Calcium sparks

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Calcium sparks are calcium release events that occur within muscle cells. They are important in a physiological process called excitation-contraction coupling, which is crucial to muscle function. In essence, electrical stimulation of the outer surface of a muscle cell triggers (via a mechanism dependent on the muscle type) thousands of calcium sparks that spatio-temporally summate to increase intracellular calcium levels. This increase in calcium subsequently activates calcium-sensitive proteins that are responsible for cell shortening, or contraction.

In muscle, action potentials lead to the opening of intracellular calcium ion channels called ryanodine receptors or RyRs located in the membrane of the sarcoplasmic reticulum (SR). This results in facilitated diffusion of calcium ions from the SR into the sarcoplasm of the muscle cell. The calcium sparks them combine to form a calcium signal. This calcium signal allows the calcium to bind to troponin and initiate contraction.

In addition to this synchronized spatially homogenous calcium signal, discrete calcium release events have been discovered using laser scanning confocal microscopy in various tissues including skeletal, cardiac and smooth muscle. These events were called calcium “sparks” because of spontaneous localized and transient elevations of fluorescence exhibited with the calcium release. Calcium sparks were discovered in 1992 by Mark B. Cannell and Peace Cheng (a graduate student). This discovery took place in the confocal microscope laboratory set up by Jon W. Lederer while Mark Cannell was on sabbatical leave from St. George's Hospital Medical School. Although initially rejected by the Journal Nature as artifacts, they were quickly recognised as being of fundamental importance to muscle physiology. It may also be notable that these events also led to the sale of a great number of confocal microscopes as only this machine was capable of detecting them at that time.