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Cardiovascular System. Physiology. Cardiac Muscle. Cardiac muscle cells are elongated, branching cells that contain one or occasionally two centrally located nuclei. CM cells contain myosin and actin organized into sarcomeres which join end to end to form myofibrils.
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Cardiovascular System Physiology
Cardiac Muscle • Cardiac muscle cells are elongated, branching cells that contain one or occasionally two centrally located nuclei. • CM cells contain myosin and actin organized into sarcomeres which join end to end to form myofibrils. • The arrangement of actin and myosin gives the striations. • Striations of cardiac muscle are less regularly arranged and less numerous than skeletal.
Diagram of muscle contraction and relaxation. The same mechanism exists in skeletal muscle.
Cardiac Muscle Continued • Another characteristic is the intercalated disks; these are specialized cell junctions – they work to increase contact between the cells. • This also serve to allow the cardiac muscle contract in waves to “massage” the blood out of the heart.
Contraction • Adenosine triphosphate (ATP) provides the energy for cardiac muscle contraction, and , as in other tissues, ATP depends on oxygen availability. Cardiac muscle cannot develop a large O2 debt. A large oxygen debt would result in muscular fatigue and the heart would eventually cease beating. What organelle must be present in large numbers?
Contraction • Action potentials are conducted through two nodes that are made of modified cardiac muscle cells. The two nodes are contained in the walls of the right atrium and are named according to their position in the atrium. • The SA (Sinoatrial) node is medial to the opening of the superior vena cava. • The AV (atrioventricular) node is medial to the right atrioventricular valve. The AV node gives rise to a conducting bundle of the heart (the atrioventricular bundle). Within the septum, this bundle divides into the right and left bundle branches.
Contraction • Cardiac muscle cells have the ability to generate spontaneous action potentials, but cells of the SA node do this at an increased frequency – PACEMAKER. • Action potentials that are produced spread from the SA node to adjacent cardiac muscle fibers of the atrium. Action potentials are conducted from the SA node to the AV node really fast. • Action potentials get conducted from the AV node to the bundle branches and then the velocity of the action really speeds up. It passes through the right and left bundle branches and penetrates the myocardium of the ventricles. Ventricular contraction begins at the apex and proceeds toward the base of the heart. During this process, the distance between the base and the apex decreases – shortening the heart. • CONTRACTION IS SYSTOLE. VENTRICULAR SYSTOLE CAUSES EJECTION OF BLOOD FROM THE HEART.
Cardiac Cycle • VD – Pressure decreases in the atria. As the AV valves open, blood flows into the ventricles. 2/3rds of the way through filling, the SA node depolarizes (the action potential is spread over the atria) the atria contracts causing AS. • VS – Ventricular contraction causes the ventricular pressure to increase – AV valves close. Pressure rises until the ventricular pressure exceeds the pressure in the blood vessels. Isometric contraction: AV valve is closed and the SL is not yet open (pressurizes the chambers). When the ventricular pressure exceeds the pressure in the vessels, the SL’s open and ejection begins. Ventricular volume decreases during ejection (VD). Shockwave
