What is that which spontaneously depolarise to trigger contraction of cardiac muscle and establishes the heart rate

               It is Na+ ions that polarized and in result the adhraring of Ca in muscles tubulin that trigger the contraction of cardiac muscles

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answer:

convection currents in the magma drive plate tectonics. heat generated from the radioactive decay of elements deep in the interior of the earth creates magma (molten rock) in the aesthenosphere. the aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the earth that extends to 2900 km.