Urgeent!! Frog dissection

[sunshine662]

I'm going to finish a lab about frog dissection.
There's a few questions to discuss after the lab, and I can't search the answers through the web, can anyone help me?
How Do sodium and calcium ions respectively affect heart beat rate and heart contraction?
I already have some answers, but I'm not quite sure about that and I need more explanations.
When there is high concentration of Na+ ions in our body, our heart beat rate and heart contraction decrease. Is it because sodium ions lengthens depolarization, so that contraction weakens?
And, will calcium ions increase with heart contraction?
Please give me detailed explanation, I've found that in many resources, but can't find enough information
THX=)