Question

Animal Physiology Question

1. A good pump always has a contraction-relaxation cycle that is completed before the next cycle. How is this achieved in cardiac muscle, providing the necessary details in terms of ion movement and tension generation in the muscle fibers? What feature of skeletal muscle would make it a terrible pump?

Answer 1

The contraction and relaxation cycle of heart is maintained by pacemaker cells in SA node which generates cardiac action potential at regular intervals and causes change in membrane potential and results in movement of ions across cell membrane and regulates contraction and relaxation of cardiac muscle.

SA node generates the cardiac action potential which changes voltage across cell membrane causing movement of ions through ion channels. to atrium, AV node, bundle of his and purkinje fibres. The action potential is generated by pacemaker cells causes depolarisation by opening sodium channels and causes influx of sodium ions changes transmembrane potential to reach -70 from -90mv which further opens fast sodium channels and further increases the transmembrane potential. Calcium channels also opens when the transmembrane potential is -40mv and causes steady movement of Calcium ions against its concentration gradient. This results in positive transmembrane potential and opening of potassium channels occurs and potassium ions escapes the cell and results in decreasing membrane potential as the Persistent outflow of K+, now exceeding Ca2+ inflow results in resting membrane potential(-90mv). Normal transmembrane ionic concentration gradients are restored by returning Na+ and Ca2+ ions to the extracellular environment, and K+ ions to the cell interior.

Cardiac muscle is involuntary and found only in the heart is striated, but the bundles are connected at branching, irregular angles called intercalated discs.Contraction in each cardiac muscle fiber is triggered by Calcium ions in a similar manner as skeletal muscle, but the Calcium ions come from Sarcoplasmic reticulum and through voltage-gated calcium channels in the sarcolemma and unlike skeletal muscle the contraction of cardiac cells is longer and continuous. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle whereas the skeletal muscle is controlled by nervous system.