What effect does high extracellular calcium have on muscle excitability?

What effect does high extracellular calcium have on muscle excitability?

• A. More excitable.
• B. Less excitable.
• C. No change.
• D. Variable by cell type.

Answer: (B)

High extracellular calcium makes muscle membranes less excitable. Ca2+ on the outside of the cell stabilizes the membrane and raises the threshold for action potential initiation. It does this by binding near voltage-gated sodium channels and making them less likely to open, so a stronger stimulus is required to reach the threshold. The net effect is a reduced probability that depolarization will trigger an action potential, meaning slower or weaker responses. Clinically, higher calcium levels are associated with decreased excitability (muscle weakness), while lower calcium levels increase excitability.

High extracellular calcium makes muscle membranes less excitable. Ca2+ on the outside of the cell stabilizes the membrane and raises the threshold for action potential initiation. It does this by binding near voltage-gated sodium channels and making them less likely to open, so a stronger stimulus is required to reach the threshold. The net effect is a reduced probability that depolarization will trigger an action potential, meaning slower or weaker responses. Clinically, higher calcium levels are associated with decreased excitability (muscle weakness), while lower calcium levels increase excitability.