A small population of immune cells—namely macrophages—reside in skeletal muscle. Under inflammatory conditions that cause musculoskeletal pain, circulating macrophages infiltrate the muscle. This image of a cross section of mouse muscle, showing macrophages (green), purinergic receptors (red), and nuclei (blue), comes from Sandra Kolker, research associate in the lab of Kathleen Sluka, PhD, UI professor of physical therapy and rehabilitation sciences. The Sluka team uses animal models to study how macrophages influence pain and how their regulation may be affected by short-term bouts of exercise. The lab also investigates how regular physical activity may decrease the response to painful and inflammatory stimuli by altering the genes macrophages express.