The function of the heart is simply to act as a pump that provides pressure to move blood to its ultimate destination – the tissues. Cardiac muscle differs from skeletal muscle both morphologically and functionally. Probably the most striking and fascinating feature of its contractility is its automaticity, that is its ability to initiate its own rhythmic contractions without requiring external stimulation. This automaticity is believed to be due to ‘leaky’ cell membranes, in which the calcium ions slowly leak into the cells. This leaking causes a slow depolarization to threshold, thus firing an action potential and initiating contractions of cardiac muscle. The cells that are most ‘leaky’ to calcium and that depolarize fastest control the rate of contraction of all other cardiac cells, thus they act as pacemakers for the rest of the heart. In the mammalian heart, the pacemaker is the sinoatrial (SA) node, a group of specialized cells near the junction of the vena cava and the right atrium. In the frog heart, the pacemaker is the sinus venosus, an enlarged region between the vena cava and the right atrium. (The mammalian SA node is believed to be an evolutionary remnant of the sinus venosus). Each region of the heart has its intrinsic rate of beating: for example SA node 72 beats/min; atrium 60 beats/mind; ventricle 25 beats/min. Only when the faster pacemaker region is blocked is it possible to observe the intrinsic rate of the slower regions.

In this series of experiments you will use a frog heart because it functions well at room temperature and will continue to beat even when excised from the body. Mammalian hearts have the same contractile characteristics but must be supplied with a constant flow of warm oxygenated blood to maintain their contractility.

The frog heart differs from the mammalian heart anatomically in that they are three