This is Video S3,which corresponds to still images in bottom row of Fig. 2D. It shows that knocking down levels of Myosin II B (MIIB) increases spine dynamics. Spines from MIIB knockdown neurons extend and retract more frequently than spines in control neurons. Note also the increased length of spine extensions in MIIB knockdown neurons. Timelapse confocal imaging was performed on DIV14 hippocampal neurons co-expressing GFP and an shRNA vector against MIIB. Images were acquired every 1-minute using confocal microscopy. 3 frames/sec shown.