Cardiovascular system lecture 4
EXCITATION-CONTRACTION COUPLING— FUNCTION OF CALCIUM IONS AND THE TRANSVERSE TUBULESThe term “excitation-contraction coupling” refers to the mechanism by which the action potential causes the myofibrils of muscle to contract.
When an action potential passes over the cardiac muscle membrane, the action potential spreads to the interior of the cardiac muscle fiber along the membranes of the transverse (T) tubules.
This mechanism of excitation-contraction coupling is the same as that for skeletal muscle, but there is a second effect that is quite different. In addition to the calcium ions that are released into the sarcoplasm from the cisternae of the sarcoplasmic reticulum, calcium ions also diffuse into the sarcoplasm from the T tubules themselves at the time of the action potential, which opens voltage-dependent calcium channels in the membrane of the T tubule .
Calcium entering the cell then activates calcium release channels, also called ryanodine receptor channels, in the sarcoplasmic reticulum membrane, triggering the release of calcium into the sarcoplasm. Calcium ions in the sarcoplasm then interact with troponin to initiate cross-bridge formation and contraction by the same basic mechanism.
The strength of contraction of cardiac muscle depends to a great extent on the concentration of calcium ions in the extracellular fluids.
In contrast, the strength of skeletal muscle contraction is hardly affected by moderate changes in extracellular fluid calcium concentration because skeletal muscle contraction is caused almost entirely by calcium ions released from the sarcoplasmic reticulum inside the skeletal muscle fiber.
At the end of the plateau of the cardiac action potential, the influx of calcium ions to the interior of the muscle fiber is suddenly cut off, and calcium ions in the sarcoplasm are rapidly pumped back out of the muscle fibers into both the sarcoplasmic reticulum and the T tubule– extracellular fluid space. Transport of calcium back into the sarcoplasmic reticulum is achieved with the help of a calcium–adenosine triphosphatase (ATPase) pump.
Duration of Contraction. Cardiac muscle begins to contract a few milliseconds after the action potential begins and continues to contract until a few milliseconds after the action potential ends. Therefore, the duration of contraction of cardiac muscle is mainly a function of the duration of the action potential, including the plateau— about 0.2 second in atrial muscle and 0.3 second in ventricular mature contraction.
jugular venous pressure
The jugular venous pressure (JVP, sometimes referred to as jugular venous pulse) is the indirectly observed pressure over the venous system via visualization of the internal jugular vein. It can be useful in the differentiation of different forms of heart and lung disease. Classically three upward deflections and two downward deflections have been described.
• The upward deflections are the "a" (atrial contraction), "c" (ventricular contraction and resulting bulging of tricuspid into the right atrium during isovolumetric systole) and "v" = venous filling
• The downward deflections of the wave are the "x" (the atrium relaxes and the tricuspid valve moves downward) and the "y" descent (filling of ventricle after tricuspid opening).
JVP waveform
The jugular venous pulsation has a biphasic waveform.
• The " a " wave corresponds to right Atrial contraction and ends synchronously with the carotid artery pulse. The peak of the 'a' wave demarcates the end of atrial systole.
• The " c " wave corresponds to right ventricular Contraction causing the triCuspid valve to bulge towards the right atrium.
• The " x' " (x prime) descent follows the 'c' wave and occurs as a result of the right ventricle pulling the tricuspid valve downward during ventricular systole. (As stroke volume is ejected, the ventricle takes up less space in pericardium, allowing relaXed atrium to enlarge). The x' (x prime) descent can be used as a measure of right ventricle contractility.
• The " x " descent follows the 'a' wave and corresponds to atrial relaXation and rapid atrial filling due to low pressure.
• The " v " wave corresponds to Venous filling when the tricuspid valve is closed and venous pressure increases from venous return
• The " y " descent corresponds to the rapid emptYing of the atrium into the ventricle following the opening of the tricuspid valve
