By Jacob Kim ’16

This video was taken during a lab exercise for BIOL 205: Physiology. In the exercise, we dissected a brain-dead frog to expose its heart and observe its behavior in different environments, such as in different temperatures. Towards the end of the lab (the part shown in the video), we completely detached the heart from the rest of the body and recorded its heart rate.

You can see the heart is beating on its own. This is possible because the heart keeps its own rhythm using a group of specialized cells, known as the pacemaker cells. In the frog, they are located near the junction of the vena cava and the right atrium, a region called the sinus venosus. These cells have an intrinsic heart rate, which can be increased by the sympathetic nervous system and decreased by the parasympathetic nervous system. The cells fire signals called action potentials, which move throughout the nerves on the heart, causing parts of the cardiac muscle to contract at the same time. When the ventricle or the left atrium is cut away, it quickly ceases to beat, while the right atrium keeps contracting as it holds the pacemaker cells. It will continue to contract until it runs out of either ATP (a type of chemical energy) or extracellular sodium.