MuscleTwitch & Latent Period

CH. 9 HW: MUSCLES AND MUSCLE TISSUE Video 1: Muscle contraction Part 3: the Cross Bridge Cycle ● The contraction of a skeletal muscle generates the force necessary to move the skeleton ● A contraction is triggered by a series of molecular events known as the cross bridge cycle ● In a skeletal muscle fiber, the functional unit of contraction is called the sarcomere ○ Sarcomere shortens when myosin heads in thick myofilaments form cross bridges with actin molecules in thin myofilaments ● The formation of a cross bridge is initiated when calcium ions, released from the sarcoplasmic reticulum, bind to troponin ○ This binding causes troponin to change shape ○ Tropomyosin moves away from the myosin binding sites on actin, allowing the myosin head to bind actin to form a cross bridge *Note: the myosin head must be activated before a cross bridge cycle can begin - This occurs when ATP binds to the myosin head and is hydrolyzed to ADP and inorganic phosphate - The energy liberated from the hydrolysis of ATP activates the myosin head, forcing it into the cocked position 4 STEPS OF CROSS BRIDGE CYCLE: Step 1: cross bridge formation ● The activated myosin head binds to actin, forming a cross bridge ● inorganic phosphate is released ● The bond between myosin and actin becomes stronger Step 2: the power stroke ● ADP is released and the activated myosin head pivots, sliding the thin myofilament toward the center of the sarcomere Step 3: Cross Bridge Detachment ● When another ATP binds to the myosin head, the link between the myosin head and actin weakens and the myosin head detaches Step 4: Reactivation of Myosin head ● ATP is hydrolyzed to ADP and inorganic phosphate ● The energy released during hydrolysis reactivates the myosin head, returning it to the cocked position As long as the binding sites on actin remain exposed, the cross bridge cycle will repeat ● As the cycle repeats, the thing myofilaments are pulled toward each other and the sarcomere shortens ● This shortening causes the whole muscle to contract Cross bridge cycling ends when calcium ions are actively transported back into the sarcoplasmic reticulum ● In the absence of calcium, troponin returns to its original, allowing tropomyosin to glide over and cover the myosin binding site on actin 1. The cross bridge cycle starts when _________. a. b. c. d. e. ATP binds to troponin and is hydrolyzed to ADP and Pi Ca2+ from the sarcoplasmic reticulum binds to troponin Ca2+ from the sarcoplasmic reticulum binds to tropomyosin Ca2+ is actively transported into the sarcoplasmic reticulum Acetylcholine diffuses away from the synaptic cleft → The release of Ca2+ is triggered by the propagation of an action potential along a skeletal muscle fiber. Ca2+ is released from the sarcoplasmic reticulum and into the sarcoplasm of the muscle fiber. When Ca2+ concentration is high in the sarcoplasm, Ca2+ binds to troponin causing change in its shape. This shape change alters the position of tropomyosin and moves it away from myosin binding sites on actin, thus allowing the myosin head to bind actin and form a cross bridge. 2. The sarcomere shortens when the myosin heads of the thick filaments, in a cocked position, form cross bridges with the actin molecules in thin filaments. This activity will test your understanding of the steps that occur in one complete cross bridge cycle. → The thin filaments are pulled toward the center of the sarcomere by the myosin heads of the thick filament. This shortens the sarcomeres in the muscle fiber and causes the whole skeletal muscle to contract. The cross bridge cycle ends when Ca2+ are actively transported back into the sarcoplasmic reticulum. The cycle repeats as long as the binding sites on actin remain exposed, and both Ca2+ and ATP are available. 3. Drag and drop the terms to their correct location in the illustration of a sarcomere. 4. Review some terms associated with a skeletal muscle fiber (also called a muscle cell) and the sarcomere. Drag and drop each term to the best description of that term. 5. Review another set of terms associated with a skeletal muscle fiber/cell and the sarcomere. Drag and drop each term to the best description of that term. ** Cross Bridge Formation In its simplest representation, you can think of cross bridge cycling as actin combining with myosin and ATP to produce force. To better understand this relationship, we will deconstruct the steps of cross bridge cycling, beginning with the formation of cross bridges. The process of cross bridge cycling begins when action potentials traveling from the central nervous system (CNS) to the muscle cell trigger an influx of calcium ions (Ca2+) into the sarcoplasm. When a muscle is at rest, actin’s binding sites are covered by tropomyosin, which is attached to troponin. When the levels of intracellular Ca2+ increase, Ca2+ binds to troponin, and the troponin-tropomyosin complex changes conformation (changes shape). This movement exposes myosin-binding sites on the actin filament. At this stage, the myosin head is said to be "energized," which is to say the myosin head is bent back and is bound to ADP and inorganic phosphate (Pi). The actual "cross bridge" is formed when the energized myosin head binds to the newly exposed binding sites on the actin filaments. 6. Drag and drop the terms to the best description of terms involving the cross bridge formation. Answers may be used once or not at all. The Power Stroke When the bond between myosin and actin forms, ADP and Pi are released. In the power, or working, stroke, the release of ADP and Pi triggers the myosin head to pivot into its low-energy conformation as the myosin neck extends. Since the two filaments are still bound, the movement of the myosin head causes the actin filament to be pulled along with it. This action ratchets the actin myofilaments toward the M line, changing the lateral boundaries of the sarcomere (i.e., the Z discs move closer to one another). It is significant that not all myosin heads and actins are bound at the same time, allowing the continual pulling of the actin in contraction shortening. 7. Drag and drop the phrases to their best description involving the power stroke. Answers may be used once or not at all. Cross Bridge Detachment The actin and myosin remain bound until ATP binds to the myosin head. This binding weakens the cross bridge bond and causes the myosin head to detach from actin. When we consider the cross bridge cycle along the entire actin filament, at any given time, only about one-half of the myosin heads are detached from actin. In this way, waves of cross bridging can propagate along the length of the fibers. 8. Drag and drop the terms to correctly complete the sentences involving cross bridge detachment. Answers may be used once or not at all. Cocking of the Myosin Head In the final step of cross bridge cycling, the bound ATP attached to the myosin head is hydrolyzed to ADP and inorganic phosphate (Pi). This triggers the myosin head to return to its energized, cocked position. 9. Drag and drop the phrases to the appropriate status during the cocking of the myosin head phase. Answers may be used once or not at all. 10. Place the phases of cross bridge cycling in order by dragging the appropriate figure to the ordered box. Begin with actin binding to myosin. 11. A muscle cell runs out of ATP. Even though these are cyclic reactions, what step of the cross bridge cycle given is most directly inhibited or terminated? a. the power stroke b. cross bridge detachment c. cross bridge formation d. cocking of myosin head → When ATP is added to the myosin, myosin detaches from actin. 12. A person dies, and within hours, the skeletal muscles develop a locked contraction known as rigor mortis. Calcium ions leak from the sarcoplasmic reticulum into cytoplasm. From your knowledge of cross bridge cycling, what best explains this rigor? a. The cross bridge is locked up because the power stroke requires ATP, which is produced only during life. b. Cross bridge formation cannot occur. ATP, which is produced only during life, must to be bound to the myosin head to permit cross bridging. c. The cross bridge is locked up because ATP directly attaches myosin to actin. d. Cross bridge detachment cannot occur. Detachment requires ATP, which is produced only during life. → After ATP attaches to the myosin head, the bond between actin and myosin is weakened and the cross bridge breaks. excitation-contraction coupling 1. Excitation-contraction coupling is a series of events that occur after the events of the neuromuscular junction have transpired. The term excitation refers to which step in the process? a. Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. b. Excitation refers to the release of calcium ions from the sarcoplasmic reticulum. c. Excitation refers to the shape change that occurs in voltage-sensitive proteins in the sarcolemma. d. Excitation refers to the propagation of action potentials along the axon of a motor neuron. → Yes! These action potentials set off a series of events that lead to a contraction. 2. Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction? a. Calcium release from the sarcoplasmic reticulum initiates the contraction. → Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle cells. It is especially abundant and convoluted in skeletal muscle cells. It functions in the storage, release, and reuptake of calcium ions. 3. A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected? a. A series of proteins that control calcium release. → When action potentials propagate along T-tubules, a voltage-sensitive protein changes shape and triggers a different protein to open it's channels, resulting in the release of calcium from the terminal cisternae. 4. What is name given to the regularly spaced infoldings of the sarcolemma? a. transverse or T tubules → T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior. 5. Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? a. Calcium ions → Action potentials propagating down the T-tubule cause a voltage-sensitive protein to change shape. This shape change opens calcium release channels in the sarcoplasmic reticulum, allowing calcium ions to flood the sarcoplasm. This flood of calcium ions is directly responsible for the coupling of excitation to contraction in skeletal muscle fibers. 6. What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated? a. Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. → There are many more skeletal muscle fibers than there are motor neurons. The ratio of neurons to fibers varies from approximately one to ten to approximately one to thousands. 1. Consider how the action potential that initiates contraction is delivered to the muscle cell. Which of the choices below correctly describes how an action potential generated at the neuromuscular junction (NMJ) is converted to excitation in the muscle fiber? a. An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh to sarcolemma receptors initiates graded potentials. → An action potential traveling down a motor neuron arrives at the axon terminal and causes exocytosis of the neurotransmitter ACh into the synaptic cleft. ACh diffuses into the synaptic cleft, binds to the receptor proteins on the junctional folds of the muscle sarcolemma (motor end plate), and initiates graded potentials. These graded potentials sum to an action potential, thus initiating excitement of the muscle fiber. 2. The action potential propagates along the sarcolemma. As the action potential spreads down the T tubules of the triads, voltage-sensitive tubule proteins change shape. How does the shape change of these proteins lead to contraction? a. It allows calcium to exit the sarcoplasmic reticulum and enter the cytosol. → As the action potential propagates, it changes the shape of T tubule proteins. These proteins are linked to calcium channels in the terminal cisterns of the sarcoplasmic reticulum. When these proteins' calcium channels open, a massive amount of calcium flows into the cytosol. 3. Drag and drop each term to the correct location on the figure of the muscle cell. 4. Which selection correctly describes the role of calcium in coupling? a. Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin. → The thin filament in a sarcomere is composed of actin, troponin, and tropomyosin. Troponin and tropomyosin are attached to one another, both overlaying actin. When a muscle is relaxed, tropomyosin blocks actin's myosin-binding sites. Calcium binds to troponin, initiating a shape change that removes the blocking action of tropomyosin. This exposes the myosin-binding sites on actin to the myosin heads for cross bridging. 5. Which selection best describes the initial event in contraction? a. Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges. → Contraction begins when myosin binds to actin and forms cross bridges. 6. Drag and drop the terms to match the best description. Some terms are not used. 7. Click and drag the description of the part of the excitation-contraction coupling to the correct order of occurrence from left to right. Do not overlap any steps. → Coupling begins with an action potential after the arrival of ACh and graded potentials, and it ends before cross bridge cycling. 8. Which of the following choices best summarizes excitation-contraction coupling? A series of events in which an electrical stimulus is conveyed to a muscle fiber to enact contraction → You have completed the final activity in this tutorial. The tutorial covered the sequence of events in excitation-contraction coupling that links the excitation of the membrane to the cross bridge cycling of the contracting proteins. Great job!