Production of spermatozoa is essential to success in reproduction. With respect to the spermatozoa tail structure, what is the unit that is essential for motility?

Spermatozoa contain a tail for motility. The structure of the tail or composition of the tail is the axoneme which is composed of several internal structures. The structures are termed dynein outer arm, dynein inner arm, A tubule, B tubule, radial spoke, radial head, central tubule (middle of wheel) and a nexin link between the A and B tubules to give circular appearance (like a spoke wheel). A and B tubules dynein arms are attached to the tubules and project toward the nexin link. The radial spokes jet inward toward the central tubule with the radial head attached to the end of the central tubule. The structure looks like a spoke wheel. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 23. Function of the Male Reproductive System. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.