Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
The upper figure, showing two sarcomeres, accounts for the usual light microscopic appearance (i.e., the staining densities) of sarcomeric cross striations in relaxed (left), contracting (middle), and fully contracted (right) skeletal and cardiac muscle fibers.
In the lower figure, also showing two sarcomeres, the comparable electron microscopic, ultrastructural configuration is shown. The appearance of cross striations or bands by both light and electron microscopy have their basis in the relative position and resulting density of the two major sets of myofilaments that constitute the sarcomere, that is, thin (actin) filaments, emanating from the Z line, and thick (myosin) filaments, held in hexagonal register at the M line, and their relative interdigitation in relaxation and in shortening (contraction).
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