An elaborate tubulolamellar membrane system that underlies the postsynaptic cell membrane. [ http://www.ncbi.nlm.nih.gov/pubmed/19244343 http://www.ncbi.nlm.nih.gov/pubmed/18171947 http://www.ncbi.nlm.nih.gov/pubmed/7946331 http://www.ncbi.nlm.nih.gov/pubmed/1460464 ]