During voiced speech, 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 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 (fo) interval [145 Hz, 315 Hz]. The interval contains the first vocal tract resonance fR1 and the first formant F1 of [i] as well as the fractions of the first resonance fR1/5, fR1/4, and fR1/3 of [A]. The glide simulations reveal a locking pattern in the fo trajectory approximately at fR1 of [i]. The resonance fractions of [α] produce perturbations in the pressure signal at the lips but no locking.