ABSTRACT
Heart rate (HR) constantly changes on a beat-to-beat basis due to autonomic influences on the pacemaker process of the sinoatrial node. These changes can be quantified as heart rate variability (HRV). In principle, HRV is caused by complex nonlinear interactions between sinoatrial node cells and the autonomic nervous system. Intracellular recordings of spontaneous action potentials were performed with glass micropipettes. Microelectrodes were connected to a battery-operated amplifier. The reference electrode for potential measurements was a chlorided silver wire immersed into the experiment chamber. Microscopic techniques are commonly used in order to visualize biological tissues or cells. Beside aspects of magnification, illumination, and resolution, the most important parameter in order to visualize beating single cells is contrast. The microscope's field of view depends on the magnification as well as additional optical elements such as mirrors or lenses in the optical path.
