Is the Klein's bottle a good analogy to the relation between T-tubule and sarcolemma?
If we're looking at diagrams for the t-tubule with respect to the sarcolemma, we see the two are actually fused:





The function therein is to allow depolarization to penetrate the interior of the cell quickly. The surface isn't transected anywhere, it simply folds inward. You can visualize this by sort of pushing a hole through some putty with a finger, and making a hole to the other side. Really no need to upvote me here, just easier to paste images in answers.