In this article we will discuss about the origin of the frog heart beat with the help of suitable diagram.
By separating atria from the ventricles of excised hearts, William Harvey in 1628 had shown that the atrial rhythm was higher than the ventricular rhythm. Keith and Flack (1907) have described that from the S.A. node the heart beat first starts.
If the impulse conduction from this area is cut off by giving a Stannius ligature between the sinus and atrium of the frog heart (first Stannius ligature), the heart beat does not cease but a new slower rhythm develops within a few seconds from a new pacemaker area.
This indicates that the S.A. node (sinus) is not the only pacemaker; there is other pacemaker area-the A.V. node which can maintain the beat in its absence. If second ligature is applied in the same heart in between the atrium and ventricle then ventricle begins to beat at a more, slower rate indicating that ventricle can function as pacemaker in the absence of S.A. node or A.V. node.
Now it shows that the three separate components of the heart beat at a descending order of frequency are represented by the sequence; sinus → atrium → ventricle. In normal heart the S.A. node controls the rest of heart muscle by its higher rhythmical activities.
This classical experiment, described above cannot be possible in mammalian heart because, S.A. node and A.V. node are both present in the atrium. So by simple ligature the S.A. node cannot be separated from the A.V. node by first stannius ligature. Ventricle however can be separated from the atrium by the second Stannius ligature (Fig. 7.26).
Experimental localisation of the pacemaker can be done by many other ways. If cold or warm saline is applied to the sinus region of the amphibian heart then rhythm is altered. If the S.A. node is damaged by thermocautery then the heart rate slows.
With the advent of recording the trans-membrane potential simultaneously from the S.A. node, the A.V. node and ventricle in intact beating heart, a more precise localisation of the pacemaker has been possible.