BIOL 1031 Mastering Biology Chapter 50
Smooth contraction of a muscle results from the simultaneous, combined pulling and sliding of millions of thick and thin filaments in thousands of muscle fibers.
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Some are bound to thin filaments while others are reaching for new binding sites.
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The ATP molecule is hydrolyzed into ADP and a phosphate group, which remain bound to the myosin head.
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A long series of closely spaced action potentials results in a sustained, maximum contraction, called tetanus.
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A nerve impulse stimulates the muscle cell. This triggers the sarcoplasmic reticulum to release stored calcium ions.
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A skeletal muscle cell is composed of a large number of contractile fibrils called myofibrils. The basic unit of contraction in a myofibril is a sarcomere. The sarcomeres are arranged end to end along the entire length of each myofibril. Each myofibril is partially surrounded by a specialized form of endoplasmic reticulum, called the sarcoplasmic reticulum.
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A tiny muscle "twitch" is caused by a single action potential.
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A typical thick filament has hundreds of heads, pulling against several surrounding thin filaments at once.
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Acetylcholine opens channels that allow sodium ions into the muscle cell, which triggers an action potential.
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Action on the molecular level-- the sliding of protein filaments-- is responsible for muscle contraction.
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An action potential causes T tubules to depolarize, which stimulates the sarcoplasmic reticulum to release calcium ions into the muscle cell.
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An action potential traveling along the motor neuron initiates an action potential in the muscle cell, which leads to muscle contraction. At the synaptic terminal of the motor neuron, an action potential causes the release of neurotransmitters, which subsequently trigger an action potential on the plasma membrane of the muscle cell. This action potential is propagated deep into the muscle cell via the T (transverse) tubules. In the interior of the cell, the action potential initiates changes in the sarcoplasmic reticulum.
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As ADP and phosphate are released, the myosin head bends. This pulls on the thin filament, causing the thin filament to slide toward the center of the sarcomere.
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As all of the thin filaments slide back to their relaxed positions, the entire muscle relaxes.
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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.
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At the intercalated disks, gap junctions provide direct electrical coupling between the cells. This allows the action potential generated by specialized cells in one part of the heart to spread to all cardiac muscle cells and causes the whole heart to contract. Read about vertebrate cardiac muscles.
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Because calcium ions are no longer available, the binding sites on actin become covered again and so the myosin head cannot bind to the actin filament.
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Calcium ions interact with other proteins on the thin filament, opening up binding sites on the actin and making it possible for myosin heads to bind to the actin.
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Calcium ions interact with proteins on the thin filament, resulting in the exposure of the myosin-binding sites on the actin.
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Contraction of a muscle cell begins with input from a motor neuron across the synaptic cleft and results in a change in the cytosolic Ca2+ level in the muscle cell. These Ca2+ changes directly control muscle contraction at the molecular level. Thus the transmission of information at the cellular level is a critical step in the overall process of muscle contraction.
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Contraction only shortens the sarcomeres; it does not change the lengths of the thick and thin filaments. What causes the thin filaments to slide across the thick filaments?
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Each myosin head is able to bind and unbind about five times per second. The heads do not work in synchrony.
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Each myosin molecule has a fibrous tail region and a globular head.
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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.
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In muscle cells, the cytosolic Ca2+ concentration is kept low by active transport of Ca2+ from the cytosol into the sarcoplasmic reticulum. When an action potential moves down the T tubules, it triggers Ca2+ channels in the sarcoplasmic reticulum to open. As a result, Ca2+ ions rush into the cytosol. Once in the cytosol, the Ca2+ ions diffuse into the myofibrils, where they enable muscle contraction to begin.
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In response to a series of closely spaced action potentials, sustained muscle contraction (tetanus) can only occur if the Ca2+ level in the sarcomere remains high throughout the contraction. If sufficient Ca2+ were taken up into the sarcoplasmic reticulum (SR) to lower the Ca2+ level in the sarcomere significantly between each action potential, tropomyosin would cover the myosin-binding sites on the actin, and the muscle would relax between each action potential.
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More ATP binds to the myosin head and the whole process repeats. The myosin head continues to move and pull on the actin filament and the muscle contracts as long as calcium ions are available to open up actin binding sites-- until the muscle fiber is fully contracted.
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Myosin heads bind to thin filaments, release their bound ADP and phosphate, and bend, sliding the thin filaments toward the center of the sarcomere.
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Only skeletal muscle contains multinucleated cells.
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Sarcomeres are the alternating light-dark units that produce the banded appearance of myofibrils, which are the strands that make up each muscle fiber.
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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.
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The Ca2+ channels in the SR conduct ions much more rapidly than the Ca2+ pumps in the SR. So even though the Ca2+ channels close between closely spaced action potentials, the Ca2+ concentration in the cytosol and the sarcomeres remains high. Consequently, Ca2+ remains bound to troponin between action potentials, the myosin-binding sites remain exposed, and muscle contraction continues until tetanus is achieved.
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The T tubules are invaginations of the muscle cell plasma membrane that extend deep into the muscle cell and are in close contact with (but not continuous with) the sarcoplasmic reticulum. The T tubules play two important roles in linking an action potential to muscle contraction. T tubules propagate the action potential from the plasma membrane into the interior of the muscle cell via voltage-gated Na+ and K+ channels. An action potential carried by a T tubule regulates the opening and closing of Ca2+ channels in the sarcoplasmic reticulum. The resulting change in cytosolic Ca2+ concentration triggers contraction of the myofibrils.
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The combined shortening of many sarcomeres in many muscle fibers results in contraction of the whole muscle.
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The head of a myosin molecule can attach to the actin of a thin filament.
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The hydrolysis of ATP causes the myosin head to change position, storing energy that will be used to contract the muscle.
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The power stroke occurs when the myosin head pivots, causing the actin filament to slide past the myosin filaments.
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The thin filaments and thick filaments in a sarcomere interact with each other when an action potential triggers the muscle to contract. The thin filament is composed of actin and regulatory proteins that respond to changes in the Ca2+ concentration in the sarcomere. The thick filament is composed of myosin proteins, whose heads bind to actin and pull the thin filament toward the center of the sarcomere.
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Thick filaments in a muscle fiber are made of many molecules of the protein myosin.
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Thin filaments are twisted chains made of the globular protein actin.
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This causes the myosin head to change position, storing energy that is later used in contraction.
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This is the first of the events that ultimately result in the relaxation of a muscle.
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Two or more closely spaced action potentials have an additive effect because the muscle does not have sufficient time to relax between action potentials.
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When a motor neuron stimulates a muscle fiber, overlapping thick and thin filaments slide along one another and sarcomeres shorten.
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When a nerve impulse arrives at a muscle fiber, calcium ions trigger muscle contraction, and ATP provides the energy.
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When an ATP molecule binds to the myosin head, it detaches from the actin filament.
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When the action potential is completed, the Ca2+ channels in the sarcoplasmic reticulum close, and Ca2+ ions are again pumped back into the sarcoplasmic reticulum. As the cytosolic level of Ca2+ drops, Ca2+ ions diffuse out of the myofibrils, stopping muscle contraction.
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When the motor neuron stops sending impulses to the muscle, calcium is pumped out of the cytoplasm and back into the sarcoplasmic reticulum.
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With the myosin head in its high-energy position, the muscle fiber is ready to contract.
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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
Which of the following statements about the stimulation of muscle cells is true?
An action potential in a muscle cell ultimately results in the release of calcium ions into the cell.
Identify the role(s) of ATP in muscle contraction.
Binds to myosin to break an actin-myosin cross-bridge Provides the energy to convert myosin to a form that forms a cross-bridge with actin
Which of the following statements correctly describe(s) the relationship between Ca2+ concentration in the cytosol and the response in the sarcomere?
Decreasing Ca2+ concentration causes dissociation of Ca2+ from troponin. Increasing Ca2+ concentration causes movement of tropomyosin, exposing myosin-binding sites on actin.
True or false? Myofibrils are the alternating light-dark units that produce the banded appearance of muscle fibers.
False
How does cardiac muscle differ from the other types of muscle?
It contains branched cells.
Which of the following interactions is the molecular basis of muscle contraction?
Myosin and thin filaments.
Which molecules form the thick filaments of sarcomeres?
Myosin.
Which muscle type is involved in the function of the digestive tract and blood vessels?
Smooth
Plasma membranes of adjacent cardiac muscle cells interlock at specialized regions called intercalated disks. What is the significance of this feature of cardiac muscles?
The intercalated disks allow coordinated contraction of the whole heart.
Which step constitutes the power stroke of muscle contraction?
The phosphate ion is released, and the myosin head moves back to its original position.
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?
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.
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.
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.
The thin filaments of sarcomeres are composed of _____.
actin
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.
Calcium ions initiate sliding of filaments in skeletal muscles by ___.
binding to the troponin complex, which then relocates tropomyosin
The release of _____ ions from the sarcoplasmic reticulum is required for skeletal muscle contraction.
calcium
An endoskeleton is the primary body support for the ___.
cartilaginous fishes, including sharks
A skeletal muscle deprived of adequate ATP supplies will ___.
enter a state where actin and myosin are unable to separate
A skeletal muscle with abnormally low levels of calcium ions would be impaired in
initiating contraction.
A single muscle cell is referred to as a _____.
muscle fiber
In a relaxed skeletal muscle, actin is not chemically bound to
myosin
The thick filaments of sarcomeres are composed of _____.
myosin
During the course of muscle contraction the potential energy stored in ATP is transferred to potential energy stored in _____.
the myosin head
Of these events, the first to occur when a motor neuron stops sending an impulse to a muscle is _____.
the pumping of calcium ions out of the cytoplasm and back into the sarcoplasmic reticulum
Myosin heads bind to _____, which they then pull and cause to slide toward the center of the sarcomere.
thin filaments
Skeletal muscle contraction begins when calcium ions bind to
troponin
The calcium ions released into the cytosol during excitation of skeletal muscle bind to ___.
troponin
The hydrostatic skeleton of the earthworm allows it to move around in its environment by
using peristaltic contractions of its circular and longitudinal muscles.
