Smooth endoplasmic reticulum (SER) is important for regulating calcium which has been shown to be present at high levels in activated dendritic spines. The dimensions and organization of the SER in rat hippocampal spines and dendrites were measured through serial electron microscopy and three-dimensional analysis. This figure illustrates the three-dimensional reconstruction of the SER (purple) in a rat hippocampal CA1 dendritic segment. In both figures, the membrane of the dendrite is not visible, although in the left figure the membrane of the attached spine is present. The left side shows that the SER in the dendrite is contiguous with the SER entering the thin neck of a large dendritic spine (grey). The SER in the head of the spine (seen in the right figure where the spine membrane is invisible) is thought to provide synapse-specific regulation of calcium and other molecules. This particular dendritic spine has a highly irregular synaptic area (red).
Reference: Spacek, J. and Harris, K.M. (1997) J. Neuroscience, 17(1):190-203