George A. Edwards

Electron microscopy of the impulse conducting system op the sheep heart

SummaryElectron micrographs of the conducting system of the sheep heart show it to be strictly cellular in nature. All protoplasmic components are restricted to areas defined by the individual plasma membranes of the cells. The individual cells are surrounded by a common interspace and groups of cells are separated from neighboring connective tissue by a common, longitudinal basement membrane. The extracytoplasmic interspace shows occasional enlargements, probably corresponding to the “intracytoplasmic vacuoles” visualized in the light microscope. The apposing plasma membranes, along which ephaptic thickenings may occur, and their interspace form the intercalated discs of the system.Myofibrils are few and are situated preferentially immediately beneath the plasma membrane. Chains of myofibrils, interrupted at the intercalated discs, appear to course through the whole tissue. In some myofibrils the pattern of long periods may be interrupted by a small-period pattern for several sarcomeres.The cytoplasmic matrix consists mainly of fine filaments of unknown composition. The relatively scarce mitochondria are found near the nucleus, myofibrils and cell surface. The endoplasmic reticulum is poorly developed and its arrangement is a comparatively loose one.The above results and their physiologic consequences are discussed, particularly with regard to the core conduction theory in conducting tissue.

A concept of intracellular transmission of excitation by means of the endoplasmic reticulum.

Die morphologische Beobachtung von getrennten cytoplasmatischen Phasen in geschlossenen Membranen führt zur Annahme von Membranpotentialen innerhalb der Zelle. Schreibt man den Membranen des endoplasmatischen Reticulums die Eigenschaften der Plasmamembran zu, so leiten sie die Erregung ins Zellinnere und dienen als Potentialspeicher.

Comparative studies on the fine structure of motor units

The problem in the consideration of the comparative aspects of motor units is that of deter-mining whether muscle response is due to one given mechanism involving components of like structure, or to several mechanisms and unlike components in both nerve and muscle. Not long ago this problem was considered essentially physiologic, i. e., that variation in muscle response depended upon differences in mode of innervation of fixed muscle types and also upon the inherent properties of contractile substances.It is now evident that modifications occur in the fundamental intracellular components in relation to specific functional differences, and that differences exist among axons and neuromuscular junctions, as well as among muscle fibers.