Actomyosin-II–dependent axon plasticity shields CNS axons from mild mechanical stress. (A) Actomyosin-II–driven radial contraction and dilation of the axon cortex (blue/red arrows) leads to rapid and reversible axonal beading (red brackets) in axon shafts exposed to mild mechanical stress. The acute shape change confines the spreading of elevated Ca²⁺ (yellow regions within beads) in the axon, thereby protecting the stressed axon from widespread Ca²⁺ (yellow bidirectional arrow) and subsequent severe injury caused by mild mechanical stress. (B) The schematic of a single bead is magnified to show how the acutely beading axon restricts both organelle trafficking and Ca²⁺ spreading. Cellular components are indicated in the graph.