Contraction Of Heart Flashcards by Karen Fouad

AP precedes contraction by

20 msec

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Mention the sources of calcium for cardiac muscle contraction

Ca entry from ECF

2. Ca triggered Ca release by SR

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DHP receptor is ….

While ryanodine receptor is …..

Voltage gated Ca-channels

Ligand-gated Ca-channels

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Mention the complex respnosible for inhibting actin-myosin interaction

Troponin-tropomyosin complex

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Mention the Ca binding component if myofibril

Troponin C

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…… moves during contraction to uncover actin binding site for myosin.

Tropomyosin

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Actin-myosin binding is broken in presence of …..

ATP

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Mention methods of Ca removal during relaxation

Pumping Ca into longitudinal portion of SR
Ca pump (active medahnism) to ECF
Na-Ca exchanger (passive mechanism)

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The mostvimportant ohysiological factor that enhances relaxation is …..

Sympathetic stimulation

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Mention the mechanism by which sympathetic stimulation inc heart rate & its important

By accelerating the rate of relaxation by increasing the activity of SR Ca ATPase pump leading to faster & more Ca re-uptake by SR.
This is important because the increased rate of relaxation compensates for the decreased time of ventricular filling during the shortened diastole

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Mention the most important pathological factor affecting rate of relaxation & its mechansim

Ischemic heart disease
The activity of SR Ca-ATPase pump & hence the Ca-reuptake by SR are decreased resulting in a depressed rate of relaxation (diastolic dysfunction)

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Contraction reaches its peak during ….

The last 1/3 of thecplateau of AP

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Mention the importance if long ARP in cardiac muscle

Almostv50% of ventricular relaxation is obligatory before it can respond to a second stimulus & this guarantees satisfactiry filling.

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Eachof atrial & ventricular muscle sheets act as ….

One big muscle unit because it is a functional syncytium (all or none rule)

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Mention intrinsic & extrinsic factors affecting force of myocardial contraction

I: preload, afterload, frequency if stimulation & contractility
E:nervous, drugs, ECF ions, neurohormonal.

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Define contractility (now)

The intrinsin ability of the cardiac muscle to generate firce at constant length

The term prload refers to …..

Its effect is describe by … law

The degree of passive stretch exerted by volume of blood in the ventricle just before its contraction
Frank-Starling

The ……. determines maximum tension , while the …… determine passive tension

Length of CE

Length of PE

Mention the effect of stretch of muscle within limits

Elongates the CE and this exposes more actin binding sites for myosin heads thus increasing the number of cross-bridges when muscle is stimulated.

Mention the effect of overstretch of muscle

Deterioration of active tension due tobpulling away of actin filaments from the myosin heads.

In human, Lmax is ….

2.2 microns

The afterload is increased when ……..

Beating against higher arterial BP, or a stenosed valve

The stages of cardiac muscle contraction are ….&….

Isometric & isotonic

Describe isometric contraction

Shortenig of CE is compensated by lengthening of PE, no lifting of load, force is generated in the form of increased tension alone.

Describe isotonic contraction

The muscle tension developed in enough to lift load and shortening of CE shortens the whole muscle, thus force generated is in the form of incteased tension then shortening.

Describe effects of increased afterloading

A) Increased tension | B) Less and less shortenening of muscle till contraction is isometric.

Greater preload can overcome a large ....., but has no effect on ....

Aftetload | Vmax

Vmax is increased by ......

Factors that increase contractility (+ve inotropic factors) such as sympathetic stimulation & vice versa.

GR: Although vagus doesn't supply the ventricles, it decreases strength of contraction.

This can be explained by the effect of vagus nerve on the SA node decreasing heart rate and hence decreasing contractility of ventricular muscle.

What is the effect of repeated stimulation on force of contraction?

As the IC Ca increases cumulatively over several beats before the active reuptake of Ca by the sarcoplasmic reticulum has lowered the IC Ca conc to original low value, leading to increased force of contraction.

Contractility depends on ......, it is severely affected by .....

Integrity of the muscle elements | Myocardial infarction

The main factor determinig inotropism directly is ....

Ca++ concentration

Mention the effect of autonomic nervous system on myocardial contraction force.

Sym: +ve inotropic by -Increase entry of Ca from ECF by activation of calcium slow channels -Increase Ca entry from ECF due to increased frequency -Increase Ca release from SR & stimulation of Ca-ATPase pump Para: -ve inotropic by opposite effects

Mention effect of neurohormones on inotropism

Epinepherine & norepinephrine +ve | Ach -ve

..... stops heart at systole, while .... stops heart at diastole.

Hypercalcemia | Hyperkalemia

Describe the mechanism of action digitalis drug in treatment of heart failure.

Inhibit Na-K pump, thus increase IC Na, and increase activity of Na-Ca exchanger, so the IC Ca conc rises aiding cardiac muscle contraction.

``` Unlike skeletal muscle cardiac muscle is NOT affected by 1..... 2..... But it is affected by: 4..... ```

1. Numer of motor units recruited 2. Summation of contraction 3. Increasing its initial length 4. Increasing the cytoplasmic Ca conc