T-tubule

**Structure:** T-tubules are extensions of the plasma membrane (sarcolemma) that run deep into the muscle fiber, perpendicular to the long axis. They are lined up with the sarcomere and lie adjacent to the terminal cisternae of the sarcoplasmic reticulum (SR). - Skeletal muscle: T-tubules lie at the A-band/I-band junction. Each T-tubule is flanked by two terminal cisternae, forming a **triad**. - Cardiac muscle: T-tubules lie at the Z-lines and are larger. Each T-tubule pairs with one SR terminal cisterna, forming a **diad**. - Smooth muscle: lacks T-tubules (it uses caveolae).

**Function (excitation-contraction coupling):** *Skeletal muscle (mechanical coupling):* 1. Acetylcholine binds nicotinic receptors at the motor end plate, producing an end plate potential and then a muscle action potential. 2. The action potential spreads along the sarcolemma and down the T-tubules. 3. The depolarization changes the conformation of the DHPR (L-type Ca2+ channel, Cav1.1) in the T-tubule membrane. 4. DHPR is physically linked to the RyR1 on the SR, so the conformational change opens RyR1 directly. Extracellular Ca2+ is NOT required. 5. Ca2+ released from the SR binds troponin C, which moves tropomyosin and exposes the myosin-binding sites on actin, so cross-bridge cycling begins. 6. Relaxation occurs when SERCA pumps Ca2+ back into the SR.

*Cardiac muscle (Ca2+-induced Ca2+ release):* 1. The action potential depolarizes the T-tubule. 2. L-type Ca2+ channels (Cav1.2) open, so a small amount of extracellular Ca2+ enters the cell. 3. This trigger Ca2+ binds RyR2 on the SR and causes a much larger Ca2+ release. 4. Extracellular Ca2+ IS required, so cardiac contraction depends on it. Ca2+ channel blockers (verapamil, diltiazem) reduce contractility for this reason.

**Clinical correlations:** - **Malignant hyperthermia:** Mutations in RYR1 (or CACNA1S, which encodes the DHPR) make the SR Ca2+ release channel hypersensitive to volatile anesthetics (halothane, sevoflurane) and succinylcholine. The result is sustained Ca2+ release, muscle rigidity, hyperthermia, rhabdomyolysis, hypercarbia and tachycardia. Treatment is **dantrolene**, which blocks RyR1, plus stopping the trigger agent. - **Central core disease:** An RYR1 mutation that causes hypotonia and a core-like loss of oxidative enzyme activity on biopsy. It is associated with malignant hyperthermia. - **Hypokalemic periodic paralysis:** CACNA1S (DHPR) mutations are the most common cause. Episodes of flaccid weakness follow carbohydrate-rich meals or rest after exercise. - **Heart failure:** Loss and disorganization of T-tubules (T-tubule remodeling) impairs Ca2+ release synchrony and contributes to reduced contractility. - **Myotonic disorders and others:** Mutations in proteins involved in T-tubule formation (e.g., BIN1/amphiphysin II in centronuclear myopathy, caveolin-3 in limb-girdle muscular dystrophy 1C) can disrupt T-tubule structure. - **Drugs:** Dihydropyridines (nifedipine, amlodipine) act on L-type Ca2+ channels, mainly in vascular smooth muscle. Verapamil and diltiazem act more on the heart. Dantrolene blocks RyR1 and treats malignant hyperthermia and neuroleptic malignant syndrome.

**High-yield memory aids:** - Skeletal = **Triad** at the A-I junction. Cardiac = **Diad** at the Z-line. - Skeletal muscle DHPR is a voltage sensor (mechanically coupled to RyR1). Cardiac DHPR is a true Ca2+ channel (Ca2+-induced Ca2+ release through RyR2). - T-tubules carry the electrical signal. The SR stores and releases Ca2+.