Physiology: Cardiac Muscle/Function Flashcards
How does the shortening of muscle occur?
By the sliding filament mechanism :
Actin filaments slide along adjacent myosin filaments by cycling of cross-bridges with myosin
What happens when cardiac muscle shortens?
Z lines come closer together and muscle cell shortens
Cardiac muscle produces force/tension
What is the trigger for cardiac muscle contraction?
Calcium
What undergoes a conformational change for cardiac muscle contraction?
Tropomyosin
This allows actin and myosin to interact
When does the excitation-contraction coupling occur?
Occurs in systole
What process occurs in diastole?
Relaxation-restitution
Describe what happens as the cell is depolarized.
Cells depolarized
Membrane potential rises
Threshold L-type calcium channels open
Ca ions enter the cell
Are the Ca ions released from the L type channels enough?
NO
Causes calcium to be released from the SR
What do we call the process whereby calcium released from the L-type channels stimulates calcium release from the SR?
Calcium Induced Calcium Release (CICR)
What receptors does the junctional SR have?
Ryanodine receptors (RYR)
Name the organization of the RYR that ensures maximal release of Calcium.
Density of RYR is maximal near the L-type Ca receptors
Distance between the RYR and L-type channels is short
This facilitates CICR
Describe what happens to tropomyosin when Ca ions bind.
Ca ions bind to troponin C in troponin complex
Tropomyosin moves
Actin-myosin interaction occurs
Muscle contraction occurs
What happens at the end of contraction to allow cardiac muscle relaxation?
Ca2+ influx ceases and SR is no longer stimulated to release Ca
Intracellular concentration of Ca is reduced:
1 - SERCA
2 - Sodium-calcium exchange pump
3 - Cell membrane ATP dependent Ca pumo
Describe the SERA mechanism.
ATP dependent Ca pump
Pumps calcium from the cytoplasm into the SR
What regulates SERCA pump?
Regulatory protein - Phospholamban (PLN) = inhibits calcium uptake into the SR (while in its dephosphorylated state)
What powers the sodium calcium exchange pump?
Sodium gradient
Where does the majority of Ca from the cytoplasm go?
Goes back into the SR by SERCA pump
Remainder goes via sodium calcium pump
Very small amount of calcium goes via Ca-ATPase pump
Why does cardiac contraction need to be adaptable?
1 - make the heart a better pump i.e. improving pumping action
2 - improve operating conditions to give it more to work with
What does sympathetic stimulation of cardiac muscle produce?
Increase in active tension
Increase in rate of tension development
Increase in rate of relaxation - shorter contraction
Describe how sympathetic stimulation has a positive inotropic effect on the heart.
Binding of noradrenaline to B1-adrenoreceptors on cardiac muscle cell
- increased concentration of cAMP
- activation of protein kinase A (PKA)
- PKA phosphorylates L-type Ca channel and RYR channel
- more Ca ions enter cell
- increases force of contraction
Describe how the sympathetic stimulation has a lusitropic effect on the heart.
Binding of noradrenaline to B1 adrenoreceptors of the heart
- increase in cAMP
- increase in PKA
- phosphorylation of phospholamban (PLN) = inhibition of SERCA
- increased uptake of Ca into SR
- promotes rate of relaxation (lusitropic effect)
- phosphorylation of troponin-I limits interaction between troponin C and Ca = relaxation
What underlies cardiac muscle’s ability to control stroke volume?
Increase its contractile force with a slight increase in its length
What happens if the heart muscle is overstretched?
Its ability to produce active tension is reduced
An overly dilated heart is not a good pump
Starlings Law
Describes responses to changes in LVEDV
Increase LVEDV = increased SV
