  Investigations over half a century have indicated that mechanical forces
induce neurite growth - with neurites elongating at a rate of
0.1-0.3{\mu}mh^{-1} per pico-Newton (pN) of applied force - when mechanical
tension exceeds a threshold, with this being identified as 400-1000 pN for
neurites of PC12 cells. Here we demonstrate that there is no threshold for
neurite elongation of PC12 cells in response to applied mechanical forces.
Instead, this proceeds at the same previously identified rate, on the
application of tensions with intensity below 1pN. This supports the idea of
mechanical tension as an endogenous signal used by neurons for promoting
neurite elongation.
