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When comparing the contractile responses in smooth and skeletal
muscle, which of the following is most different?
a. The source of activator calcium
b. The role of calcium in initiating contraction
c. The mechanism of force generation
d. The source of energy used during contraction
e. The nature of the contractile proteins
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for this i thought A too.... but ans is BBBB
The greatest difference in
excitation-contraction coupling involves the role of calcium in initiating contraction.
In smooth muscle, calcium binds to and activates calmodulin,
which, by activating myosin light chain kinase (MLCK), catalyzes the phosphorylation
of the 20,000-Da myosin light chain (LC20). Once the light chains
are phosphorylated, myosin cross-bridges bind to actin on the thin filaments,
which initiates contraction. In skeletal muscle, calcium binds to troponin,
which removes the tropomyosin-mediated inhibition of the actin-myosin
interactions. Once the inhibition is removed, cross-bridge cycling (and contraction)
begins. In both smooth and skeletal muscle, force is generated by the
cycling of cross-bridges. ATP provides the energy for the cycling of the crossbridges
in both muscles. In skeletal muscle, activator calcium comes exclusively
from the sarcoplasmic reticulum (SR), while in smooth muscle, calcium
can come from both the SR and the extracellular fluid.
