What is the difference between a dyad and triad in skeletal cardiac muscle?

Dyad

A dyad is a specialized junctional complex found in skeletal muscle, where the sarcolemma (muscle cell membrane) is closely associated with the transverse tubules (T-tubules). Dyads are fundamental for the excitation-contraction coupling process in skeletal muscles, facilitating rapid signal transmission and coordination between the electrical excitation and the mechanical contraction of the muscle cells.

When an action potential propagates along the sarcolemma, it also spreads into the T-tubules, which extend deep into the muscle fiber. The close proximity between the sarcolemma and the T-tubules at the dyadic clefts allows for the efficient spread of depolarization into the muscle fiber's interior.

The dyads are structurally composed of two main components:

1. Sarcolemma Cisternae: These are infoldings of the sarcolemma that form sac-like structures closely apposed to the T-tubules.

2. Transverse Tubules (T-tubules): These are deep invaginations of the sarcolemma that extend into the muscle fiber, carrying the action potential deep into the interior.

The space between the sarcolemma cisternae and the T-tubules within the dyad is called the dyadic cleft. This gap is typically filled with extracellular fluid.

Triad

Triads are analogous structures to dyads, but they are found in cardiac muscle, not skeletal muscle. Triads serve a similar function in facilitating excitation-contraction coupling, but they have a distinct structural arrangement.

In cardiac muscle, the T-tubules also penetrate deep into the muscle fibers, but they are not directly adjacent to the sarcolemma. Instead, they are sandwiched between two terminal cisternae of the sarcoplasmic reticulum (SR), forming a "triadic" arrangement.

The components of a cardiac triad include:

1. Sarcolemma Cisternae: These cisternae form the outer boundary of the triad and are continuous with the sarcolemma.

2. Transverse Tubules (T-tubules): As in skeletal muscle, T-tubules are deep invaginations of the sarcolemma that carry the electrical signals deep into the cardiac muscle fibers.

3. Terminal Cisternae: These are specialized portions of the sarcoplasmic reticulum (SR), which is the intracellular calcium storage organelle in muscle cells. The terminal cisternae are located on either side of the T-tubules within the triad.

The spatial arrangement of the triad ensures efficient communication between the electrical signal and the release of calcium ions from the SR, initiating muscle contraction in cardiac muscle.

In summary, dyads are found in skeletal muscle and consist of a pair of cisternae and a T-tubule, while triads are found in cardiac muscle and have a central T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum. Both dyads and triads are essential for the rapid and synchronized excitation-contraction coupling in muscle cells.