Papillary muscle

In anatomy, the papillary muscles of the heart serve to limit the movements of the mitral and tricuspid valves and prevent them from being inverted. They do not close or open the valves, which close passively in response to pressure gradients. Instead they brace the valves against the high pressure.
The U wave in an ECG represents papillary muscle repolarization. It usually does not appear unless a patient's electrolytes are imbalanced.