Myocardium

Myocardium

• Myocardium: It is the heart’s main component and serves as the contractile component. It is thinner in the atria and thicker in the ventricles. The left ventricle’s myocardium is thicker than the right ventricle’s. (2)
  • Vortex of the heart: The vortex of the heart is situated at the apex, where the two ventricles meet, and the muscle fibres create a swirl. This intricate arrangement resembles a vortex, suggesting a dynamic and spiralling pattern within the heart. (1)
  • Conducting system of the heart: It is specialized tissue on the inner aspect of the myocardium that conducts the heart’s electrical impulses. (3)

• Sinu-atrial node: The SA node is shaped like a tadpole and is situated in the right atrium near the superior vena cava, in the terminal groove. Multiple interdigitations arise from the SA node, serving for a better connection between the node and the myocardium. (1,3)
• Atrioventricular node: The AV node is a particular structure of cells located in the right atrium, somewhere near the centre of the heart, in a landmark known as the Triangle of Koch. (1,3)
• Atrioventricular bundle (Bundle of His) is a bundle of fibres that penetrate the fibrous body, to conduct the electrical impulses from the AV node to the ventricles. (1,3)

• Right bundle branch: The RBB advances through the subendocardial along the septal wall of the right ventricle. (1) As it runs down the right ventricle, it reaches a point where it crosses the ventricular cavity through the moderator band and branches out to reach the apex and the base of the right ventricle. (3)
• Left bundle branch: located beneath the endocardium, is a segment of the bundle of His that extends toward the middle of the left ventricle. It divides into smaller bundles and spreads throughout the left ventricle. (6)
  • Left anterior branch of atrioventricular bundle: It courses in the subendocardium along the superior aspect of the left ventricle towards the anterolateral papillary muscle. It spreads over the anterolateral papillary muscle in every direction. (3,4,5)
  • Left intermediate branch of atrioventricular bundle: It courses superficially through the subendocardium along the septum of the left ventricle towards the apex. (3,4)
  • Left posterior branch of atrioventricular bundle: It courses superficially through the subendocardium along the inferior aspect of the left ventricle towards the posteromedial papillary muscle. It spreads over the posteromedial papillary muscle in every direction. (3,4,5)
• Subendocardial branches: The Purkinje fibres continue the bundle branches like a mesh that lines the subendocardium (inner part of the myocardium). The Purkinje fibres interface with the working cardiac muscle cells. (1,2,3,4)

Bibliography

1 Gray H, Lewis W. Angiology. In: Anatomy of the human body. 1918. p. 526–42

2 Ross MH, Pawlina W. Cardiovascular system. In: Histology : a text and atlas : with correlated cell and molecular biology. Phioladelphia: Wolters Kluwer; 2020. P. 432–70.

3. Ho SYen. Anatomy for cardiac electrophysiologists : a practical handbook. Cardiotext Pub; 2012. 67–97 p.

4 Pérez-Riera AR, Barbosa-Barros R, Baranchuk A. Left septal fascicular block: Characterization, differential diagnosis and clinical significance. Left Septal Fascicular Block: Characterization, Differential Diagnosis and Clinical Significance. Springer International Publishing; 2016. 1–19 p.

5 Ho SYen. Anatomy for cardiac electrophysiologists : a practical handbook. Minneapolis: Cardiotext Pub; 2012. 195–213 p.

6 Howard SA, Bateman MG, Laske TG, Iaizzo PA. The Cardiac Conduction System. In: Iaizzo PA, editor. Handbook of Cardiac Anatomy, Physiology, and Devices, Third Edition. Third. Minneapolis: Springer International Publishing; 2015. p. 215–35.

The structure and organization of anatomical terms used in this text follow the guidelines provided by FIPAT (2019) in their publication: FIPAT. (2019). Terminologia Anatomica (2nd ed.). FIPAT.library.dal.ca. Federative International Programme for Anatomical Terminology. Retrieved May 7, 2023, from https://fipat.library.dal.ca/TA2/