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Hyperkalaemia, which refers to elevated potassium levels in the blood, leads to significant changes in the excitability of myocytes, particularly cardiac muscle cells. The initial effect of hyperkalaemia on myocyte excitability is an increase in excitability.

This occurs because increased extracellular potassium reduces the resting membrane potential, making it less negative (depolarized). When the resting potential is higher (closer to zero), myocytes become more likely to reach the threshold for action potential generation. In the early phases of hyperkalaemia, the depolarized state enhances the propensity for spontaneous depolarization and can prime myocytes to fire more readily in response to stimuli, thereby increasing excitability.

As hyperkalaemia progresses, however, myocyte function can become impaired due to further depolarization, which leads to refractory periods and decreased excitability overall. However, during the initial stages, the enhanced excitability clearly stands out.