the sa node begins electrical signaling when it is spontaneously generated.
the sa node begins electrical signaling when it is spontaneously generated.
when does the sa node begin electrical signaling?
spontaneously generated
under normal conditions, electrical activity is spontaneously generated by the sa node, the cardiac pacemaker. this electrical impulse is propagated throughout the right atrium, and through bachmann's bundle to the left atrium, stimulating the myocardium of the atria to contract. the conduction of the electrical impulses throughout the atria is seen on the ecg as the p wave.









































as the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as "internodal tracts", from the sa node to the av node.









































section::::ecg.:av node and bundles: pr interval.
electrical conduction system of the heart
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.
sinoatrial node