The primary duty of transverse tubules, or T-tubules, is to allow the conduction of electrical impulses. Transverse tubules exist as invaginations of sarcolemma, which are muscle fiber membranes. Transverse tubules comprise a triad, along with two parallel strands of sarcoplasmic reticulum, which varies in size and shape depending on muscle cell.
Transverse tubules exist in plasma membranes within skeletal muscle and cardiac muscle cells. They appear as deep in invaginations within surrounding sheaths of sarcoplasmic reticulum, which is further broken down into myoglobin and mitochondria. In the process of creating invaginations, transverse tubules allow impulses to permeate the cell membrane, and ultimately excite, or activate, the sarcoplasmic reticulum. The sarcoplasmic reticulum, in turn, creates a network around sensitive myofibrils, which store and release critical supplies of calcium that enable muscle contractions. Transverse tubules act as transmitters and receivers of information during the process of conducting electrical impulses. As receivers, they allow the infiltration of calcium into chambers called L-type calcium channels. These channels then absorb and transmit calcium through the cell into calcium-release channels, which are essential for enabling proper skeletal muscle contractions. While transverse tubules help promote proper movement of skeletal tissues and fibers, they only appear in skeletal muscles, and are absent in skeletal muscles.