The backbone conformation of a comblike polymethacrylate, PMA-CH3, was studied by small-angle neutron scattering in the nematic phase. The backbone is perpendicular to the mesogenic side chain moieties, corresponding to the oblate ellipsoid of the NI phase of Wang and Warner's theory. We have measured the temperature dependence of the conformation of the polymer backbone on five well-defined molecular weights. In the direction perpendicular to the hanging groups, the backbone adopts a weakly confined random walk conformation, compared to that of the isotropic phase. In the direction of the nematic field, the chain conformation is extended: the exponents of the scaling laws vary between 0.60 and 0.50. At low temperature, smectic fluctuations freeze the backbone anisotropy. When increasing the temperature, the nematic order exerts a weaker force on the polymer backbone, which tends to an isotropic Gaussian conformation. The scaling laws for the backbone conformation and their variations with temperature are compared to the theoretical predictions.