Cardiac Muscle| structure|

                                              Cardiac Muscle

 Keywords: Excitability, Cardiomyocytes, Intercalated disc, Extracellular matrix, Fibroblasts.

 Types of muscles

1. Smooth muscle

2. Skeletal muscle, and

3. Cardiac muscle

 The cardiac muscle is the primary tissue of the heart wall, known as the myocardium. The other tissues are fibroblasts and the extracellular matrix. It is present in between the outer epicardium and inner endocardium.

The epicardium or visceral pericardium is continuous with the peripheral pericardium and forms a pericardial sac. The sac performs specific functions 1. Surround the heart 2. Lubricate the heart, and 3. Protect the heart.

In the pericardial sac, a small volume of fluid is present, facilitating the heart's movements.

The heart chambers and valves are lined with the endocardium, which continues with the endothelial lining of the connected blood vessels.

The cardiac muscle cells or cardiomyocytes are involuntary and striated muscles; their size is 100-150 microns x30-50 microns and almost rectangular. A cardiomyocyte is a complete unit surrounded by sarcolemma. The cardiac muscles are the contracting cells that allow the heart to pump. Therefore, the cells must contract in coordination with the adjacent cells, which the intercalated discs achieve. Therefore, individual heart muscles are joined together at their ends by an intercalated disc that connects the single cardiomyocyte to an electrochemical syncytium responsible for force transmission during muscle contraction.

The intercalated disc is a complex adhering structure formed due to the extensive folding of sarcolemma at the junction point of two cardiac muscle cells. Individual cardiomyocytes are joined together at their ends by intercalated disc, which provides a strong binding between them. The intercalated discs are at a right angle to the direction of the muscle fibers and have three different types of cell-cell junctions 1. Fascia adherens 2. Desmosomes, and 3. Gap junctions. The three junctions together are called  'Area composite.' They provide low resistance bridges, so excitation spreads rapidly and behaves as a 'functional syncytium.'

In the heart, there are two syncytia: atrial syncytium and ventricular syncytium, separated by the fibrous tissues but connected with the atrioventricular bundle.

The cardiomyocytes are vascular with plenty of cytoplasms (=sarcoplasm), mitochondria(=sarcosomes), and glycogen.

The structure of cardiomyocyte 

The striations are due to differences in the refractory indices of various parts of muscle fiber and show alternate light and dark cross bands.

A sarcomere is the muscle's contractile unit extending from one Z line to an adjacent Z line. 

The Z line (Zwischenscheibe is a german word between disc) is a dark narrow line of highly refractile material, on both sides of which a light band is present. The light band is due to lower refractile material and is known as the I band.

After the I band, a dark band of highly refractile material is called the A band. 

The h band is a slightly less refractile region in the center of each A band, and an M line is seen in the middle of this H band.

The Pseudo-H zone is the M line and the light area on either side of the M line.

The I band is due to actin only, part of the A band is expected actin plus myosin, and the H band is expected to myosin only. The T system penetrates the sarcomere at the junction of the I and A band; therefore, each sarcomere has two transverse tubules in skeletal muscles, but in cardiac muscles, it penetrates the sarcomere at the Z line. Therefore cardiac muscles have one triad only.

Fibroblasts are present in the myocardium. They are numerous and of small size. They provide support to the cardiac muscles and electrical insulation. After the cardiac injury, the fibroblasts change into myofibroblasts.

Extracellular matrix( ECM) surrounds cardiac muscles and fibroblasts. They are composed of collagen, elastin, and polysaccharides. The extracellular matrix outside the cardiac muscle is a basement membrane composed of type IV collagen. They support and hydrate the cardiac muscles. In addition, they create elasticity in the cardiomyocytes.

Some proteins form the cytoskeleton of the muscle cell. 

  Sr. no

Muscle protein                              Binds 

 

1.         Actinin                        Z lines to actin

2.         Titin                            Z lines to the M lines

3.         Desmin                       Z lines to the cell membrane.

 

 Properties of cardiomyocytes:

1.    Excitability: The cardiomyocytes are excitable. They form action potential in response to a stimulus.

2.    Autorhythmicity: The heart beats for some time without any stimulation.

3.    Contractility: The cardiomyocytes contract in response to a stimulus-action potential.

4.    Conductivity: The cardiomyocytes regularly spread the cardiac impulse.

5.    Cardiomyocytes follow all or none law.

6.    Cardiomyocytes have a refractory period.

Heart and major blood vessels

Internal link: https://blog.totalphysiology.com

External link: https://en.m.wikipedia.org>wiki

Hashtags: #Excitability#Cardiomyocytes# Intercalateddisc#, Extracellular matrix# Fibroblasts#