Cardiac Muscle: Contraction, Relaxation & The Cardiac Cycle

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UMKC SOD

What are the two pumps/sensations of the heart?

pumps = Rt - pulmonary; Lf - systemic
sensations = atria and ventricles (the atria contract simulaneous like the ventricles do)

What is the purpose of the Atrioventricular (AV) valves? What are the two AV valves of the heart?

They prevent back flow from ventricles to atria during systole. Tricuspid and Bicuspid (mitral)

What is the purpose of the Semilunar valves? What are the two semilunar valves of the heart?

The semilunar valves prevent backflow from the arteries to the ventricles during diastole. Pulmonary and aortic

Which of the two valve types is flimsier and why?

The AV are flimsier and thus take lower pressure to close, the semilunar valves are more rigid and thus take higher pressure to close.

What are valves?

Fibrous tissues covered with epithelium; closure of valves is due to a change in pressure.

What are the two types of cardiac myocytes?

- contractile muscle fibers/cells
- conductive muscle fibers (i.e. autorhythmic)

What are contractile muscle fibers? What types are there?

These are cells responsible for pumping; two types atrial and ventricular myocytes.

Which of the Contractile muscle fibers are more forceful?

The ventricular are stronger because they have more T tubules, mitochondria and sacromeres. These use 70% of the O2. The left ventricles has lots and losts of mitochondria.

What are conductive muscle fibers?

These are the autorhythmic/pacemaker cells. They are "puny" and have even fewer mitochondria, SR etc than atrial contractile cells.

What is stenosis?

Narrowing of the valves we heard stenosis when the valves are open.

What is regurgitation?

Backflow, inability to close the valve completely so black moves in a backwards direction. We hear this when the valves are closed.

What is a heart murmur?

Any abnormal sound.

What is happening every time you hear heart sounds?

The turbulence (right before closing) - signals that a valve is closing.

What is the job of papillary muscles?

They are responsible for preventing prolapse, NOT assisting with closure.

Where are t tububles located?

On the z line in dyads.

When the the tri and bicupsid valves close?

When ventricular pressure is higher than atrial pressure.

What is characteristic of cardiac muscle?

- striated
- lots of branches
- tons of mitochondria and SR

What is the job of troponin, tropomyosin, and mysosin in muscle contraction?

Myosin had a head which binds to the C subunit of troponin once the tropomyosin releases troponin. Ca+ binding to tropomyosin causes the release of troponin.

What are intercalated discs?

Specialized cell membranes containing gap junctions and desmosomes. The cardiac muscle is a syncytium.

What is the purpose of gap junctions/desmosomes?

gap junctions = rapid Na+ movement
desmosomes = prevent the cardiac cells from ripping each other apart during contraction; also allow free transfer of force from myocyte to myocyte

T/F: Different types of cardiac myocytes have difference in intercalated discs.

True

What kind of intercalated disc do autorhythmic cells, atrial myocytes and ventricular myocytes have?

Both ventricular and autorhythmic cells have end-to-end transmission, while atrial myocytes have end-to-end and side-to-side transmission.

T/F: The atria and ventricles share signals due to gap junctions.

False, the atria and ventricles operate as separate entities because of the thick fibrous tissue separating them.

Why is is important than atria and ventricles not share signals?

It allows time for blood to fill the ventricles from the atria, otherwise the contraction would be so sudden that no blood would be transferred ~ AV nodal delay.

How do Action Potentials in skeletal m. compare to those in cardiac muscle?

APs in skeletal muscles are much shorter (15X shorter)

What ions are responsible for the AP of cardiac muscle?

Na, K, Ca

The cardiac AP is numbered 0-4 what ion is responsible for each number of the curve?

0 - Na depolarization (voltage gated Na channels)
1 - K slight repolarization (voltage gated K channels)
2 - K and Ca plateau effect (loss of K same as intake of Ca) held at depolarized state
3 - K full repolarization
4 - resting state
* we always start at 4

What is the absolute refractory period in cardiac mm?

due to inactive Na+ channels, last most of the duration of the AP

What is the effective refractory period in cardiac mm?

includes and is longer than the effective refractory period; when possible to contract but a CONDUCTED AP cannot be generated.

What is the relative refractory period in cardiac mm?

period when an AP is possible but it needs a larger excitatory stimulus than normal

What is the supranormal refractory period in cardiac mm?

at the end of the AP, easier because of a smaller than normal EPSP is needed, but blood can't fill ventricles in that small amount of time

Why is the longer AP of cardiac myocytes vital?

allows time for blood to enter ventricles and prevents temporal summation of cardiac contraction

Where are the most t tubules found? Why?

In ventricular myocytes, rare in atrial myocytes and absent in autorhythmic cells. They enable more forceful contractions.

Where do we get Ca+ from?

SR and ECS

What is the strength of contraction dependent on?

The Ca+ in the ECS - strenght of cardiac contraction is graded based on extracellular Ca+ content

What type of Ca+ channels are there in cardiac cells?

L type

What do T-tubules have lots of which bind Ca+ to keep an extracellular store near and ready?

muscopolysaccharides - negatively charged and attract Ca+

How is Ca+ released from the sarcoplasmic reticulum?

By calcium-induced calcium release through ryanodine receptors channels. These are a sort of positive feedback system.

What initiates contraction?

Ca in the presence of ATP

How is relaxation of cardiac muscle accomplished?

By removing Ca+ using either Ca+ ATPases (1' active transport) or Na+/Ca+ exchangers (2' active transport)

What is digitalis?

a cardiac glycoside that inbhits the Na+/K+ ATPase by binding to K+ site => increases the force of contractions by diminishing the Na+ gradient so that not as much Ca+ can be moved

What are inotrophic agents?

those which alter the force of contractions
+ = increase force of contractions (i.e. digitalis)
- = decrease force of contractions

Which side of the heart does more work?

The left side

What happens during systole?

Contraction = increase in pressure

What happens in diastole?

relaxation = decrease in pressure

Over, where is pressure highest for longest?

In atria (ventricular pressure is higher value wise, but over an extended time pressure is highest in the atria.

T/F: Valves on the same side of the heart, like the pulmonary semilunar and the tricuspid are never closed or open at the same time.

True

Which atria contracts first? Ventricle?

Right Atria first, left ventricle first

What is true of the cardiac cycle?

What is happening on the right side is also happeing on the left side with minor differences.

Atrial filling is ______. Ventricular filling mostly occurs during ______.

Atrial filling is passive and ventricular filling mostly occurs during atrial diastole.

What percentage of blood in the ventricles is due to atrial systole (i.e. contraction)?

only 20% rest comes from passive filling.

T/F: Changes is pressure determine the state of valves.

True

What are the two stages of ventricular systole and diastole?

systole = contraction and ejection
diastole = isovolumic relaxation and ventricular filling

T/F: The pressure in the ventricles is higher than the atria in systole.

False, ventricles are lower so that blood blows from high pressure to low

When is atrial volume the greatest?

end of arterial diastole

When is ventricular volume the greatest? Smallest?

G = end of diastole
S = end of systole

What happens when the AV valves open?

increase in ventricular volume

Why do the semilunar valves stay a bit apart?

to allow supply to coronary blood flow, this is because it takes a while for valves to overcome initial lag => once aortic pressure is higher = close

In ventricular filling what are the three stages? Describe them?

1. rapid ventricular filling - after AV valves open, blood enters ventricles
2. diastasis - slow changes in ventricular pressure and volume
3. atrial contraction - flows p wave of ECG

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