Pluto is considered a dwarf planet because it does not pass around its orbit. It orbits a disc-shaped zone beyond the orbit of Neptune called the Kuiper Belt, a distant region occupied by frozen corpses left over from the formation of the solar system.
Transverse tubules (T tubules) are extensions of the sarcolemma that penetrate into the interior of the muscle transmitting electrical signals to the sarcoplasmic reticulum.
