What is cardiac muscle and its function?
Written by Elisha Kristin Pasco
The cardiac muscles are also known as the myocardium. They make up the muscular middle layer of your heart and enable it to circulate the blood in your body. The myocardium is a muscle that you can only locate in your heart. Surrounding it is a thin outer layer called visceral pericardium or your epicardium. Your myocardium then covers the inner layer called the endocardium.
The myocardium is responsible for the involuntary contraction and relaxation of your heart. It can make your heartbeat because of cardiomyocytes. Cardiomyocytes are cardiac muscles that compose your myocardium. The primary function of these heart cells is to contract, thus enabling your heart to pump your blood.
The cardiomyocytes that make up your myocardium have a rectangular shape. They are branching cells with only one nucleus found at the center. It also contains mitochondria. The mitochondria provide your heart with the energy needed for contraction.
The cardiomyocytes need a sufficient amount of adenosine triphosphate. It is why mitochondria are present within the cells.
When cardiomyocytes organize themselves into repetitive units, it forms a sarcomere. The sarcomere is the repeated overlapping of the muscle filaments. The thick and thin myofilaments arrangement gives your myocardium its signature striated appearance.
Sarcomeres, however, are not exclusive to only your myocardium. They are also present in your skeletal muscles. And, like in the myocardium, they are also responsible for their striated appearance.
A sarcolemma surrounds your cardiomyocyte. It is a plasma membrane that regulates what comes in and out of the cells. The sarcolemma contains invaginations called T-tubules. The tubules hold the numerous proteins necessary for cardiomyocyte function. The proteins found within the cavity are as follows:
- L-type calcium channels
- sodium-calcium exchanger
- calcium ATPases
- beta-adrenergic receptors
The t-tubules are invaginations that excite the contraction of your cells. They ensure that your body’s pump can circulate the blood in your body.
Intercalated discs link each cardiomyocyte through three different cell junctions. The discs are a distinct feature of your heart tissues and contain the following:
- fascia adherens
- gap junctions.
Fascia adherens serve as anchoring junctions. They enable actin filaments to attach to the thin filaments of your sarcomeres to your cells.
Desmosomes are adhesion sites that keep your muscles cells together during a contraction.
Gap junctions are responsible for direct contact between the cells of your heart. It enables electric communication, enabling your heart to beat.
The myocardium also holds pacemaker cells called sinoatrial (SA) nodes. The SA nodes are responsible for regulating your heart rate.
The myocardium is composed of individual muscle cells called cardiomyocytes. Cardiomyocytes can make your heart contract due to the myofibrils that make up the cell. Your myofibrils are the specialized cytoskeletal structure that enables your heart to beat.
As a cytoskeletal structure, your myofibrils help the cardiomyocytes maintain their shape. They are composed of myofilaments. They are rod-like tubules that overlap and organize themselves in repeating units called sarcomeres.
Sarcomeres are the contractile units of your muscle cells. Two types of myofilaments overlap to form your sarcomeres. Thick and thin myofilaments containing unique proteins make your sarcomeres.
The thick myofilaments contain the protein myosin, whereas the thin ones hold actin.
Actin is the major cytoskeletal protein of your cardiomyocytes. It is the protein responsible for the cells’ movement. On the other hand, myosin is a protein that converts adenosine triphosphate (ATP) to energy. It provides your muscles the fuel to move.
The cardiac muscle is involuntary. AF Huxley and R Niedergerke and HE Huxley and J Hanson’s first described the sliding filament theory. They discussed their theory in two research papers published in 1954. The papers explain how the muscles in your heart can beat on their own.
The research describes the molecular basis behind the muscle contraction of your heart. In the paper, it notes that the sarcomeres have two different zones. The “A band” is an area that maintains its length during the contraction. On the other hand, the “I band” is the zone that changes its span when the sarcomere contracts.
The A band contains thick myofilaments composed of myosin. The constant length of this zone suggests that though the myosin participates in the beating of your heart, it is central and does not move. The I band is thin actin filaments that shorten whenever the heart contracts. Observations between the myosin and actin during the contraction of your heart enabled the development of the sliding filament theory.
The sliding filament theory describes actin sliding past the myosin. The movement creates tension within your muscles. This action would cause the sarcomere to shorten because actin is bound to the z bands. Z bands are structures at the lateral end of your sarcomeres. It transmits the tension from one sarcomere to the next.
Since the beating of your heart is involuntary, it would have to be regulated by some processes in your body. For your myocardium to contract, it uses calcium and ATP cofactors. ATP provides your muscles with the energy to move, whereas calcium is responsible for regulating muscle contraction.
Calcium, however, isn’t what controls the contraction of your heart. However, the proteins that manage it, troponin and tropomyosin, require it as a trigger.
The tropomyosin keeps the myosin from binding with the actin if the sarcomere is at rest. On the other hand, troponin is the protein that moves the tropomyosin from the myosin-binding sites of actin, enabling contraction. The action between troponin and tropomyosin is only possible when there is calcium. Without calcium, the myosin-binding areas will remain blocked by the tropomyosin, keeping actin and myosin from sliding against each other.
How do you identify cardiac muscle tissue?
The arrangement of the sarcomeres in your myocardium and striations can help you identify the cardiac muscle at first glance. However, skeletal muscles are also striated, so to further differentiate one from the other, look into its defining characteristics.
Aside from the striations, you will need to look into the general shape of the cells that make up your tissue. Cardiomyocytes are branching rectangular cells.
The next step would be to locate and count how many nuclei are present within the cells. Your myocardium is mononucleated. It only has one core situated in the center of the cell.
The striations in your heart are due to the arrangement of your thin and thick myofilaments in your sarcomeres. Your sarcomeres have different zones divided due to their composition and behavior during contraction.
The thickness of the A-bands gives them a darker color than the I-bands, with a relatively bright area in the middle. The Z disc presents itself as a dark line that connects the actin filaments of your muscles.
The myocardium is rectangular, but each muscle cell has a tubular structure. The shape is because of the repeating chains of myofibrils that form the sarcomeres, which have a rod-like profile.
Skeletal, smooth, and cardiac muscles differ in control, composition, shape, function, and location.
Skeletal muscles are attached to your bone and are in charge of your body’s skeletal movements and posture. Nerves from your somatic nervous system innervate the fibers enabling your central nervous system to control them.
As your central nervous system controls them, the muscles are under conscious and voluntary control. It is composed of repeating multinucleated cells that form sarcomeres. The sarcomeres are responsible for the striations in this muscle.
Aside from movement and posture, the skeletal muscles also play a hand in the following functions:
- Heat production
- Irritability – enables you to respond to stimuli from the external environment.
- Conductivity – can transmit impulses.
- Extensibility – gives you the ability to stretch without tearing yourself apart.
- Contractility – ability to shorten and create movement
The smooth muscles are in the walls of hollow internal organs such as:
- blood vessels
- gastrointestinal tract
- urinary bladder
You do not have voluntary control over your smooth muscles. Your autonomic nervous system controls them without your conscious input. Unlike your cardiac and skeletal muscles, the smooth muscles lack striations.
The lack of striations is because the thick and thin filaments do not form sarcomeres. Sarcomeres are responsible for the striated appearance of other tissues of your body.
Like the cardiac muscle, it can contract independently and with rhythm.
The cardiac muscles are muscles that are only present in your heart. The striations are due to the myofilament arranging themselves into sarcomeres as the skeletal muscle. The striations on the myocardium are shorter than those in your musculoskeletal system.
Unlike the musculoskeletal system, the cells of your cardiac muscle have one nucleus. The nucleus rests at the very center of the cell.
The autonomous nervous system controls them and enables them to contract involuntarily. The myocardium’s contractions have rhythm and are very strong.
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