Electron microscopic and electrophysiologic study of the pacemaker in the sino-atrial node of the rabbit heart

Summary

Within the sino-atrial node, which is a thin structure of 30–50 mm², surrounded by atrial myocardium, the pacemaker, latent pacemaker areas and occasionally atrial myocardium were located by electrophysiological methods. Tissue from these areas was studied with the phase-contrast and the electron microscope.

Pacemaker cells can be observed isolated, but are mostly an aggregate of cells. Usually there are no end to end connections; the cells, presumably spindle shaped, lie side by side. They are separated only by their respective membranes and by a narrow intercellular space of approximately 200 Å. A basement membrane surrounds the fiber bundle but not the individual cell. Larger areas of the cell surface do not face the extracellular space if one does not consider the small cleft at the contact areas an extracellular space. Pacemaker cells of the rabbit sinus are characterized by the scarcity of myofibrils, the mitochondria which are scattered at random in the cell, and the endoplasmic reticulum which is predominantly found close to the cell membrane. Clear cytoplasm occupies the bulk of the cross section. The organization of the myofibrils is generally similar to that in the atrial myocardial cell. Two types of nerve muscle fiber relation were observed in the sino-atrial node. One type shows nerve endings containing abundant vesicles situated 0.5 μ, from the post-synaptic muscular surface which is enlarged by fingerlike processes. The second type is characterized by the close contact between naked axon endings and post-synaptic muscular structure.