  We report the results of comprehensive study of the critical dynamics of the
prototype perovskite antiferroelectric PbZrO$_3$. The combination of inelastic
X-ray and diffuse X-ray scattering techniques and Brillouin light scattering
was used. It is found that the dispersion of the TA phonons is strongly
anisotropic. The dispersion curve of the in-plane polarized TA phonons
propagating in [1 1 0] direction demonstrates pronounced softening. Slowing
down of the excitations at R-point is found, it is manifested in growing of the
central peak. This slowing down is too weak to be considered as a primary
origin of the corresponding order parameter. Obtained results are treated in
terms of TA--TO flexoelectric mode coupling. It is demonstrated that the
structural phase transformation in PbZrO$_3$ can be considered as the result of
the only intrinsic instability associated with the ferroelectric soft mode.
