Events Underlying Contraction and Relaxation B

The signal triggering contraction is a propagated action potential (AP) generated in the sinoatrial node. Depolarization of the plasmalemma leads to a rapid rise in cytosolic Ca2+ levels, which causes the contractile filaments to shorten (electromechanical coupling). The level of Ca2+ concentration attained determines the degree of shortening, i.e., the force of contraction. Sources of Ca2+ are: a) extracellular Ca2+ entering the cell through voltage-gated Ca2+ channels; b) Ca2+ stored in membranous sacs of the sarcoplasmic reticulum (SR); c) Ca2+ bound to the inside of the plasmalemma. The plasmalemma of cardio-myocytes extends into the cell interior in the form of tubular invaginations (transverse tubuli).

The trigger signal for relaxation is the return of the membrane potential to its resting level. During repolarization, Ca2+ levels fall below the threshold for activation of the myofilaments (3x10-7 M), as the plasmalemmal binding sites regain their binding capacity; the SR pumps Ca2+ into its interior; and Ca2+ that entered the cytosol during systole is again extruded by plasmalemmal Ca2+-ATPases with expenditure of energy. In addition, a carrier (antiporter), utilizing the transmembrane Na+ gradient as energy source, transports Ca2+ out of the cell in exchange for Na+ moving down its transmembrane gradient (Na+/Ca2+ exchange).

Psychotropic drugs

Sympatholytics

Ganglionic blockers

Parasympathetic

Sympathetic Epinephrine

Drugs with indirect action

Psychotropic drugs

Sympatholytics

Ganglionic blockers

Parasympathetic

Sympathetic Epinephrine

Drugs with direct action Nutrient solution

Drugs with direct action Nutrient solution

Force Rate

ß-Sympathomimetics Cardiac Phosphodiesterase inhibitors glycosides

Force Rate

ß-Sympathomimetics Cardiac Phosphodiesterase inhibitors glycosides

Force Rate

Parasympathomimetics Catamphiphilic Ca-antagonists Local anesthetics

A. Possible mechanisms for influencing heart function

Parasympathomimetics Catamphiphilic Ca-antagonists Local anesthetics

A. Possible mechanisms for influencing heart function

Phosphodiesterase Inhibitors Heart
B. Processes in myocardial contraction and relaxation
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