[Music] the current theory of how a muscle cell contracts is the sliding filament theory which states that the contraction of a muscle cell occurs as the Thin filaments slide past the thick filaments during contraction the sarcom shortens and the Thin and Thick filaments overlap to a greater degree your goals for learning are to explore the molecular structure and functional features of the Thin and Thick filaments to understand the sequence of events in a single crossbridge cycle to examine the sequence of events in multiple crossbridge cycling here's what you need to know the anatomy of a skeletal muscle cell the arrangement of myofilaments within a skeletal muscle cell the events occurring at the neuromuscular Junction definitions of thick filament thin filament sarcomere binding sight terminal cisterna ATP and action potential to review the anatomy of a skeletal muscle cell the arrangement of myofilaments or the events occurring at the neuromuscular Junction click a link button if you use a link button you can return to the page you started started from by clicking the return button to see definitions of terms click the bold red words the sliding filament theory of how a skeletal muscle contracts involves the activities of five different molecules Plus calcium ions the five molecules include myosin actin tropomyosin tropinin and ATP in addition to these five molecules calcium ions are also involved in the process of muscle contraction now we will explore how each of these molecules participates in the contraction of a sarcomere click a thick filament to examine its structure the first molecule we will look at is the protein mein in skeletal muscle cells the myosin molecules are bundled together to form the thick filaments click the thick filament to see an individual myosin molecule the shape of an individual mein molecule is similar to a golf club with two heads mein has several important functional features the head or crossbridge has the ability to move back and forth the flexing movement of the mein head provides the power stroke for muscle contraction click the head of the myosin molecule to see it move another feature of the myosin molecule is the hinge portion of the linear tail this allows vertical movement so that the crossbridge can bind to actin the thin filament click the tail to see this movement click anywhere on the mein molecule to see both activities the crossbridge has two important binding sites one site specifically binds ATP adenosine triphosphate a high energy molecule notice the position of the crossbridge this is called the low energy confirmation click the ATP molecule to see it bind to the crossbridge The Binding of at TP transfers energy to the mein crossbridge as ATP is hydrolized into ADP and inorganic phosphate click the ATP molecule to transfer energy to mein now the crossbridge is in its high energy confirmation the second binding site on the mein crossbridge Bridge has a strong attraction for binding to Acton as we'll see later in the crossbridge cycling animation now that we've examined the structure of thick filaments let's look more closely at thin filaments thin filaments are composed of three molecules Acton tropomyosin and tropinin click a thin filament to examine the arrangement of these three molecules actin is the major component of the thin filament the actin portion of the thin filament is composed of actin subunits Twisted into a double helical chain each actin subunit has a specific binding site to which the mein crossbridge binds the regulatory protein tropomyosin is also part of the thin filament tropomyosin entwines around the actin in the unstimulated muscle the position of the tropomyosin covers The Binding SES on the actin subunits and prevents myosin crossbridge binding to expose The Binding SES for binding with mein the tropomyosin molecule must be moved aside this is facilitated by the presence of a third molecular complex called tropinin tropinin is attached and spaced periodically along the tropomyosin Strand tropinin by itself is not able to move the tropomyosin away from The Binding sites this process requires calcium ions after an action potential calcium ions are released from the terminal CNA and bind to tropinin this causes a confirmational change in the tropomyosin tropinin complex dragging the tropomyosin strands off The Binding SES click the terminal sna to release calcium ions and see this effect before seeing how all these components work together in a complete cross Bridge cycle click the link buttons to review any of the organic molecules or calcium ions the five organic molecules and the calcium ions function together in a coordinated manner to cause the thin filament to slide past the thick filament we will first show an animation of a single crossbridge cycle and then describe this process step by step on the following pages to see the entire process of crossbridge cycling click the terminal S as you've just seen crossbridge cycling is a continuous event for the purposes of easier understanding however let's break it down into six separate steps one the influx of calcium triggering the exposure of binding SES on actin two The Binding of mein to actin three the power stroke of the crossbridge that causes the sliding of the thin filaments four The Binding of ATP to the crossbridge which results in the crossbridge disconnecting from actin five the hydrolysis of ATP which leads to the re-energizing and repositioning of the crossbridge and six the transport of calcium ions back into the sarcoplasmic reticulum the first step is exposing Acton's binding SES when a muscle cell is stimulated the action potential brings about the release of calcium ions from the terminal CNA of the sarcoplasmic reticulum the calcium ions flood into the cytool and bind to the tropinin causing a change in confirmation of the tropinin tropomyosin complex this confirmational change exposes The Binding SES on click the terminal C to start the process when a binding site on Acton is exposed an energized crossbridge can bind to it click the energized crossbridge to begin the The Binding The Binding of mein to actin brings about a change in the confirmation of the crossbridge resulting in the release of ADP and inorganic phosphate at the same time the crossbridge flexes pulling the thin filament inward toward the center of the sarcomere this movement is called called the power stroke click the energized crossbridge to see this activity the chemical energy of ATP has been transformed into the mechanical energy of a muscle cell contraction in order to disconnect the crossbridge from Acton an ATP molecule must bind to its sight on the mein crossbridge click on the ATP molecule to disconnect the crossbridge the release of the mein crossbridge from Acton triggers the hydrolysis of the ATP molecule into ADP and inorganic phosphate energy is transferred from ATP to the mein crossbridge which returns to its high energy confirmation click the bound ATP to trigger hydrolysis in the final step calcium is actively transported from the cytool back into the sarcoplasmic reticulum by ion pumps as the calcium is removed the tropinin tropomyosin complex again covers The Binding sights on actin click a calcium ion to see this process the active transport of calcium involves specialized ion pumps in the membrane of the sarcoplasmic reticulum these pumps must be energized by ATP click the calcium ion pump to see active transport to see the entire process of crossbridge cycling click the terminal sisterna in Contracting muscle we see multiple crossbridge Cycles here are four cross Bridges which cycle in a coordinated manner note that during a contraction all cross bridges are neither bound nor disconnected at the same time click the terminal cisterna to begin the process the in this view several mein and actin filaments are interacting to demonstrate the sliding filament theory of muscle contraction notice that although the sarir shortens the length of each mile filament does not change however watch as the width of the H zone changes click a thin filament to start the contraction we've seen that ATP plays a key role in the contraction of muscle before we study muscle metabolism let's review atp's role in one energizing the power stroke of the mein cross bridge two disconnecting the mein crossbridge from The Binding site on Acton at the conclusion of a power stroke three actively transporting calcium ions into the sarcoplasmic reticulum click the link buttons to review these rols here's a summary of what we've covered the sequence of events in a single crossbridge cycle includes one the influx of calcium triggering the exposure of binding sites on actin two The Binding of myosin to actin three the power stroke of the crossbridge that causes the sliding of the thin filaments four The Binding of ATP to the crossbridge which results in the crossbridge disconnecting from actin five the hydris of ATP which leads to the re-energizing Ing and repositioning of the crossbridge and six the transport of calcium ions back into the sarcoplasmic reticulum multiple crossbridge cycling is coordinated sequentially to prevent all cross Bridges from either being connected or disconnected at the same time to test your knowledge click the quiz button to go to the self quiz e