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

- [Instructeur] Een spier trekt samen wanneer de overlap van de dunne en dikke draden toeneemt, waardoor de sarcomeerlengte afneemt. Op moleculair niveau treedt samentrekking op wanneer ATP gebonden aan het bolvormige hoofdgebied van myosine wordt gehydrolyseerd tot ADP, waardoor de myosinekop wordt omgezet in een hoge-energietoestand waarin het zich bindt tot actine en een overbrug creëert. De vrijlating van ADP zorgt ervoor dat het myosinekopje terugkeert naar een energiezuinige toestand, waarbij actine naar het centrum van de sarcomeer komt.

De binding van een nieuwe ATP-molecule aan de myosinekop zorgt ervoor dat deze zich losmaakt van actine. De volgende keer dat deze myosinekop zich bindt aan actine, zal het op een gedeelte zijn dat dichter bij de Z-lijn ligt. Dit bindingsproces wordt gecontroleerd door twee regulerende eiwitten, tropomyosine en troponine, en de concentratie van calcium, dat wordt opgeslagen en vrijkomt uit het sarcoplasmatische reticulum.

Tropomyosine bedekt de myosine bindingsplaats op actine, en troponine bindt aan calcium wanneer het beschikbaar is, waardoor tropomyosine weg van de myosine bindingsplaats op actine. In deze vorm kan zich een crossbridge vormen en de spieren samentrekken. Deze cyclus gaat door totdat calcium en ATP niet meer aanwezig zijn in de spiervezel.

kruisbrugcyclus

Naarmate de spier samentrekt, neemt de overlap tussen de dunne en dikke filamenten toe, waardoor de lengte van de sarcomeer – de contractiele eenheid van de spier – afneemt, waarbij energie in de vorm van ATP wordt gebruikt. Op moleculair niveau is dit een cyclisch proces dat uit meerdere stappen bestaat, waarbij binding en hydrolyse van ATP en de beweging van actine door myosine betrokken zijn.

Wanneer ATP, dat aan de myosinekop vastzit, wordt gehydrolyseerd tot ADP, komt myosine in een toestand van hoge energie gebonden aan actine, waardoor een kruisbrug ontstaat. Wanneer ADP wordt vrijgegeven, beweegt de myosinekop naar een lage energietoestand, waardoor actine naar het midden van de sarcomeer wordt verplaatst. Binding van een nieuw ATP-molecuul dissocieert myosine van actine. Wanneer dit ATP wordt gehydrolyseerd, zal de myosinekop zich binden aan actine, dit keer op een deel van de actine dichter bij het uiteinde van de sarcomeer. Regulerende eiwitten troponine en tropomyosine werken samen met calcium om de myosine-actine-interactie onder controle te houden. Wanneer troponine zich aan calcium bindt, wordt tropomyosine weggevoerd van de myosinebindingsplaats op actine, waardoor myosine en actine kunnen interageren en spiercontractie kan optreden.

<h4>Calcium</h4>

Als regulator van de spiercontractie wordt de calciumconcentratie in de spiervezels zeer nauwkeurig gecontroleerd. Spiervezels staan in nauw contact met motorneuronen. Actiepotentialen in motorneuronen veroorzaken de afgifte van de neurotransmitter acetylcholine in de buurt van spiervezels. Dit genereert een actiepotentiaal (depolarisatie) in de spiercel, dat langs het plasmamembraan wordt gevoerd en via invaginaties van het plasmamembraan, transversale of T-tubuli genoemd.

T-tubuli lopen diep in de spier en grenzen aan gespecialiseerde endoplasmatisch reticulumorganellen die sarcoplasmatisch reticulum of SR worden genoemd. Calcium dat in de SR is opgeslagen, komt vrij wanneer spanningsafhankelijke ionkanalen (ionenkanalen die openen en sluiten op basis van lokale ladingen) zich openen als reactie op depolarisatie, waardoor calciumionen het cytoplasma kunnen binnendringen en de spieren kunnen samentrekken.

Wanneer de signalering van motorneuronen stopt, begint de ontspanning van de spier terwijl calcium terug in de SR wordt gepompt, waardoor de cytoplasmatische calciumniveaus worden verlaagd en de calciumvoorraden in de SR worden aangevuld ter voorbereiding op de volgende contractie.

<h4>Spierdegeneratie</h4>

Gezonde spieren kunnen samentrekken, maar zieke spieren verliezen dit vermogen vaak. Ziekten zoals myasthenia gravis verhinderen de stimulatie van de spieren door motorneuronen, wat resulteert in spieratrofie en een afname van de spiermassa. Amyotrofische laterale sclerose (ALS of de ziekte van Lou Gehrig) zorgt ervoor dat motorneuronen degenereren, waardoor.

Kruisbrugcyclus

As muscle contracts, the overlap between the thin and thick filaments increases, decreasing the length of the sarcomere&#8212;the contractile unit of the ...

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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.

Calcium Control in Muscle Contraction | JoVE K12

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Calcium Control in Muscle Contraction

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