Cardiac Muscle Mechanics

Question 1

Cardiac and skeletal muscle is very similar but have one major difference, what is that difference?

Answer 1

Cardiac muscle has intercalated discs that separate adjacent myocytes. They are connected by desmosomes and gap junctions. Action potentials can trace across an intercalated disc.

Question 2

Why is cardiac muscle called a functional syncitium?

Answer 2

The cardiac muscle cells are mechanically and electrically connected to one another the entire tissue resembles a single giant muscle cell. It is almost two syncitium the atrial and ventricular portions.

Question 3

Where is the T-tubule in the cardiac myocyte?

Answer 3

At the Z line. It is a different triad than in skeletal muscle.

Question 4

How is Ryanodine receptor opening different for cardiomyocytes?

Answer 4

Instead of a physical attachment between the L-type calcium channel and the Ryanodine receptor as is found in skeletal muscle, cardiac muscle uses Ca induced Ca release (CICR). A small amount of Ca influx through the sarcolemma is amplified into a large amount of Ca release from the SR and triggers muscle contraction.

Question 5

In terms of AP duration how does cardiac muscle differ from skeletal?

Answer 5

The cardiac AP lasts ~250 msec while skeletal is less than 5 msec. The length of the cardiac AP and its refractory period prevent summation, therefore the heart contracts by twitch only.

Question 6

How does cardiac muscle AP initiation differ from skeletal?

Answer 6

The cardiac AP is calcium-dependent and allows Ca entry into the myofiber, the skeletal AP is Na-dependent and does not allow much Ca entry.

Question 7

How do APs move between adjacent cardiac cells?

Answer 7

Via charge displacement through gap junction.

Question 8

Does external Ca contribute to cardiomyocyte twitch? What about skeletal muscle?

Answer 8

Yes about 20%. In skeletal muscle external Ca does not contribute to the Ca transients and twitch.

Question 9

What are the major Ca clearance mechanisms for Cardiomyocytes?

Answer 9

The sarcoplasmic and endoplasmic reticulum ATPase (SERCA) and the Na-Ca exchanger (NCX).

Question 10

Is the heart sensitive to L-type Ca channel blockers?

Answer 10

Yes, extremely. Cardiac muscle is much more sensitive to extracellular [Ca] than skeletal muscle and therefore also more sensitive to L-type channels. CICR is important.

Question 11

The high affinity Ca binding site on the Ryanodine receptor does what?

Answer 11

It is the on switch and it causes the channel to open.

Question 12

The low affinity Ca binding site on the Ryanodine receptor does what?

Answer 12

It is the off switch and it causes the channel to close.

Question 13

Which of these three proteins are shared by skeletal and cardiac muscle cells: TroponinC, tropomyosin, heavy chain of myosin?

Answer 13

They both use TroponinC and tropomyosin but the heavy chains differ. In cardiac muscle there is only twitch contraction while in skeletal summation can occur.

Question 14

Heart rate is increased by sympathetic innervation and norepinephrine release, what effect does this have on the SA node?

Answer 14

It changes the K conductances

Question 15

What is the Frank-Starling law?

Answer 15

The force of contraction increases at increased end diastolic length. The more the ventricle is filled with blood during diastole the greater the volume of ejected blood will be drink the resulting systolic contraction.

Question 16

What accounts for the change in force of the heart of rate Frank-Starling law?

Answer 16

Increased load stretches the myocardium and increases the affinity of troponin C for Ca, leading to an increase in contractile force for a given level of sarcoplasmic Ca. Increase is sensitivity. (The force is not accounted for by changing amounts of thick and thin filament overlap or thin filament interference)

Question 17

What happens to the velocity of shortening, the amount of shortening, the work, and the power of the heart as length increases?

Answer 17

The velocity of shortening increases
The amount of shortening increases
The work of the heart increases
The power delivered by the heart increases.

Question 18

Describe the effect of an Inotropic agent (norepinephrine) on cardiac muscle

Answer 18

Norepi bind to Beta-1 adrenergic receptor leading to an increase in cAMP, PKA, Ca, and tension. The amount of Ca released increases the force of contraction and is an example of an increase in contractility. The increase in contractility decreases the duration of the twitch resulting from a greater Ca flux.

Question 19

Which of the following happens immediately upon the addition of norepi; an increase in action potential amplitude and duration, an increase in tension?

Answer 19

The AP changes immediately, whereas the associated increase in contractility takes approximately 8 beats to reach a new steady state. The rise in intracellular Ca takes longer that the rise in the AP.

Question 20

cAMP activates a protein kinase which phosphorylates phospholamban, what does phospholamban do?

Answer 20

It is a regulatory protein located in the SR membrane of cardiac muscle (and smooth) resulting in increased sensitivity of the SR Ca pump. It increases the rate of uptake of Ca into the SR and reduces the duration of the muscle contraction.

Question 21

Beta-adrenergic stimulation results in what? Discuss Inotropic and chronotropic effect

Answer 21

It has a positive inotropic effect and results in a contraction which is increased in magnitude and decreased in duration. It also has a positive chronotropic effect in that it increases the heart rate.

Question 22

What is the effect of an increase in contractility on the work done by the heart muscle on any load?

Answer 22

It increases in the presence of Norepi