An action potential is a rapid change in membrane potential, produced by the movement of charged atoms ( ions). The main role of a sinoatrial node cell is to initiate action potentials of the heart that can pass through cardiac muscle cells and cause contraction. Instead, smaller venules drain the blood directly into the right atrium. There are no large veins that drain blood away from the SA node. Despite these many differences, there doesn’t appear to be any advantage to how many sinoatrial nodal arteries an individual has, or where they originate. Finally, the SA node artery commonly passes behind the superior vena cava, before reaching the SA node however in some instances it passes in front. Also, the SA node artery mainly originates as a branch of the right coronary artery however in some individuals it has arisen from the circumflex artery, which is a branch of the left coronary artery. For example, in most humans, this is a single artery, although in some cases there have been either 2 or 3 sinoatrial node arteries supplying the SA node. This blood supply, however, can differ hugely between individuals. The sinoatrial node receives its blood supply from the sinoatrial nodal artery. This is again important in insulating the SA node from the surrounding atrial cells. There are fewer gap junctions within the SA node and they are smaller in size. These gap junctions are made of proteins called connexins. Īction potentials pass from one cardiac cell to the next through pores known as gap junctions. This means that the SA node cells are less equipped to contract compared to the atrial and ventricular cells. Unlike the atrial cells, SA node cells contain fewer mitochondria and myofibers, as well as a smaller sarcoplasmic reticulum. 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). 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. These cells have structures intermediate between that of the SA node cells and the rest of the atrium. Immediately surrounding the SA node cells are paranodal cells. The cells of the SA node are spread out within a mesh of connective tissue, containing nerves, blood vessels, collagen and fat. Cardiac muscle cells of the right atrium can be seen to the left of the node, and fat tissue to the right. The SA node surrounds the sinoatrial nodal artery, seen as the open lumen. These grooves run between the entrance of the superior vena cava and the inferior vena cava.įigure 2: Low magnification stained image of the SA node (center-right on image) and its surrounding tissue. It is positioned roughly between a groove called the crista terminalis located on the internal surface of the heart and the corresponding sulcus terminalis, on the external surface. The SA node is located in the wall ( epicardium) of the right atrium, laterally to the entrance of the superior vena cava in a region called the sinus venarum (hence sino- + atrial). The size can vary but is usually between 10-30 mm long, 5–7 mm wide, and 1–2 mm deep. The sinoatrial node is an oval-shaped structure that is approximately 15 mm long, 3 mm wide, and 1 mm thick, located directly below and to the side of the superior vena cava. The rate of action potentials produced (and therefore the heart rate) is influenced by the nerves that supply it. In a healthy heart, the SA node continuously produces action potentials, setting the rhythm of the heart ( sinus rhythm), and so is known as the heart's natural pacemaker. These cells can produce an electrical impulse known as a cardiac action potential that travels through the electrical conduction system of the heart, causing it to contract. The sinus node is approximately 15 mm long, 3 mm wide, and 1 mm thick, located directly below and to the side of the superior vena cava. The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells.
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