The image shows a diagram of the transverse tubule and sarcoplasmic reticulum network of skeletal or cardiac muscle cells. By OpenStax – https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30015052