the cells of the sa node are spread out within a mesh of connective tissue, containing nerves, blood vessels, collagen and fat. immediately surrounding the sa node cells are paranodal cells. these cells have structures intermediate between that of the sa node cells and the rest of the atrium. the connective tissue, along with the paranodal cells, insulate the sa node from the rest of the atrium, preventing the electrical activity of the atrial cells from affecting the sa node cells. the sa node cells are smaller and paler than the surrounding atrial cells, with the average cell being around 8 micrometers in diameter and 20-30 micrometers in length (1 micrometer= 0.000001 meter). unlike the atrial cells, sa node cells contain fewer mitochondria (the power plant of the cell), fewer myofibers (the contractile machinery of the cell), and a smaller sarcoplasmic reticulum (a calcium storage organelle that releases calcium for contraction). this means that the sa node cells are less equipped to contract compared to the atrial and ventricular cells.