The critical concentration is the concentration of either G-actin (actin) or the alpha,beta- tubulin complex (microtubules) at which the end will remain in an equilibrium state with no net growth or shrinkage. What determines whether the ends grow or shrink is entirely dependent on the cytosolic concentration of available monomer subunits in the surrounding area. Critical concentration differs from the plus (C) and the minus end (C), and under normal physiological conditions, the critical concentration is lower at the plus end than the minus end. Examples of how the cytosolic concentration relates to the critical concentration and polymerization are as follows: 
* A cytosolic concentration of subunits above both the C and C ends results in subunit addition at both ends
* A cytosolic concentration of subunits below both the C and C ends results in subunit removal at both ends
Note that the cytosolic concentration of the monomer subunit between the C and C ends is what is defined as treadmilling in which there is growth at the plus end, and shrinking on the minus end.
The cell attempts to maintain a subunit concentration between the dissociation constants at the plus and minus ends of the polymer.