  Neurites, the long cellular protrusions that form the routes of the neuronal
network are capable to actively extend during early morphogenesis or to
regenerate after trauma. To perform this task, they rely on their cytoskeleton
for mechanical support. In this paper, we present a three-component active gel
model that describes neurites in the three robust mechanical states observed
experimentally: collapsed, static, and motile. These states arise from an
interplay between the physical forces driven by growth of the microtubule-rich
inner core of the neurite and the acto-myosin contractility of its surrounding
cortical membrane. In particular, static states appear as a mechanical
traction/compression balance of these two parallel structures. The model
predicts how the response of a neurite to a towing force depends on the force
magnitude and recovers the response of neurites to several drug treatments that
modulate the cytoskeleton active and passive properties.
