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Content: This reverses the normal imbalance of charged particles and is referred to as depolarisation. One region of membrane depolarises adjacent regions, and the resulting wave of depolarisation then spreads along the cell membrane.[8] The polarisation of the membrane is restored as potassium ions flow back across the membrane from the inside to the outside of the cell. In cardiac muscle cells, as the action potential passes down the T-tubules it activates L-type calcium channels in the T-tubular membrane. Activation of the L-type calcium channel  allows calcium to pass into the cell. T-tubules contain a higher concentration of L-type calcium channels than the rest of the sarcolemma and therefore the majority of the calcium that enters the cell occurs via T-tubules.[9] This calcium binds to and activates a receptor, known as a ryanodine receptor, located on the cell's own internal calcium store, the sarcoplasmic reticulum. Activation of the ryanodine receptor causes calcium to be released from the sarcoplasmic reticulum, causing the muscle cell to contract.[10] In skeletal musclecells, however, the L-type calcium channel is directly attached to the ryanodine receptor on the sarcoplasmic reticulum allowing activation of the ryanodine receptor directly without the need for an influx of calcium. [ 11]
The importance of T-tubules is not solely due to their concentration of L-type calcium channels, but lies also within their ability to synchronise calcium release within the cell. The rapid spread of the action potential along the T-tubule network activates all of the L-type calcium channels near-simultaneously. As T-tubules bring the sarcolemma very close to the sarcoplasmic reticulum at all regions throughout the cell,  calcium can then be released from the sarcoplasmic reticulum across the whole cell at the same time. This synchronisation of calcium release allows muscle cells to contract more forcefully.[12] In cells lacking T-tubules such as smooth muscle cells, diseased cardiomyocytes, or muscle cells in which T-tubules have been artificially removed, the calcium that enters at the sarcolemma has to diffuse gradually throughout the cell, activating the ryanodine receptors much more slowly as a wave of calcium leading to less forceful contraction. [