What is the Difference Between Dyad and Triad Muscle

The key difference between dyad and triad muscle is that dyad muscle is a structure present in cardiac myocyte in the Z-line of sarcomere, while triad muscle is a structure present in skeletal muscles in between the junctions of A and I band in the sarcomere.

The internal membrane systems, transverse tubular system (t-tubule), and sarcoplasmic reticulum are interrelated systems that influence the excitation of muscle fibers during the contraction and relaxation of muscles. The t-tubule system is a branched network of tubules that run transversely across the muscle fibers. Sarcoplasmic reticulum releases calcium ions when t-tubules bring sarcolemma closer to the sarcoplasmic reticulum from all regions of the cell. T-tubules are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. The muscles responsible for contraction are dyad and triad muscles.

CONTENTS

1. Overview and Key Difference
2. What is a Dyad Muscle
3. What is a Triad Muscle
4. Similarities – Dyad and Triad Muscle
5. Dyad vs Triad Muscle in Tabular Form
6. Summary – Dyad vs Triad Muscle

What is a Dyad Muscle?

A dyad muscle is a structure in the cardiac myocyte that is located in the sarcomere Z-line. It is made up of a single t-tubule paired with a terminal cisterna in the sarcoplasmic reticulum. This muscle is important in excitation and contraction coupling for the action potential in the presence of calcium ions. The wave of depolarization is therefore coupled to calcium-mediated muscle contraction in the heart through sliding filament mechanisms.

Dyad muscles are a type of intracellular synapse that transmit excitatory signals from the plasmalemma to the juxtaposed terminal cisternae of the sarcoplasmic reticulum. The structure and the distribution of dyad muscle depend on the myocyte. The calcium influx through plasmalemma by the incoming action potential during the systole is sufficient in small myocytes such as the embryonic myocardium. However, complex myocytes such as adult myocardium require an additional force for the influx of calcium. Therefore, cytosolic calcium increase is produced by the release of calcium ions at dyad muscles. In atrial tissues, which are thin myocytes, dyads exist on the myocyte surface, while in ventricular tissues, which are thick myocytes, dyads are present very close to contractile myofibrils and in myocytes. In ventricular tissues, the plasmalemma develops a network of tubules to transmit excitation signals.

What is a Triad Muscle?

Triad muscle is a structure formed by a t-tubule with a sarcoplasmic reticulum on either side. Triad muscles are usually located in the junction between the A and I band of the sarcomere. Triad muscles form and function on the anatomical basis of excitation-contraction coupling theory, where a stimulus excites the muscle and causes a contraction.

A stimulus transmits from the neuromuscular junction along the t-tubules and activates the dihydropyridine receptors (DHPRs). This activation causes a negligible amount of influx of calcium ions and mechanical interaction between calcium conduction ryanodine receptors (RyRs) on the adjacent sarcoplasmic reticulum. This, in turn, initiates the series of events that leads to muscle contraction. These muscle contractions are caused due to the binding of calcium with troponin, unmasking the binding sites that cover the troponin-tropomyosin complex located on actin myofilament. This allows the myosin cross-bridges to connect with actin and cause muscle contraction.

What are the Similarities Between Dyad and Triad Muscle?

• Dyad and triad muscle structures are formed by the t-tubule with a sarcoplasmic reticulum.
• They belong to a muscle type.
• Both function under excitation-contraction coupling.
• Moreover, their actions are simulated by calcium ion influx.

What is the Difference Between Dyad and Triad Muscle?

Dyad muscle is a structure present in cardiac myocyte in the Z-line of the sarcomere, whereas triad muscle is a structure present in skeletal muscles in between the junctions of A and I band in the sarcomere. Thus, this is the key difference between dyad and triad muscle. Dyad muscle is found in the cardiac muscle, while triad muscles are found in the skeletal muscle. Moreover, dyad muscles help in cardiac muscle contraction, while triad muscle helps in skeletal muscle contraction.

The below infographic presents the differences between dyad and triad muscle in tabular form for side by side comparison.

Summary – Dyad vs Triad Muscle

The t-tubule system is a branched network of tubules that run transversely across the muscle fibers. The muscles responsible for contraction are dyad and triad muscles. Dyad muscle is a structure present in cardiac myocyte in the Z-line of the sarcomere. Triad muscle is a structure present in skeletal muscles in between the junctions of the A and I band in the sarcomere. So, this is the key difference between dyad and triad muscle.

Reference:

1. Lu, Fujian, and William T. Pu. “The Architecture and Function of Cardiac Dyads.” Biophysical Reviews, vol. 12, no. 4, 2020, pp. 1007–1017., https://doi.org/10.1007/s12551-020-00729-x.
2. “Triad.” Encyclopædia Britannica, Encyclopædia Britannica, Inc.

Image Courtesy:

1. “Cardiac Muscle” By BruceBlaus – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Blausen 0801 SkeletalMuscle” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia