Axonemal dynein, 9+2 microtubule structure, beat cycle and fluid flow
Motile cilia contain a 9+2 axoneme: nine outer doublet microtubule pairs surrounding a central pair. Axonemal dynein motor complexes slide adjacent doublets relative to each other; since doublets are anchored at the basal body, sliding converts to bending. Nexin links and radial spokes coordinate dynein activity across the axoneme to produce an asymmetric beat cycle: a fast power stroke that propels fluid, followed by a slower recovery stroke close to the cell surface.