  Cardiac muscle contraction is initiated by an elementary Ca signal (called Ca
spark) which is achieved by collective action of Ca release channels in a
cluster. The mechanism of this synchronization remains uncertain. This paper
approaches Ca spark activation as an emergent phenomenon of an interactive
system of release channels. We construct a Markov chain that applies an Ising
model formalism to such release channel clusters and realistic open channel
configurations to demonstrate that spark activation is described as a system
transition from a metastable to an absorbing state, analogous to the pressure
required to overcome surface tension in bubble formation. This yields
quantitative estimates of the spark generation probability as a function of
various system parameters. Our model of the release channel system yields
similar results for the sarcoplasmic reticulum Ca concentration threshold for
spark activation as previous experimental results, providing a mechanistic
explanation of the spark initiation. Additionally, we perform numerical
simulations to find spark probabilities as a function of sarcoplasmic reticulum
Ca concentration obtaining similar values for spark activation threshold as our
analytic model, as well as those reported in experimental studies.
