USMLE/COMLEX - 1 - Skeletal Muscle Organization

The information in this tutorial is core for preparation for USMLE/COMLEX 1. Additional, requisite information is included here. Muscle cells are multinucleated (meaning, each cell contains many nuclei), which reflects the actual process of muscle cell formation. Mature muscle cells form from fused myoblasts (embryonic cells) and each myoblast contributes its nucleus to the adult muscle cell. Myostatin and testosterone help regulate muscle growth and recovery. Myostatin slows muscle growth.

• Specifically, myostatin inhibits satellite cells and promotes protein degradation, which regulates the formation of mature skeletal muscle cells.

Testosterone promotes muscle growth.

• Testosterone encourages the synthesis of proteins, thus athletes abuse anabolic steroids that mimic testosterone to promote muscle growth.

Satellite cells lie within the external lamina. They are skeletal muscle stem cells: inactive myoblasts, lying in-wait. Think of them as army reserves.

• Upon muscle injury, they enter mitosis, fuse with other satellite cells to form differentiated muscle fibers. Centrally located nuclei are a hallmark of regenerating muscle cells; whereas mature muscle cells contain peripherally located nuclei.

The A Band and I Band polarize light differently which gives muscle a striated (light/dark) appearance. The *A Band of myosin is the dark band seen in striated muscle. Think "A" for d-a-rk. The "A" can also help you remember that the thick filament is aniosotropic (or birefringent) in polarized light. The I Band is the region along the thin filaments (between the thick filaments). Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).