Fig. 6. Relative direction of shear forces or microtubule sliding within the axoneme during bend formation. (A) The basis for curvature controlled models for flagella by Brokaw (Brokaw, 2001) with some modifications. The black arrows indicate the direction of shear forces or of sliding in the flagellum. For bends propagating from left to right, the direction of internal sliding must reverse at the points shown by the open arrows when the curvature exceeds critical curvatures. (B) Localized cyclical bending induced by iontophoretic applications of ATP to a demembranated or to a demembranated and elastase-treated sea urchin sperm flagellum (Shingyoji and Takahashi, 1995). (C) Interpretation of the backward sliding observed in the present study. (1) In the elastase-treated quiescent flagella severed at the base (Fig. 2A,B, left), P-sliding is induced by ATP application (left). Bending in the P-bend direction (middle) induces backward sliding (right) mainly in the region proximal to the bending. (2,3), In the elastase-treated quiescent axoneme (Fig. 3C,E), P-sliding occurs in the distal part of the basal P-bend (left). Bending the distal region of the flagellum in the R-bend direction (middle) induces backward sliding in the whole region of the bending (2, right) or only in the proximal region of the bending (3).