Layers Of Wall Of The Heart:
Heart is made up of three layers of tissues:
- Outer Pericardium
- Middle Myocardium
- Inner Endocardium
Pericardium is the outer covering of the heart. It is up of two layers:
- Outer Parietal Pericardium
- Inner Visceral Pericardium
The space between the two layers is called pericardial cavity or pericardial space and it contains a thin film of fluid.
- Outer Parietal Pericardium: Parietal pericardium forms a strong protective sac for the heart. It helps also to anchor the heart within the mediastinum. Parietal Pericardium is made up two layers:
- Outer Fibrous Layer: Fibrous layer of the parietal pericardium is formed by thick fibrous connective tissue. It is attached to the diaphragm end it is continuous with tunica adventitia (outer wall) of the blood vessels, entering and leaving the heart, It is attached with diaphragm below. Because of the fibrous nature, it protects the heart from over stretching.
- Inner Serous Layer: Serous layer is formed by mesothelium. together with a small amount of connective tissue. Mesothelium contains squamous epithelial cells which secrete a small amount of fluid, which lines the pericardial space. This fluid prevents friction and allows free movement of heart within pericardium, when it contracts and relaxes. The total volume of this fluid is only about 25 to 35 mL.
- Inner Visceral Pericardium: Inner visceral pericardium lines the surface of myocardium. It is made up of flattened epithelial cells. This layer is also known as epicardium.
Myocardium is the middle layer of wall of the heart and it is formed by cardiac muscle fibers or cardiac myocytes. Myocardium forms the bulk of the heart and it is responsible for pumping action of the heart. Unlike skeletal muscle fibers, the cardiac muscle fibers are involuntary in nature. Myocardium has three types of muscle fibers:
i- Muscle Fibers which Form Contractile Unit of Heart:
These cardiac muscle fibers are striated and resemble the skeletal muscle fibers in structure. Cardiac muscle fiber is bound by sarcolemma. It has a centrally placed nucleus. Myofibrils are embedded in the sarcoplasm. Sarcomere of the cardiac muscle has all the contractile Proteins, namely actin, myosin, troponin and tr0P0myosin. Sarcotubular system in cardiac muscle is similar to that of skeletal muscle.
Important difference between skeletal muscle and cardiac muscle is that the cardiac muscle fiber is branched and the skeletal muscle is not branched.
- Intercalated Disk: Intercalated disk is a tough double membranous structure, situated at the junction between the branches of neighboring cardiac muscle fibers. It is formed by the fusion of the membrane of the cardiac muscle branches. Intercalated disks form adherens Junctions, which play an important role in the contraction of cardiac muscle as a single unit.
- Syncytium: Syncytium means tissue with cytoplasmic continuity between adjacent cells, However, cardiac muscle is like a physiological syncytium, since there is no continuity of the cytoplasm and the muscle fibers are separated from each other by cell membrane. At the sides, the membranes of the adjacent muscle fibers fuse together to form gap junctions. Gap junction is permeable to ions and it facilitates the rapid conduction of action potential from one fiber to another. Because of this, all the cardiac muscle fibers act like a single unit, which is referred as syncytium. Syncytium in human heart has two portions, syncytium of atria and the syncytium of ventricles. Both the portions of syncytium are connected by a thick non-conducting fibrous ring called the atrioventricular ring.
ii. Muscle Fibers Which Form the Pacemaker:
Some of the muscle fibers of heart are modified into a specialized structure known as pacemaker. These muscle fibers forming the pacemaker have less striation.
- Pacemaker: Pacemaker is structure in the heart that generates the impulses for heart beat. It is formed by pacemaker cells called P cells. Sinoatrial (SA) node forms the pacemaker in human hear.
iii. Muscle Fibers Which Form Conductive System:
Conductive system of the heart is formed by modified cardiac muscle fibers. Impulses from SA node are transmitted to the atria directly. However, the impulses are transmitted to ventricles through various components of conducting system.
Endocardium is the inner most layer of heart wall. It is a thin, smooth and glistening membrane, It is formed by a single layer of endothelial cells, lining the inner surface of the heart. Endocardium continues as endothelium of the blood vessels.