Relaxed and contracted muscle fibers
Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
Human; Helly's fluid;
A., C., phosphotungstic acid hematoxylin,
D. H. & E.; 4416 x.
In this plate, the structural basis of skeletal muscle fiber contraction is shown.
A: Relaxed fiber showing distinct cross striations, the darker staining A band and the lighter staining I band. Note that the I band is bisected by a thin but deeply staining line (Z line), while the A band is bisected by a lightly staining line (H zone).
B: A fiber seen in the relaxed state except for a small segment of localized contraction. Note the change in the band pattern in this segment. Two adjacent A bands are in contact, and the I band has disappeared.
C: A fiber shown with both a relaxed and contracted segment. The A and I bands are clearly outlined in the relaxed segment but not in the contracted segment. In contraction, the I band becomes narrower and disappears. The A band does not normally become shorter except in extreme contraction. Contraction bands appear as a result of an increase in density and staining of the Z line.
D: A portion of a fully contracted muscle fiber is shown. The changes here are similar to those described in C for a contracted segment except that the normal distance between the thickened Z lines (contraction bands) is reduced, denoting extreme contraction.
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