Flashcards in Cardiac Muscle Mechanics Deck (22)
Cardiac and skeletal muscle is very similar but have one major difference, what is that difference?
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.
Why is cardiac muscle called a functional syncitium?
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.
Where is the T-tubule in the cardiac myocyte?
At the Z line. It is a different triad than in skeletal muscle.
How is Ryanodine receptor opening different for cardiomyocytes?
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.
In terms of AP duration how does cardiac muscle differ from skeletal?
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.
How does cardiac muscle AP initiation differ from skeletal?
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.
How do APs move between adjacent cardiac cells?
Via charge displacement through gap junction.
Does external Ca contribute to cardiomyocyte twitch? What about skeletal muscle?
Yes about 20%. In skeletal muscle external Ca does not contribute to the Ca transients and twitch.
What are the major Ca clearance mechanisms for Cardiomyocytes?
The sarcoplasmic and endoplasmic reticulum ATPase (SERCA) and the Na-Ca exchanger (NCX).
Is the heart sensitive to L-type Ca channel blockers?
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.
The high affinity Ca binding site on the Ryanodine receptor does what?
It is the on switch and it causes the channel to open.
The low affinity Ca binding site on the Ryanodine receptor does what?
It is the off switch and it causes the channel to close.
Which of these three proteins are shared by skeletal and cardiac muscle cells: TroponinC, tropomyosin, heavy chain of myosin?
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.
Heart rate is increased by sympathetic innervation and norepinephrine release, what effect does this have on the SA node?
It changes the K conductances
What is the Frank-Starling law?
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.
What accounts for the change in force of the heart of rate Frank-Starling law?
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)
What happens to the velocity of shortening, the amount of shortening, the work, and the power of the heart as length increases?
The velocity of shortening increases
The amount of shortening increases
The work of the heart increases
The power delivered by the heart increases.
Describe the effect of an Inotropic agent (norepinephrine) on cardiac muscle
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.
Which of the following happens immediately upon the addition of norepi; an increase in action potential amplitude and duration, an increase in tension?
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.
cAMP activates a protein kinase which phosphorylates phospholamban, what does phospholamban do?
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.
Beta-adrenergic stimulation results in what? Discuss Inotropic and chronotropic effect
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.