Ch 50 Muscle Physiology

Which of the following is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber? 1. Tropomyosin shifts and unblocks the cross-bridge binding sites. 2. Calcium is released and binds to the troponin complex. 3. Transverse tubules depolarize the sarcoplasmic reticulum. 4. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP. 5. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane.

5 → 3 → 2 → 1 → 4 5. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane. 3. Transverse tubules depolarize the sarcoplasmic reticulum. 2. Calcium is released and binds to the troponin complex. 1. Tropomyosin shifts and unblocks the cross-bridge binding sites. 4. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.

Myosin heads have binding sites for

ATP and actin

The muscles of a recently deceased human can remain in a contracted state, termed rigor mortis, for several hours, due to the lack of

ATP needed to break actin-myosin bonds

During the contraction of a vertebrate skeletal muscle fiber, calcium ions

Bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.

When an action potential from a motor neuron arrives at the neuromuscular junction (NMJ), a series of events occurs that leads to muscle contraction. Which of the following events will occur last (that is, after all of the others listed below)?

Conformational change in troponin

A patient is hospitalized with muscle spasms caused by failure of back muscles to relax after contraction. Which of the following would be most likely to help?

Induce tropomyosin and troponin to bind to the myosin binding sites on actin.

Action potentials in the heart move from one contractile cell to the next via

Intercalated disks

Compared to oxidative skeletal muscle fibers, those classified as glycolytic typically have

Less resistance to fatigue

Actin forms microfilaments that contain

Many identical subunits assembled into a double helix.

The venom of cobras contains a mixture of substances that have a variety of physiological effects. One substance in the venom works by preventing acetylcholine from binding to muscle receptors. How does the venom affect the prey of the cobra?

Muscle contractions would be prevented, causing paralysis.

The contraction of skeletal muscles is based on

Myosin cross-bridges binding to actin and transitioning from a high-energy to a low-energy state

The motor unit in vertebrate skeletal muscle refers to

One motor neuron and all of the muscle fibers on which it has synapses

Most of the ATP supplies for a skeletal muscle undergoing one hour of sustained exercise come from

Oxidative phosphorylation

Part A: Skeletal muscle consists of many bundles of fibers that run the entire length of the muscle. Each fiber is an individual muscle cell. Each muscle cell is in contact with a single motor neuron that controls the contraction of that cell. The diagram below shows a single muscle fiber and its motor neuron. Understanding the unique structural components of a muscle cell and its interaction with its motor neuron is a prerequisite for understanding muscle contraction and how it is regulated. Drag the labels to their appropriate locations on the diagram below. Part B: Propagation of an action potential in a skeletal muscle cell links the signal from a motor neuron to contraction of the muscle cell. An action potential in a muscle cell is propagated by the same mechanism as in neurons, the sequential opening and closing of voltage-gated Na+ and K+ channels in the plasma membrane. However, in muscle cells, the topography of the plasma membrane is quite different than in neurons, and this difference is critical to the function of muscle cells. Which of the following statements correctly describe(s) T tubules and their role in conducting action potentials in muscle cells? Part C: The sarcoplasmic reticulum is a specialized form of ER that surrounds each myofibril. The sarcoplasmic reticulum functions to control cytosolic Ca2+ levels in the muscle cell. Changes in cytosolic Ca2+ concentrations couple action potentials to muscle contraction. The concentration of Ca2+ ions in the sarcoplasmic reticulum is typically much higher than the Ca2+ concentration in the cytosol. This concentration gradient is key to the movement of Ca2+ in the muscle cell in response to an action potential. The cycle diagram below shows the sequence of events that affect Ca2+ levels in a muscle cell, beginning with the propagation of an action potential down a T tubule (top of the diagram). Drag the labels to their appropriate locations on the cycle diagram below. Note that SR stands for sarcoplasmic reticulum.

Part A: a. Motor Neuron Axon b. T-tubule c. Sacromere d. Synaptic Terminal e. Sarcoplasmic Reticulum f. Myofibril g. Plasma Membrane Part B: Without T tubules, the muscle cell would not be able to contract. T tubules carry action potentials into the interior of the muscle cell via voltage-gated Na+ and K+ channels. T tubules are infoldings of the plasma membrane that encircle the myofibrils and are in contact with the sarcoplasmic reticulum. Part C: Action potential propagates down T-tublues a. Ca2+ channels in SR open b. Ca2+ diffueses out of SR c. Cytosolic Ca2+ level rises d. Ca2+ diffuses into myofibril Action potential completed e. Ca2+ channels in SR close f. Ca2+ pumped into SR g. Cytosolic Ca2+ level drops h. Ca2+ diffuses out of myofibril

Part A: The thin filaments and thick filaments and their associated proteins are the key functional components of the sarcomere. It is important to be able to identify these components in order to understand their roles in muscle contraction. Drag the labels to their appropriate locations on the diagram below, indicating the function of each structure during muscle contraction. Part B: Contraction of a sarcomere, and thus contraction of the muscle as a whole, is accomplished by a cyclic sequence of interactions between actin (thin filaments) and myosin (thick filaments) in the sarcomere. Understanding this sequence of events is essential to understanding the mechanism of muscle contraction at the cellular and molecular levels. Begin by considering a relaxed muscle. In this state, the myosin molecules of the thick filaments are not in contact with the actin of the thin filaments, and ADP and Pi are bound to the myosin heads. An action potential on the muscle cell plasma membrane initiates contraction of the sarcomeres from this relaxed state. Drag the labels to their appropriate locations on the cycle diagram below. Part C: In a relaxed muscle, the myosin heads of the sarcomeres' thick filaments are extended and ready to bind to the actin strands of the thin filaments. But this binding does not occur until an action potential is triggered in the muscle cell. An action potential results in the release of Ca2+ ions from the sarcoplasmic reticulum into the cytosol of the muscle cell. As the concentration of Ca2+ rises in the cytosol, so does the concentration of Ca2+ in the sarcomeres. In response to changes in the Ca2+ concentration in the sarcomeres, two protein components of the thin filaments, troponin and tropomyosin, control access to actin's myosin-binding sites. In this way, Ca2+ concentration in the cytosol and sarcomeres regulates muscle contraction. Which of the following statements correctly describe(s) the relationship between Ca2+ concentration in the cytosol and the response in the sarcomere? Select all that apply. Part D: Skeletal muscle contraction is a graded process, meaning that you can voluntarily alter the strength and extent of contraction of your skeletal muscles, such as your biceps. Increasing the strength and extent of contraction occurs by increasing the number of muscle cells that receive action potentials. In addition, increasing the number of action potentials sent to a muscle cell can also increase muscle tension, as shown in the graph.A tiny muscle "twitch" is caused by a single action potential.Two or more closely spaced action potentials have an additive effect because the muscle does not have sufficient time to relax between action potentials.A long series of closely spaced action potentials results in a sustained, maximum contraction, called tetanus. Which of the following statements correctly describes why a series of closely spaced action potentials causes a sustained contraction rather than a series of closely spaced twitches?

Part A: a: Filament that is pulled toward the center of the sarcomere b: Filament that pulls the other filament toward the center of the sarcomere c: Motor protein head that hydrolyzes ATP and drives contraction d: Sites at which Ca2+ ions are bound e: Binding site for motor protein head f: Protein that controls access to motor protein binding sites Part B: relaxed muscle action potential and Ca2+ release Bind a: Myosin head forms a cross-bridge with actin b: Myosin releases P Power Stroke c: Myosisn pulls actin toward center of sarcomere d: Myosin binds ATP Detach e: Cross-bridge between myosin and actin is broken f: Myosin hydrolyzes ATP to ADP and P Extend g: Myosin head extends (pivots) Part C: Decreasing Ca2+ concentration causes dissociation of Ca2+ from troponin. Increasing Ca2+ concentration causes movement of tropomyosin, exposing myosin-binding sites on actin. Part D: When a series of action potentials is closely spaced, there is not sufficient time for Ca2+ uptake into the sarcoplasmic reticulum between action potentials, and Ca2+ remains bound to troponin throughout the series.

Part A: Which muscle type is involved in the function of the digestive tract and blood vessels? Part B: How does cardiac muscle differ from the other types of muscle? Part C: True or false? Myofibrils are the alternating light-dark units that produce the banded appearance of muscle fibers. Part D: Which molecules form the thick filaments of sarcomeres? Part E: Which of the following interactions is the molecular basis of muscle contraction? Part F: Which step constitutes the power stroke of muscle contraction? Part G: Which of the following statements about the stimulation of muscle cells is true?

Part A: Smooth Part B: It contains branch cells Part C: False Part D: Myosin Part E: Myosin and thin filaments Part F: The phosphate ion is released, and the myosin head moves back to its original position. Part G: An action potential in a muscle cell ultimately results in the release of calcium ions into the cell.

Part A: During the course of muscle contraction the potential energy stored in ATP is transferred to potential energy stored in Part B: The release of _____ ions from the sarcoplasmic reticulum is required for skeletal muscle contraction. Part C: Myosin heads bind to _____, which they then pull and cause to slide toward the center of the sarcomere. Part D: Of these events, the first to occur when a motor neuron stops sending an impulse to a muscle is

Part A: The myosin head Part B: Calcium Part C: Thin filaments Part D: The pumping of calcium ions out of the cytoplasm and back into the sarcoplasmic reticulum

Part A: What do the orange bars on the graph represent? Part B: According to the graph, which type of muscle fiber predominates in endurance athletes (marathon runners and extreme endurance athletes)? Part C: The percentage of intermediate muscle fibers plus fast muscle fibers in world-class sprinters is approximately the same as the percentage of slow muscle fibers in Part D: When an inactive person becomes active, the percentage of slow muscle fibers ______, and the percentage of fast muscle fibers Part E: World-class sprinters are born with a higher percentage of fast and intermediate muscle fibers than other people have. Part F: To train as a middle-distance runner, an average person should increase the percentage of slow fibers at the expense of fast fibers.

Part A: The percentage of slow muscle fibers in individuals with different athletic abilities Part B: Slow Part C: A world-class marathon runner Part D: Increases; decreases Part E: Cannot be determined from the graph Part F: Supported

The leg muscles of a sprinter would differ from a marathon runner in that

Per gram, the marathon runner's muscles would contain more myoglobin than the sprinter's muscles

Identify the role(s) of ATP in muscle contraction.

Provides the energy to convert myosin to a form that forms a cross-bridge with actin Binds to myosin to break an actin-myosin cross-bridge