Question 1

Which of the following best describes a major source of calcium that initiates contraction of smooth muscle?

Accepted Answer

Calcium ions move into the smooth muscle cell from the extracellular fluid, which initiates muscle contraction.

Question 2

The contraction of smooth muscle can be attenuated even when intracellular calcium levels are high. This unique ability of smooth muscle can be attributed to variations in the activity of which of the following?

Accepted Answer

Myosin light chain phosphatase (MLCP) is able to dephosphorylate myosin, which leads to relaxation of the smooth muscle even when intracellular calcium levels are high. This is in contrast to skeletal muscle, where calcium must be removed from the cytosol in order for relaxation to occur. The other choices are important for smooth muscle contraction, but they do not explain how relaxation can occur even when calcium levels are high.

Question 3

The effect of autonomic nerve stimulation on smooth muscle contractile behavior depends on the types of receptors on the smooth muscle fibers. Which of the following best describes the effects of acetylcholine and norepinephrine on various types of smooth muscle? $\begin{array}{ll}
\underline{\text { A. Acetylcholine} } &\underline{ \text { Norepinephrine }} \
\text { B. Contraction only } & \text { Relaxation or contraction } \
\text { C. Contraction only } & \text { Relaxation only } \
\text { D. Relaxation only } & \text { Contraction only } \
\text { E. Relaxation only } & \text { Relaxation only } \
\text { F. Relaxation or contraction } & \text { Contraction only } \
\text { G. Relaxation or contraction } & \text { Relaxation or contraction }
\end{array}$

Accepted Answer

F

Question 4

Multiple elements can regulate the contraction of smooth muscle, including endocrine, neurocrine, and paracrine factors. This level of diversity in the control of smooth muscle contraction can be attributed to a diversity of which of the following?

Accepted Answer

Smooth muscle contraction is regulated by various hormones, neurotransmitters, and locally produced mediators that bind to specific cell-surface receptors on smooth muscle cells. This explains the diversity in the control of smooth muscle contraction.

Question 5

Smooth muscle can maintain a contraction for a relatively long time with minimal energy usage because the latch mechanism is present in smooth muscle.

Accepted Answer

Smooth muscle cells have a unique mechanism called the latch mechanism, which allows the cells to maintain a contraction for an extended period with minimal energy usage. The reasoning behind it is also correct as the latch mechanism helps the cells maintain contractile force without the need for ATP hydrolysis. Thus, both the statement and the reason are correct and related.

Question 6

Which of the following factors are important for initiating smooth muscle contractions? $$\begin{array} { l l l } 
\underline{\text {  Hormones }} &\underline{ \text { Paracrine Factors }} &\underline{ \text { Autonomic Nervous System} } \
\text { A. No } & \text { No } & \text { No } \
\text { B. No } & \text { No } & \text { Yes } \
\text { C. Yes } & \text { No } & \text { Yes } \
\text { D. Yes } & \text { Yes } & \text { No } \
\text { E. Yes } & \text { Yes } & \text { Yes }
\end{array}$$

Accepted Answer

The answer of Which of the following factors are important...

Question 7

The sensitivity of the smooth muscle contractile apparatus to calcium can increase. This increase in calcium sensitivity can be attributed to a decrease in the levels of which of the following substances?

Accepted Answer

Myosin light chain phosphatase is responsible for the dephosphorylation of myosin, which decreases the calcium sensitivity of the contractile apparatus. Therefore, a decrease in the levels of myosin light chain phosphatase would result in an increase in calcium sensitivity.

Question 8

Unlike skeletal muscle, the contraction of smooth muscle requires which of the following?

Accepted Answer

Smooth muscle contraction is initiated by the phosphorylation of myosin light chains, which is different from skeletal muscle contraction that involves the troponin-tropomyosin complex and does not require myosin light chain phosphorylation.

Question 9

The control mechanism of smooth muscle contraction is considered to be myosin-based because the calcium-calmodulin complex binds to myosin light chain kinase.

Accepted Answer

The statement correctly describes the control mechanism of smooth muscle contraction, which involves the binding of the calcium-calmodulin complex to myosin light chain kinase. The reason provided is also correct and relevant to the statement, as it explains why the control mechanism is considered myosin-based. Therefore, both the statement and reason are correct and related.

Quiz 8: Excitation and Contraction of Smooth Muscle