Ciliary function in transport of mucus.

Mucus is propelled by short cilia which rest during each beat cycle. Cilia move from rest by bending sideways and backwards through a recovery stroke in which they keep near the cell surface. This is followed by an effective stroke, in a plane nearly perpendicular to the cell surface, which ends with the cilium bent over in its rest position and with its tip pointing in the direction of propulsion. The cilium moves in a layer of periciliary fluid whose depth is a little less than the ciliary length, so that the overlying mucus is only penetrated by the ciliary tips in the effective stroke. The thickness of the periciliary layer is critical for effective propulsion of mucus. The cilia are coordinated by visco-mechanical interaction between the moving units to produce short oblique metachronal waves which pass across a few ciliated cells before dying away. Many small areas of independently coordinated activity collaborate to propel the overlying mucus. The activity of the cilia can respond to the load of mucus and control of ciliary rate may be exerted indirectly by varying the load rather than by any direct neural mechanism.