Question

functions of the atria as essentially blood reservoirs and primer pumps for the ventricles. Thus, the ventricles are "forcibly filled" slightly by mild atrial contractions. Given what you know about the molecular "micromechanics" of muscle cell contraction, comment on how this atrial contraction might facilitate the subsequent contraction of the ventricle. Try to consider both the passive effects (e.g. natural cell elasticity) as well as cellular and molecular level mechanisms involved in generating contractile force when answering this question.

Answer 1

Myocardial contraction and hearbeat is regulated by the process called "excitation-contraction coupling.During atrial diastole blood flows down the atria,increasing the pressure inside atrial chambers,this leads the passive flow of blood from atria to the ventricles.when the action potential is generated at SA node it induces atrial muscles to contract(atrial systole) pushes the blood towards venticular chamber to get filled through mittral and tricuspid valve. Further on,impulse is passed to the AV nodes located at the bottom of right atrium sends signals to lower ventricles through Bundle of His and Purkinje fibres causing ventricular contraction after which the blood leaves the heart through arotic valve into the arota and then to the body.

As,the myocardium (cardiac muscles) found in ventricles is much thicker as compared to the one present in atria,which helps to allow forceful contractions during ventricular systole(depolarisation).Also the myocytes(cardiac muscle cells) of ventricles are different from the myocytes of atria,as the former have longer and wider myocytes with a denser T-tubule network.These network helps to conduct impulses from sarcolemma(cell surface) to sarcoplasmic reticulum (inside the cell).Due to which sarcolemma(plasma membrane of muscle cell) is depolarised and the calcium channels open, further the ca+2 influxes rapidly inside the cell increasing the ca+2 amount in sarcoplasm.Now the ca+2 binds with the troponin component of actin,this uncoveres the active binding site of myosin present on actin as soon as it is uncovered actin and myosin bind together results in pulling of myosin fillament towars actin filament which surrounds either end of the myosin filament,with the utilisation of ATP.

Thank you.