The urodele amphibians, Amphiuma and Necturus, provide heart fibers large enough to serve for microelectrode recording under visual control with the microscope. Bundles containing as few as 5 to 10 fibers yield spike potentials, rather than the plateau forms generally considered to be characteristic of heart muscle. These spikes fail to overshoot. The plateau form, and only the plateau form, is recorded exclusively from large tissue masses. An intermingling of spikes and plateau-shaped action potentials is obtained from bundles of intermediate size. These data are confirmed in experiments in which the myocardium is sliced into adhering strips of unequal sizes. The conclusion is drawn that the configuration of the recorded action potential curve is contingent upon the mass and geometry of the tissue impaled by the microelectrode. The crucial experiment of recording from an isolated single heart fiber is not possible, because of the attendant injury. Our proposal that the spike form is the elemental heart action potential is, to this extent, an extrapolation. Attempts to explain the nature of the spike along classical lines are not entirely satisfactory. Other theories are considered which, in their turn, are generally unacceptable. Evidently only further experimentation can clarify the situation.
Engineering the Microenvironment for Heart Muscle Cell Mechanobiology