In smooth muscle, coordination of contraction throughout the muscle is facilitated by the
a. thick filaments that run through the plasma membrane of one cell into the next.
b. electrical connections between muscle fibers.
c. parallel arrangement of striations in neighboring fibers.
d. reflex arcs stimulating groups of fibers simultaneously.
e. elements of the sarcoplasmic reticulum connecting one fiber with another.
B; Gap junctions connect smooth muscle fibers, communicating a depolarization in one cell directly to the other. Neither the thick filaments nor the sarcoplasmic reticulum run across the plasma membrane (choices a and e). Striations and control by reflex arcs are properties of skeletal muscle, not smooth (choices c and d).
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