Smooth Muscle Study Questions Bank

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.

kinase for contraction
phosphatase for relaxation

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.

kinase for contraction
phosphatase for relaxation

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.

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

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.

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.

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.

there are no nicotinic receptors on smooth muscle

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.