Neurons never sit still for long. Receptors move in and out of the cell surface. Signals surge, fade, then surge again. Beneath that activity, a fine lattice made of actin and spectrin quietly lines the inner membrane. In a new study, researchers used super-resolution imaging to watch how that lattice, known as the membrane-associated periodic skeleton, or MPS, shapes what enters a neuron and what stays out. Their findings suggest the MPS does more than hold structure. It acts as a physical barrier that regulates endocytosis, the process cells use to pull material inside. The team visualized four major endocytic pathways in mature neurons: clathrin-mediated endocytosis, caveolin-mediated endocytosis, flotillin-mediated endocytosis, and fast endophilin-mediated endocytosis. Using structured illumination microscopy and 3D STORM imaging, they mapped where these pathways operate across axons, dendrites, and the axon initial segment. Ruobo Zhou, assistant professor of chemistry, of biochemistry and molecular biology, and of biomedical engineering, on left, co-led the study with Jinyu Fei, a graduate student in the chemistry department in Penn State’s Eberly College of Science. (CREDIT: Jaydyn …