Depolarization, potassium influencing cell

The level of potassium in the cells and in the extracellular fluid will influence cell depolarization—the movement of the resting potential closer to the threshold at which an action potential can occur, causing more cell excitability and hyperpolarization—decreased resting membrane potential to a point far away from the threshold at which an action potential can occur, causing less cell excitability. Potassium imbalance thus can affect nerve and muscle cells by causing hyperexcitability or depressed excitability. 

Automaticity can be defined as the ability of a cell to alter its resting membrane potential toward the excitation threshold without the influence of an external stimulus. The characteristic feature of cells with automaticity is a slow decrease in the membrane potential during diastole (phase 4) such that the membrane potential reaches threshold (Figure 16.2). During phase 4 in these pacemaker cells, the background potassium leak current decreases and an inward depolarizing current (h) is... 

The morphology of an action potential is dictated by the flow of ions across the cell membrane (Figure 2a). An inward flow of sodium and calcium ions has a depolarizing influence on the membrane potential, while an outward flow of potassium has a repolarizing effect. The upstroke of the cardiac action potential (phase 0) is due to an inward flux of sodium ions and the plateau phase (phase 2) is maintained... 

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