During voiced speech, the human vocal folds interact with the vocal tract acoustics. The resulting glottal source-resonator coupling has been observed using mathematical and physical models as well as in in vivo phonation. We propose a computational time-domain model of the full speech apparatus that, in particular, contains a feedback mechanism from the vocal tract acoustics to the vocal fold oscillations. It is based on numerical solution of ordinary and partial differential equations defined on vocal tract geometries that have been obtained by Magnetic Resonance Imaging. The model is used to simulate rising and falling pitch glides of [a, i] in the fundamental frequency (f_o) interval [150 Hz, 320 Hz]. The interval contains the first vocal tract resonance f_R1 and the first formant F1 of [i] as well as the fractions of the first resonance f_R1/4 and fR1/3 of [a]. The simulations reveal a locking pattern of the fo-trajectory at f_R1 of [i] in falling and rising glides. The resonance fractions of [a] produce perturbations in the pressure signal at the lips but no locking. All these observations from the model behaviour are consistent and robust within a wide range of feasible model parameter values and under exclusion of secondary model components.