Mechanical Activity of the Heart - PGY300

Rewatch 44:58

!!!!!!!! important summary Blue: Pressure in Atrium Green: Pressure in Ventricle Red: Pressure in Aorta Yellow: Ventricular Volume Pink: Heart sounds

1) Excitation-Contraction Coupling in Cardiac Muscles

Action potential enters from adjacent cell. Action potential travels along the cell membrane, down T-Tubules & reaches a channel & the membrane

Isovolumetric Relaxation Phase - Wiggers Diagram

After Ventricular Repolarization - T wave Causes Ventricular Pressure to drop Ventricles enter Diastole HOWEVER, Still Pv > Pat Pressure in Ventricle is still higher than Pressure in the Atrium AV/Mital/Bicuspid Valve will remain closed -- But, Pv < Pao Pressure in the Ventricle falls below the Pressure in the Aorta Aortic Semilunar Valve closes --2nd Heart sound With both Valves closed (Mitral & Aortic Valve) -Volume remains the same even though the pressure is falling in the Ventricle due to relaxation -Constant Volume Will repeat again from the beginning- the pressure will eventually fall below the Atrial Pressure

Diastole - Wiggers Diagram - Change in Volume

As the ventricles fill, the volume is at its highest --130 mL during Atrial Contraction which follows the P Wave (Atrial Depolarization)

P Wave - Atrial Depolarization - Wiggers Diagram

Atrial Pressure rises is caused by: the contraction of the Atria triggered by the P Wave (Atrial Depolarization)

2) Mechanical Events of the Cardiac Cycle Atrial Systole

Atrial Systole - atrial contraction forces a small amount of additional blood into ventricles. Initiation of a heartbeat by depolarization of the autorhythmic cells of the SA Node & the spread of that action potential throughout the Atria, which causes atrial depolarization & then atrial systole. Atrial Systole causes Atrial Contraction which pushes a small amount of additional blood into the ventricles. As this happens, the electrical signal is spreading down into the ventricles to lead to ventricular depolarization

3) Excitation-Contraction Coupling in Cardiac Muscles

Ca2+ induces Ca2+ release through Ryanodine Receptor channels (RyR). Calcium ions bind to RyR & cause them to open to allow Ca2+ to exit the Sarcoplasmic Reticulum & accumulate in the cytosol. In cardiac muscles, there is chemical process: A chemical signaling event in the form of Ca2+ influx that triggers release of more calcium from the sarcoplasmic plasma

6) Excitation-Contraction Coupling in Cardiac Muscles

Ca2+ ions bind to troponin to initiate contraction. Mechanism underlying this contraction: Cytosolic Ca2+ levels increase allowing Ca2+ to bind to troponin, which causes a conformational shift in the troponin tropomyosin complex, which exposes the myosin-binding site on actin. As actin & myosin bind, there's ATP hydrolysis: Energy which allows the power stroke & the actin & myosin filaments to slide past one another, creating shortening in sarcomere & the generation of tension. **As long at Ca2+ & ATP are present, the power stroke will continue to cause muscle contraction

8) Excitation-Contraction Coupling in Cardiac Muscles

Ca2+ is pumped back into the sarcoplasmic reticulum for storage. The Ca2+ will move in cardiac cells into the sarcoplasmic reticulum through Ca2+-ATPase-Pumps or (step 9)

T Wave - Ventricular Repolarization / Ventricular Relaxation - Wiggers Diagram

Causes Ventricular Pressure to fall below the Aortic Pressure Triggers: Closure of the Aortic Valve --S2 heart sound "dub" Isovolumic Relaxation Followed by a period of time where the Ventricular pressure is between the Aortic pressure & the Atrial pressure -->Aortic Valve = Closed --> Mitral Valve = Closed With these Valves closed, there's no where for blood to come or go from the Ventricles --> Volume remains the same

As the Ventricular Pressure Rises above Aortic Pressure - Wiggers Diagram

Causes an Opening of the Aortic Valve --Ejection of blood (Ejection Phase)

Ventricular Pressure Falls below the Atrial Pressure - Wiggers Diagram

Causes opening of the Mitral Valve --Allows blood to move from the Left Atria to the Left Ventricle

Stroke Volume

Difference between End Diastolic Volume & End Systolic Volume The amount of blood ejected from the heart in one contraction. The amount of blood ejected in the heartbeat, out into the aorta from the left ventricle

Ejection gradient - Wiggers Diagram - Change in Volume

Ejection gradient- represents the pressure gradient between, Left Ventricle & the Aorta -- associated with drop in volume Drop in volume as volume is ejected from the Left Ventricle into the Aorta Volume falls to about 50 mL

How does the timing of the mechanical events of the cardiac cycle relate to the timing of the electrical events shown on and ECG

Electrical events are followed by Mechanical events: -Depolarization precedes Contraction -Repolarization precedes Relaxation

Depolarization -> Contraction via Excitation-Contraction Coupling

Events of excitation-contraction coupling is what link depolarization to contraction & depolarization to relaxation

Filling gradient - Volume begins to increase - Wiggers Diagram - Change in Volume

Filling gradient- represents the pressure gradient between the Left Atrium & the Left Ventricle -- Facilitates movement of blood from the atrium into the ventricle -- associates with the rise in volume Volume begins to increase during the filling (diastolic period before the next systolic period) Ventricle fills with blood in the atrium

QRS Complex - Ventricular Depolarization - Wiggers Diagram

Isovolumic Contraction Period The electrical depolarization is followed shortly by a rapid rise in Ventricular Pressure --The Ventricular pressure exceeds the Atrial pressure Which causes: The Mitral Valve (left AV Valve) to close -->which causes S1 heart sound "lub" During the time the Ventricular pressure is higher than the Atrial pressure, but lower than Aortic pressure -- the Mitral Valve will be closed & Aortic Valve will be closed --the volume in the Ventricles will remain the same

5) Mechanical Events of the Cardiac Cycle Isovolumic Ventricular Relaxtion (Repolarization)

Isovolumic Ventricular Relaxtion - As ventricles relax, pressure in ventricles falls. Blood flows back into cusps of semilunar valves & snaps them closed. Relaxation as the ventricular muscle relaxes which causes a drop in pressure. When the pressure in the ventricles fall below the pressure in the arteries, Semilunar Valves Close: 2nd heart sound (S2) - "dub" As ventricular pressure falls, it takes some time before it falls below the pressure & the atria during all of the valves are close - volume in the chambers remain the same during ventricular relaxation -- But when the ventricular pressure falls below the atrial pressure, the AV Valves open & blood can now move from the Atria to the Ventricles & the process repeats itself

1) Mechanical Events of the Cardiac Cycle Late Diastole

Late Diastole - both sets of chambers are relaxed & ventricles fill passively. Both the ventricles & the atria are relaxed -- these chambers are filling passively w/ blood. There's blood moving into the atria & then moving through open AV valves into the ventricles.

4) Excitation-Contraction Coupling in Cardiac Muscles

Local release causes Ca2+ spark. initial release of Ca2+ sparks

Electrical Events (depolarization & repolarization) Precede Mechanical Events Throughout the Cardiac Cycle

Mechanical events lag behind electrical events: Contractions follows depolarization *ECG begins w/ the atrial depolarization (P wave): Atrial contraction occurs at the end of P wave. Simultaneously measuring the mechanical activity & the electrical activity, the mechanical contraction of the atria will occur slowly after the P Wave *P-R Segment: electrical signals slows as it moves through AV Node & AV Bundle. P-R segment that separates the P Wave & the QRS Complex represents the time that the action potential spread slows as it moves through the AV Node & AV Bundle into the ventricles *QRS Complex = ventricular depolarization: ventricular contraction begins shortly after Q wave and continues through T wave. QRS Complex (ventricular depolarization) triggers ventricular contraction which begins shortly after the QRS Complex because of the delay between the electrical mechanical events.

10) Excitation-Contraction Coupling in Cardiac Muscles

Na+ gradient is maintained by the Na+-K+-ATPase pump. & prevents the accumulation of Na+ inside the cell

Filling Phase - Wiggers Diagram

Pat > Pv Pressure in Atrium exceeds Pressure in Ventricle Blue line is above the Green line AV/Mital/Bicuspid Valve will be open With the pressure gradient, & the open mitral valve: -Blood moves in the left ventricle -- Pv < Pao Pressure in Aorta exceeds Pressure in Ventricle Red line is above the Green line Aortic Semilunar Valve will be closed *If this wasn't closed, it would allow blood to move from higher pressure aorta into the lower pressure ventricle With the Aortic Semilunar valve closed, & the open Mitral Valve: Volume in the ventricle will increase

Ejection Phase - Wiggers Diagram

Pv > Pat Pressure in Ventricle remains elevated above the Pressure in the Atrium AV/Mital/Bicuspid Valve will remain closed -- Pv > Pao Pressure in Ventricle rises above the Pressure in the Aorta Green line rises above Red line Aortic Semilunar Valve will open *To allow blood to be ejected from the Ventricle through the Aorta --Decrease in Ventricular Volume --Yellow line is slopes down

Phonocardiogram - Wiggers Diagram

Recording of heart sounds 1st "Lub": Closure of the Mitral Valve 2nd "Dub": Closure of the Aortic Valve

7) Excitation-Contraction Coupling in Cardiac Muscles

Relaxation occurs when Ca2+ unbind from troponin. When Ca2+ levels decrease in the cytosol, as Ca2+ moves either into the sarcoplasmic reticulum or out of the cell, calcium unbinds troponin to cause muscle relaxation

5) Excitation-Contraction Coupling in Cardiac Muscles

Summed Ca2+ sparks create a Ca2+ signal. Initial release of Ca2+ sparks (from step 4) would add up with additional Ca2+ release from the sarcoplasmic reticulum to form a Ca2+ signal

End Diastolic Volume - Wiggers Diagram

The highest volume in the left ventricle before the onset of Systole

Wiggers Diagram

This diagram covers many of the topics from this lecture

Isovolumetric Contraction Phase - Wiggers Diagram

Triggered by the electrical depolarization As the Ventricle begins to contract, Pv > Pat Pressure in Ventricle rises above Pressure in Atrium Green line is higher than Blue line AV/Mital/Bicuspid Valve will close to prevent the back flow of blood from Ventricle to Atrium --1st Heart sound -- Still, Pv < Pao Pressure in Aorta exceeds Pressure in Ventricle Red line is above the Green line Aortic Semilunar Valve will remain close -- With the Aortic Semilunar valve still closed, & the closure of the Mitral Valve: There's nowhere for blood to enter or leave the Ventricle --Ventricular Volume remains Constant --Yellow line is flat

4) Mechanical Events of the Cardiac Cycle Ventricular Ejection

Ventricular Ejection - ventricular pressure rises & exceeds pressure in the arteries, the semilunar valves open & blood is ejected. -Right Ventricular pressure exceeds the pressure in the Pulmonary Arteries -Left Ventricular pressure exceeds the pressure in the Aorta Causes: Opening of the Semilunar Valves & allows ejection of blood from the high pressure ventricles into the lower pressure arteries

3) Mechanical Events of the Cardiac Cycle Isovolumic Ventricular Contraction

Ventricular depolarization is followed soon by a ventricular contraction through excitation-contraction coupling events Isovolumic Ventricular Contraction - 1st phase of ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves. Ventricular Contraction causes the pressure in the Ventricular chambers to rise. As soon as the ventricular pressure rises above the pressure in the atria Causes: Closure of the AV valves that separate the atria & ventricles & ensures that blood does not move backwards from the ventricles into the atria. Closure of Valves is associated with: 1st heart sound (S1) - "lub" Initially, after closure of the AV Valves, the pressure in the ventricles is not yet high enough to cause opening in the Semilunar Valves. --So during the time it takes for pressure to rise further, there's no change in volume while the ventricle is contracting

2) Excitation-Contraction Coupling in Cardiac Muscles

Voltage-gated Ca2+ channels open. Ca2+ enters cell. L-type Calcium channels open in response to the Action Potential to allow Calcium ions to move into the cell. The Ca2+ ions that move into the cell are the same ions responsible for the plateau phase of the cardiac contractile cell, action potential

End Systolic Volume - Wiggers Diagram

Volume of blood remaining (left over) in ventricle after systole

Ventricular Systole - Wiggers Diagram - Change in Volume

follows the QRS Complex (Ventricular depolarization) Isovolumic Contraction Period Represented by flat line of the volume curve since volume isn't changing

9) Excitation-Contraction Coupling in Cardiac Muscles

or the Ca2+ out of the cell Ca2+ is exchanged with Na+ by the NCX aniporter (Sodium-Calcium Exchanger). The pump moves Ca2+ out in exchange for Na+ with the resulting Na+ that moves in exchange for K+ through the Na-K-ATPase Pump

Wiggers Diagram

relates electrical events of heart cycle to the mechanical, pressure, and sound changes that occur during a cardiac cycle