Name: calcium control in muscle contraction
Uploaded: 2025-10-23T11:47:01.000Z
Duration: 1 min 26 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Life Science

SC.912.L.14

SC.912.L.14.17

Cross-bridge Cycle

- Un muscle se contractequand le chevauchement des filaments fins et épais augmente,faisant décroître la longueur du sarcomère. 
Au niveau moléculaire,la contraction a lieu quand l'ATP,lié à la zone de la tête de la myosine, globulaire,est hydrolysé en ADP, convertissant la tête de la myosineen un état de haute énergielors duquel elle se lie à l'actineet crée un pont croisé. 
La libération de l'ADP fait retourner la tête de la myosineà un état de faible énergie,faisant avancer l'actine vers le centre du sarcomère. 
La liaison d'une nouvelle molécule d'ATPà la tête de la myosinela dissocie ensuite de l'actine. 
Quand la tête de la myosine se lie à nouveau à l'actine,c'est sur la portion plus proche de la ligne Z. 
Ce processus de liaison est contrôlépar deux protéines régulatrices,la tropomyosine et la troponine,et par la concentration en calcium,qui est stocké et libéré depuis le réticulum sarcoplasmique. 
La tropomyosine couvre, sur l'actine,le site de liaison à la myosine,et la troponine se lie au calcium lorsqu'il est disponible,éloignant la tropomyosinedu site de liaison à la myosine de l'actine. 
Dans cette conformation, un pont croisé peut se formeret le muscle se contracte. 
Ce cycle continue jusqu'à ce que le calcium et l'ATPne soient plus présents dans la fibre musculaire.

Guellich, Aziz, Elisa Negroni, Valérie Decostre, Alexandre Demoule et Catherine Coirault. « Propriétés altérées des ponts croisés dans les dystrophies musculaires squelettiques. » Frontiers in Physiology 5 (14 octobre 2014). [<a href="https://doi.org/10.3389/fphys.2014.00393 » target="__blank">Source</a>]

Debold, Edward P. « Aperçus récents de la fatigue musculaire au niveau du pont croisé. » Frontiers in Physiology 3 (1er juin 2012). [<a href="https://doi.org/10.3389/fphys.2012.00151 » target="__blank">Source</a>]

Rall, Jack A. "Qu’est-ce qui rend le muscle squelettique strié ? Découvertes dans le cytosquelette endosarcomérique et exosarcomérique. Avancées dans l’enseignement de la physiologie 42, n° 4 (15 novembre 2018) : 672-84. [<a href="https://doi.org/10.1152/advan.00152.2018 » target="__blank">Source</a>]

ponts actine-myosine

À mesure que le muscle se contracte, le chevauchement entre les filaments minces et épais augmente, diminuant la longueur du sarcomère — l’unité contractile du muscle — en utilisant de l’énergie sous forme d’ATP. Au niveau moléculaire, il s’agit d’un processus cyclique en plusieurs étapes qui implique la liaison et l’hydrolyse de l’ATP, et le mouvement de l’actine par la myosine.
 
 Lorsque l’ATP, qui est fixé à la tête de myosine, est hydrolysé en ADP, la myosine passe dans un état lié à l'actine de haute énergie, créant un pont transversal. Lorsque l’ADP est libéré, la tête de myosine se déplace vers un état de faible énergie, déplaçant l’actine vers le centre du sarcomère. La liaison d’une nouvelle molécule d’ATP dissocie la myosine de l’actine. Lorsque cet ATP est hydrolysé, la tête de myosine se lie à l’actine, cette fois sur une partie de l’actine plus près de la fin du sarcomère. Les protéines régulatrices troponine et tropomyosine, ainsi que le calcium, agissent ensemble pour contrôler l’interaction myosine-actine. Lorsque la troponine se lie au calcium, la tropomyosine est déplacée loin du site de liaison à la la myosine sur l’actine, permettant à la myosine et à l’actine d’interagir et que la contraction musculaire ait lieu.
 
 <h4>Le calcium</h4>
 
 En tant que régulateur de la contraction musculaire, la concentration de calcium est très étroitement contrôlée dans les fibres musculaires. Les fibres musculaires sont en contact étroit avec les neurones moteurs. Les potentiels d’action dans les neurones moteurs provoquent la libération du neurotransmetteur acétylcholine à proximité des fibres musculaires. Cela génère un potentiel d’action (dépolarisation) dans la cellule musculaire, qui est transporté le long de la membrane plasmique et à travers les invaginations de la membrane plasmique appelées tubules transverses, ou tubules en T.
 
 Les tubules en T s’enfoncent profondément dans le muscle et sont adjacents aux organites spécialisés du réticulum endoplasmique appelés réticulum sarcoplasmique, ou RS. Le calcium séquestré à l’intérieur du RS est libéré lorsque les canaux ioniques tensiodépendants (les canaux ioniques qui s’ouvrent et se ferment sur la base des charges locales) s’ouvrent en réponse à la dépolarisation, ce qui permet aux ions calcium d’entrer dans le cytoplasme et aux muscles de se contracter.
 
 Lorsque la signalisation des neurones moteurs s’arrête, la relaxation du muscle commence lorsque le calcium est pompé à nouveau dans le RS, diminuant les niveaux cytoplasmiques de calcium et réapprovisionnant les réserves de calcium du RS en vue de la prochaine contraction.
 
 <h4>La dégénérescence musculaire</h4>
 
 Un muscle sain peut se contracter, mais un muscle malade perd souvent cette faculté. Des maladies comme la myasthénie empêchent la stimulation du muscle par des neurones moteurs, ce qui entraîne une atrophie musculaire et une diminution de la masse musculaire. La sclérose latérale amyotrophique (SLA ou maladie de Lou Gehrig) provoque la dégénérescence des neurones moteurs, ce qui entraîne de même une dégénérescence musculaire et une atrophie.

Ponts actine-myosine

Au fur et à mesure que le muscle se contracte, le chevauchement entre les filaments fins et épais augmente, diminuant la longueur du sarcomère - l’unité contractile du ...

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calcium control in muscle contraction

Calcium control in muscle contraction depends on ATP, motor neurons, and the sliding action of actin and myosin. As a muscle contracts, the thin and thick filaments overlap more, and the sarcomere, which is the contractile unit of the muscle, becomes shorter. This process uses energy from ATP and happens in a repeating cycle at the molecular level.

Myosin drives the movement of actin through a cross-bridge cycle. When ATP attached to the myosin head is hydrolyzed to ADP, myosin shifts into a high-energy state and binds actin. This forms a cross-bridge. When ADP is released, the myosin head moves to a lower-energy state and pulls actin toward the center of the sarcomere. A new ATP molecule then binds to myosin and causes it to detach from actin. After that ATP is hydrolyzed, myosin can bind again, this time to a new site on actin closer to the end of the sarcomere.

Troponin, tropomyosin, and calcium regulate this interaction. When troponin binds calcium, tropomyosin moves away from the myosin-binding site on actin. That shift lets myosin and actin interact, and muscle contraction can occur. Calcium concentration in muscle fibers is tightly controlled so this switch can happen at the right time.

Motor neurons start the process by signaling nearby muscle fibers. Their action potentials trigger release of the neurotransmitter acetylcholine near the muscle cell. This produces an action potential, or depolarization, in the muscle cell membrane. The signal travels along the plasma membrane and through T-tubules, which are inward folds of the membrane that carry the signal deep into the muscle. T-tubules sit next to the sarcoplasmic reticulum, or SR, a specialized endoplasmic reticulum that stores calcium.

When depolarization reaches the SR, voltage-gated ion channels open and release calcium into the cytoplasm. The rise in calcium allows the muscle to contract. When signaling from the motor neuron stops, relaxation begins. Calcium is pumped back into the SR, which lowers calcium in the cytoplasm and restores the SR calcium supply for the next contraction.

Healthy muscle can contract, but diseased muscle may lose that ability. Myasthenia gravis can prevent motor neuron stimulation of muscle, which can lead to muscle atrophy and a decrease in muscle mass. Amyotrophic lateral sclerosis, or ALS, also called Lou Gehrig's disease, causes motor neurons to degenerate. That loss of motor neurons can likewise lead to muscle degeneration and atrophy.

Calcium Control in Muscle Contraction

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cross-bridge cycle

Calcium control in muscle contraction depends on ATP, motor neurons, and the sliding action of actin and myosin. As a muscle contracts, the thin and thick filaments overlap more, and the sarcomere, which is the contractile unit of the muscle, becomes shorter. This process uses energy from ATP and happens in a repeating cycle at the molecular level. Myosin drives the movement of actin through a cross-bridge cycle. When ATP attached to the myosin head is hydrolyzed to ADP, myosin shifts into a...

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key terms and concepts

Learn how calcium, troponin, tropomyosin, and ATP control muscle contraction, from motor neuron signaling to relaxation and muscle degeneration.

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Calcium Control in Muscle Contraction

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