Figure 5. The cistern as a calcium capacitor.

A, Under normal operating conditions (low levels of efferent or hair cell activity) calcium influx through hair cell AChRs (grey) is rapidly absorbed and bound by the synaptic cistern – perhaps through calcium-gated calcium channels such as the ryanodine receptor (green channels). B, overloading of the cistern (e.g. from prolonged efferent activity, or by voltage-gated calcium influx at nearby ribbons, pumped into the cistern by sarco(endo)plasmic reticulum calcium-ATPase (SERCA)) slows the uptake of efferent calcium and leads to calcium-induced calcium release that amplifies and extends activation of SK channels (black channels). The spread of calcium is also thought to stimulate synthesis of nitric oxide (NO) to drive retrograde facilitation.