Question 1

Which of the following statements best explains why smooth muscle can sustain contractions for extended periods without fatiguing?

Accepted Answer

Smooth muscle uses less energy to generate and maintain a given amount of force, and can enter a latch state where myosin remains attached to actin even after dephosphorylation.

Question 2

Which structural feature distinguishes smooth muscle from skeletal muscle at the cellular level?

Accepted Answer

Smooth muscle cells are small, spindle-shaped, and have a single nucleus.

Question 3

A patient is given a drug that blocks the phosphorylation of myosin light chains in smooth muscle. What is the most likely effect on smooth muscle contraction?

Accepted Answer

Contraction will be inhibited because phosphorylation of myosin light chains is necessary to activate myosin ATPase and initiate crossbridge cycling.

Question 4

Which of the following best describes the source(s) of Ca$$^{2+}$$ for smooth muscle contraction?

Accepted Answer

Ca$$^{2+}$$ for contraction comes from both the extracellular fluid and the sarcoplasmic reticulum.

Question 5

Which of the following mechanisms is responsible for initiating contraction in smooth muscle in response to a chemical signal?

Accepted Answer

Pharmacomechanical coupling

Question 6

In smooth muscle, which protein directly activates myosin light chain kinase (MLCK) after binding Ca$$^{2+}$$?

Accepted Answer

Calmodulin

Question 7

Which of the following is TRUE regarding the organization of actin and myosin filaments in smooth muscle?

Accepted Answer

Smooth muscle has more actin relative to myosin (10-15:1), and myosin heads are distributed along the entire filament.

Question 8

A researcher observes that a smooth muscle cell contracts in response to stretch. Which type of channel is most likely responsible for this response?

Accepted Answer

Stretch-activated calcium channel

Question 9

Which of the following statements best explains why smooth muscle contraction and relaxation are slower than those of skeletal muscle?

Accepted Answer

Smooth muscle lacks t-tubules, and its contractile proteins interact more slowly due to differences in regulatory mechanisms.

Question 10

Which of the following is a correct sequence of events leading to smooth muscle contraction after an increase in cytosolic Ca$$^{2+}$$?

Accepted Answer

Ca$$^{2+}$$ binds calmodulin → calmodulin activates MLCK → MLCK phosphorylates myosin light chains → myosin ATPase activity increases → contraction

Question 11

Which of the following is a consequence of increased myosin light chain phosphatase (MLCP) activity in smooth muscle?

Accepted Answer

Decreased Ca$$^{2+}$$ sensitivity and reduced contraction

Question 12

Which of the following best describes the effect of nitric oxide on smooth muscle in blood vessels?

Accepted Answer

Nitric oxide relaxes smooth muscle, causing vasodilation.

Question 13

Which type of receptor is most commonly involved in the action of neurotransmitters and hormones on smooth muscle?

Accepted Answer

G protein-linked receptors

Question 14

A patient with asthma is exposed to high levels of histamine. What is the expected effect on their airway smooth muscle?

Accepted Answer

Constriction of airway smooth muscle, leading to bronchoconstriction

Question 15

Which of the following best describes the role of caveolae in smooth muscle cells?

Accepted Answer

Caveolae function as invaginations of the cell membrane that help organize signaling molecules and facilitate Ca$$^{2+}$$ entry.

Question 16

Which of the following is a key difference between single-unit and multiunit smooth muscle?

Accepted Answer

Single-unit smooth muscle cells are electrically linked by gap junctions, allowing coordinated contraction, while multiunit smooth muscle cells are not.

Question 17

Which of the following best describes the effect of autonomic neurotransmitters on smooth muscle?

Accepted Answer

They can cause either contraction or relaxation depending on the receptor subtype present in the tissue.

Question 18

Which of the following is TRUE regarding the duration of muscle twitch in smooth muscle compared to skeletal and cardiac muscle?

Accepted Answer

Smooth muscle twitches are the slowest to contract and relax.

Question 19

Which of the following is a correct description of the cytoskeletal organization in smooth muscle?

Accepted Answer

Smooth muscle contains intermediate filaments and dense bodies that anchor actin filaments.

Smooth Muscle: Structure, Function, and Contraction Mechanisms

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