Discrete-State Stochastic Models of Calcium-Regulated Calcium Influx and Subspace Dynamics Are Not Well-Approximated by ODEs That Neglect Concentration Fluctuations

Figure 6

Illustration of two possible volume scalings. For the single channel volume scaling, calcium influx 𝛼 increases proportional to the increase in 𝑣, but the single channel has only two conductance levels, 𝛼₀ and 𝛼₁, depending on whether calcium is free or bound. In the alternative scaling, the number of channels increases in proportion to the volume 𝑣, and when there are many channels the calcium influx rate may take many values between 𝛼₀ and 𝛼₁.