Sect12b2 fig1210 fig1211 the sa node is the pacemaker

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(Sect12B.2, Fig12.10, Fig12.11) The SA node is the pacemaker for the entire heart & its discharge rate determines heart rate. The action potential initiated and originated in the SA node spreads throughout myocardium, passing from cell to cell via gap junctions. Depolarization first spreads through muscle cells of the atria , w/conduction fast enough that right & left atria contract at same time. Next, excitation spreads through ventricles. The action potential spreading through the right atrium causes depolarization of the AV node (the propagation of action pot. through the AV node is slow). This delay allows atrial contraction to be completed before ventricular excitation occurs. That’s why there is a pause after excitation through atria. After leaving AV node, action potential propagates down interventricular septum . This pathway has conducting system fibers called bundle of His . (The AV node & bundle of His are the only electrical connection between atria and ventricles. Except for this pathway, the atria are separated from ventricles by layer of nonconducting connective tissue) Within the interventricular septum, the bundle of His divides into right and left bundle branches, which separate bottom (apex) of heart and enter walls of both ventricles. These make contact w/ Purkinje fibers, which are large diameter conducting cells that quickly distribute the impulse throughout ventricles. (Rapid conduction of Purkinje fibers causes depolarization of right & left ventricles to occur nearly simultaneously and in a single contraction). 2. Describe the ventricular action potential. Include a discussion of which currents are responsible for its various phases. (Sect12B.2, Fig12.12) Diagram p. 363 The ventricular muscle action potential has a stable resting potential ; it is close to K+ equilibrium potential, -90 mV. It also has a relatively long duration ; depolarization plateau , which is due to sodium channels inactivating quickly and depolarization is sustained due to potassium permeability staying below resting value & increase in membrane permeability to calcium; Long refractory period also occurs. Currents: upstroke occurs when voltage-gated Na + are opened, which leads to depolarization. As more sodium enters the cell, it continues to depolarize and sustains the opening of more Na+ channels, positive feedback. The plateau occurs because of L-type Ca 2+ ; long lasting; calcium channels open and calcium rushes into cell; but they open slower than sodium channels, and remain open for long period; The flow of positive
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