15.4: Muscle Contraction (2023)

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    Arm wrestling

    A sport like arm wrestling relies on muscle contractions. Arm wrestlers must contract the muscles in their hands and arms andto maintainthey contracted to resist the opposing force of their opponent. The fighter whose muscles can contract with the greatest force wins the fight.

    15.4: Muscle Contraction (2)

    Muscular contraction

    How does a skeletal muscle contraction begin?

    Excluding reflexes, all skeletal muscle contractions occur as a result of conscious effort originating in the brain. The brain sends electrochemical signals through the somatic nervous system to motor neurons that innervate muscle fibers (to review how the brain and neurons work, see chapterNervous system). A single motor neuron with multiple axon terminals can innervate multiple muscle fibers, causing them to contract at the same time. The connection between a motor neuron axon terminal and a muscle fiber occurs at a neuromuscular junction site.This is a chemical synapse where a motor neuron transmits a signal to the muscle fiber to initiate a muscle contraction.

    The process by which a signal is transmitted at a neuromuscular junction is illustrated in Figure \(\PageIndex{2}\). The sequence of events begins when an action potential is initiated in the cell body of a motor neuron and the action potential is propagated along the neuron's axon to the neuromuscular junction. Once the action potential reaches the end of the axon terminal, it causes the neurotransmitter acetylcholine (ACh) to vesicle out of synaptic vesicles at the axon terminal. ACh molecules diffuse across the synaptic cleft and bind to muscle fiber receptors, thereby initiating a muscle contraction. Muscle contraction is initiated with the depolarization of the sarcolemma caused by the entry of sodium ions through sodium channels associated with ACh receptors.

    15.4: Muscle Contraction (3)

    Things happen very quickly in the world of excitable membranes (think how quickly you can snap your fingers once you decide to do so). Immediately after the membrane depolarizes, it repolarizes, restoring the negative membrane potential. Meanwhile, ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE). ACh cannot rebind to a receptor and reopen its channel, which would cause excitation and unwanted extended muscle contraction.

    Propagation of an action potential along the sarcolemma enters theTubules T. For the action potential to reach the Sarcoplasmic Reticulum (SR) membrane, periodic invaginations occur in the sarcolemma, calledTubules T(“T” stands for “cross”). The arrangement of a T tubule with the RS membranes on either side is called atriad(Figure \(\PageIndex{3}\)). The triad surrounds the cylindrical structure calledmyofibrils, which contains actin and myosin. T tubules carry the action potential into the cell, which triggers the opening of calcium channels in the adjacent SR membrane, causing \(\text{Ca}^{++}\) to diffuse out of the SR and in the sarcoplasm. It is the arrival of \(\text{Ca}^{++}\) into the sarcoplasm that initiates contraction of the muscle fiber by its contractile units, or sarcomeres.

    (Video) Chap 15 (Part 2) Striated Muscle Contraction | Cambridge A-Level 9700 Biology

    15.4: Muscle Contraction (4)

    alternation excitation-contraction

    although the termalternation excitation-contractionconfuses or frightens some students, it boils down to this: In order for a skeletal muscle fiber to contract, its membrane must first be "excited"—in other words, it must be stimulated to fire an action potential. The action potential of the muscle fiber, which travels through the sarcolemma like a wave, is “coupled” to the actual contraction through the release of calcium ions (\(\text{Ca}^{++}\)) from the SR. Once released, \(\text{Ca}^{++}\) interacts with the protective proteins, troponin and tropomyosin complex, forcing them apart so that actin binding sites are available for attachment by myosin heads. Myosin then pulls the actin filaments toward the center, shortening the muscle fiber.

    15.4: Muscle Contraction (5)

    In skeletal muscle, this sequence begins with signals from the somatic motor division of the nervous system. In other words, the “arousal” step in skeletal muscles is always triggered by signaling from the nervous system.

    Sliding Filament Theory of Muscle Contraction

    Once the muscle fiber is stimulated by the motor neuron, the actin and myosin protein filaments within the skeletal muscle fiber slide over each other to produce a contraction. Osliding filament theoryis the most widely accepted explanation of how this occurs. According to this theory, muscle contraction is a cycle of molecular events in which thick myosin filaments repeatedly bind and pull on thin actin filaments so that they slide past one another. Actin filaments are attached to Z disks, each of which marks the end of a sarcomere. Sliding of the filaments brings the Z disks closer to a sarcomere, thus shortening the sarcomere. When this occurs, the muscle contracts.

    (Video) A2 Biology - Structure of the sliding filament model (OCR A Chapter 13.9-10)

    15.4: Muscle Contraction (6)

    Crossbridge Cycling

    crossbridge cyclingit is a sequence of molecular events underlying the theory of sliding filaments. There are many projections of the myosin thick filaments, each of which consists of two myosin heads (you can see the projections and heads in Figures \(\PageIndex{5}\) and \(\PageIndex{3}\)). Each myosin head has binding sites for ATP (or ATP hydrolysis products: ADP and Peu) and actin. Thin actin filaments also have binding sites for myosin heads—a cross-bridge forms when a myosin head attaches to an actin filament.

    The cross-bridge cycling process is shown in Figure \(\PageIndex{6}\). A cross-bridge cycle begins when the myosin head attaches to an actin filament. ADP and Peuare also attached to the myosin head at this stage. Then a force stroke moves the actin filament inward toward the center of the sarcomere, thus shortening the sarcomere. At the end of the power stroke, ADP and Peuare released from the myosin head, leaving the myosin head attached to the thin filament until another ATP binds to the myosin head. When ATP binds to the myosin head, it causes the myosin head to detach from the actin filament. ATP is again split into ADP and Peuand the released energy is used to move the myosin head into an "armed" position. Once in this position, the myosin head can reattach to the actin filament and another cross-bridge cycle begins.

    15.4: Muscle Contraction (7)
    Feature: Human Biology in the Journal

    Interesting and hopeful background research on muscle contraction is often in the news because muscle contractions are involved in many different bodily processes and disorders, including heart failure and stroke.

    • Cardiac insufficiencyIt is a chronic condition in which the heart muscle cells cannot contract strongly enough to keep the body's cells adequately supplied with oxygen. In 2016, researchers at the University of Texas Southwestern Medical Center identified a potential new target for drug development to increase the strength of heart muscle contractions in patients with heart failure. The UT researchers found a previously unidentified protein involved in muscle contraction. Minimal protein turns off the heart's "brake", causing it to pump blood more vigorously. At the molecular level, the protein affects the calcium ion pump that controls muscle contraction. This result will likely lead to searches for such additional proteins.
    • AAVCIt occurs when a blood clot lodges in an artery in the brain and cuts off blood flow to a part of the brain. Clot damage would be reduced if the smooth muscles lining the cerebral arteries relaxed after a stroke because the arteries would dilate and allow more blood flow to the brain. In a recent study conducted at the Yale University School of Medicine, researchers determined that the muscles that line the blood vessels in the brain actually constrict after a stroke. This constricts blood vessels, reduces blood flow to the brain, and appears to contribute to permanent brain damage. The hopeful conclusion of this finding is that it suggests a new target for stroke therapy.
    (Video) Muscle contraction
    (Video) Chapter 15.5: Muscle Contractions


    1. What is skeletal muscle contraction?
    2. Distinguish between isometric and isotonic contractions of skeletal muscle.
    3. How does a motor neuron stimulate skeletal muscle to contract?
    4. What is the sliding filament theory?
    5. Describe cross-bridge cycling.
    6. Where does the ATP needed for muscle contraction come from?
    7. Explain why an action potential in a single motor neuron can cause multiple muscle fibers to contract.
    8. The name of the synapse between a motor neuron and a muscle fiber is ____________.
    9. If a drug blocks acetylcholine receptors on muscle fibers, what do you think it would do to muscle contraction? Explain your answer.
    10. True or false:According to the sliding filament theory, actin filaments actively bind and pull on myosin filaments.
    11. True or false:When a motor neuron produces an action potential, the sarcomeres in the muscle fiber it innervates become shorter as a result.
    12. Explain how cross-bridge cycling and the theory of sliding filaments are related to each other.
    13. When does anaerobic respiration typically occur in human muscle cells?
    14. If there were no ATP available in a muscle, how would that affect the cross-bridge cycle? What would that do to muscle contraction?

    Explore more


    (Video) Movie 15.4 Protein Translocation


    1. Arm wrestlingby US Navy photo by Lieutenant Colonel. Kenneth Honek, public domain: Wikimedia Commons
    2. Motor plate and innervationbyOpenStaxName,CC POR 4.0via Wikimedia Commons
    3. Skeletal muscleby the Blausen.com team (2014). "Blausen Medical Medical Gallery 2014". WikiJournal of Medicine 1 (2).DOI:10.15347/wjm/2014.010.ISSN 2002-4436. licensedCC POR 3.0via Wikimedia Commons
    4. Actin-tropomyosin-troponby Daniel Walsh and Alan Sved,CC POR 4.0via Wikimedia Commons
    5. sliding filament modelbyOpenStaxName,CC POR 4.0via Wikimedia Commons
    6. crossbridge cyclingbyOpenStaxName,CC POR 4.0via Wikimedia Commons
    7. Text adapted fromhuman biologybyCK-12licensedCC BY-NC 3.0


    What is muscle contraction A level? ›

    Muscle contraction involves two protein fibres - myosin and actin. During muscle contraction, these slide over each other in a process which requires the ATP produced in respiration. The more we exercise or move about, the more glucose is converted into ATP during aerobic respiration .

    How many percent is needed for a maximal force contraction of muscle? ›

    The Ideal Length of a Sarcomere: Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80 percent to 120 percent, approximately 1.6 to 2.6 micrometers.

    What results in a strong muscle contraction? ›

    ‌Your muscles contain fibers called myosin. Depending on how you need to use your muscles, the myosin fibers either tighten up and shorten or loosen up and stretch out. Myosin is also responsible for muscle contractions like your heartbeat that happens at regular intervals.

    What do muscle contractions require large amounts of? ›

    Muscle contractions consume energy, which is provided by carbohydrates, lipids, and rarely proteins. High intensity training demands large amounts of blood sugar and glycogen from muscles to produce ATP, whereas low intensity training consumes free fatty acids.

    What are low level muscle contractions? ›

    This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon—the force generated by a 95% contraction of all fibers is sufficient to damage the body.

    Why is muscle contraction graded? ›

    In other words, muscles contractions are graded (unlike the action potentials which regulate them, which are all-or-nothing events). The grading of muscle contractions enables the same muscles to move very light objects and very heavy objects.

    What percent do muscles contract? ›

    Because a muscle is attached to bones, muscle contraction is restricted to lengths that are between 60 percent and 175 percent of the length that produces optimal strength.

    What is maximum contraction? ›

    Maximal voluntary contraction (MVC) is the maximal force-generating capacity of a muscle or group of muscles in humans.

    How is the strength of a muscle contraction measured? ›

    Measurement of muscle contraction is mainly achieved through electromyography (EMG) and is an area of interest for many biomedical applications, including prosthesis control and human machine interface.

    What is strength of contraction? ›

    The Definition of Strength Speed Contractions

    Strength Speed is the ability of the neuromuscular system to produce the greatest possible impulse in the shortest possible time.

    What are the 5 things that affect muscle contraction? ›

    • Depolarisation and Calcium Ion Release.
    • Actin and Myosin Cross-Bridge Formation.
    • Sliding Mechanism of Actin and Myosin.
    • Sarcomere Shortening.

    What are the 4 muscle contractions? ›

    Concentric, Isometric, and Eccentric Contractions Serve Different Functions
    Types of ContractionsDistance ChangeFunction
    ConcentricShortening (+D)Acceleration
    IsometricNo change (0 D)Fixation
    EccentricLengthening (−D)Deceleration

    What are the 3 requirements for muscle contraction? ›

    In order for a skeletal muscle contraction to occur;
    • There must be a neural stimulus.
    • There must be calcium in the muscle cells.
    • ATP must be available for energy.

    What is the most common muscle contraction? ›

    A concentric contraction is a type of muscle activation that causes tension on your muscle as it shortens. As your muscle shortens, it generates enough force to move an object. This is the most popular type of muscle contraction. In weight training, a bicep curl is an easy-to-recognize concentric movement.

    When should I be worried about muscle contraction? ›

    Muscle spasms, twitches, and cramps are not usually a cause for concern. They are perfectly normal, particularly in athletes and other people who exercise regularly. In some cases, however, they can indicate an underlying health condition, such as multiple sclerosis, thyroid disease, or cirrhosis of the liver.

    What is abnormal contractions of muscles? ›

    Dystonia is a movement disorder that causes the muscles to contract involuntarily. This can cause repetitive or twisting movements. The condition can affect one part of your body (focal dystonia), two or more adjacent parts (segmental dystonia), or all parts of your body (general dystonia).

    What are abnormal muscle contractions known as? ›

    Dystonia is a neurological movement disorder that results in unwanted muscle contractions or spasms. The involuntary twisting, repetitive motions, or abnormal postures associated with dystonia can affect anyone at any age.

    What does 4 +/ 5 muscle strength mean? ›

    For example, a grade of 4+/5 indicates that a muscle yielded to maximum resistance but was able to provide some resistance during the test. A 4-/5 grade means that a muscle was not on the verge of collapse during testing.

    What is a 3+ muscle strength? ›

    3+ Ability to move through full range of motion against gravity and to resist against minimal pressure through partial range of motion, then contraction breaks abruptly. 3+ The muscle is capable of transient resistance but collapses abruptly.

    What is the level of muscle strength? ›

    Muscle strength is scored on a scale of 0 to 5. 0 – No contraction; the patient is unable to even contract the muscle. This is scored as a zero. 1 – No movement but slight visible/palpable muscle contraction is present; contraction without movement is scored as grade 1 strength.

    Is it good for muscles to contract? ›

    Muscle contraction allows athletes to apply force and tension during a workout. There are different types of muscle contractions that all help you build strength and mass. While it might not be something we often think about, muscle contraction is used in all kinds of movements and especially in functional fitness.

    What are fatigue muscles? ›

    Muscle fatigue is defined as a decrease in maximal force or power production in response to contractile activity. 5. It can originate at different levels of the motor pathway and is usually divided into central and peripheral components.

    Do we only use 60% of your muscles? ›

    Estimates vary, but researchers have pegged the amount of muscle mass recruited during maximal exercise at around 60%; even elite athletes who have trained to get more output from their musculature might only harness around 80% of their theoretical strength.

    How many contractions is too many? ›

    For most international obstetrical societies, 5 contractions per 10 min averaged over 30 min is considered as the upper limit of normal uterine activity. We hypothesize that it might be safer to adopt an upper limit of 4 contractions per 10 min.

    What is the number range for contractions? ›

    In the first stage of labor uterine contraction (UC) gradually increases from 25 mmHg to about 50 mmHg. Frequency of contraction is 3–5 per 10 min and the basal tone is 8–12 mmHg. During the second stage there further increase in the UC and it typically reaches 80–100 mmHg.

    What is the range of contraction? ›

    Contractions in active labor generally last between 45 to 60 seconds, with three to five minutes of rest in between. In transition, when the cervix dilates from 7 to 10 centimeters, the pattern changes to where contractions last 60 to 90 seconds, with just 30 seconds to 2 minutes of rest between.

    What strength is measured during a muscle strength test? ›

    Muscle strength can be measured by estimating a person's one repetition maximum (1RM) - a measurement of the greatest load (in kg) that can be fully moved (lifted, pushed, or pulled) once without failure or injury.

    How are contractions measured? ›

    Duration is timed from when you first feel a contraction until it is over. This time is usually measured in seconds. Frequency is timed from the start of one contraction to the start of the next. It includes the contraction as well as the rest period until the next contraction begins.

    What is muscle contraction in biology? ›

    According to this theory, muscle contraction is a cycle of molecular events in which thick myosin filaments repeatedly attach to and pull on thin actin filaments, so they slide over one another. The actin filaments are attached to Z discs, each of which marks the end of a sarcomere.

    What is muscle contraction in science? ›

    muscle contraction. A process leading to shortening and/or development of tension in muscle tissue.

    What happens during a muscle contraction quizlet? ›

    When a muscle contracts, it attempts to shorten toward its center, thereby creating a pulling force upon its bony attachments. If this pulling force is sufficiently strong, one or both of the bones to which the muscle is attached will be pulled toward the center of the muscle.

    How do muscles repair themselves? ›

    Three main phases have been identified in the process of muscle regeneration; a destruction phase with the initial inflammatory response, a regeneration phase with activation and proliferation of satellite cells and a remodeling phase with maturation of the regenerated myofibers.

    What are the three types of muscle contractions? ›

    2.1. 1 Types of Contractions. There are three types of muscle contraction: concentric, isometric, and eccentric.

    What is an example of muscle contraction? ›

    For example, when carrying an object in front of you. The weight of the object pulls your arms down, but your muscles contract to hold the object at the same level. Another example is gripping a tennis racket. There is no movement in the joints of the hand, but the muscles are contracting.

    What causes muscle fatigue? ›

    Improper exercise, long time combat, military training and some related diseases (for example, cancer and stroke) can cause muscle fatigue, which negatively affects athletic achievement, military combat ability and patient recovery.

    How fast can a muscle contract? ›

    The speed record holders among vertebrates are the so-called superfast muscles, which can move up to 250 times per second.

    What triggers a muscle spasm? ›

    What causes muscle spasms? Muscle spasms can occur due to several causes, including a lack of nutrients, muscular tension, overuse of the muscle, increased demand of blood flow, or various underlying medical conditions.

    Why would calcium deficiency limit muscle contractions? ›

    Calcium triggers contraction by reaction with regulatory proteins that in the absence of calcium prevent interaction of actin and myosin.

    What is muscle tone? ›

    Muscle tone is the amount of tension (or resistance to movement) in muscles. Our muscle tone helps us to hold our bodies upright when we are sitting and standing.

    How does a muscle contract step by step? ›

    Skeletal Muscle Contraction. (a) The active site on actin is exposed as calcium binds to troponin. (b) The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the previous contraction cycle is released.

    What muscle that can't consciously be controlled? ›

    Smooth muscle cannot be controlled consciously and thus acts involuntarily. The non-striated (smooth) muscle cell is spindle-shaped and has one central nucleus. Smooth muscle contracts slowly and rhythmically.

    What changes during muscle contraction? ›

    During muscle contraction, chemical energy is converted to mechanical energy when ATP is hydrolysed during cross-bridge cycling. This mechanical energy is then distributed and stored in the tissue as the muscle deforms or is used to perform external work.


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