T-tubules (transverse tubules)
Tubuli transversi
Also known as: transverse tubules, T system, DHPR, dihydropyridine receptor
Tube-shaped inward folds of the sarcolemma that carry the action potential into the depth of the fibre. In skeletal muscle they lie at every A–I junction.
Overview
T-tubules are continuous with the sarcolemma and open to the extracellular fluid. They penetrate the fibre transversely and encircle every myofibril. In mammalian skeletal muscle there are two per sarcomere, one at each A–I junction, and each pairs with two SR terminal cisternae to form a triad. In cardiac muscle they are wider, occur once per sarcomere at the Z line, and form dyads.
The T-tubule membrane contains L-type Ca²⁺ channels / dihydropyridine receptors (DHPR, Caᵥ1.1) arranged in groups of four ("tetrads") facing RyR1 channels. In skeletal muscle these act mainly as voltage sensors rather than as channels that let Ca²⁺ in. Because the T-system brings the action potential to within ≈1 µm of every myofibril, Ca²⁺ is released almost simultaneously throughout the fibre. Without it, diffusion would be far too slow for a thick fibre.
Function
- Conducts the action potential deep into the fibre
- Holds the voltage sensors (DHPR) that trigger Ca²⁺ release from the SR
- Synchronises contraction of all the myofibrils
Clinical significance
Hypokalaemic periodic paralysis
Mutations in the CACNA1S gene (encoding DHPR Caᵥ1.1) or in SCN4A cause episodic flaccid weakness with low serum K⁺. Attacks are often triggered by carbohydrate-rich meals or rest after exercise.
Test yourself
In skeletal muscle, what directly opens the ryanodine receptor (RyR1) of the sarcoplasmic reticulum?
- Ca²⁺ entering through L-type channels (Ca²⁺-induced Ca²⁺ release)
- Mechanical coupling to voltage-sensing dihydropyridine receptors in the T-tubule
- IP₃ generated by muscarinic receptors
- cAMP-dependent phosphorylation by protein kinase A
Answer: Mechanical coupling to voltage-sensing dihydropyridine receptors in the T-tubule
In skeletal muscle, the DHPR (Caᵥ1.1) acts as a voltage sensor that is physically linked to RyR1, so depolarisation alone opens it. Ca²⁺-induced Ca²⁺ release through RyR2 is the mechanism in cardiac muscle.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.