5.20: Mechanism of Ciliary Motion

The ciliary structures were first seen in 1647 by Antonie Leeuwenhoek while observing the protozoans. In lower organisms, these appendages are responsible for cell movement, while in higher organisms, these appendages help in the movement of the extracellular fluids within the body cavities.

The cilia are made up of microtubules in a 9+2 arrangement, with nine microtubule doublet ring bundles, surrounding a pair of central singlet microtubule bundles. The doublet microtubule bundles are connected by nexin protein and axonemal dyneins. Radial spokes connect these outer doublet microtubules to the inner central pair. The coordinated movement of axonemal dyneins is responsible for the characteristic whip-like movement of the cilia. This characteristic ciliary movement is explained by the switch inhibition or switch-point mechanism proposed by Wais-Steider and Satir in 1979. The model suggests that during the ciliary motion, only half of the dyneins are active at a given time, while the other remains inactive. The axonemal dyneins on either side alternately switch between active and inactive forms to propel the ciliary motion. The sliding microtubules within the cilia require energy from ATP hydrolysis within the heavy chain domain of the axonemal dyneins.

Cilia in humans move rhythmically; they constantly remove waste materials such as dust, mucus, and bacteria through the airways, away from the lungs, and toward the mouth. Beating cilia on cells in the female fallopian tubes move egg cells from the ovary towards the uterus. A flagellum is an appendage larger than a cilium and specialized for cell locomotion. In humans, sperms are the only flagellated cell that must propel themselves towards female egg cells during fertilization.

Ciliary motion can be explained through the microtubule sliding theory involving axonemal dyneins.

When axonemal dyneins are activated, they attempt to move along neighboring microtubule doublets, forcing them to slide relative to one another.

As the doublets are cross-linked to one another by nexins and to the central microtubules by spokes, this crosslinking limits the extent of microtubule sliding by axonemal dyneins.

Microtubule sliding is active only on a single side due to the switch inhibition mechanism, which ensures that dynein is active on one side and inactive on the other. This results in the bending of cilia in one direction.

Switch inhibition also regulates the cycling between active and inactive phases of axonemal dynein and is responsible for alternate bending of the cilium. This results in a characteristic whip-like movement that is responsible for cell motility.