L19-myocardial Contractility Flashcards
Define myocardial contractility
Ability of cardiac muscle to contract
Fundamental contractile unit in both skeletal and cardiac muscle
Sarcomere
What are the main contractile elements and describe them
Myosin thick filaments with global your heads evenly spaced along their heads and contain myosin ATPaSe
Actin: smaller molecules thin filaments consisting of two strands arranged as an alpha helix woven between myosin filaments
What are the regulatory elements
Tropomyosin:double helix that lies in the groove between actin filaments and prevents contraction in the resting state by inhibiting the interaction between myosin heads and actin
Troponin: complex with three subunits that at sit at regular intervals on the Actin strands
Types of troponin
TnT=ties troponin complex to actin and Tropomyosin molecules
TI= inhibits activity of atpase in actin and myosin interaction
TnC=binds calcium ions that regulate contractile process
What is excitation contraction coupling
Process by which an electrical action potential leads to contraction of cardiac muscle cells and is achieved by converting a chemical signal in to mechanical energy via the action of the contractile proteins
Calcium is the mediator
Why do we need the CICR
AP results in an inward flow of calcium ions from extracellular fluid
Initial influx in to myocytes through L type channels during phase 2 is insufficient to trigger contraction
Signal is amplified by CICR which triggers greater release of Ca2+ from SR
Describe CICR
▪️Sarcolemma contains t tubules that bring L type channels in close contact with ryanodine receptors
▪️When Ca2+ enter L type channels ryanodine receptors change conformation and induce a larger release of Ca2+ from abundant SR stores
Describe excitation contraction coupling process
1-Ca2+ binds to TnC and TnI is inhibited,conformational change in Tropomyosin exposes active site between actin and myosin
2-myosin heads interact with active sites on actin filaments and flex
3-hydrolysis of ATP by ATPase on myosin induces crossbridge formation between myosin head and active site on actin
4-interaction between myosin head and actin trigger firing of myosin head causing it pull itself along the actin filament in a process known as the power stroke
5-ADP is released from myosin head binds a new ATP and releases the actin filament
What inhibits the action of TnI
Calcium
Relation between the action potential
& the mechanical response
Cardiac ms begin to contract few milliseconds after AP begins and continues to contract until few milliseconds after AP ends
Diastole begins at the end of plateau
2nd rapid depolarization is completed at about middle of diastole
Duration of contraction in atria and ventricles
- 2 sec in atria
0. 3 in ventricles
Describe the process of myocyte relaxation
Ca2+ is sequestered back in to SR by SERCA and pumped out of the cell
Ca2+ dissociate from TnC as their intracellular conc falls
Ca2+ is expelled from myocyte via Na-Ca ion channel
ATP used to pump out and in to SR stores
Stored as ionized Ca
Attached to calsequestrin
Effect of calcium excess
Spastic contraction of heart
Caused by direct effect of calcium ions to initiate cardiac contractile process
How does sympathetic innervation affect contractility
Increases it by
Carecholamines acting in B1 receptors
Phosphorylation of proteins as sarcolemmal calcium channels in SR inc calcium release from SR
Phosphorylation of L type calcium channels will inc their permeability to calcium allowing entry of more calcium to cardio myocytes
