Dominic Stone
The sarcoplasmic reticulum is an intracellular system of membranes that is involved in the storage and regulation of calcium ions in muscle cells. It plays a crucial role in muscle contraction and relaxation by controlling the release and reabsorption of calcium. The sarcoplasmic reticulum is composed of tubules and cisternae that surround each myofibril in skeletal and cardiac muscle cells. Its structure and function are essential for maintaining muscle function and calcium homeostasis, making it a key component in the biology of muscle cells and their contraction mechanisms.
| Characteristics | Values |
|---|---|
| Description | Sarcoplasmic reticulum is a network of tubules that extend throughout muscle cells, wrapping around the myofibrils (contractile units of the cell). |
| Forms | Rough ER, smooth ER |
| Functions | Control of muscle contraction, regulation of intracellular calcium concentration, regulation of calcium homeostasis in the context of muscle contraction, calcium storage and release |
| Muscle types | Skeletal muscle, cardiac muscle, smooth muscle |
| Calcium release | Occurs through a ryanodine receptor (RyR) and is known as a calcium spark |
| Calcium release sites | Junctional SR/terminal cisternae |
| Ryanodine receptors | RyR1 (skeletal muscle), RyR2 (cardiac muscle), RyR3 (brain) |
| Calcium release triggers | In cardiac and smooth muscle, an electrical impulse triggers calcium ions to enter the cell through an L-type calcium channel |
| Calcium ion target | Troponin, a protein of skeletal and cardiac muscle |
| Calcium ion effect | Binding of calcium to troponin results in a change in its conformation, causing contraction |
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Calcium ion release
Calcium ions play a crucial role in muscle contraction and relaxation. The sarcoplasmic reticulum (SR) is a network of tubules that extend throughout muscle cells, wrapping around the myofibrils (the contractile units of the cell). Cardiac and skeletal muscle cells contain transverse tubules (T-tubules), which are extensions of the cell membrane that travel into the centre of the cell.
The SR is responsible for regulating calcium ion levels within the cell. Calcium ions (Ca2+) are stored within the SR and released into the cell through calcium release channels called ryanodine receptors (RyR). This release of calcium ions triggers muscle contraction. The ryanodine receptor is a protein channel that opens in response to specific stimuli, allowing calcium ions to flow from the SR into the cytoplasm of the cell. This process is known as a "calcium spark."
During muscle contraction, calcium ions are released from the SR and bind to the protein troponin, which is part of the contractile filament. This binding causes a change in the conformation of troponin, leading to muscle contraction. The release of calcium ions from the SR is carefully regulated to ensure that the correct amount of calcium is released to trigger contraction without causing harmful calcium accumulation within the cell.
Following muscle contraction, it is essential for calcium ions to be removed from the cytoplasm to allow muscle relaxation. Calcium ions are reabsorbed into the SR through calcium pumps called Ca2+-ATPase. This process reduces the concentration of calcium ions in the cytoplasm, allowing the muscle to relax. The Ca2+-ATPase pump plays a critical role in maintaining low calcium levels in the sarcoplasm surrounding the myofibrils, ensuring that the muscle remains relaxed until the next stimulation for contraction occurs.
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Cardiac muscle
The sarcoplasmic reticulum (SR) is a network of tubules that extend throughout muscle cells. It is involved in the coupling of excitation to contraction, and it regulates intracellular calcium concentration. Calcium ions bind to the M1-M10 transmembrane region of the SR, and when two calcium ions bind to the cytosolic side of the pump, along with a molecule of ATP, the pump opens. This releases the calcium ions into the SR.
The SR is found in skeletal and cardiac muscle cells. In skeletal muscle, the SR is highly ordered and structured, consisting of an intricate network of tubules and cisternae. In cardiac muscle, the SR is less extensive and less precisely arranged in relation to the cross-banded pattern of the myofibrils. However, it is believed that it may have a significant role in the physiology of the myocardium.
The SR in cardiac muscle contains RyR2 channels, which are involved in cardiac ECC through a CICR mechanism. The CICR mechanism is where an increase in cytosolic Ca2+ causes the opening of RyR channels and the release of Ca2+ from intracellular stores. The release of calcium ions from the SR in cardiac muscle is triggered by an electrical impulse (action potential) that causes calcium ions to enter the cell through an L-type calcium channel located in the cell membrane. This increase in intracellular calcium activates the RyR, producing a larger increase in intracellular calcium, which is necessary for cardiac contraction.
The SR in cardiac muscle also contains the protein SERCA2, which is a Ca2+-pump ATPase. SERCA2 is involved in the removal of Ca2+ from the cytosol and the maintenance of cardiac function under biomechanical stresses and endurance exercise training. The phosphorylation of SR Ca2+-cycling proteins, such as SERCA2, by protein kinase A or Ca2+-calmodulin kinase, has been shown to increase SR Ca2+-release and Ca2+-uptake activities, promoting cardiac contraction and relaxation functions.
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Skeletal muscle
The sarcoplasmic reticulum is a membrane-bound structure found within muscle cells, including skeletal muscle cells. It is a network of tubules that extend throughout the muscle cells, wrapping around the myofibrils (contractile units of the cell).
In skeletal muscle, the sarcoplasmic reticulum is a highly ordered structure, consisting of an intricate network of tubules and cisternae. These tubules and cisternae surround each myofibril, and the region of the terminal cisternae that opposes the transverse tubule (TT) membrane is called the junctional SR (j-SR). The transverse tubules, along with the two neighbouring terminal cisternae, form a functional association called a triad. The sarcoplasmic reticulum contains several proteins, some of which support calcium storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fibre.
The sarcoplasmic reticulum is physically separate from the sarcolemma and surrounds each myofibril in a highly repetitive pattern. The SR membranes contain a high concentration of calcium ATPase, the protein calsequestrin, and the calcium release channel called the ryanodine receptor (RyR). Calsequestrin can bind to around 50 Ca2+, decreasing the amount of free Ca2+ within the SR and allowing for more calcium to be stored.
The sarcoplasmic reticulum is essential for muscle contraction and relaxation. It regulates intracellular calcium concentration, keeping calcium ion levels relatively constant. Calcium release through ryanodine receptors in the SR is triggered differently in various muscles. In skeletal muscle, the L-type calcium channel is bound to the RyR. Activation of the L-type calcium channel, via an action potential, activates the RyR directly, causing calcium release, which is known as a calcium spark.
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Muscle contraction
The physiological concept of muscle contraction is based on two key variables: length and tension. Muscle shortening and contraction are not interchangeable terms. Tension within a muscle can be generated without any change in its length, such as when holding a weight or an object. Upon relaxation, the muscle fibres return to a low-tension state. Mammals have three types of muscles: skeletal, cardiac, and smooth muscles, each with distinct functions and contraction mechanisms.
Skeletal muscles are attached to bones and provide structure and strength to the body. They are under voluntary control, requiring synaptic input from motor neurons to contract. When innervated, the protein filaments within skeletal muscles slide past each other, producing a contraction as described by the sliding filament theory. Cardiac muscle, on the other hand, forms the walls of the heart and is responsible for pumping blood. Smooth muscle is found in blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder, and it uses actin and myosin contraction to constrict blood vessels and move the contents of hollow organs.
The process of muscle contraction, known as excitation-contraction coupling, begins with an action potential that causes depolarization in the myocyte membrane. This depolarization spreads through the transverse (T) tubules, leading to a conformational change in dihydropyridine receptors. This, in turn, triggers the opening of nearby ryanodine receptors on the sarcoplasmic reticulum (SR), the calcium storage site within muscle cells. The release of calcium from the SR results in calcium binding to troponin C, causing a conformational change that shifts tropomyosin and allows the myosin heads to attach to the actin filaments, forming cross-bridges.
The cross-bridge cycling is initiated when ATP binds to an ATP-binding domain on the myosin head. Subsequently, myosin dissociates from actin, breaking the cross-bridge. ATP is hydrolysed into ADP and P, leading to a conformational change in the myosin heads, which then move towards the positive end of the actin. The phosphate is released, and ADP-bound myosin binds to a new site on the actin filament. With the release of ADP, myosin returns to its original position, pulling on the actin filament and causing the sarcomere and muscle fibre to contract. These cycles continue until calcium levels in the myocyte decrease, leading to tropomyosin covering the actin filaments' myosin-binding sites.
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Troponin
The binding of calcium ions (Ca2+) to troponin results in a change in its conformation, which causes contraction. Troponin is the main target of calcium ions in the Ca signalling pathway. The sarcoplasmic reticulum is the major Ca2+ transport protein that regulates intracellular calcium.
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Frequently asked questions
The sarcoplasmic reticulum is an intracellular system of closed sac-like membranes involved in the storage of intracellular calcium in striated (skeletal) muscle cells.
The sarcoplasmic reticulum is found in skeletal muscle cells and cardiac muscle cells.
The sarcoplasmic reticulum regulates the concentration of calcium in the sarcoplasm (the cytoplasm of striated muscle cells). This determines whether muscle contraction occurs.
