As mentioned above, Ca2+ sparks depend on the opening of ryanodine receptors, of which there are three types: •
Type 1 – found mainly in skeletal muscle •
Type 2 – found mainly in the heart •
Type 3 – found mainly in the brain 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.
Evoked 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. In
cardiac and
smooth muscle, activation of the DHPR results in it forming an
ion channel. This allows Ca2+ to pass into the
cell, increasing the local Ca2+ concentration, around the RyR. When four Ca2+ molecules bind to the RyR, it opens, resulting in a larger release of Ca2+, from the SR . This process, of using
Ca2+ to activate release of
Ca2+ from the
SR is known as
calcium-induced calcium release. However, in skeletal muscle the DHPR touches the RyR. Therefore, the shape change of the DHPR activates the RyR directly, without the need for Ca2+ to flood into the cell first. This causes the RyR to open, allowing Ca2+ to be released from the SR.
Spontaneous Ca2+ sparks can also occur in cells at rest (i.e. cells that have not been stimulated by an action potential). This occurs roughly 100 times every second in each cell == Calcium after release ==