Myocardium|Cardiac Muscle.
As explained in anatomy of heart, myocardium is the middle layer of the heart and it is composed of specialized striated muscle fibers. It is an involuntary heart muscle which is not under our control and functions on its own.
Myocardium forms the bulk of the heart wall and is responsible for the coordinated pumping action of contraction and relaxation of the heart, which enables it to supply blood to all parts of the body. Myocardial is the word used to describe of the myocardium, such as myocardial infarct meaning infarct of the myocardium.
Picture of heart showing myocardium. (Click on image to enlarge)
Myocardium|Cardiac Muscle|Heart Muscle.
The cardiac muscle or the heart muscle which forms the myocardium is one of the three major muscles in the body, the other two being the smooth muscle and the skeletal muscle.
- Smooth muscles are also involuntary muscles found in the walls of the blood vessels, the G.I. tract, the reproductive tract, the respiratory tract, etc.
- The skeletal muscles are voluntary muscles (under your control) which help to perform movements such as standing, walking, lifting, etc. Examples include the biceps, triceps, calf muscles, etc.
- Cardiac muscle tissue is specialized and present only in the myocardium in view of its unique function.
The Incredible Myocardium.
Pumping at 72 beats per minute, 24×7, throughout life makes one wonder how this heart muscle never gets fatigued. This is because the heart muscle is resistant to fatigue. It boasts of this property because of the high number of mitochondria present in it.
Mitochondria are subunits present in cells and within their own lipid membranes. They are sometimes referred to as cellular power plants because they are a great source of chemical energy. Mitochondria enable aerobic or cellular respiration which is further helped because of good supply of blood and nutrients to the myocardium. Cellular respiration is one of the important ways a cell gains useful energy.
Under basal aerobic conditions, the energy of the myocardium is derived as follows:
- 60% from free fatty acids and triglycerides,
- 35% from carbohydrates,
- 5% from amino acids and ketones.
However, the above percentages vary according to the nutritional state of the person and presence of diseases like diabetes. The heart muscle is unique without comparison. Read fascinating facts about the heart.
Function of Myocardium.
Though the other layers of the heart, namely the pericardium and the endocardium, have their own functions to perform, the myocardium comprising of the cardiac muscle is singularly responsible for bringing about the coordinated relaxation and contraction of the atria and ventricles of the heart. This process which constitutes the heart beat is brought about at the rate of 72 beats per minute.
It is this action which helps to bring about purification of blood in the lungs and supply of this purified blood to all parts of the body so that all body functions are performed to the optimum. The myocardium plays an important role in maintaining the sequence of blood flow through the heart, with systole alternating with diastole.
Histology.
As mentioned above, the cardiac muscle is an involuntary muscle composed of specialized striated muscles. These striations are due to alternating thick and thin protein filaments. The thin filaments are made up of the protein actin while the thick filaments are made up of the protein myosin.
These long fibrous proteins slide past each other when the muscles relax and contract. The thin actin striations form major portion of the I band while the thick myosin filaments form the A band which appears darker under the electron microscope. The cardiac muscle cells are branched while the muscle cells of the skeletal muscle are longitudinal and linear.
Calcium and Heart.
Calcium is essential to the muscles to contract. Unlike skeletal muscles which require intracellular calcium, cardiac muscles require extra cellular calcium ions for contraction.
Blood Supply of the Myocardium.
Considering its non stop working throughout life, the myocardium or the heart muscle needs and deserves a rich supply of oxygenated blood. This is brought about by the left and right coronary arteries which are branches of the ascending aorta. Drainage of blood is by the coronary veins.
Most of the blood supply of the myocardium occurs during its relaxation (diastole) as during contraction, the arteries get compressed. Read coronary circulation for a detailed report.
Nerve Supply to The Myocardium.
Being an involuntary muscle, myocardium is controlled by the autonomic nervous system. Both the sympathetic and the parasympathetic nervous systems (which are part of the autonomic nervous system) supply nerves to the myocardium with both afferent (sensory) and the efferent (motor) innervations.
Sympathetic innervation.
This is from the cardiac fibers coming from the superior, middle and inferior cervical ganglia and the thoracic cardiac nerves coming from T1 to T4 segments of the spinal cord.
Parasympathetic innervation.
This is from the cardiac branches of the vagus nerve. These nerves of the parasympathetic nervous system which innervate the heart form several cardiac plexuses. They are:-
- Superficial cardiac plexus
- Deep cardiac plexus.
- Right coronary plexus
- Left coronary plexus.
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