Ca2+-CaM Activation domain. (A) A snapshot of a simulation showing that CaM molecules become fully Ca2+ saturated only within close proximity of ion channels with low Ca2+ injection rates. The probability of CaM entering into a fully Ca2+ saturated state depends sharply on the distance of CaM molecules from the source of Ca2+ entry. (B) A snapshot of a simulation having a higher Ca2+ injection rate. The CaM molecules become fully Ca2+ saturated throughout the entire spine. The red spots in the figures indicate fully Ca2+ saturated CaM molecules while dark blue are partially Ca2+-saturated CaM. The light brown particles represent Ca2+ channels. The small yellow particles are Ca2+ ions.