Functional Morphology of the Heart

Abstract

The primary function of muscles is the generation of force or movement in response to physiological stimulus. The myocardium is a heterogeneous mass of muscle tissue which represents a transition between the involuntary (smooth) and the voluntary (striated) muscles. While the individual muscle fibers in a voluntary muscle are supplied by the nerve endings and can be recruited at will across the spectrum of forces, the cardiomyocytes are electrically connected and function as a single unit, a syncytium that contracts in unison with each impulse autonomously generated by a specialized group of pacemaker cells within the heart itself. Unlike the skeletal muscle and its fibers, the myocardium lacks origin and insertion. Despite hundreds of years of anatomical dissection, many details of the heart's structure, as related to its function, remain unresolved. Pettigrew demonstrated that the myocardial wall in sheep consists of seven layers which ``overlap externally and internally and equilibrate each other according to mathematical law'' and maintained that the arrangement of myocardial fibers holds the key to its function. Carl Ludwig introduced the concept of a muscular cylinder as the principal structure of the left ventricle, enveloped by endocardial and epicardial layers, crossing at the right angles of its longitudinal axis in the form of an X. Hunter and Smaill proposed that methods of continuum mechanics provide a suitable theoretical framework for the analysis of the complex interaction between mechanical, metabolic, and electrical functions of the heart. Diffusion tensor magnetic resonance imaging (DTMRI), a novel technique which enables visual ``tracking'' of the aggregated myocyte chains within ventricular walls, has confirmed the three-dimensional helical fiber patterns proposed by Pettigrew.