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Maintaining Muscle Mass in Space

Author(s): Kenneth M. Baldwin, PhD

Myosin, the “Motor” Molecule

This slide illustrates the contractile process. When the muscle is electrically activated by the nervous system, calcium molecules are released into the vicinity of the myofilaments. The calcium then binds to special sites on the actin filament so that the myosin head can attach to the actin. When these two proteins interact, energy molecules (called adenosine triphosphate) are broken down, as depicted by the “sparking action” in the animation. This energy transformation process enables the myosin to slide along the actin filament, thus producing force and movement in the muscle fiber, as depicted on the right side of the illustration. It is important to appreciate that every gravity-opposing action we carry out each day is mediated through the recruitment of the slow type motor units.

Can you predict what would happen if we lost expression of these slow units, in terms of how effectively we would be able to oppose the gravity field on Earth?

Suggested Reading:
Brooks, G. A., Fahey, T. D., White, T. D., & Baldwin, K. M. (2005). Exercise Physiology 4th Edition: Human Bioenergetics and Its Applications. McGraw Hill, New York, NY. pp. 363-395.

During a muscle contraction, calcium is released into the muscle fiber, which allows the myosin to interact with the actin. An explosion occurs and the myosin drives the actin along the chain. This creates the movement process. On the right side of the slide, we can see that dynamic of motion whereby the myosin, labeled in pink, is actually sliding those actin molecules along, and you can see the shortening action of that schematic representation. With a slow fiber, that whole dynamic action takes place much more slowly. It is not as explosive, but this type of movement is more economical, and the fibers that we recruit during postural adjustment allow the individual to sustain this anti-gravity capability in a more economical way. With a fast fiber, there is a much more dynamic action, which favors types of activities involving a lot of movement, in which the muscle fibers have to contract with more speed and explosiveness. The type of myosin being expressed in these fibers really drives the essence of the diversity in the contraction process that can occur in our muscles. What kind of fibers would you predict exist in the leg muscles of the lion, which must chase down its prey? In contrast, what type of fibers would you expect to find in a sloth, which hangs onto trees for very long durations?

Funded by the following grant(s)

National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.