The regulation of airway ciliary beat frequency by calcium.
Introduction
During normal breathing, the
airways transport large quantities of air in to the lungs. Unfortunately
this air is frequently contaminated with a variety of pollutants,
particles and bacteria that become deposited in the
airways. To fight this constant accumulation of material, the airways have
developed the defense mechanism of mucociliary clearance 
.
The beat pattern of airway cilia. Positions 1 - 9: Recovery stroke, moving right to left. Positions 9 - 12: Effective stroke moving, left to right. Data obtained from high-speed movies.
The
Mucociliary Interface
The trachea to the terminal
bronchioles are lined with a ciliated epithelium. The cilia of each
cell project towards the airway lumen and are bathed in a
"watery" ionic solution called the periciliary layer. Each
cilium performs a repetitive beat cycle consisting of a rest, recovery and
effective stroke. During the effective stroke the cilium makes contact
with the overlying mucus and propels it forward to transport it, together
with entrapped particles, from the airways.
A scanning electron micrograph showing the profile of the
mucociliary interface.
Most studies of cilia have been performed in culture with tissues from large airways. Under these conditions, the cells lose their in vivo orientation and organization. However, by examining freshly prepared lung slices, the cilia of the small airways can easily be seen.
Low mag profile movie of cilia in small airways
(lumen at the top)
In our recent research we found the cilia of the small airways of mice to beat at a high frequency of about 20 -25 Hz. Importantly, this beat frequency appeared maximal and could not be increased.
High Speed movie - re-played at x8 slower (30 fps)
Ciliary Metachrony
To avoid interference and to
enhance cooperative activity, the densely-packed cilia beat with a
metachronal organization. This activity consists of many areas
in which the cilia perform their beat cycle in a coordinated manner. In
the video (right), the effective stoke moves towards the top of the image
while the recovery stroke moves towards the lower right. The
recovery stroke generates the appearance of a wave moving across the field
of cilia. By watching a small area, the progress of several metachronal
waves can be followed .
Video: Ciliary activity of rabbit tracheal cilia viewed
from above with DIC optics
A SEM of rabbit tracheal airway cilia
Advantages of
metachrony for mucus transport
Because mucus is a
viscous, elastic secretion, it cannot be moved by individual cilia.
Consequently, ciliary metachrony is used to recruit the cooperative
efforts of multiple cilia. By relying on the recovery stoke of the cilia
to initiate the activity of adjacent resting cilia, greater numbers
of cilia can be recruited to enable mucus transport even though many
of the cilia will be slowed during contact with the mucus load .