[en] Neutral particles of hydrogen isotopes, released locally and impulsively into the plasma of fusion devices, can significantly affect local plasma properties. A model, allowing to describe self-consistently the spreading of neutrals from the source and their effect on the local and global plasma conditions is developed. It is based on the separation on each flux surface of two zones, the ‘cold’ cloud, comprising neutral molecules injected and atoms generated in collisions of molecules with electrons and ions, and the ‘hot’ environment affected by flows along the magnetic field of newly produced charged particles outward and heat conduction toward the cloud. Computations are done for the conditions of laser induced desorption spectroscopy applied in Ohmically heated plasmas in the TEXTOR tokamak and foreseen for the ITER fusion reactor. In both cases the local plasma state is strongly changed by the desorption pulse, and this effect is increasing with the growing isotope mass. As a result the total number of photons emitted is reduced noticeably, up to 4 times in the case of tritium injection in ITER, and the necessity to take into account the plasma response by interpreting measurements is demonstrated. (paper)