Steps Of Muscle Contraction

Force

Muscle contracts

Muscle relaxes

RESULTS Autonomic neurotransmitters alter membrane resting potential and thereby determine the rate that smooth muscle cells fire action potentials.

Conclusion: Smooth muscle contraction is stimulated by stretch and by the parasympathetic neurotransmitter acetylcholine.

47.2 Mechanisms of Smooth Muscle Activation

Stretching depolarizes the membrane of smooth muscle cells, and this depolarization causes action potentials that activate the contractile mechanism.The neurotransmitters acetylcholine and norepinephrine also alter the membrane potential of smooth muscle, making it more or less likely to contract.

Sliding filaments cause skeletal muscle to contract

Skeletal muscle carries out, or effects, all voluntary movements, such as running or playing a piano, and generates the movements of breathing. Skeletal muscle is also called striated muscle because of its striped appearance (Figure 47.1, bottom). Skeletal muscle cells, called muscle fibers, are large. Unlike smooth muscle and cardiac muscle cells, each of which has a single nucleus, skeletal muscle fibers have many nuclei because they develop through the fusion of many individual cells. A muscle such as your biceps (which bends your arm) is composed of many muscle fibers bundled together by connective tissue.

What is the relation between a skeletal muscle fiber and the actin and myosin filaments responsible for its contraction? Each muscle fiber is packed with myofibrils—bundles of contractile filaments made up of actin and myosin (Figure 47.3). Within each myofibril are thin actin filaments and thick myosin filaments. If we cut across the myofibril at certain locations, we see only thick filaments; if we cut at other loca

Bundle of muscle fibers

✓ Connective tissue

47.3 The Structure of Skeletal Muscle A

skeletal muscle is made up of bundles of muscle fibers. Each muscle fiber is a multinucleate cell containing numerous myofibrils, which are highly ordered assemblages of thick myosin and thin actin filaments.The structure of the myofibrils gives muscle fibers their characteristic striated appearance.

tions, we see only thin filaments. But, in most regions of the myofibril, each thick myosin filament is surrounded by six thin actin filaments, and conversely, each thin actin filament sits within a triangle of three thick myosin filaments.

A longitudinal view of a myofibril re veals the reason for the striated appearance of skeletal muscle (and cardiac muscle). The myofibril consists of repeating units, called sarcomeres, which are the units of contraction. Each sarcomere is made of overlapping filaments of actin and myosin, which create a distinct band pattern. As the muscle contracts, the sarcomeres shorten, and the appearance of the band pattern changes.

The observation that the widths of the bands in the sar-comeres change when a muscle contracts led two British biologists, Hugh Huxley and Andrew Huxley, to propose a

Single myofibril

Bundle of muscle fibers

✓ Connective tissue

Myofibrils Micrograph

47.3 The Structure of Skeletal Muscle A

skeletal muscle is made up of bundles of muscle fibers. Each muscle fiber is a multinucleate cell containing numerous myofibrils, which are highly ordered assemblages of thick myosin and thin actin filaments.The structure of the myofibrils gives muscle fibers their characteristic striated appearance.

Single myofibril

Z line

Where there are only actin filaments the myofibril appears light; where there are both actin and myosin filaments the myofibril appears dark.

Z line

Where there are only actin filaments the myofibril appears light; where there are both actin and myosin filaments the myofibril appears dark.

molecular mechanism of muscle contraction. Let's look at the band pattern of a sarcomere in detail (see the micrograph in Figure 47.3). Each sarcomere is bounded by Z lines, which are structures that anchor the thin actin filaments. Centered in the sarcomere is the A band, which contains all the myosin filaments. The H zone and the I band, which appear light, are regions where actin and myosin filaments do not overlap in the relaxed muscle. The dark stripe within the H zone is called the M band; it contains proteins that help hold the myosin filaments in their regular arrangement.

The bundles of myosin filaments are held in a centered position within the sarcomere by a protein called titin. Titin is probably the longest polypeptide in the body; it runs the full length of the sarcomere from Z line to Z line, and each titin molecule runs right through a myosin bundle. Between the ends of the myosin bundles and the Z lines, titin molecules have the properties of a bungee cord—they are very stretch-able. In a relaxed skeletal muscle, resistance to stretch is mostly due to the elasticity of the titin molecules.

When the muscle contracts, the sarcomere shortens. The H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments were sliding into the region occupied by the myosin filaments. This observation led Huxley and Huxley to propose the sliding filament theory of muscle contraction: Actin and myosin filaments slide past each other as the muscle contracts.

Actin-myosin interactions cause filaments to slide

To understand what makes the filaments slide, we must examine the structures of actin and myosin (Figure 47.4 ). Each myosin molecule consists of two long polypeptide chains coiled together, each ending in a large globular head. A myosin filament is made up of many myosin molecules arranged in parallel, with their heads projecting laterally from one or the other end of the filament. An actin filament consists of a helical arrangement of two chains of actin monomers twisted together like two strands of pearls. Twisting around the actin chains is another protein, tropomyosin, and attached to it at intervals are molecules of troponin. We'll discuss the roles of these last two proteins in the following section.

The myosin heads have sites that can bind to actin and thereby form cross-bridges between the myosin and the actin filaments. The myosin heads also have ATPase activity; that is, they bind and hydrolyze ATP. The energy released when this happens changes the conformation, and therefore the orientation, of the myosin head.

Myosin filament

47.4 Actin and Myosin Filaments Overlap to Form Myofibrils Myosin filaments are bundles of molecules with globular heads and polypeptide tails. Actin filaments consist of two chains of actin monomers twisted together. They are wrapped by chains of the polypeptide tropomyosin and studded at intervals with another protein,troponin.

Together, these details explain the cycle of events that cause the actin and myosin filaments to slide past each other and shorten the sarcomere. A myosin head binds to an actin filament (see Figure 47.6). Upon binding, the head changes its orientation with respect to the myosin filament, thus exerting a force that causes the actin filament to slide about 5 to 10 nm relative to the myosin filament. Next, the myosin head binds a molecule of ATP, which causes it to release the actin. When the ATP is hydrolyzed, the energy released causes the myosin head to return to its original conformation, in which it can bind again to actin. It is as if the energy from ATP hydrolysis is being used to cock the hammer of a pistol, and contact of the myosin head with an actin binding site pulls the trigger.

We have been discussing the cycle of contraction in terms of a single myosin head. Don't forget that each myosin filament has many myosin heads at both ends and is surrounded by six actin filaments; thus the contraction of the sarcomere involves a great many cycles of interaction between actin and myosin molecules. That is why when a single myosin head breaks its contact with actin, the actin filaments do not slip backward.

An interesting aspect of this contractile mechanism is that ATP is needed to break the actin-myosin bonds, but not to form them. Thus muscles require ATP to stop contracting. This fact explains why muscles stiffen soon after animals die, a condition known as rigor mortis. Death stops the replenishment of the ATP stores of muscle cells, so the actin-myosin bonds cannot be broken, and the muscles stiffen. Eventually the proteins begin to lose their integrity, and the muscles soften. These events have regular time courses that differ somewhat for different regions of the body; therefore, an examination of the stiffness of the muscles of a corpse can help a coroner estimate the time of death.

Actin-myosin interactions are controlled by calcium ions

Muscle contractions are initiated by action potentials from motor neurons arriving at the neuromuscular junction (see

Troponin has three subunits: one binds actin, one binds tropomyosin, and one binds Ca2+.

Actin filament

Actin monomer

Tropomyosin Troponin

Troponin has three subunits: one binds actin, one binds tropomyosin, and one binds Ca2+.

Actin filament

Actin monomer

Tropomyosin Troponin

Tropomyosin Conformation

Linear polypeptide chain

Linear polypeptide chain

Figure 44.13). The axons of motor neurons are generally highly branched and can synapse with up to a hundred muscle fibers each. All the fibers activated by a single motor neuron constitute a motor unit and contract simultaneously in response to action potentials fired by that motor neuron.

Like neurons, muscle cells are excitable; that is, their plasma membranes can generate and conduct action potentials. In the case of skeletal muscle fibers (but not smooth or cardiac muscle fibers), all action potentials are initiated by motor neurons. When an action potential arrives at the neu-romuscular junction, the neurotransmitter acetylcholine is released from the motor neuron, diffuses across the synaptic cleft, binds to receptors in the postsynaptic membrane, and causes ion channels in the motor end plate to open. Most of the ions that flow through these channels are Na+, and therefore the motor end plate is depolarized. The depolarization spreads to the surrounding plasma membrane of the muscle fiber, which contains voltage-gated sodium channels. When threshold is reached, the plasma membrane fires an action potential that is conducted rapidly to all points on the surface of the muscle fiber.

An action potential in a muscle fiber also travels deep within the cell. The plasma membrane is continuous with a system of tubules that descends into and branches throughout the cytoplasm of the muscle fiber (also called the sarcoplasm) (Figure 47.5). The action potential that spreads over the plasma membrane also spreads through this system of transverse tubules, or T tubules.

The T tubules come very close to a network of intracellu-lar membranes called the sarcoplasmic reticulum. The sarcoplasmic reticulum forms a membrane-enclosed compartment that surrounds every myofibril. Calcium pumps in the sarcoplasmic reticulum cause it to take up Ca2+ ions from the sarcoplasm. Therefore, when the muscle fiber is at rest, there is a high concentration of Ca2+ in the sarcoplasmic reticulum and a low concentration of Ca2+ in the sarcoplasm.

Spanning the space between the membranes of the T tubules and the membranes of the sarcoplasmic reticulum are two proteins. One protein, which is located in the T tubule membrane, is voltage-sensitive and changes its conformation when an action potential reaches it. The other protein is lo-

47.5 T Tubules in Action An action potential at the neuromuscular junction spreads throughout the muscle fiber via a network of T tubules, triggering the release of Ca2+ from the sarcoplasmic reticulum.

cated in the sarcoplasmic reticulum membrane and is a Ca2+ channel. When it is activated by an action potential, the voltage-sensitive protein opens the Ca2+ channel, and Ca2+ ions diffuse out of the sarcoplasmic reticulum and into the sar-coplasm surrounding the actin and myosin filaments. It is these Ca2+ ions that trigger the interaction of actin and myosin and the sliding of the filaments. How do the Ca2+ ions do this?

An actin filament, as we have seen, is a helical arrangement of two strands of actin monomers. Lying in the grooves between the two actin strands is the two-stranded protein tropomyosin (see Figure 47.4). At regular intervals, the filament also includes a globular protein, troponin. The troponin molecule has three subunits: One binds actin, one binds tropomyosin, and one binds Ca2+.

When Ca2+ is sequestered in the sarcoplasmic reticulum, the tropomyosin strands block the sites on the actin filament where myosin heads can bind. When the T tubule system depolarizes, Ca2+ is released into the sarcoplasm, where it binds to troponin, changing its conformation. Because the troponin is bound to the tropomyosin, this conformational change of the troponin twists the tropomyosin enough to expose the actin-myosin binding sites. Thus the cycle of making and breaking actin-myosin bonds is initiated, the filaments are pulled past each other, and the muscle fiber contracts. When the T tubule system repolarizes, the calcium pumps remove the Ca2+ ions from the sarcoplasm, causing the tropomyosin to return to the position in which it blocks the binding of myosin heads to actin, and the muscle fiber returns to its resting condition. Figure 47.6 summarizes this cycle.

Motor neuron ffi An action potential (black arrows) arrives at the motor neuron terminal.

47.5 T Tubules in Action An action potential at the neuromuscular junction spreads throughout the muscle fiber via a network of T tubules, triggering the release of Ca2+ from the sarcoplasmic reticulum.

Motor neuron ffi An action potential (black arrows) arrives at the motor neuron terminal.

Myofibril Motor Neuron

The muscle fiber plasma membrane generates an action potential that spreads down T tubules.

.which causes the release of Ca2+ stored in the sarcoplasmic reticulum.

^Myofibril

Released Ca2+ stimulates muscle contraction.

Plasma membrane

Sarcoplasmic reticulum

The muscle fiber plasma membrane generates an action potential that spreads down T tubules.

.which causes the release of Ca2+ stored in the sarcoplasmic reticulum.

^Myofibril

Released Ca2+ stimulates muscle contraction.

Sarcoplasmic reticulum

Plasma membrane

6 ATP is hydrolyzed and the myosin head returns to its resting conformation.

6 ATP is hydrolyzed and the myosin head returns to its resting conformation.

Sarcoplasmic Reticulum Cycles
47.6 The Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction When Ca2+ binds to troponin, it exposes actin-myosin binding sites. As long as binding sites and ATP are available, the cycle of actin and myosin interactions continues, and the filaments slide past each other.

Calmodulin mediates Ca2+ control of contraction in smooth muscle

Smooth muscle cells do not have the troponin-tropomyosin mechanism for controlling contraction, but Ca2+ still plays a critical role. A Ca2+ influx into the sarcoplasm of a smooth muscle cell can be stimulated by action potentials, by hormones, or by stretching. The Ca2+ that enters the sarcoplasm combines with a protein called calmodulin. The calmod-ulin-Ca2+ complex activates an enzyme called myosin kinase, which can phosphorylate myosin heads. When the myosin heads in smooth muscle are phosphorylated, they can undergo cycles of binding and releasing actin, causing muscle contraction. As Ca2+ is removed from the sarcoplasm, it dissociates from calmodulin, and the activity of myosin kinase falls. In addition, another enzyme, myosin phosphatase, de-phosphorylates the myosin and helps stop the actin-myosin interactions.

Single skeletal muscle twitches are summed into graded contractions

In skeletal muscle, the arrival of an action potential at a neu-romuscular junction causes an action potential in a muscle fiber. The spread of that action potential through the T tubule system of the muscle fiber causes a minimum unit of contraction, called a twitch. A twitch can be measured in terms of the tension, or force, it generates (Figure 47.7a). A single action potential stimulates a single twitch, but the ultimate force generated by a muscle can vary enormously depending on how many muscle fibers are in its motor units. In muscles responsible for fine movements, such as those of the fingers, a motor neuron may innervate only one or a few muscle fibers, but in a muscle that produces large forces, such as the biceps, a motor neuron innervates a large number of muscle fibers. Still, however, at the level of the single muscle fiber, a single action potential stimulates a single twitch.

If action potentials reaching the muscle fiber are adequately separated in time, each twitch is a discrete, all-or-

Force

Stimulus

Force Stimulus

A stimulus elicits a twitch, the minimum unit of contraction of a muscle fiber.

Two twitches in quick succession have a summed effect.

A stimulus elicits a twitch, the minimum unit of contraction of a muscle fiber.

Two twitches in quick succession have a summed effect.

47.7 Twitches and Tetanus

(a) Action potentials from a motor neuron cause a muscle fiber to twitch. Twitches in quick succession can be summed. (b) Summation of many twitches can bring the muscle fiber to the maximum level of contraction, known as tetanus.

i Muscles relax when stimulation stops.

i Muscles relax when stimulation stops.

47.7 Twitches and Tetanus

(a) Action potentials from a motor neuron cause a muscle fiber to twitch. Twitches in quick succession can be summed. (b) Summation of many twitches can bring the muscle fiber to the maximum level of contraction, known as tetanus.

Images Muscle Contractions Tetanus
Eight summed twitches bring the muscle fiber to maximum contraction, known as tetanus.

Tetanus is sustained by a high rate of stimulation.

Tetanus is sustained by a high rate of stimulation.

t t t t t t t t tttttttttttttt

Time none phenomenon. If action potentials are fired more rapidly, however, new twitches are triggered before the myofibrils have had a chance to return to their resting condition. As a result, the twitches sum, and the tension generated by the fiber increases and becomes more sustained. Thus an individual muscle fiber can show a graded response to increased levels of stimulation by its motor neuron.

At high levels of stimulation, the calcium pumps in the sarcoplasmic reticulum can no longer remove Ca2+ ions from the sarcoplasm between action potentials, and the contractile machinery generates maximum tension—a condition known as tetanus (Figure 47.7b). (Do not confuse this condition with the disease tetanus, which is caused by a bacterial toxin and is characterized by spastic contractions of skeletal muscles.)

How long a muscle fiber can maintain a tetanic contraction depends on its supply of ATP. Eventually the fiber will become fatigued. It may seem paradoxical that the lack of ATP causes fatigue, since the action of ATP is to break actin-myosin bonds. But remember that the energy released from the hydrolysis of ATP "re-cocks" the myosin heads, allowing them to cycle through another power stroke. When a muscle is contracting against a load, the cycle of making and breaking actin-myosin bonds must continue to prevent the load from stretching the muscle. The situation is like rowing a boat upstream: You cannot maintain your position relative to the stream bank by just holding the oars out against the current; you have to keep rowing. Likewise, actin-myosin bonds have to keep cycling to maintain tension in the muscle.

The level of tension generated by a muscle depends on how many motor units in that muscle are activated. Whether a muscle contraction is strong or weak depends both on how many of the motor neurons that synapse with that muscle are firing and on the rate at which those neurons are firing. These two factors can be thought of as spatial summation and temporal summation, respectively.

Many muscles of the body maintain a low level of tension even when the body is at rest. For example, the muscles of the neck, trunk, and limbs that maintain our posture against the pull of gravity are always working, even when we are standing or sitting still. Muscle tone comes from the activity of a small but changing number of motor units in a muscle; at any one time, some of the muscle's fibers are contracting and others are relaxed. Muscle tone is constantly being readjusted by the nervous system.

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Responses

  • KATIE
    How does an action potential get to a t tubules?
    5 years ago
  • Adiam
    Where there are only actin filaments the myofibril appears?
    5 years ago
  • VALENTIN
    Where would you find stored Ca2?
    5 years ago
  • Wanda
    What is the difference between a myofibril and muscle fiber?
    5 years ago
  • Wanda
    When a muscle contracts which structure does not shorten?
    5 years ago
  • Gimja
    What stimulates muscle contraction?
    5 years ago
  • bowman
    How do action potentials trigger muscle contraction?
    5 years ago
  • lena
    How long does it take sarcoplasmic reticulum to take up calcium after a contraction?
    5 years ago
  • isaias
    Which filaments have two twisted chains of monomers bound by tropomyosin polypeptide chains?
    5 years ago
  • regan
    What does cell memberanes have to do with muscle contractions?
    5 years ago
  • laura
    Where does the ca2 released from the sarcoplasmic membrane bind to?
    4 years ago
  • Toby
    How a muscle contracts steps?
    4 years ago
  • tewolde
    How long does it take for the sacroplasmic reticulum to take up calcium after a contraction?
    4 years ago
  • hanna
    What causes an action potential of the muscle cell membrane?
    4 years ago
  • IRENE
    How long doe it take for the sarcoplasmic reticulmto take up calcium after a contraction?
    3 years ago
  • gina
    How does the plasma membrane help the contraction of skeletal?
    3 years ago
  • Sanna-Leen
    What causes depolarization of the tubule membrane?
    3 years ago

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