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Calcium is crucial for muscle function, and its concentration within muscle cells determines whether they contract or relax. Calcium ions are released from the sarcoplasmic reticulum when the membrane of the T-tubular system is excited, and they bind to Troponin C, causing muscle contraction. Calcium is also involved in the regulation of cardiac muscle contraction, although the mechanism differs from that of skeletal muscles. Problems with calcium handling in heart muscle cells can lead to heart rhythm disorders and heart failure.
| Characteristics | Values |
|---|---|
| Calcium ions | Released from the sarcoplasmic reticulum through calcium ion channels when the membrane of the T-tubular system is excited |
| Calcium ions | Bind to Troponin C, causing it to conform and allowing the myosin head to latch onto the actin filament, triggering muscle contraction |
| Calcium concentration | Controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm |
| Calcium concentration | Determines the contraction and relaxation properties of a muscle fibre |
| Calcium | Crucial for muscle function, plasticity, and disease |
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What You'll Learn
- Calcium triggers muscle contraction
- Calcium ions are pumped back into the sarcoplasmic reticulum to allow muscle relaxation
- Calcium is the main regulatory and signalling molecule in muscle fibres
- Calcium-handling abnormalities in the heart can cause heart rhythm disorders and/or heart failure
- Calcium regulates muscle contraction through two different systems: actin-linked and myosin-linked regulation
Calcium triggers muscle contraction
During stimulation of the muscle cell, the motor neuron releases the neurotransmitter acetylcholine, which then binds to a post-synaptic nicotinic acetylcholine receptor. A change in the receptor conformation causes an action potential, activating voltage-gated L-type calcium channels, which are present in the plasma membrane. The inward flow of calcium from the L-type calcium channels activates ryanodine receptors to release calcium ions from the sarcoplasmic reticulum.
Calcium is released from the sarcoplasmic reticulum through calcium ion channels when the membrane of the T-tubular system is excited. It binds to Troponin C, causing it to conform and permitting the myosin head to latch onto the actin filament, onsetting muscle contraction.
The contractile properties of muscle fibres are dependent on the variable expression of proteins involved in Ca2+ signalling and handling. The molecular diversity of the main proteins in the Ca2+ signalling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fibre.
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Calcium ions are pumped back into the sarcoplasmic reticulum to allow muscle relaxation
Calcium is the main regulatory and signalling molecule in muscle fibres. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm.
Calcium triggers muscle contraction by reacting with regulatory proteins. In the absence of calcium, these proteins prevent the interaction of actin and myosin. When calcium is present, it binds to Troponin C, allowing the myosin head to latch onto the actin filament, causing muscle contraction.
The sarcoplasmic reticulum releases calcium ions when the membrane of the T-tubular system is excited. This causes muscle contraction. When calcium ions are pumped back into the sarcoplasmic reticulum, the muscle cell can relax.
The calcium cycle, or Ca2+ signalling apparatus, includes the ryanodine receptor, which is the sarcoplasmic reticulum Ca2+ release channel, the troponin protein complex, which mediates the Ca2+ effect to the myofibrillar structures leading to contraction, the Ca2+ pump responsible for Ca2+ reuptake into the sarcoplasmic reticulum, and calsequestrin, the Ca2+ storage protein in the sarcoplasmic reticulum.
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Calcium is the main regulatory and signalling molecule in muscle fibres
The sarcoplasmic reticulum is a calcium cycle, which includes the ryanodine receptor that is the sarcoplasmic reticulum Ca2+ release channel, the troponin protein complex that mediates the Ca2+ effect to the myofibrillar structures leading to contraction, the Ca2+ pump responsible for Ca2+ reuptake into the sarcoplasmic reticulum, and calsequestrin, the Ca2+ storage protein in the sarcoplasmic reticulum.
Calcium triggers contraction by reacting with regulatory proteins that, in the absence of calcium, prevent interaction of actin and myosin. In actin-linked regulation, troponin and tropomyosin regulate actin by blocking sites on actin required for complex formation with myosin; in myosin-linked regulation, sites on myosin are blocked in the absence of calcium.
The contraction of cardiac muscles is also regulated by the concentration of calcium ions. However, there are some main differences in contraction mechanisms, such as the T-tubular system in cardiac muscles having much greater invaginations on the cell surface and the sarcoplasmic reticulum being much less complex in comparison to that in the skeletal muscle.
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Calcium-handling abnormalities in the heart can cause heart rhythm disorders and/or heart failure
Calcium is released from the sarcoplasmic reticulum through calcium ion channels when the membrane of the T-tubular system is excited. This causes muscle contraction. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle cell to relax.
Calcium is the main regulatory and signalling molecule for all muscle fibres. The contractile properties of muscle fibres are dependent on the variable expression of proteins involved in calcium signalling and handling. The calcium cycle includes the ryanodine receptor that is the sarcoplasmic reticulum calcium release channel, the troponin protein complex that mediates the calcium effect to the myofibrillar structures leading to contraction, the calcium pump responsible for calcium reuptake into the sarcoplasmic reticulum, and calsequestrin, the calcium storage protein in the sarcoplasmic reticulum.
In some cases, the doors controlling the movement of calcium malfunction, causing too much or too little calcium to enter the cell. Sometimes, this malfunction is caused by advancing age or other diseases. Alternatively, changes/variations in our genes (called genetic mutations) can change the shape of the ion channel which, in extreme cases, may prevent the channel from opening or closing properly. This can lead to abnormal electrical signals, which may cause a group of heart diseases called heart rhythm disorders.
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Calcium regulates muscle contraction through two different systems: actin-linked and myosin-linked regulation
Calcium ions are released from the sarcoplasmic reticulum when the membrane of the T-tubular system is excited. This causes the calcium to bind to Troponin C, allowing the myosin head to latch onto the actin filament, which causes muscle contraction.
The actin-linked regulation system involves troponin and tropomyosin regulating actin by blocking sites on actin required for complex formation with myosin. The myosin-linked regulation system involves sites on myosin being blocked in the absence of calcium.
The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle cell to relax.
The molecular diversity of the main proteins in the Ca2+ signalling apparatus (the calcium cycle) determines the contraction and relaxation properties of a muscle fibre.
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Frequently asked questions
Calcium triggers muscle contraction by binding to Troponin C, allowing the myosin head to latch onto the actin filament.
The myosin head latches onto the actin filament, causing the muscle to contract.
Calcium is released from the sarcoplasmic reticulum through calcium ion channels when the membrane of the T-tubular system is excited.
The muscle cell relaxes.
Problems with calcium handling in heart muscle cells can cause heart rhythm disorders and/or heart failure.
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