the action potential, stimulates both muscle contraction and calcium release from the SR. The Ca2+ released during the spark, then activates two other ion channels on the membrane. One channel allows potassium ions to exit the cell, whereas the other allows chloride ions to leave the cell. The result of this movement of ions, is that the membrane voltage becomes more negative. This deactivates the DHPR (which was activated by the positive membrane potential produced by the action potential), causing it to close and stopping the flow of Ca2+into the cell, leading to relaxation. Termination The mechanism by which SR Ca2+ release terminates is still not fully understood. Current main theories are outlined below: Local depletion of SR Ca2+ This theory suggests that during a calcium spark, as calcium flows out of the SR, the concentration of Ca2+ within the SR becomes too low. However, this was not thought to be the case for spontaneous sparks as the total release during a Ca2+ spark is small compared to total SR Ca2+ content and researchers have produced sparks lasting longer than 200 milliseconds, therefore showing that there is still enough Ca2+ left within the SR after a 'normal' (200ms) spark. However local depletion in the junctional SR may be much larger than previously thought (see ). During the activation of a large number of ryanodine receptors however, as is the case during electrically evoked Ca2+ release , the entire SR is about 50% depleted of Ca2+ and this mechanism will play an important role in repriming of release. Stochastic attrition Despite the complicated name, this idea simply suggests that all ryanodine receptors in a cluster, and the associated dihydropyridine receptors happen to randomly close at the same time. This would not only prevent calcium release from the SR, but it would also stop the stimulus for calcium release (i.e. the flow of calcium through the DHPR). However, due to the large numbers of RyRs and DHPRs in a single cell, this theory seems to be unrealistic, as there is a very small probability that they would all close together at exactly the same time. Inactivation/adaptation This theory suggests that after activation of the RyR and the subsequent release of Ca2+, the channel closes briefly to recover. During this time, either the channel cannot be reopened, even if calcium is present (i.e. the RyR is inactivated) or the channel can be reopened,