Ultrastructure of primary pacemaking cells in rabbit sino‐atrial node cells indicates limited sarcoplasmic reticulum content

Abstract

The main mammalian heart pacemakers are spindle-shaped cells compressed into tangles within protective layers of collagen in the sino-atrial node (SAN). Two cell types, "dark" and "light," differ on their high or low content of intermediate filaments, but share scarcity of myofibrils and a high content of glycogen. Sarcoplasmic reticulum (SR) is scarce. The free SR (fSR) occupies 0.04% of the cell volume within ~0.4 µm wide peripheral band. The junctional SR (jSR), constituting peripheral couplings (PCs), occupies 0.03% of the cell volume. Total fSR + jSR volume is 0.07% of cell volume, lower than the SR content of ventricular myocytes. The average distance between PCs is 7.6 µm along the periphery. On the average, 30% of the SAN cells surfaces is in close proximity to others. Identifiable gap junctions are extremely rare, but small sites of close membrane-to-membrane contacts are observed. Possibly communication occurs via these very small sites of contact if conducting channels (connexons) are located within them. There is no obvious anatomical detail that might support ephaptic coupling. These observations have implications for understanding of SAN cell physiology, and require incorporation into biophysically detailed models of SAN cell behavior that currently do not include such features. K E Y W O R D S gap junctions, pacemaker cells, sarcoplasmic reticulum, sino-atrial node c Bossen et al, 1978, 33 d Mobley and Eisenberg, 1975 34 e (Number of hearts/Number of cells/Number of measurements)

Slowing down as we age: aging of the cardiac pacemaker’s neural control

“…As such, pacemaker cells are connected through gap junctions formed by connexins 45, 40, and 30.2 to secure the rapid spread of the signal [18,[28][29][30]. Interestingly, cells at the center of the pacemaker do not connect to a vast extent, suggesting that they have a very specialized and efficient electrical coupling [31]. Given the high metabolic activity of this tissue, the pacemaker is highly vascularized, being supplied directly by the sinoatrial node artery and a vast arteriole and capillary network (Fig.…”