Smooth Myocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 400

that are too fast or delayed remain ineffective. In severe asthmatic crises, slow and
complete inspirations do not relieve bronchospasm. The failure of deep inspiration-
generated bronchodilation may cause airway hyperresponsiveness.

8.4.6.1 Contractile Cytoskeleton Adaptivity

Mural smooth myocytes have a large functioning length range, in opposition to
striated myocytes that works at most at 20% of their resting length. The malleability
of the myofilament lattice of smooth myocytes results from remodeling of contrac-
tile filaments of the cytoskeleton in response to stress and strain exerted on these
cells [ 742 ].
Smooth muscle length is determined not only by the balance between active force
that it generates and passive reaction developed by the external load, but also, due
to the breathing time scale, by an outside-equilibrium behavior [ 743 ]. In normal
conditions, tidal fluctuations of muscle length suffice to avoid bronchoconstriction.
The malleable cytoskeleton behavior is similar to that of soft, colloidal materials
[ 743 ]. 27 The elastic modulus ranges from the order of 1 Pa to 1 kPa. The stiffness
over a wide range of frequencies does not exhibit any characteristic molecular
relaxation time or resonant frequency. Frictional stress within the network is
typically smaller than elastic stress. Both stress types are scale-free, as material
rheological responses are not associated with any particular relaxation time and rise
with frequency. On application of shear of sufficient magnitude, these materials
become fluid-like bodies. When shear is stopped, they return to their original
solid-like state. Colloidal dispersions can actually exhibit both fluid-like and solid-
like behavior, with various types of transition between these two states. Colloidal
particles with an attractive interparticle energy display a fluid-to-solid transforma-
tion due to gelation, at a critical volume fraction that depends on the magnitude
of attractive interparticle energy on contact. Gelation occurs when interconnected
colloid-rich regions solidify. Other features of soft, glassy materials include the
rheological pattern that results from the competition between the spontaneous
restructuration (slow scale-free recovery of rheological properties, the so-called
aging) and destruction of the internal structure by shear (“shear rejuvenation”).
Intermittent motions result from nanoscale rearrangements. Sub- and superdiffusive
displacements rely on interactions between material constituents. Material transfer
creates to low- and high-density regions of given types of constituents. Slowing of
dynamics and more prominent intermittency of structural rearrangements indicate
approach of a transition and kinetic arrest of colloidal particles, when crowded
particles are trapped by their nearest neighbors (gelation).

27 A colloidal solid, liquid, or gaseous body consists of a dispersed phase in a continuous dispersion
medium. Dispersed-phase particles have a typical size that ranges from 5 to 200 nm. Blood
illustrates a solid dispersed phase in a continuous liquid medium. Inhaled aerosols are mostly
liquid dispersed phase in a continuous gaseous medium.