There are two sets of tubules within skeletal muscles fibers that carry out critical functions during muscle contractions: the sarcoplasmic reticulum and the T-tubules.

T-tubules (transverse tubules) are invaginations, or indentations, of the sarcolemma. They are formed much like a young picky-eater poking holes in his mashed potatoes. T-tubules communicate with the extracellular space and are filled with extracellular fluid. They are located on the sarcomere at the point where the A band and I band overlap. The T-tubules are flanked on either side by dilated regions of the cell's endoplasmic reticulum—the sarcoplasmic reticulum.

Image by Hannah Crowder and JS F24

The Sarcoplasmic reticulum (SR) is an elaborate network of smooth endoplasmic reticulum that surrounds and encases each myofibril, much like a loosely knitted sweater that covers your arms. It stores calcium which can then be released into the sarcoplasm when an action potential is conducted along the sarcolemma of the T-tubule. Most of the sarcoplasmic reticulum runs parallel to the myofibrils, but there are right-angle enlargements of the SR at the A band/I band junctions that flank the T-tubules. These enlargements are known as terminal cisternae ("end sacs"). One T-tubule inbetween the two terminal cisternae that parallel it form the triad. The triad is critical in skeletal muscle function. At each triad, the T-tubule membrane contains large numbers of voltage-dependent proteins called dihydropyridine (DHP) channels or L-type calcium channels. Although these are called channels, they do not allow calcium to move through them; rather, they are physically linked to calcium release channels on the terminal cisternae known as ryanodine receptor channels (RyR). When the membrane is depolarized by an action potential, the DHP channel detects a depolarization and causes the RyR channels to open, resulting in the release of calcium from the terminal cisternae of the SR (more on this in the next unit). To ensure a large concentration gradient for calcium is present at the terminal citsterna, each ryanodine channel as an addition protein linked to it called calsequestrin. This protein binds up calcium ions so that they don’t diffuse throughout the long network of tubes of the sarcoplasmic reticulum. The binding is very low affinity, but enough to keep calcium ions near the channel. This arrangement ensures that once the ryanodine channel is triggered to open, there will be a substantial calcium gradient to exit through the channel and into the sarcoplasm. Calsequestrin is not frequently shown in illustrations of the terminal cisternae, but we will illustrate it in this section for the previous and next images to help you see where it is. 

Image by Hannah Crowder and JS F24

In the image above we have a very close up image of the SR and the T tubules. Notice how the SR lies above the myofilaments proteins including actin and myosin. Notice that the DHPR (Dihydropyridine and Ryanodine Complex) proteins are activated in this image for the channels illustrated at the top while they are represented as still being in the resting stage at the bottom. The membrane potential will have to deploarize to activate the DHPR complexs which is the subject of the next section. Notice that when the DHPR complex is open, this allows calcium to escape the SR and enter the cytoplasm where it can diffuse quickly down to the myofibrils. Calcium will not have to travel far to find a troponin where it will bind. Finally, notice that no calcium enters the cytoplasm from the extracellular fluids which is found in the T-tubules.