Smooth Muscle Study Questions Bank

1. In excitation-contraction of smooth muscle, calcium binds to what protein after influx into the cytoplasm?

Calmodulin

Myosin light chains

Troponin

Tropomyosin

Protein kinase A

In excitation-contraction of smooth muscle, calcium binds to calmodulin after influx into the cytoplasm. Calmodulin is a calcium-binding protein that plays a crucial role in regulating various cellular processes, including muscle contraction. When calcium ions enter the cytoplasm, they bind to calmodulin, causing a conformational change in the protein. This activated calmodulin then interacts with other proteins, such as myosin light chains, to initiate muscle contraction. Therefore, calmodulin acts as a key mediator in the calcium signaling pathway that leads to smooth muscle contraction.

Explanation

In excitation-contraction of smooth muscle, calcium binds to calmodulin after influx into the cytoplasm. Calmodulin is a calcium-binding protein that plays a crucial role in regulating various cellular processes, including muscle contraction. When calcium ions enter the cytoplasm, they bind to calmodulin, causing a conformational change in the protein. This activated calmodulin then interacts with other proteins, such as myosin light chains, to initiate muscle contraction. Therefore, calmodulin acts as a key mediator in the calcium signaling pathway that leads to smooth muscle contraction.

2. Which of the following enzymes is responsible for phosphorylating myosin light chains in order to activate smooth muscle contraction?

Calmodulin

Protein kinase A

Myosin light chain kinase

Phospholipase C

Actomyosin ATPase

kinase for contraction
phosphatase for relaxation

Explanation

kinase for contraction
phosphatase for relaxation

3. Single-unit smooth muscle differs from multiunit smooth muscles how?

Single-unit muscle contraction speed is slow, while multiunit is fast

Single-unit muscle has T-tubules, while multiunit does not

Single-unit muscles are not innervated by autonomic nerves

Single-unit muscle produces action potentials spontaneously that spreads to neighboring cells, while multiunit does not

Single-unit smooth muscle differs from multiunit smooth muscle in that it produces action potentials spontaneously that spread to neighboring cells, while multiunit smooth muscle does not. This means that in single-unit smooth muscle, the cells are electrically connected and can contract as a coordinated unit, whereas in multiunit smooth muscle, each cell contracts independently. This allows single-unit smooth muscle to generate slow, rhythmic contractions, while multiunit smooth muscle can contract rapidly and with more precision. Single-unit smooth muscle also has T-tubules, while multiunit smooth muscle does not, but this is not the main distinguishing factor between the two types of muscle.

Explanation

Single-unit smooth muscle differs from multiunit smooth muscle in that it produces action potentials spontaneously that spread to neighboring cells, while multiunit smooth muscle does not. This means that in single-unit smooth muscle, the cells are electrically connected and can contract as a coordinated unit, whereas in multiunit smooth muscle, each cell contracts independently. This allows single-unit smooth muscle to generate slow, rhythmic contractions, while multiunit smooth muscle can contract rapidly and with more precision. Single-unit smooth muscle also has T-tubules, while multiunit smooth muscle does not, but this is not the main distinguishing factor between the two types of muscle.

4. Which of the following enzymes is responsible for dephosphorylating myosin light chains, thereby causing smooth muscle relaxation?

Calmodulin

Protein kinase A

Myosin light chain kinase

Myosin light chain phosphatase

Phospholipase C

Actomyosin ATPase

kinase for contraction
phosphatase for relaxation

Explanation

kinase for contraction
phosphatase for relaxation

5. Which characteristic or component is shared by skeletal muscle and smooth muscle?

Thick and thin filaments arranged in sarcomeres

Troponin

Elevation of intracellular [Ca2+] for excitation-contraction coupling

Spontaneous depolarization of the membrane potential

High degree of electrical coupling between cells

Both skeletal muscle and smooth muscle share the characteristic of elevation of intracellular [Ca2+] for excitation-contraction coupling. This means that an increase in the concentration of calcium ions inside the muscle cells is necessary for the muscle fibers to contract. Calcium ions bind to specific proteins, such as troponin, which then triggers a series of molecular events leading to muscle contraction. This process is essential for both types of muscle to generate force and perform their respective functions.

Explanation

Both skeletal muscle and smooth muscle share the characteristic of elevation of intracellular [Ca2+] for excitation-contraction coupling. This means that an increase in the concentration of calcium ions inside the muscle cells is necessary for the muscle fibers to contract. Calcium ions bind to specific proteins, such as troponin, which then triggers a series of molecular events leading to muscle contraction. This process is essential for both types of muscle to generate force and perform their respective functions.

6. During the process of excitation-contraction coupling in smooth muscle, intracellular [Ca²⁺] is increased through all of the following methods except which?

Ca2+ influx from extracellular stores through voltage-activated Ca2+ channels

Ca2+ influx from the sarcoplasmic reticulum through IP3 receptors

Ca2+ influx from extracellular stores through ryanodine receptors

Ca2+ influx from extracellular stores through ligand-gated Ca2+ channels

ryanodine receptors are on the SR, DHP receptors are on the sarcoplasma membrane.

Explanation

ryanodine receptors are on the SR, DHP receptors are on the sarcoplasma membrane.

7. Which of the following muscle proteins plays a critical role in contraction of both smooth and striated muscle?

Calmodulin

Troponin

Tropomyosin

Actin

Myosin light chains

Actin is a muscle protein that plays a critical role in the contraction of both smooth and striated muscle. It is a major component of the thin filaments in muscle fibers and interacts with myosin to generate force during muscle contraction. Actin filaments slide past myosin filaments, causing the muscle to contract. This process occurs in both smooth and striated muscle, making actin an essential protein for muscle contraction.

Explanation

Actin is a muscle protein that plays a critical role in the contraction of both smooth and striated muscle. It is a major component of the thin filaments in muscle fibers and interacts with myosin to generate force during muscle contraction. Actin filaments slide past myosin filaments, causing the muscle to contract. This process occurs in both smooth and striated muscle, making actin an essential protein for muscle contraction.

8. The role of myosin light-chain protein in smooth muscle is what?

Bind to calcium ions to initiate excitation-contraction coupling

Phosphorylate cross-bridges, thus driving them to bind with the thin filament

Split ATP to provide the energy for the power stroke of the cross-bridge cycle

Dephosphorylate myosin light-chains of the cross-bridge, thus relaxing the muscle

Pump calcium from the cytosol back into the sarcoplasmic reticulum

Myosin light-chain protein in smooth muscle phosphorylates cross-bridges, which allows them to bind with the thin filament. This phosphorylation process is essential for the contraction of smooth muscle.

Explanation

Myosin light-chain protein in smooth muscle phosphorylates cross-bridges, which allows them to bind with the thin filament. This phosphorylation process is essential for the contraction of smooth muscle.

9. Which of the following temporal sequences is correct for excitation-contraction coupling in smooth muscle?

The correct temporal sequence for excitation-contraction coupling in smooth muscle is as follows: Ca2+ influx through L-type Ca2+ channels, Ca2+ activated Ca2+ release from RyR in the SR, Ca-calmodulin activation of MLCK, cross-bridge cycling, Ca2+ removal from SR by SERCA, PMCA, and Na/Ca exchanger. This sequence starts with the influx of Ca2+ through L-type Ca2+ channels, which triggers the release of more Ca2+ from the sarcoplasmic reticulum (SR) through RyR. The released Ca2+ then activates MLCK through its binding to calmodulin. MLCK activation leads to cross-bridge cycling, resulting in muscle contraction. Finally, Ca2+ is removed from the SR by SERCA, pumped out of the cell by PMCA, and exchanged with Na+ by the Na/Ca exchanger.

Explanation

The correct temporal sequence for excitation-contraction coupling in smooth muscle is as follows: Ca2+ influx through L-type Ca2+ channels, Ca2+ activated Ca2+ release from RyR in the SR, Ca-calmodulin activation of MLCK, cross-bridge cycling, Ca2+ removal from SR by SERCA, PMCA, and Na/Ca exchanger. This sequence starts with the influx of Ca2+ through L-type Ca2+ channels, which triggers the release of more Ca2+ from the sarcoplasmic reticulum (SR) through RyR. The released Ca2+ then activates MLCK through its binding to calmodulin. MLCK activation leads to cross-bridge cycling, resulting in muscle contraction. Finally, Ca2+ is removed from the SR by SERCA, pumped out of the cell by PMCA, and exchanged with Na+ by the Na/Ca exchanger.

10. Smooth muscle cells are initiated to contract by all of the following ways except which?

Depolarization from a neighboring cell spread through connexon channels

Norepinephrine binding to α1-adrenergic receptors

Acetylcholine binding to nicotinic acetylcholine receptors

The movement of a bolus of food through the small intestine

there are no nicotinic receptors on smooth muscle

Explanation

there are no nicotinic receptors on smooth muscle

11. When comparing the contractile responses in smooth and skeletal muscle, which of the following is most different?

The source of activator calcium

The role of calcium in initiating contraction

The mechanism of force generation

The source of energy used during contraction

The nature of the contractile proteins

In smooth muscle, calcium is required for contraction to occur, but it does not directly initiate contraction. Instead, calcium binds to calmodulin, which then activates myosin light chain kinase, leading to phosphorylation of myosin and initiation of contraction. In skeletal muscle, however, calcium directly binds to troponin, causing a conformational change that exposes the myosin binding sites on actin and initiates contraction. Therefore, the role of calcium in initiating contraction is most different between smooth and skeletal muscle.

Explanation

In smooth muscle, calcium is required for contraction to occur, but it does not directly initiate contraction. Instead, calcium binds to calmodulin, which then activates myosin light chain kinase, leading to phosphorylation of myosin and initiation of contraction. In skeletal muscle, however, calcium directly binds to troponin, causing a conformational change that exposes the myosin binding sites on actin and initiates contraction. Therefore, the role of calcium in initiating contraction is most different between smooth and skeletal muscle.