, K⁺ channels	HCN (If), T-type & L-type Ca²⁺, K⁺ channels
Function	Force generation	Impulse generation & conduction

4. Distribution Across the Heart

4.1 Atria

Atrial walls: mostly contractile myocytes
SA node and internodal pathways contain pacemaker and conduction cells
Atrial myocytes have less developed T-tubules, allowing rapid electrical propagation

4.2 Ventricles

Ventricular myocardium: dense network of contractile myocytes
Purkinje network runs in subendocardial layer → triggers contraction from apex upward
Thick myocardium requires efficient conduction for synchronized contraction

4.3 Nodal Regions

SA node (RA wall near SVC): pacemaker cells
AV node (interatrial septum): slow-conducting nodal cells, relay to His bundle
Together, form the “AV conduction axis”

5. Clinical Relevance

5.1 Arrhythmias

Damage to SA node → sinus node dysfunction (bradycardia)
AV nodal disease → heart block
Purkinje fiber dysfunction → bundle branch block, QRS widening

5.2 Myocardial Infarction

Ischemia kills contractile myocytes → reduces pump function
Purkinje network involvement → ventricular arrhythmias

5.3 Target of Drugs

Beta-blockers: reduce SA node automaticity and AV node conduction
Antiarrhythmics: block Na⁺, K⁺, Ca²⁺ channels in specific cell types

6. Histological Identification

Cell Type	Key Histological Feature
Contractile Myocyte	Striated, branching fibers, central nuclei
SA/AV Node Cell	Small, pale, fewer striations
Purkinje Fiber	Very large, pale cytoplasm, glycogen-rich
Bundle Branch Fiber	Intermediate size, located in septum

7. Summary Table

Location	Dominant Cell Type	Function
Atrial Walls	Contractile myocytes	Pump blood from atria to ventricles
SA Node	Pacemaker cells	Generate impulse
AV Node	Pacemaker/relay cells	Delay conduction
His-Purkinje System	Conduction fibers	Rapid impulse delivery
Ventricular Walls	Contractile myocytes	Generate force for systemic/pulmonary circulation