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When a brief current pulse is incident on cells in cardiac and other nervous tissue, a change in phase of the cell's response is usually observed. In cardiac tissue, the cells are exposed to external stimulation of mainly positive currents, which depolarise the cells (except for some ACh interactions). There are a number of factors that influence the phase response of the cell. These include the tinting of the stimulus, its magnitude, duration and polarity. The interbeat interval of the cell may be prolonged or shortened, causing a steady state phase offset from the original cycle. The particular phase response is dependent on the stimulus parameters. We investigate the phase response of a model sinoatrial node cell when subjected to single depolarising stimuli as a function of the stimulus parameters. The model used was developed by Dokos  which defines the ionic currents that cause the spontaneous electrical activity of the cells.