T-Tubule

T-tubules (transverse tubules) are deep invaginations of a muscle fiber's plasma membrane (sarcolemma) that tunnel into the cell's interior. They allow the muscle cell's action potential (electrical impulse) to travel rapidly into the fiber, ensuring that the excitation signal reaches all myofibrils simultaneously.

• In skeletal muscle, T-tubules are positioned at each A-I band junction of the sarcomere and are flanked by the sarcoplasmic reticulum (SR) on both sides. One T-tubule plus two adjacent SR terminal cisternae form a <u>triad</u>. This anatomical arrangement allows the electrical signal in the T-tubule to immediately trigger calcium release from the SR for contraction.
• During <u>excitation-contraction coupling</u>, the depolarization of the T-tubule membrane activates voltage-sensitive receptors (DHP receptors) in the T-tubule, which are mechanically linked to calcium release channels (ryanodine receptors) in the SR. As a result, Ca2+ floods out of the SR near the myofilaments, initiating muscle contraction.
• Because of T-tubules, even large muscle fibers contract quickly and uniformly. Without functional T-tubules, the interior of a muscle fiber would receive the membrane depolarization more slowly or not at all, leading to weak or uncoordinated contraction.

• Classic question setup: How does a muscle action potential lead to calcium release? The answer involves the T-tubules carrying the action potential inward, causing the SR to release Ca2+. If you see a question about an impulse traveling into a muscle fiber, the key structure is the <u>T-tubule</u>.
• Identify on diagrams: A triad in muscle histology consists of a T-tubule with two flanking SR cisternae. An exam might show an electron micrograph or schematic and ask what the transverse tubule's role is - the correct explanation: it propagates the depolarization from the sarcolemma into the fiber to trigger calcium release from the SR.