Difference between dromotropic vs. chronotropic?

[beatsbydre]

Dromotropic and chronotropic both relies on SA/AV nodal action potential (Phase 0,3,4). I'm wondering why chronotropic relies on inward Na+ current during Phase 4, and why dromotropic relies on inward Ca++ current during Phase 0. I think they all do the same thing?

Can someone explain it to me like I'm five?

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[withrye]

I couldn't tell you why one channel is preferred in one node versus the other, but chronotropic effects come mainly from the SA node and dromotropic from the AV node. The chrono- effect is about a rate of firing, so it's more relevant to think of the SA node as it's in control of the basal firing rate. The dromo- effect is more of a speed/conduction effect, so it's best to think of that at its rate-limiting location, the AV node (the place in the heart with the slowest conduction).