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Depiction of smooth muscle contraction. Muscle contraction is the activation of tension-generating sites within muscle cells. [1] [2] In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. [1]
A calcium spark is the microscopic release of calcium (Ca 2+) from a store known as the sarcoplasmic reticulum (SR), located within muscle cells. [1] This release occurs through an ion channel within the membrane of the SR, known as a ryanodine receptor (RyR), which opens upon activation. [2] This process is important as it helps to maintain Ca ...
In the low calcium environment present during diastole (~100 nM), [26] tropomyosin is anchored into the "blocked" position along the actin thin filament through the binding of the troponin I inhibitory (cTnI 128-147) and C-terminal (cTnI 160-209) regions. [27] [28] This prevents actin-myosin cross-bridging and effectively shuts off muscle ...
Excitation-contraction coupling in myocardium relies on sarcolemma depolarization and subsequent Ca 2+ entry to trigger Ca 2+ release from the sarcoplasmic reticulum. When an action potential depolarizes the cell membrane, voltage-gated Ca 2+ channels (e.g., L-type calcium channels) are activated.
The calcium binds to the calcium release channels (RYRs) in the SR, opening them; this phenomenon is called "calcium-induced calcium release", or CICR. However the RYRs are opened, either through mechanical-gating or CICR, Ca 2+ is released from the SR and is able to bind to troponin C on the actin filaments.
This is similar to a voltage-gated calcium channel, but is not actually an ionotropic channel. Instead, it serves to activate ryanodine, which will let calcium ions pass into the sarcoplasmic reticulum, and triggers calcium release to the muscle fiber itself. A T-tubule surrounded by two terminal cisternae is called a triad.
The thin filaments are placed between 2 myosin filaments and contain only the actin filaments of neighboring sarcomeres. Bisecting the I band and serving as an anchoring point for the two adjacent actin filaments is the Z disc. During muscle contraction, the I band will shorten, while an A band will maintain its width. [2]
[1] [2] [3] The calcium ion concentration in sarcoplasm is also a special element of the muscle fiber; it is the means by which muscle contractions take place and are regulated. [ 4 ] [ 5 ] The sarcoplasm plays a critical role in muscle contraction as an increase in Ca 2+ concentration in the sarcoplasm begins the process of filament sliding.