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We have developed a minimum kinetic model for cross-bridge interactions with the thin filament in smooth muscle. The model hypothesizes two types of cross-bridge interactions: 1) cycling phosphorylated cross bridges and 2) noncycling dephosphorylated cross bridges ("latch bridges"). The major assumptions are that 1) Ca2+-dependent myosin phosphorylation is the only postulated regulatory mechanism, 2) each myosin head acts independently, and 3) latch bridges are formed by dephosphorylation of an attached cross bridge. Rate constants were resolved by fitting data on the time courses of myosin phosphorylation and stress development. Comparison of the rate constants indicates that latch-bridge detachment is the rate-limiting step. Model simulations predicted a hyperbolic dependence of steady-state stress on myosin phosphorylation, which corresponded with the experimental observation of high values of stress with low levels of phosphorylation in intact tissues. Model simulations also predicted the experimental observation that an initial phosphorylation transient only accelerates stress development, with no effect on the final steady-state levels of stress. Because the only Ca2+-dependent regulatory mechanism in this model was activation of myosin light chain kinase, these results are consistent with the hypothesis that myosin phosphorylation is both necessary and sufficient for the development of the latch state.

High-energy or attached state: This occurs when a cross-bridge between actin and myosin is generated. Myosin is filled with potential energy from the phosphate ion in ATP. This phase is driven by the entry of calcium ions, which exposes actin-binding sites.

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The cross-bridge formed from the binding of actin with myosin. The formation shows muscle shortening caused by the movement of the contractile protein. The cross-bridge muscular contraction cycle is identified in all muscle types, including cardiac, smooth, and skeletal muscles. Cross-bridge cycling is caused by the cyclic attachment and detachment of contractile proteins.

Andrew Huxley developed the cross-bridge hypothesis quantitatively for the first time in 1957. The cross-bridge cycling demonstrates the shortening of muscles due to the movement of the contractile proteins. The cross-bridge muscle contraction cycle is recognized in all types of muscles, such as cardiac, smooth, and skeletal muscles. This means that all muscle types contract by cross-bridge cycling. The cycling of cross-bridge is due to the cyclic attachment and detachment of contractile proteins.

The contractile proteins are proteins that mediate the contraction of muscles which are actin and myosin. Actin and myosin heads create a cross-bridge, causing muscle contraction. When the thin actin and thick myosin filaments slip past each other, muscle contraction takes place. This mechanism is thought to be influenced by cross-bridges that stretch from myosin filaments and cyclically engage with actin filaments upon the hydrolyzation of ATP.

The sarcomere is made up of a mesh of thin actin and thick myosin filaments. Myosin pulls thin actin filaments towards the middle on each side and shortens the sarcomere. Actin-myosin cross-bridge is formed when actin and myosin heads become linked together. In general, a myosin head flexes and delivers a power stroke when it binds to an actin filament. The force which is generated from the power stroke is due to the moving of the actin filament past the myosin. This leads to the shortening of the sarcomere. There are many heads within the myosin filaments and many binding sites within the actin filaments. The number of heads and binding sites is significant because several power strokes are required for the sarcomere to optimally contract and shorten. In reality, the quantity of actin-myosin cross-bridges created determines the power of a muscle contraction.

While this is clearly not what happens inside of our muscle cells, what you see in the picture above can help you understand how they contract. This is how the contractile proteins that we call actin and myosin interact with each other to cause contraction. So, you can think of the guys as the myosin and the rope as actin as we move into discussing how the contractile proteins interact with one another during cross-bridge cycling.

ATP stands for adenosine triphosphate. It is an energy-carrying molecule made up of adenine, ribose sugar, and three phosphate groups. ATP is in charge of pulling back the myosin head in preparation for the next cycle. When ATP attaches to the myosin head, it leads to the detachment of the actin-myosin cross-bridge. The detachment of actin and myosin takes place by hydrolyzing ATP to ADP+Pi. ATP has hydrolyzed into ADP + Pi by myosin ATPase, restoring the cross-bridge cycle to a high-energy state.

As mentioned earlier, the cross-bridge cycle is triggered by the influx of calcium ions. The calcium ions bind to a protein called troponin which leads to exposing the actin active site. Upon exposing the actin active site, the myosin head binds with the actin. When calcium ions are pumped back to the sarcoplasmic reticulum, myosin cannot bind with actin as the binding site of actin is covered. As a result, the muscle relaxes. The pumping of calcium ions requires ATP.

The cross-bridge cycling shows muscle shortening caused by the movement of the contractile proteins. The cross-bridge muscular contraction cycle is identified in all muscle types, including cardiac, smooth, and skeletal muscles. The contractile proteins are actin and myosin, which are proteins that mediate muscle contraction. Actin is a thin contractile protein found in the muscle cells, whereas myosin is a thick contractile protein found in muscle cells. The heads of actin and myosin form a cross-bridge, generating muscle contraction. Muscle contraction occurs when the thin actin and thick myosin filaments slide past each other. Cross-bridges that extend from myosin filaments and cyclically interact with actin filaments upon ATP hydrolysis are hypothesized to regulate this process.

As you can see, actin makes up the thin filaments, and they're attached to the Z lines. Myosin makes up the thick filaments, which overlap the thin filaments in the middle of a sarcomere. Perhaps you can imagine myosin forming a cross-bridge with actin much like a person would grab a rope and pull on it. Myosin pulls the thin filaments towards the middle on each side, thus shortening the sarcomere and causing contraction.

In the context of muscular contraction, a cross-bridge refers to the attachment of myosin with actin within the muscle cell. All muscle types - whether we're talking about skeletal, cardiac, or smooth - contract by cross-bridge cycling - that is, repeated attachment of actin and myosin within the cell. Let's get out that trusty magnifying glass again and focus now on a single cross-bridge within a sarcomere.

Let's start at the top with what we call the high-energy or attached state of the cross-bridge. In this stage of the cycle, myosin is loaded with potential energy and attached to actin, just as a mouse trap is loaded with potential energy when we set it to hopefully catch a mouse.

Much like a mouse trap tripping, myosin binding releases the stored energy and the myosin head changes its shape, pulling the thin filament towards the middle of the sarcomere. This is referred to as the working stroke of the cross-bridge cycle, as work requires movement, and now movement is being done.

After myosin changes its shape, ATP binds to the myosin head. That binding of ATP to myosin releases the myosin from actin, and that changes the cross-bridge to its detached state. The myosin head is pushed back into its high-energy state using energy from the hydrolysis of ATP - the ATP that just bound to the myosin. Myosin can now attach to actin and form the attached state once again. The cross-bridge will continue to cycle and cause contraction as long as the muscle is stimulated.

In summary, cross-bridge cycling between actin and myosin is responsible for muscular contraction. A single cross-bridge cycle consists of four basic stages. First, myosin binds actin, forming the high-energy/attached state. The power stroke occurs when myosin changes its shape, pulling the thin filaments towards the middle of the sarcomere - that's what causes sarcomere shortening in muscular contraction.

After this power stroke, ATP binds myosin, causing it to be released from actin and form the detached state. Myosin hydrolyzes the ATP, thus releasing energy that is used to push the myosin back into its high-energy state. Once myosin is loaded with that potential energy, it binds to actin again, reforming the high-energy/attached state of the cross bridge. Cross-bridge cycling will continue as long as the muscle is stimulated.

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The functional correlates of fatigue observed in both animals and humans during exercise include a decline in peak force (P0), maximal velocity, and peak power. Establishing the extent to which these deleterious functional changes result from direct effects on the myofilaments is facilitated through understanding the molecular mechanisms of the cross-bridge cycle. With actin-myosin binding, the cross-bridge transitions from a weakly bound low-force state to a strongly bound high-force state. Low pH reduces the number of high-force cross bridges in fast fibers, and the force per cross bridge in both fast and slow fibers. The former is thought to involve a direct inhibition of the forward rate constant for transition to the strong cross-bridge state. In contrast, inorganic phosphate (Pi) is thought to reduce P0 by accelerating the reversal of this step. Both H+ and Pi decrease myofibrillar Ca2+ sensitivity. This effect is particularly important as the amplitude of the Ca2+ transient falls with fatigue. The inhibitory effects of low pH and high Pi on P0 are reduced as temperature increases from 10 to 30 degrees C. However, the H+-induced depression of peak power in the slow fiber type, and Pi inhibition of myofibrillar Ca2+ sensitivity in slow and fast fibers, are greater at high compared with low temperature. Thus the depressive effects of H+ and Pi at in vivo temperatures cannot easily be predicted from data collected below 25 degrees C. In vitro, reactive oxygen species reduce myofibrillar Ca2+ sensitivity; however, the importance of this mechanism during in vivo exercise is unknown. 006ab0faaa