The segmental dynamics of amorphous poly(dimethylsiloxane) (PDMS) in a lamellar poly(styrene)-poly(dimethylsiloxane) diblock (PS-PDMS) is investigated by means of broadband dielectric spectroscopy. Because of the absence of normal mode in PDMS, we have been able to analyze the low-frequency side of the alpha-relaxation in detail. The characteristic time of the segmental dynamics is found to be significantly higher than that in a PDMS homopolymer. In addition, the dielectric loss peak displays a pronounced broadening in the low-frequency side as compared with that observed in pure PDMS. We showed that the epsilon"(omega) curves could be well described assuming two types of PDMS segments depicting qualitatively distinct segmental dynamics. In one case, the dielectric response is similar to that one of the amorphous constrained phase in a cold-crystallized PDMS homopolymer. For the other segments, the dielectric behavior appears close to that one of an amorphous PDMS homopolymer. However, some differences remain indicating a significant gradient of mobility within this second type of PDMS segments. We explain all these findings by considering a simple picture of the PDMS segments in the PS-PDMS lamellar mesophase.