Feedforwards and global system failure: A general mechanism for senescence

Although senescence is almost a ubiquitous property of organisms, it remains one of the most poorly understood of biological phenomena. The many mechanisms which have been proposed to account for senescence, however much they differ in detail, are alike in regarding senescence as a kind of system failure arising from local failures in specific subsystems. If senescence is indeed identified with system failure, then we must consider the possibility that there exist modes of failure which do not arise from local subsystem failures. We show that any system manifesting a feed-forward mode of control will in general exhibit such a non-local failure; or more generally, that any property of such a system will be temporally spanned. Since physiological systems are replete with feedforward loops, it is suggested that the temporal spanning arising in this way is responsible for senescence. A number of suggestive results are drawn from this argument, bearing on the possibility of system “rejuvenation”, and on the inability of experimental techniques entirely concerned with local sub-system behavior to bear directly on senescence mechanisms.