Smooth muscle fibers do not have striations like skeletal and cardiac muscle cells do. This is because:
a) the sarcomeres are smaller than in cardiac muscle cells and skeletal fibers, and aren’t visible even when smooth muscle fibers are examined microscopically
b) smooth muscle fibers contain only intermediate filaments, which aren’t arranged in sarcomeres
c) smooth muscle fibers contain thin and thick filaments, as well as intermediate filaments, but none of them are arranged in sarcomeres
d) visceral smooth muscle fibers don’t have striations, but multiunit smooth muscle cells do, but these cells are more rare, and so we rarely have a chance to view them with a microscope e) both b and d are correct
The correct answer and explanation is:
The correct answer is c) smooth muscle fibers contain thin and thick filaments, as well as intermediate filaments, but none of them are arranged in sarcomeres.
Smooth muscle fibers lack striations because their internal structure differs from that of skeletal and cardiac muscle. In both skeletal and cardiac muscles, the myofilaments (thin and thick filaments) are organized into highly structured units called sarcomeres, which are responsible for the striated appearance when viewed under a microscope. These sarcomeres are the repeating contractile units that provide the visible striations.
However, in smooth muscle, the arrangement of myofilaments is not as regular. While smooth muscle does contain actin (thin filaments) and myosin (thick filaments), these filaments are not organized into sarcomeres. Instead, the filaments are arranged in a more dispersed manner across the cell, which allows smooth muscle cells to contract in a more gradual and sustained manner. The actin and myosin filaments in smooth muscle are attached to dense bodies, which serve a similar role to Z-discs in striated muscle, but the overall structure remains unstriated.
In addition to actin and myosin, smooth muscle cells also contain intermediate filaments, which provide structural support but do not play a direct role in contraction. These intermediate filaments help maintain the shape of the cell and are not involved in the regular, repeating structure seen in striated muscles.
This lack of sarcomere arrangement is why smooth muscle fibers do not exhibit striations when examined under a microscope. The more irregular, non-striated organization allows smooth muscle to perform its function of slow, sustained contractions, which is essential for processes like peristalsis in the digestive tract and vasoconstriction in blood vessels.