Page 3 - The Myocardial Grain
Large groupings of myocytes might be defined by classic dissection. However, the myocardium is notoriously difficult to dissect, having no visible tissue planes of any consequence once the epicardium is removed. This infers that, although the myocytes lie in reasonably coherent planes, the degree of branch linking is significant and the muscle tends to function as a single mass. By patient blunt dissection, a pattern of general myocyte stranding, or grain, can be made out. Such a specimen is shown above courtesy Dr Paul Lunkenheimer: the illustration in the background is Pettigrew's dissection of 1864, and you can see that they look very similar.
The dissection exposes deeper layers as it works to the left. The outer, subepicardial layer has the grain oblique to the long axis of the ventricle. The angle it forms with the long axis is called its helical angle, as the way the grain passes around the ventricle is best described as a helix.
As the dissection progresses deeper, the helical angle gradually diminishes until the grain in the middle of the ventricular wall is in the plane of the short axis. Moving on toward the endocardium, the grain becomes oblique/helical in the reverse direction to the outer layer. Finally a few subendocardial longitudinal strands can be identified.
This pattern of grain is generally accepted, and most of the content of these pages will rely quite heavily on its accuracy.
Given the branching of the myocytes, and the collagenous network around them, it is no surprise that the helical angle changes gradually. The limitation of movement between the strands of grain increases the coherence of contraction, and yet, as we will see in later pages, some movement will be required in order to allow the muscle to change its shape when it contracts.
All the grain seems to sit at a tangent to the circumference, or close to it. (Degrees of intrusion or imbrication (inward sloping) have been described but seldom appear to exceed 30 degrees in the relaxed heart.) As a result, myocyte contraction will diminish the diameter of the cavity in all directions. Most of the power of the ventricular wall appears to be directed towards reducing the diameter of the short axis of the ventricular cavity. This is not the whole story, however.