Chapter 11 - Section 11.7 - Study Guide - Assess Your Learning Outcomes - Page 433: 14

The latch-bridge mechanism in smooth muscle is a unique feature that allows smooth muscle to sustain prolonged contractions with minimal energy expenditure. This mechanism is distinct from the rapid cross-bridge cycling observed in skeletal muscle. The latch-bridge mechanism involves the maintenance of force and tension in the absence of continuous ATP consumption. Here's an explanation of the nature and effect of the latch-bridge mechanism in smooth muscle: **1. Cross-Bridge Cycling in Smooth Muscle**: - Like skeletal muscle, smooth muscle contraction involves the interaction between myosin and actin filaments through cross-bridge cycling. - Myosin, a motor protein, hydrolyzes ATP to ADP and inorganic phosphate (Pi), releasing energy. This energy is used to power the cycling of myosin heads, leading to the sliding of actin and myosin filaments and muscle contraction. **2. The Latch-Bridge Mechanism**: - In smooth muscle, once the cross-bridges are formed between myosin and actin and force is generated, the latch-bridge mechanism comes into play. - Unlike skeletal muscle, where the cross-bridge cycle is rapid and ATP consumption is continuous, the latch-bridge mechanism allows smooth muscle to maintain force with minimal ATP consumption. - In this mechanism, after myosin heads initially attach to actin and generate force, they enter a "latched" state. - In the latched state, myosin remains bound to actin without undergoing the ATP hydrolysis and detachment phase seen in skeletal muscle. **3. Effect of the Latch-Bridge Mechanism**: - The latch-bridge mechanism allows smooth muscle to: - Sustain prolonged contractions with little energy expenditure: Since myosin remains bound to actin without constant ATP hydrolysis, the muscle can maintain tension over extended periods with minimal ATP consumption. This is especially beneficial in situations where sustained muscle tone is required, such as in maintaining vascular tone or the tone of hollow organs. - Gradually adjust force: Smooth muscle can slowly and finely regulate force by modulating the proportion of cross-bridges in the latched state. - Achieve plasticity: The latch-bridge mechanism provides smooth muscle with a high degree of plasticity, allowing it to adapt to changing mechanical and physiological conditions while maintaining contractile tone. **4. Regulation of the Latch-Bridge Mechanism**: - The latch-bridge state is regulated by factors such as the level of calcium ions (Ca2+) in the cytoplasm and the phosphorylation state of myosin light chains. - An increase in cytoplasmic calcium levels activates myosin light-chain kinase (MLCK), which phosphorylates myosin light chains and initiates cross-bridge formation. - The balance between phosphorylated and dephosphorylated myosin light chains influences the transition between the latch-bridge state and active cycling. In summary, the latch-bridge mechanism in smooth muscle enables sustained contractions with minimal ATP consumption, making smooth muscle well-suited for functions requiring prolonged tension maintenance, such as maintaining vascular tone and regulating organ functions. This mechanism allows smooth muscle to adapt to varying mechanical loads and physiological demands efficiently.

The latch-bridge mechanism in smooth muscle is a unique feature that allows smooth muscle to sustain prolonged contractions with minimal energy expenditure. This mechanism is distinct from the rapid cross-bridge cycling observed in skeletal muscle. The latch-bridge mechanism involves the maintenance of force and tension in the absence of continuous ATP consumption. Here's an explanation of the nature and effect of the latch-bridge mechanism in smooth muscle: **1. Cross-Bridge Cycling in Smooth Muscle**: - Like skeletal muscle, smooth muscle contraction involves the interaction between myosin and actin filaments through cross-bridge cycling. - Myosin, a motor protein, hydrolyzes ATP to ADP and inorganic phosphate (Pi), releasing energy. This energy is used to power the cycling of myosin heads, leading to the sliding of actin and myosin filaments and muscle contraction. **2. The Latch-Bridge Mechanism**: - In smooth muscle, once the cross-bridges are formed between myosin and actin and force is generated, the latch-bridge mechanism comes into play. - Unlike skeletal muscle, where the cross-bridge cycle is rapid and ATP consumption is continuous, the latch-bridge mechanism allows smooth muscle to maintain force with minimal ATP consumption. - In this mechanism, after myosin heads initially attach to actin and generate force, they enter a "latched" state. - In the latched state, myosin remains bound to actin without undergoing the ATP hydrolysis and detachment phase seen in skeletal muscle. **3. Effect of the Latch-Bridge Mechanism**: - The latch-bridge mechanism allows smooth muscle to: - Sustain prolonged contractions with little energy expenditure: Since myosin remains bound to actin without constant ATP hydrolysis, the muscle can maintain tension over extended periods with minimal ATP consumption. This is especially beneficial in situations where sustained muscle tone is required, such as in maintaining vascular tone or the tone of hollow organs. - Gradually adjust force: Smooth muscle can slowly and finely regulate force by modulating the proportion of cross-bridges in the latched state. - Achieve plasticity: The latch-bridge mechanism provides smooth muscle with a high degree of plasticity, allowing it to adapt to changing mechanical and physiological conditions while maintaining contractile tone. **4. Regulation of the Latch-Bridge Mechanism**: - The latch-bridge state is regulated by factors such as the level of calcium ions (Ca2+) in the cytoplasm and the phosphorylation state of myosin light chains. - An increase in cytoplasmic calcium levels activates myosin light-chain kinase (MLCK), which phosphorylates myosin light chains and initiates cross-bridge formation. - The balance between phosphorylated and dephosphorylated myosin light chains influences the transition between the latch-bridge state and active cycling. In summary, the latch-bridge mechanism in smooth muscle enables sustained contractions with minimal ATP consumption, making smooth muscle well-suited for functions requiring prolonged tension maintenance, such as maintaining vascular tone and regulating organ functions. This mechanism allows smooth muscle to adapt to varying mechanical loads and physiological demands efficiently.