  We present a physical mechanism that can cause the mitotic spindle to
oscillate. The driving force for this mechanism emerges from the polymerization
of astral microtubules interacting with the cell cortex. We show that Brownian
ratchet model for growing microtubules reaching the cell cortex, mediate an
effective mass to the spindle body and therefore force it to oscillate. We
compare the predictions of this mechanism with the previous mechanisms which
were based on the effects of motor proteins. Finally we combine the effects of
microtubules polymerization and motor proteins, and present the detailed phase
diagram for possible oscillating states.
