- Excitability of neuronal and cardiac tissue is inversely proportional to extracellular calcium levels
- Due to the direct effect calcium plays in binding to voltage-gated sodium and potassium channels and slowing their opening
- Results in altered membrane threshold potentials
- Low calcium levels cause increased membrane excitability:
- Reduces the membrane threshold potential, thus narrowing the gap between the resting and threshold potentials
- Can result in arrhythmias, tetany and seizures
- High calcium levels cause decreased membrane excitability:
- Reduces the membrane threshold potential, thus widening the gap between the resting and threshold potentials
- Is used in hyperkalaemia (through calcium administration) to restore a normal gradient between the membrane threshold potential and pathologically elevated resting membrane potential
