HOW CONNECTIVE TISSUES TEMPORALLY PROCESS MECHANICAL STIMULI

Connective tissues have the capacity to adapt to a changing mechanical environment. Considerable experimental evidence suggests, however, that tissues respond to only a small portion of mechanical loads they experience and thus ignore the majority of the load they experience. Evidence suggests that tissues 'temporally process' these external mechanical signals in specific ways: 1) they respond in a trigger-like manner after a relatively few events or cycles of loading; 2) they respond only to some window of strain magnitude; 3) they exhibit a refractory period after a response; 4) they have a memory for previous stimuli. These characteristics do not necessarily mirror distinct cellular phenomena but rather reflect typical features of experimental and theoretical models. The manner in which tissues temporally process mechanical signals can be explained by energy transfer over short periods of time from the matrix to the second messenger systems. Much of the energy input into connective tissues during deformation is recovered, and relatively little is 'lost' and thus potentially transferable to the cells. Since evolutionary principles suggest that a pathway of response needs to be defined by biologically optimal use of energy, it is logical that cells would have evolved mechanisms to respond to a small finite level of integrated stimuli in order to rapidly respond (fitness) especially in times of environmental duress.