Skip Navigation

Maintaining Muscle Mass in Space

Author(s): Kenneth M. Baldwin, PhD

Motor Units and the Musculoskeletal System II

Myofibers express a motor protein called myosin heavy chain (MHC). Focus on the bottom box in the center of the slide. MHC is the most abundant protein expressed in muscle. It is both a structural and regulatory protein that forms the backbone of the fiber (see next slide), and also drives the contraction process of the fiber. Slow motor units contain fibers that express what is termed a slow, type I MHC, which intrinsically causes the fiber, when activated electrically by the nerve, to contract slowly.

In contrast, the other two motor units express faster types of MHC, called isoforms (i.e., different forms of a nearly identical protein), and are designated as either IIa, IIx, or Iib, depending on the individual properties of the motor units. It is important to note that in the faster units, many of the fibers express two or more types of MHC. We don’t know why this is the case, but it is postulated that this characteristic enables the fibers to change their contractile properties quickly (over a period of a few days), depending on the physiological conditions imposed on the muscle.

Can you think of a type of activity that may serve as a stimulus to induce expression of the slower myosin isoforms? What might induce faster isoforms?

Suggested Reading:
Caiozzo, V. J., Baker, M. J., Huang, K., Chou, H., Wu, Y. Z., & Baldwin, K.M. (2003). Single fiber myosin heavy chain polymorphism: How many patterns and what proportions? Am J Physiol Regul Integr Comp Physiol. 285: R570-R580.

Look at the middle of the slide, and focus first of all on the fibers that are designated in yellow. These fibers express a certain motor protein that we call myosin heavy chain (MHC). Think of myosin heavy chain as the motor that drives the contraction process. The unique feature of these fibers is that they express only one kind of a motor protein, which we refer to as slow or type I myosin. The green and red fibers in the middle are the ones that the individual was using in the last slide, while cycling the bicycle. This motor unit, or this grouping of fibers, consists of two faster types of myosin (isoforms called IIa and IIx), one that is expressed as green, and another expressed as red. If we move to the right, we see another type of fiber that is really fast. These fibers express the fast myosin heavy chain and allow the muscle to shorten with a much more explosive capability, as illustrated on the right with the individual executing the burst power. Myosin heavy chains have evolved and been conserved over millions of years, and through many species. While I show these fibers to you in the human body, just about every animal system has the same types of myosin.

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.