Figure 2. Simplified schematic representation of the mechanism by which axonemal dynein arms generate the flagellar beat. The parallel lines in the region of the flagellum represent adjacent outer microtubule doublets of the axoneme (OMDA). To simplify the figure, only 2 outer microtubule doublets are represented (rather than the actual 9). The central pair of microtubule doublets is not shown, nor are the any of the accessory structures. Dynein arms between the pair of OMDAs are numbered. Each symbol represents a pair of inner and outer dynein arms originating from the microtubule on the right and extending toward the microtubule on the left. (A) None of the dynein arms are active; as such, the adjacent microtubule doublets are straight and there is no flagellar bend. (B) The first dynein arm has engaged the adjacent microtubule doublet. (C) The first dynein arm has generated a downward stroke, resulting in the adjacent microtubules sliding past one another (large arrows). Because both microtubule doublets are anchored to the sperm head, this sliding force is translated into a bend in the axoneme—the start of a flagellar beat. Also, the second dynein arm has engaged the adjacent microtubule doublet. (D) The first dynein arm has released the adjacent microtubule doublet, whereas the second dynein arm has generated a downward stroke. This results in the propagation of the flagellar bend down the length of the microtubules. The third dynein arm has engaged the adjacent microtubule. (E) The flagellar beat is being propagated by the downward stroke of the third dynein arm, whereas the second dynein arm has released the adjacent microtubule. The first dynein arm has returned to its original position in preparation for another stroke. This sequence will be repeated by the fourth dynein arm, and so on along the length of the microtubules. One must imagine a similar sequence occurring for all 9 dynein arm pairs and their associated 9 outer microtubule doublets. Additionally, one must imagine this sequence occurring in an asynchronous but coordinated fashion around the circumference and along the entire length of the flagellum (ie, in 3 dimensions). It is this coordinated use of dynein arms and the associated bending of the outer microtubule doublets that result in a normal flagellar beat. The ODFs and the FS add structural support. In Chlamydomonas, it has been shown that the central pair of microtubules, together with the radial spokes emanating from the 9 outer microtubule doublets, coordinate and regulate the actions of the dynein arms and so control the size and shape of the axonemal bend.