Cross-striated cardiac muscle tissue, intercalated disc

Introduction

Cardiac muscle tissue is the muscle type that forms the wall of the heart. Even though it is striated like skeletal muscle, it behaves quite differently. It contracts automatically, rhythmically and usually without our conscious control. In histology, recognising cardiac muscle is very important because it has some unique features, especially the intercalated discs, which connect the muscle cells together.

General Structure of Cardiac Muscle

Cardiac muscle is made of cardiomyocytes, which are short, branched cells. Unlike skeletal muscle fibres, which are long cylinders, cardiomyocytes look more like irregular, branching pieces that join with each other. They have one central nucleus, sometimes two, and the cytoplasm contains many mitochondria because the heart needs a lot of energy all the time.

When you look at cardiac muscle under the microscope, you can see faint cross-striations, similar to skeletal muscle. These are from the repeating sarcomeres. But the striations in cardiac tissue are usually not as sharp, and the fibres do not run perfectly parallel, so the tissue looks a little bit “messy” compared to skeletal muscle.

Intercalated Discs

One of the most important features are the intercalated discs. These are special junctions that connect cardiomyocytes end-to-end. They look like dark, zig-zag or step-like lines crossing the fibres. Because the cells branch and join in different angles, the discs do not form one straight line; they appear irregular.

Intercalated discs are made of different types of junctions:

• Desmosomes, which keep the cells strongly attached so they don't pull apart during contraction.
• Fascia adherens, which helps transmit the contractile force from one cell to the next.
• Gap junctions, allowing ions to pass quickly between cells so the heart can contract as a coordinated unit.

Thanks to these structures, the heart muscle acts almost like one big cell electrically. This is a key reason why the heartbeat is so synchronised.

Functional Properties

Cardiac muscle contracts without nervous stimulation, because the heart has its own pacemaker cells. These generate impulses that spread through gap junctions. The branching shape of the cells and presence of intercalated discs makes signal spreading much easier and faster.

Cardiomyocytes cannot divide very well, so the heart has limited ability to repair itself. After injury, like myocardial infarction, damaged muscle is replaced by scar tissue instead of new muscle.

Citation

Morphologia, E. O. O. J. (2019). Junqueira’s Basic Histology : Text & Atlas (15th ed.), 2018. Morphologia, 13(2), 101–104. https://doi.org/10.26641/1997-9665.2019.3.101-104

slides from Ph.D. Lucia Fraser Lantová