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Refractory period of cardiac muscle Learning Objectives At the end of the practical, the student should be able to: • • • • Define Refractory Period? Differentiate between absolute and relative refractory periods? Explain what is a cardiac cycle? Determine the extent to which it is possible to re-stimulate cardiac contraction during a cardiac cycle? Refractory Period Definition: The period that follows effective stimulation, during which excitable tissue fails to respond to a stimulus of threshold intensity. • • Absolute Refractory Period. Relative Refractory Period. Absolute Refractory Period: During the absolute refractory period, a second stimulus (no matter how strong) will not excite the muscle fiber. Relative Refractory Period: During the relative refractory period, the muscle fiber can respond only to a strong stimulus. Cardiac Cycle Definition: A complete contraction and relaxation episode in the heart. The cardiac cycle consists of: • • A Systolic Phase and A Diastolic Phase. One complete cardiac cycle forms the common "lub-dub" pattern recognized as the heart beat. Exercise : Refractory period of cardiac muscle • The aim is to determine the extent to which it is possible to re-stimulate cardiac contraction during a cardiac cycle. • To be performed only if time allows and instructed to do so. • If you are using the ADInstruments Bridge Amp please use this 'Zero Bridge Amp' link to correct for any residual offset voltage again. Procedure • Connect the positive and negative BNC connectors of the Animal Nerve Stimulating Electrode to the analog Outputs on the Power Lab. • Check that the number of pulses in the Stimulator panel is set to 1. • Click Start to begin recording. • Gently place the tips of the stimulating electrode on the surface of the frog heart. • By clicking the Stimulate button on the Stimulator panel, attempt to elicit a second contraction from the frog heart at each of the points in the cardiac cycle illustrated in the figure below. • Approximate points during the cardiac cycle at which you should attempt to stimulate the heart. Stimulator Panel • The Stimulator panel allows the control of the voltage (current in some experiments) of a pulse, the number of pulses and the interval between pulses applied to a tissue. • In some experiments a Stimulate button may be visible. This allows the control of stimulation independently from sampling. Stimulate Button • The Stimulate button is only available while recording. It allows the user to stimulate at any time during a recording. Stimulation can be halted by clicking the Stop button in the LabTutor Data Panel. Analysis • In the table, enter "yes" if stimulation resulted in an extra contraction (extra-systole) with a stimulus delivered at the point in the contraction cycle or "no" if it did not.