The Incessant Beat
The human heart, powered by cardiac muscles, beats ceaselessly about 100,000 times a day, 35 million times a year, and more than 2.5 billion times over an average human lifetime.
The heart is a special organ and unique in its functioning. Cardiac muscles are devoted to keep it ceaselessly functioning, for your entire lifetime. It is largely made possible by the structure and functioning of these muscles. They make the most sophisticated bio-electric pump, which is capable of continuously functioning for about 70 to 80 years, on an average, to deliver oxygen and nutrients through blood, to every part of the body.
Weighing 250 gm to 350 gm, the heart is enclosed within a sac called pericardium, that is double-walled, to protect the vital organ from its surroundings. The heart wall is made up of three layers of tissues. The outer tissue is known as epicardium. The innermost layer is known as endocardium, which comes in contact with blood.
The middle layer is the myocardium, which consists contracting muscles. We explore this layer in the following lines. Let us look at the complex structure that orchestrates the symphony of the heart rhythm.
Muscles - An Overview of the Special Tissue
Muscles are the only cell tissues that can contract and expand. This facility for elongation and contraction is enabled by their filamentous nature. By functionality, they come in three types: skeletal, smooth, and cardiac muscles.
The skeletal variety are voluntary muscles. That is, their action is governed by your own conscious will, or rather your brain's will, whereas, smooth and cardiac muscles are of an involuntary nature. They keep functioning, without the force of will, as they control the functioning of all vital internal organs, including the heart.
Structure of Cardiac Muscles
These muscles are striated (They have multiple sarcomeres, which are basic contractile units of a muscle.) and they constitute the myocardium. Though, these muscles are involuntary, they resemble skeletal muscles in a lot of respects. Both are similar with regards to appearance, striations, and contraction properties.
One distinctive feature of these muscles is that they are branched, unlike skeletal muscles, which tend to be linear. They are made up of alternate thick (Myosin) and thin (Actin) sliding protein filaments. Actin and Myosin are the two primary proteins that build these muscle fibers, which are called myofibrils.
The sub-units of myofibrils are called sarcomeres. Each one of them is made up of A bands and I bands. Actin filaments form the light-colored I bands and Myosin filaments form the dark-colored A bands. The A bands are subdivided by the M line and the H-zone, while I band is subdivided by the z-discs.
The myofibril branches are connected by Adherens junctions, which help the heart in contracting forcefully.
The 'T-tubules' are another feature of these muscles, which are comparatively broader than those of skeletal muscles. These structures run along the z-discs of these muscles. They play an important role in excitation-contraction coupling, which drives the heart.
Another feature that you see, when you observe a cardiac muscle, under an electron microscope are the intercalated discs. They are dark thin lines that divide the muscle cells and they are perpendicular to the direction of the individual muscle fibers. These discs enable the transmission of muscle contraction signals. It is because of them that action potentials spread fast and the myocardium contracts in a synchronized fashion.
Cardiac Muscle Cell (Cardiomyocyte) Structure
One of the basic functional units of the cardiac muscle is the myocardiocyte. It is built out of myofibrils, the fibers created out of alternating Actin and Myosin filaments. These myofibrils in turn, are made out of sarcomeres, the contractile units.
The cardiac muscles, unlike skeletal ones, cannot rest, even for a moment. They must work continuously. Each myocardiocyte contains one nucleus. The secret behind the unfailing and ceaseless working of the heart lies in this unit cell. The density of mitochondria (the energy generators of the body) in these cells is high, which lets them produce an abundance of ATP molecules through aerobic respiration, to drive the muscle's functioning. This is the reason why a heart's muscle tissue can work without fatigue and ensures a lifetime of service.
The flexibility of the cell, along with the integrity of the cytosol is maintained by two proteins, called vimentin and desmin.
The second set of basic cells that drive the functioning of the heart muscle are the pacemaker cells. They actuate the beating of the heart. They are placed at various positions throughout the heart to primarily generate and transmit the electrical signals that drive its functioning. Besides this prime function, they also communicate electrical signals between cells. They also receive electrical signals from the brain. The electrical impulses are generated through the cardiac action potential, that involves sodium, potassium, and calcium ions.
The whole muscle structure is built in such a way, that it can easily contract and expand with the action potential signal, spreading through it periodically. The signal is transmitted through every fiber of this structure, through gap junctions. It takes a greater amount of time for the structure to expand, than it takes for it to contract.
These muscles have a very marginal store of glycogen, which is the raw material for energy production in anaerobic conditions. So, in case of a heart attack, when the heart muscles fall short of oxygen, they simply die. Recent studies have shown that regeneration in heart cells is possible.
Knowing the beautifully complex and ingenious mechanism that drives our hearts, generates a renewed respect in us, for the marvel that is the human body.