We investigate the effect of additional cosmic-ray electron (CRE) sources on the Galactic
synchrotron emission distribution along the line of sight (LOS) through the Galaxy. In this
way, we try to explain the differences between low-frequency synchrotron measurements and
predictions from current Galactic magnetic field and CRE models. We use stochastic differential
equations to describe electron CR propagation in the Galaxy. These specific CRE density models
are used to simulate synchrotron emissivity at low radio frequencies. We consider individual
young supernova remnants as additional sources of CREs, as well as an enhanced CRE density
in an extended ring in the outer Galaxy. We compare the observed low-frequency synchrotron
emission in the direction of optically thick HII regions with synchrotron emission as predicted
by these models. We found that neither a single supernova remnant emitting CRE in addition to
the Galactic CRE background, nor additional CREs emitted in a spiral arm, can explain current
discrepancies between low-frequency observations of synchrotron emission in the direction of
optically thick HII regions. This indicates the need for other solutions to bring synchrotron data
in agreement with CRE and GMF models