Calcium spark

A calcium spark is the microscopic release of calcium (Ca²⁺) from a store known as the sarcoplasmic reticulum (SR), located within muscle cells. This release occurs through an ion channel within the membrane of the SR, known as a ryanodine receptor (RyR), which opens upon activation. This process is important as it helps to maintain Ca²⁺ concentration within the cell. It also initiates muscle contraction in skeletal and cardiac muscles and muscle relaxation in smooth muscles. Ca²⁺ sparks are important in physiology as they show how Ca²⁺ can be used at a subcellular level, to signal both local changes, known as local control, as well as whole cell changes.

As mentioned above, Ca²⁺ sparks depend on the opening of ryanodine receptors, of which there are three types:

Opening of the channel allows Ca²⁺ to pass from the SR, into the cell. This increases the local Ca²⁺ concentration around the RyR, by a factor of 10. Calcium sparks can either be evoked or spontaneous, as described below.

Electrical impulses, known as action potentials, travel along the cell membrane (sarcolemma) of muscle cells. Located in the sarcolemma of smooth muscle cells are receptors, called dihydropyridine receptors (DHPR). In skeletal and cardiac muscle cells, however, these receptors are located within structures known as T-tubules, that are extensions of the plasma membrane penetrating deep into the cell (see figure 1). These DHPRs are located directly opposite to the ryanodine receptors, located on the sarcoplasmic reticulum and activation, by the action potential causes the DHPRs to change shape.

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