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

A muscle contracts when the overlap of the thin and thick filaments increases causing the sarcomere length to decrease.

On the molecular level, contraction occurs when ATP, bound to the globular head region of myosin, is hydrolyzed to ADP, converting the myosin head to a high energy state in which it binds to actin and creates a cross bridge.

The release of ADP causes the myosin head to return to a low energy state, moving actin towards the center of the sarcomere.

Binding of a new ATP molecule to the myosin head then dissociates it from actin. The next time that this myosin head binds to actin it will be on a portion that is closer to the Z line.

This binding process is controlled by two regulatory proteins, tropomyosin and troponin, and the concentration of calcium, which is stored and released from the sarcoplasmic reticulum. Tropomyosin covers the myosin binding site on actin and troponin binds to calcium when it's available, moving tropomyosin away from the myosin binding site on actin. In this confirmation a cross bridge can form and the muscle contracts.

This cycle continues until calcium and ATP are no longer present in the muscle fiber.

cross-bridge cycle

As muscle contracts, the overlap between the thin and thick filaments increases, decreasing the length of the sarcomere&#8212;the contractile unit of the muscle&#8212;using energy in the form of ATP. At the molecular level, this is a cyclic, multistep process that involves binding and hydrolysis of ATP, and movement of actin by myosin.

When ATP, that is attached to the myosin head, is hydrolyzed to ADP, myosin moves into a high energy state bound to actin, creating a cross-bridge. When ADP is released, the myosin head moves to a low energy state, moving actin toward the center of the sarcomere. Binding of a new ATP molecule dissociates myosin from actin. When this ATP is hydrolyzed, the myosin head will bind to actin, this time on a portion of actin closer to the end of the sarcomere. Regulatory proteins troponin and tropomyosin, along with calcium, work together to control the myosin-actin interaction. When troponin binds to calcium, tropomyosin is moved away from the myosin-binding site on actin, allowing myosin and actin to interact and muscle contraction to occur.

<h4>Calcium</h4>

As a regulator of muscle contraction, calcium concentration is very closely controlled in muscle fibers. Muscle fibers are in close contact with motor neurons. Action potentials in motor neurons cause the release of the neurotransmitter acetylcholine in the vicinity of muscle fibers. This generates an action potential (depolarization) in the muscle cell, that is carried along the plasma membrane and through invaginations of the plasma membrane called transverse, or T-tubules.

T-tubules run deep into the muscle and are adjacent to specialized endoplasmic reticulum organelles called sarcoplasmic reticulum, or SR. Calcium sequestered inside the SR is released when voltage-gated ion channels (ion channels that open and close based on local charges) open in response to depolarization, allowing calcium ions to enter the cytoplasm, and muscles to contract.

When signaling from motor neurons stops, relaxation of the muscle begins as calcium is pumped back into the SR, decreasing the cytoplasmic levels of calcium and replenishing the SR calcium stores in preparation for the next contraction.

<h4>Muscle Degeneration</h4>

Healthy muscle can contract but diseased muscle often loses this ability. Diseases like myasthenia gravis prevent motor neuron stimulation of muscle which results in muscle atrophy and a decrease in muscle mass. Amyotrophic lateral sclerosis (ALS or Lou Gehrig&#8217;s disease) causes motor neurons to degenerate, which similarly leads to muscle degeneration and atrophy.

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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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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