The intercalated discs of atrophied heart muscle

Abstract

In two earlier papers 1 I presented evidence in support of my interpretation of intercalated discs as irreversible contraction bands. In a more recent paper the idea was tested by appearances in a natural experiment, namely, extremely hypertrophied heart muscle. 2 The conditions here obtaining were in perfect accord with, and confirmed the plausibility of, my previous interpretation. Since then I have had opportunity to study lesser degrees of hypertrophy, as well as an excellent specimen of atrophy 3 (weight of heart 180 grams). It is the purpose of this note to complete my report of observations on intercalated discs by a record of my findings in atrophied heart muscle, and to reëmphasize the point that all the evidence, including ontogenetic, comparative anatomical and pathological (experimental—including the phenomena of fragmentation and segmentation) data points to the same conclusion, namely, that the so-called “discs” are aggregations of irreversible contraction foci on the myofibrillæ in the form of bands variously modified by a variety of normal mechanical and pathological, both chemical and mechanical, factors.

The first step, and central fact, in the chain of observations leading to the above conclusion respecting the origin and nature of the discs is the close similarity, amounting practically to an identity, between the normal contraction band in contracted muscle fibers and the simplest type of disc. This correspondence of appearance is very striking in the humming-bird's heart. In the relaxed fiber the Z-line is very conspicuous but delicated The Q-disc is wide and pale. In the contracted fibers the appearance is one of an alternation of robust dark (Q) and light (J)bands of approximately equal thickness. These are the same conditions that obtain in striped muscle generally in relaxed and contracted states respectively.