Maximus Gage
Muscle contraction is the activation of tension-generating sites within muscle cells. The complex process leading to muscle contraction is called excitation-contraction coupling, and it begins when an action potential causes depolarization in the myocyte membrane. Calcium ions play a key role in muscle contraction, acting as the on switch. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state. Calcium concentration declines, causing the force to decline and relaxation to occur. Acetylcholine at the neuromuscular junction is broken down by acetylcholinesterase, terminating the signal for muscle contraction.
| Characteristics | Values |
|---|---|
| Muscle contraction | The activation of tension-generating sites within muscle cells |
| Termination of muscle contraction | Depletion of acetylcholine (Ach) in the neuromuscular junction |
| The enzyme acetylcholinesterase breaks down ACh into inactive components, terminating the signal for muscle contraction | |
| The sarcoplasmic reticulum ceases to release calcium ions | |
| Calcium concentration declines to resting levels, causing the force to decline and relaxation to occur | |
| The cGMP pathway can be activated by nitric oxide (NO) or natriuretic peptides (NPs) |
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What You'll Learn
Depletion of acetylcholine (Ach)
Muscle contraction is the activation of tension-generating sites within muscle cells. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibres to their low tension-generating state.
Acetylcholine (ACh) is a neurotransmitter that acts as a chemical messenger released by neurons to communicate with one another and other specialised cells. It is involved in many important functions in the body, including muscle movement. Nerve cells stimulate muscle nerve cells, causing muscles to contract.
Another cause of acetylcholine depletion is the bite of a black widow spider, which causes the cells to release all their acetylcholine, leading to severe muscle contractions, spasms, and possible paralysis. If all acetylcholine supplies are exhausted due to the venom, paralysis occurs.
In the neuromuscular junction, acetylcholine is released by motor neurons in the ventral spinal cord to activate muscle cells. If action potentials stop arriving at the neuromuscular junction, acetylcholine ceases to be released. The remaining acetylcholine in the synaptic cleft is then degraded by acetylcholinesterase or reabsorbed by the synaptic knob, and none is left to replace the degraded acetylcholine.
Cholinesterase inhibitors are used to treat Alzheimer's disease and myasthenia gravis, conditions in which there is a severe decrease in acetylcholine receptor stimulation. These inhibitors block the breakdown of acetylcholine, leading to a buildup in the synapse and continuous activation of the cholinergic receptors.
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Calcium reuptake
Calcium ions play a crucial role in muscle contraction. An increase in cytosolic calcium concentration triggers muscle contraction. Calcium has an extracellular concentration of 2-4 mm and a resting cytosolic concentration of around 100 nm. Calcium is stored inside cells within the sarcoplasmic reticulum at a concentration of about 0.4 mm. An increase in calcium levels is due to its release from the sarcoplasmic reticulum via ryanodine receptors. Neurotransmitters such as acetylcholine bind to receptors on the muscle surface, eliciting a depolarization that causes sodium and calcium ions to enter through associated channels. This shift in the resting membrane potential activates voltage-gated channels, resulting in an action potential.
The process of muscle relaxation is essential for ending muscle contraction. Muscle relaxation involves the removal of calcium ions from the cytosol and their reuptake into the sarcoplasmic reticulum. This calcium reuptake is facilitated by the sarco/endoplasmic reticulum calcium-ATPase (SERCA) pump, which actively pumps calcium back into the sarcoplasmic reticulum. SERCA plays a key role in permanently relaxing the muscle until the next action potential arrives. Mitochondria also participate in calcium reuptake, delivering the gathered calcium to SERCA for storage.
Additionally, calcium buffers, such as cytoplasmic proteins, contribute to muscle relaxation by binding to calcium ions with high affinity, allowing for quick relaxation in fast-twitch muscles. While slower than calcium buffers, SERCA ensures a complete return to resting calcium levels, allowing the troponin complex to dissociate from the actin filament, thereby ending contraction. This dissociation of the troponin complex releases the myosin heads, leading to muscle relaxation.
Phospholamban, an enzyme, acts as a brake for SERCA. At low heart rates, phospholamban is active and slows down the activity of SERCA, preventing the complete removal of calcium from the cytoplasm. Conversely, at high heart rates, phospholamban is phosphorylated and deactivated, allowing a more rapid reuptake of calcium into the sarcoplasmic reticulum. This dynamic regulation of calcium levels by phospholamban ensures that small changes in total calcium concentration result in relatively minor decreases in free calcium concentration.
In summary, calcium reuptake into the sarcoplasmic reticulum through SERCA, mitochondria, and calcium buffers is a critical process in ending muscle contraction and initiating muscle relaxation. The regulation of calcium levels by phospholamban further refines this process, ensuring a controlled decrease in calcium concentration during muscle relaxation.
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Troponin and tropomyosin block myosin
Muscle contraction is the activation of tension-generating sites within muscle cells. Muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The thick filaments are made from the protein myosin, which has one pair of heavy chains and two pairs of light chains.
Troponin and tropomyosin are regulatory proteins that help to regulate the interaction between actin and myosin filaments. Troponin is a complex of three proteins, attached to tropomyosin. Tropomyosin is an actin-binding protein that regulates muscle contraction. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin cross-bridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic membrane and release calcium into the sarcoplasm.
Some of this calcium attaches to troponin, which causes it to change shape, exposing binding sites for myosin (active sites) on the actin filaments. Myosin's binding to actin causes cross-bridge formation, and contraction of the muscle begins. Troponin is found in both skeletal muscle and cardiac muscle, but the specific versions of troponin differ between types of muscle. The main difference is that the TnC subunit of troponin in skeletal muscle has four calcium ion-binding sites, whereas in cardiac muscle there are only three.
The troponin-tropomyosin complex has the ability to weaken the binding of S-1 ADP to actin, which may cause relaxation by blocking the binding of myosin with bound ATP or ADP + Pi to actin. This is known as the steric blocking model.
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Muscle relaxation
Muscle contraction is the activation of tension-generating sites within muscle cells. The complex process leading to muscle contraction is called excitation-contraction coupling (ECC). ECC begins when an action potential causes depolarisation in the myocyte membrane. This leads to the opening of ryanodine receptors on the sarcoplasmic reticulum (SR)—the storage site for calcium within muscle cells.
The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibres to their low tension-generating state. Calcium plays a key role in muscle relaxation. As calcium levels decline to resting levels, calcium is released from Troponin C, disallowing cross-bridge cycling, causing the force to decline and relaxation to occur.
There are several mechanisms that contribute to muscle relaxation by lowering calcium levels. One mechanism involves the SERCA pump, which actively pumps calcium back into the SR, resulting in a permanent relaxation until the next action potential arrives. Another mechanism involves the sodium-calcium exchanger (NCX) and, to a lesser extent, a plasma membrane calcium ATPase, which eject calcium from the cell. Mitochondria also participate in calcium reuptake, delivering their gathered calcium to SERCA for storage in the SR.
In addition to these calcium-dependent mechanisms, muscle relaxation is also influenced by the second messengers cAMP and cyclic guanosine monophosphate (cGMP). While cAMP generally promotes contraction in cardiac muscle, it causes relaxation in smooth muscle. On the other hand, cGMP activation leads to muscle relaxation in blood vessels and other smooth muscles.
Furthermore, muscle relaxation can be achieved through calcium buffers, which bind to calcium with high affinity, allowing for quick relaxation in fast-twitch muscles. These calcium buffers moderate the fall in calcium concentration, permitting a relatively small decrease in response to a large change in total calcium.
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Excitation-contraction coupling
ECC can be categorized into three phases. It begins with depolarization and the spread of an action potential along the sarcolemma and continues with the propagation of the action potential into the T tubules. The T tubules are invaginations of the muscle cell membrane that help spread depolarization signals to the entire muscle fiber.
The second phase involves the release of calcium from the sarcoplasmic reticulum. The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells, which serves as the storage site for calcium within muscle cells. When calcium is released, it binds to the troponin molecules on the thin filament. This binding causes a configurational change in the troponin, removing tropomyosin from its blocking position on the actin filament.
The third phase of ECC is the cross-bridging cycle, which describes the cyclic events necessary for the generation of force or tension within the myosin heads during muscle contraction. The generation of tension within the contractile elements results from the binding of the myosin heads to actin and the subsequent release of stored energy in the myosin heads.
The cross-bridge cycle continues until calcium levels in the myocyte fall, causing tropomyosin to cover the actin filaments' myosin-binding sites. This decline in calcium concentration allows the troponin complex to dissociate from the actin filament, thereby ending contraction. Muscle relaxation occurs, allowing the muscle to return to its low tension-generating state and contract again.
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