Context : The gas distribution and dynamics in the inner Galaxy present many unknowns as the origin of the asymmetry of the lv -diagram of the Central Molecular Zone ( CMZ ) .
On the other hand , there are recent evidences in the stellar component of the presence of a nuclear bar that could be slightly lopsided .

Aims : Our goal is to characterize the nuclear bar observed in 2MASS maps and to study the gas dynamics in the inner Milky Way taking into account this secondary bar .

Methods : We have derived a realistic mass distribution by fitting the 2MASS star counts map of Alard ( 2001 ) with a model including three components ( disk , bulge and nuclear bar ) and we have simulated the gas dynamics , in the deduced gravitational potential , using a sticky-particles code .

Results : Our simulations of the gas dynamics reproduce successfully the main characteristics of the Milky Way for a bulge orientation of 20 ^ { \circ } -35 ^ { \circ } with respect to the Sun-Galactic Center ( GC ) line and a pattern speed of 30-40 km s ^ { -1 } kpc ^ { -1 } .
In our models the Galactic Molecular Ring ( GMR ) is not an actual ring but the inner parts of the spiral arms , while the 3-kpc arm and its far side counterpart are lateral arms that contour the bar .
Our simulations reproduce , for the first time , the parallelogram shape of the lv -diagram of the CMZ as the gas response to the nuclear bar .
This bar should be oriented by an angle of \sim 60 ^ { \circ } -75 ^ { \circ } with respect to the Sun-GC line and its mass amounts to ( 2 - 5.5 ) 10 ^ { 9 } M _ { \odot } .
We show that the observed asymmetry of the CMZ can not be due to lopsidedness of the nuclear bar as suggested by the 2MASS maps .

Conclusions : We do not find clear evidences of lopsidedness in the stellar potential .
We propose that the observed asymmetry of the central gas layer can be due to the infalling of gas into the CMZ in the l=1.3 ^ { \circ } -complex .