6.3 – Muscle Contraction

6.3. Describe steps associated in muscle contraction.

You are watching: Which of the following statements about excitation-contraction coupling is incorrect?

Sliding filament model of contraction

For a muscle cell to contract, the sarcomere have to shorten. However, thick and also thin filaments—the materials of sarcomeres—do not shorten. Instead, they on slide by one another, causing the sarcomere come shorten if the filaments continue to be the same length. The sliding filament theory of muscle contraction was developed to right the distinctions observed in the called bands top top the sarcomere in ~ different levels of muscle contraction and relaxation. The system of convulsion is the binding of myosin to actin, creating cross-bridges that generate filament activity (Figure 6.7).


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Figure 6.7. Once (a) a sarcomere (b) contracts, the Z lines move closer together and the ns band gets smaller. The A band continues to be the same width and, at complete contraction, the slim filaments overlap.


When a sarcomere shortens, some areas shorten whereas others stay the same length. A sarcomere is identified as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance in between the Z discs is reduced. The H zone—the central region of the A zone—contains just thick filaments and also is shortened during contraction. The i band consists of only slim filaments and likewise shortens. The A band does no shorten—it continues to be the same length—but A bands of different sarcomeres relocate closer together throughout contraction, at some point disappearing. Slim filaments room pulled by the special filaments toward the center of the sarcomere until the Z discs approach the special filaments. The zone the overlap, in which slim filaments and thick filaments occupy the same area, increases as the slim filaments relocate inward.

ATP and also muscle contraction

The movement of muscle shortening occurs as myosin heads tie to actin and pull the actin inwards. This action requires energy, i beg your pardon is detailed by ATP. Myosin binding to actin at a binding site on the globular actin protein. Myosin has another binding website for ATP at which enzymatic activity hydrolyzes ATP come ADP, publication an inorganic phosphate molecule and energy.

ATP binding reasons myosin to release actin, permitting actin and myosin come detach from each other. After this happens, the recently bound ATP is convert to ADP and inorganic phosphate, Pi. The enzyme in ~ the binding website on myosin is dubbed ATPase. The power released during ATP hydrolysis alters the edge of the myosin head into a “cocked” position. The myosin head is then in a position for additional movement, possessing potential energy, but ADP and also Pi are tho attached. If actin binding sites space covered and unavailable, the myosin will remain in the high energy configuration with ATP hydrolyzed however still attached.

If the actin binding sites space uncovered, a cross-bridge will certainly form; that is, the myosin head spans the distance between the actin and myosin molecules. Pi is then released, allowing myosin to expend the stored energy as a conformational change. The myosin head moves towards the M line, pulling the actin in addition to it. Together the actin is pulled, the filaments move about 10 nm towards the M line. This motion is referred to as the strength stroke, as it is the step at which pressure is produced. As the actin is pulled toward the M line, the sarcomere shortens and also the muscle contracts.

When the myosin head is “cocked,” it contains energy and is in a high-energy configuration. This energy is expended together the myosin head moves through the power stroke; at the finish of the power stroke, the myosin head is in a low-energy position. ~ the strength stroke, ADP is released; however, the cross-bridge formed is quiet in place, and also actin and also myosin space bound together. ATP can then connect to myosin, which enables the cross-bridge bicycle to start again and also further muscle contraction can take place (Figure 6.8).

Watch this video clip explaining how a muscle convulsion is signaled.

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Figure 6.8. The cross-bridge muscle convulsion cycle, which is prompted by Ca2+ binding to the actin active site, is shown. With each contraction cycle, actin moves relative to myosin.


Question 6.6

Which of the complying with statements around muscle convulsion is true?a. The strength stroke occurs once ATP is hydrolyzed to ADP and phosphate.b. The strength stroke occurs when ADP and phosphate dissociate native the myosin head.c. The strength stroke occurs when ADP and phosphate dissociate native the actin active site.d. The strength stroke occurs when Ca2+ binds the calcium head.

View this video animation of the cross-bridge muscle contraction.


Regulatory proteins

When a muscle is in a relaxing state, actin and also myosin space separated. To save actin from binding to the active site ~ above myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding website on actin molecules, staying clear of cross-bridge formation and preventing convulsion in a muscle there is no nervous input. Troponin binds come tropomyosin and also helps to place it on the actin molecule; it likewise binds calcium ions.

To permit a muscle contraction, tropomyosin must change conformation, uncovering the myosin-binding website on one actin molecule and enabling cross-bridge formation. This can only take place in the existence of calcium, which is maintained at extremely low concentration in the sarcoplasm. If present, calcium ions bind to troponin, causing conformational transforms in troponin that allow tropomyosin to move away from the myosin binding website on actin. As soon as the tropomyosin is removed, a cross-bridge can kind between actin and also myosin, triggering contraction. Cross-bridge cycling proceeds until Ca2+ ions and ATP room no longer easily accessible and tropomyosin again covers the binding sites on actin.

Excitation-contraction coupling

Excitation-contraction coupling is the attach (transduction) in between the activity potential created in the sarcolemma and also the begin of a muscle contraction. The create for calcium release from the sarcoplasmic reticulum into the sarcoplasm is a neural signal. Every skeletal muscle fiber is regulated by a motor neuron, which conducts signal from the mind or spinal cord to the muscle. The area that the sarcolemma top top the muscle fiber the interacts through the neuron is called the motor finish plate. The finish of the neuron’s axon is referred to as the synaptic terminal, and it does no actually contact the motor finish plate. A little space dubbed the synaptic slot separates the synaptic terminal indigenous the motor finish plate. Electric signals travel along the neuron’s axon, i beg your pardon branches v the muscle and connects to separation, personal, instance muscle fibers at a neuromuscular junction.

The capacity of cell to communicate electrically requires that the cell expend energy to develop an electrical gradient throughout their cabinet membranes. This fee gradient is carried by ions, which space differentially distributed throughout the membrane. Each ion exerts an electrical influence and also a concentration influence. Simply as milk will eventually mix with coffee there is no the need to stir, ions additionally distribute themselves evenly, if castle are allowed to perform so. In this case, they room not allowed to go back to an evenly combined state.

The sodium-potassium ATPase uses cellular energy to move K+ ions inside the cell and also Na+ ions outside. This alone accumulation a small electrical charge, however a large concentration gradient. Over there is numerous K+ in the cell and lots of Na+ outside the cell. Potassium is maybe to leave the cell v K+ channels that are open 90% of the time, and also it does. However, Na+ channels are rarely open, so Na+remains external the cell. When K+ leaves the cell, obeying that is concentration gradient, that properly leaves a an unfavorable charge behind. So at rest, over there is a large concentration gradient because that Na+ to enter the cell, and also there is an accumulation of an unfavorable charges left behind in the cell. This is the relaxing membrane potential. The potential in this context way a separation of electric charge that is qualified of law work. The is measured in volts, as with a battery. However, the transmembrane potential is considerably smaller (0.07 V); therefore, the little value is expressed together millivolts (mV) or 70 mV. Due to the fact that the inside of a cell is an adverse compared through the outside, a minus sign signifies the excess of negative charges inside the cell, −70 mV.

If an event changes the permeability the the membrane come Na+ ions, they will get in the cell. The will readjust the voltage. This is an electrical event, called an activity potential, that have the right to be provided as a moving signal. Communication occurs in between nerves and also muscles with neurotransmitters. Neuron action potentials reason the relax of neurotransmitters from the synaptic terminal into the synaptic cleft, where they deserve to then diffuse across the synaptic cleft and also bind to a receptor molecule on the motor end plate. The motor finish plate possesses junctional folds—folds in the sarcolemma that produce a large surface area because that the neurotransmitter to tie to receptors. The receptors are actually sodium networks that open up to permit the passage of Na+ into the cell once they receive neurotransmitter signal.

Acetylcholine (ACh) is a neurotransmitter exit by motor neurons that binding to receptor in the motor end plate. Neurotransmitter relax occurs once an action potential travels down the engine neuron’s axon, resulting in changed permeability that the synaptic terminal membrane and an flow of calcium. The Ca2+ ions permit synaptic motor to relocate to and also bind through the presynaptic membrane (on the neuron) and also release neurotransmitter native the vesicles into the synaptic cleft. When released by the synaptic terminal, ACh diffuses across the synaptic cleft come the motor end plate, whereby it binds v ACh receptors. As a neurotransmitter binds, this ion channels open, and Na+ ions overcome the membrane right into the muscle cell. This to reduce the voltage difference between the inside and also outside that the cell, which is called depolarization. As ACh binds at the motor end plate, this depolarization is called an end-plate potential. The depolarization climate spreads follow me the sarcolemma, developing an activity potential as salt channels surrounding to the initial depolarization website detect the readjust in voltage and open. The action potential moves across the entire cell, producing a wave of depolarization.

ACh is broken down through the enzyme acetylcholinesterase (AChE) right into acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh so the it go not remain bound to ACh receptors, i m sorry would cause unwanted expanded muscle contraction (Figure 6.9).


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Figure 6.9. This diagram shows excitation-contraction coupling in a skeletal muscle contraction. The sarcoplasmic illusion is a committed endoplasmic reticulum uncovered in muscle cells.


Question 6.7

The fatal nerve gas Sarin irreversibly inhibits acetylcholinesterase. What effect would Sarin have actually on muscle contraction?


After depolarization, the membrane return to its relaxing state. This is dubbed repolarization, during which voltage-gated sodium networks close. Potassium networks continue at 90% conductance. Since the plasma membrane sodium–potassium ATPase always transports ions, the resting state (negatively charged inside relative to the outside) is restored. The period immediately adhering to the transmission of an advertise in a nerve or muscle, in i beg your pardon a neuron or muscle cabinet regains its capability to transmit one more impulse, is referred to as the refractory period. Throughout the refractory period, the membrane cannot generate another action potential. The refractory period allows the voltage-sensitive ion networks to return to their relaxing configurations. The sodium-potassium ATPase continually moves Na+ back out of the cell and K+ back right into the cell, and also the K+ leaks out leaving an adverse charge behind. An extremely quickly, the membrane repolarizes, so that it deserve to again it is in depolarized.

Control of muscle tension

Neural regulate initiates the formation of actin-myosin cross-bridges, resulting in the sarcomere shortening involved in muscle contraction. This contractions extend from the muscle fiber through connective organization to traction on bones, causing skeletal movement. The traction exerted by a muscle is called tension, and also the quantity of force produced by this tension have the right to vary. This permits the very same muscles to move really light objects and really heavy objects. In separation, personal, instance muscle fibers, the amount of tension developed depends ~ above the cross-sectional area of the muscle fiber and also the frequency that neural stimulation.

The number of cross-bridges formed in between actin and also myosin identify the quantity of stress that a muscle fiber can produce. Cross-bridges can only type where thick and also thin filaments overlap, enabling myosin to tie to actin. If much more cross-bridges room formed, an ext myosin will pull on actin, and more tension will be produced.

The ideal size of a sarcomere during the production of maximal tension occurs when thick and thin filaments overlap to the biggest degree. If a sarcomere at rest is extended past perfect resting length, thick and thin filaments execute not overlap to the best degree, and also fewer cross-bridges have the right to form. This outcomes in under myosin heads pulling top top actin, and less tension is produced. Together a sarcomere is shortened, the zone the overlap is diminished as the slim filaments with the H zone, i beg your pardon is written of myosin tails. Due to the fact that it is myosin top that type cross-bridges, actin will not tie to myosin in this zone, to reduce the tension produced by this myofiber. If the sarcomere is shortened, even more, slim filaments begin to overlap v each other—reducing cross-bridge formation even further and producing also less tension. Whereas if the sarcomere is stretched to the point at i beg your pardon thick and thin filaments carry out not overlap at all, no cross-bridges space formed and also no tension is produced. This lot of extending does not typically occur since accessory proteins, interior sensory nerves, and also connective tissue oppose extreme stretching.

The major variable determining force production is the number of myofibers within the muscle that obtain an action potential native the neuron the controls the fiber. When using the biceps to choose up a pencil, the motor cortex that the mind only signals a few neurons the the biceps, and also only a couple of myofibers respond. In vertebrates, every myofiber responds totally if stimulated. As soon as picking increase a piano, the engine cortex signals every one of the neurons in the biceps and also every myofiber participates. This is close to the maximum pressure the muscle deserve to produce. As discussed above, enhancing the frequency of action potentials (the variety of signals per second) deserve to increase the force a bit more, due to the fact that the tropomyosin is flooded through calcium.

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Question 6.8

Teach her peer about the events during muscle contraction, from the come of the neural signal come generation of activity powered through the muscle. When you room done, ask her peer what state or measures you to let go or go not describe well. Let your peer fill the gaps. If there were no gaps, her peer can difficulty you v some questions about your explanation. Remember that one way that you can test whether you are finding out is to be able to transmit your expertise to another person.