Jeffrey Hord, B.S.
Skeletal muscles come in a variety of sizes and shapes so that they suit the particular function that they perform. Some of these muscles work virtually all of the time while others are necessary only part of the time. Locomotor muscles that help us physically move our bodies are not as essential as our hearts or other vital organs but they are crucial for us to go about our everyday activities. Our skeletal muscles have matured and essentially been trained to function in the presence of gravitational forces, and in the absence of gravity, the job of these muscles becomes much easier. Some of our muscles function by creating movement that opposes the gravitational force that we experience here on Earth – these are the anti-gravity or postural muscles. The postural muscles extend from the lower back down to the feet. Since the postural/antigravity muscles are trained to perform in the presence of gravity, when gravity is reduced we lose some of the muscle mass that comprises these muscles. Loss of muscle mass not only occurs by decreasing gravitational forces acting on the body, such as occurs during spaceflight, but also by decreasing our physical work load, such as when we are bed-ridden or have a limb in a cast. Basically, skeletal muscles are very dynamic and capable of adapting to the given situation.
Our lab and others have observed a loss of muscle mass to occur during inactivity, and the fibers of postural muscles are altered at the cellular level. We have noticed that a protein known as neuronal nitric oxide synthase, or nNOS for short, moves away from the membrane of the muscle fibers towards the center of the muscle fibers. The function that nNOS performs at the muscle fiber membrane is crucial for the maintenance of force output, power output, and muscle endurance. When nNOS is absent from the muscle fiber membrane, muscle force, power, and endurance all decline. So, what cellular events are causing nNOS to move? Our lab at Texas A&M University will hope to answer this question in the near future.
1. Adams, GR, Caiozzo VJ, Baldwin KM. Skeletal muscle unweighting: Spaceflight and ground-based models. J Appl Physiol. 09;2185-2201,2003.
2. Suzuki N, Motohashi N, Uezumi A, Fukada S, Yoshimura T, Itoyama Y, Aoki M, Miyagoe-Suzuki Y, and Takeda S. NO production results in suspension-induced muscle atrophy through dislocation of neuronal NOS. J Clin Invest. 117:2468-2476,2007.