Introduction to Myofibril

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A myofibril is also known as a muscle fibril. The myofibril is a basic rod-like unit of the muscle cell. Myocytes are the tubular cells that the muscles are made up of. The myocytes are also known as the muscle fibers in the striated muscle and these cells also contain many chains of myofibrils. During the process of myogenesis, the myocytes are considered to be created at the embryonic development stage. 

The myofibrils are mainly composed of many long proteins such as myosin, actin, and titin, and also there are other proteins that hold them together. All these proteins are organized into thick and thin filaments which are called myofilaments. The myofilaments keep repeating themselves along the length of the myofibril in the section called the sarcomeres. The contraction of muscles happens when there is sliding between the thick (myosin)and thin (actin) filaments along with each other. 

In this article, we are going to discuss the muscle, the definition of myofibril, its structure, function, and also a few frequently asked questions will be answered.

The Muscle 

A large fraction of the total body weight in humans is constituted by the striped or the striated muscles. the maintenance of the body posture and the movements of the libs is done when there is a contraction in the striated muscles. The striated muscles are also called the skeletal muscle as both ends of the striated muscles articulate the skeleton. The skeletal or striated muscles are attached to the bones by the tendons which have the elasticity that is provided by the protein collagen and elastic, which is considered to be the major components of the tendons. 

There are blood vessels and nerves which are associated with each striated muscle. These blood vessels help in the transportation of the blood to and from the muscle and also supply oxygen and other nutrients and also remove the unnecessary carbon dioxide and other waste from the blood. Through the motor nerves, there are signals that are sent from the central nervous system to the muscles that initiate the contraction of the muscles. There is also a response that the muscle gives to the hormones that are produced by the endocrine glands. These hormones also interact with the complementary receptors which are present on the surface of the cells which helps in initiating the specific reaction. 

There are important sensory structures associated with each muscle called the stretch receptors that help in monitoring the state of the muscle and also return all the information to the central nervous system. The change in the length of the muscle and the velocity of the movement of the muscle are the two things that the stretch receptors are sensitive to. The stretch receptors are responsible for completing a feedback system that allows the central nervous system to assess the movement of the muscles and also to adjust the motor signals in the light of the movement.

Myofibril Definition 

The myofibril is a component of the animal skeletal muscle. Myofibrils are very fine contractile fibers and many groups of myofibrils are extended in the parallel columns along the length of the striated muscle fibers. The myofibrils and the resulting myofibers may be several centimeters in length. The myofibrils are composed of many thick and thin myofilaments that help in giving the muscle its striated appearance. The thick filaments are composed of myosin and the thin filaments are composed of actin and also have other muscle proteins such as tropomyosin and troponin. The myofibrils are made up of repeated subunits which are called the sarcomeres. 

The muscular contraction happens due to the interaction between the actin and the myosin filaments when they are temporarily bound and released. The muscle fibres are single multinucleated cells that usually combine to form a muscle. 

Structure of Myofibril 

There are Two Types of Myofilaments That the Myofibrils are Made of -

  • Thin Filaments: The thin filaments primarily consist of the protein actin which is coiled with the nebulin filaments. When the polymerization of the actin filament happens, there is a formation of a ladder along which the myosin filament climbs to generate the motion. 

  • The Thick Filament: The thick filament is primarily composed of the protein called myosin. The responsibility of the protein myosin is the forced generation. Myosin is composed of a globular head that has both Adenosine triphosphate(ATP) and actin-binding sites and also a long tail which is involved in its polymerization into the myosin filaments.  

  • Acrinomyosin is the name given to the protein complex which is composed of both actin and myosin. 

  • The striated muscle such as that of the skeletal and the cardiac muscle have actin and myosin filaments that are of specific and constant length which are of the order of a few millimetres. These are very small when compared to the elongated muscle cells which are of the length in the centimetres. 

  • The myofilaments are organized into repeated subunits along the length of the myofibrils. These subunits are called the sarcomeres.

  • Myofibrils fill the muscle cells which run parallel to each other on the long axis of the cell.

  • The sarcomeric subunits of one myofibril are in perfect alignment with the myofibrils which are next to it and this alignment causes the cells to look striated or striped.

  • In the case of smooth muscle cells, there is no alignment, hence there are no striations and hence the cells are called smooth.

How to Label the Myofibril and Its Components?

The Diagram Given below Shows How to Label the Myofibril and Its Components.

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  • The skeletal muscle cells are long and cylindrical and they are also referred to as the muscle fibres or the myofibers. The skeletal muscle fibres are very large when compared to other cells. They have lengths up to 30 cm and diameter which is up to 100 micrometres and this is the example of the sartorius of the upper leg. 

  • The skeletal muscle fibres contain cellular organelles found in other cells such as mitochondria which are also called the powerhouse of the cell membrane.

  • The plasma membrane of the muscle fibres is called the sarcolemma. The sarcolemma is derived from the Greek word ‘sarco’ which means flesh. The cytoplasm of the sarcolemma is called the sarcoplasm. 

  • Within the muscle fibres, the myofibrils are present that stores the proteins in an organized manner. The myofibrils that run the length of the cell contain sarcomere which is connected in series.

  • There are hundreds and thousands of myofibrils which are only about 1.2 micrometres in diameter which have thousands of sarcomeres and that can be found inside one muscle fibre.

  • In the muscle fibre, the sarcomere is considered to be the smallest functional unit and it also has a very organized arrangement of the contractile, regulatory, and structural proteins. 

  • The shortening of the individual sarcomeres leads to the contraction of the individual muscle fibres.

The Appearance of the Myofibrils 

  • When viewed through the light microscope, it is seen that there are various sub-regions of the sarcomeres. The names of the sub-regions in the sarcomere are based on their relative darker or lighter colour appearance when viewed through the light microscope. 

  • There are two dark-coloured bands called the Z-disc or the Z line which each sarcomere is delimited with. 

  • The Z-disc or the Z-lines are dense protein discs that do not allow the passage of light. The T-tubules are present in the Z-disc area.

  • The area present between the Z-disc is further divided into two light-coloured bands at either end which is called the I-bands and in the middle region, there is a presence of a darker, greyish band called the A-band. 

  • The I bands appear to be very light because this region of the sarcomere is mainly contained very thin actin filaments that have a very small diameter as a result of which allows the passage of light between them. The I in the I-band stand for isotropic. Isotropy means to have identical values of a property in all directions. 

  • The A band is mainly composed of the myosin filaments. The myosin filament has a very large diameter and as a result, it restricts the passage of light. The A in the A-band stands for anisotropic. Anisotropy is the property of the material that allows it to change or assume different properties in a direction. 

  • The part between the A-band and the I-band is occupied by both the actin and the myosin filaments.

  • There is a presence of a relatively brighter central region in the A-band called the H-zone. There is no overlapping of the action and the myosin in the H-zone when the muscle is in a relaxed position. H in the H-zone is the German word helle which means bright. 

  • M-line is a dark central line that bisects the H-zone. The M in M-line comes from the German word mittel which means middle. The M-line also contains the enzymes that help in energy metabolism. 

Functions of Myofibril 

  • The sarcomeres are considered to be the building blocks of the myofibrils which are the functional unit of the muscle.

  • The main function of myofibrils is to perform muscle contraction. There is an incomplete overlap between the thin and the thick filaments when the muscle is at rest.

  • When there is a contraction of the muscles, there is a shortening in the length of the sarcomeres due to the thick and thin filaments sliding over each other, which results in the overlap between the filaments and there is also a shortening of the H-zone and the I band. The length of the myofilaments does not change even when the sarcomere length decreases during the muscle contraction.

  • The hydrolysis of the Adenosine triphosphate(ATP) to Adenosine diphosphate (ADP) and other inorganic phosphates powers the movement of the myofilament. 

  • The ATP molecule is attached to the globular myosin head on the filaments when it is at rest and when the ATP is hydrolyzed, the myosin head changes conformation and forms an attachment known as the cross-bridge with the thin filament.

  • The myosin head again changes conformation and pushes the thin filament towards the centre of the sarcomere as soon as the ADP and the phosphate molecules are released. 

  • The myosin head then again gets bonded to the new ATP molecule, after that the head returns to its initial conformation and then releases the thin filament in its new position nearer to the central M-line.

  • There is the repetition of this cycle where the ATP molecule gets hydrolyzed into the ADP and inorganic phosphate and the myosin head changes conformation which results in the thin filament that gets pushed towards the centre of the sarcomere.

  • Each thick filament has hundreds of myosin heads that are capable of forming cross-bridges with the thin filaments about five times per second. The muscle contraction happens due to the continuous contractions of the myofibrils. 

  • The ATP is powered by muscle contractions. The muscle fibres are capable of storing only a very small amount of ATP and as a result of which there is a requirement of energy and that requirement is fulfilled by two other compounds stored in the muscles which are creatine phosphate and glycogen.

  • To provide a short-term burst of energy for almost twenty seconds, the ATP that is stored in the muscle fibres and the ATP that is formed by creatine phosphate is used.

  • Glycogen can also be used as the long-term energy source as the glycogen gets broken down to glucose which is later converted to ATP through the process of glycolysis and aerobic respiration. 

FAQ (Frequently Asked Questions)

Q1: How Many Types of Myofilaments are Found in the Myofibril?

Ans: There are two types of myofilaments that are found in the myofibril, the thick and the thin filament. The thick is made up of myosin and the thin filaments are made up of actin. 

Q2: What is the Function of Myofibrils?

Ans: Sarcomeres are the functional unit of the muscles and the myofibrils are made of sarcomeres. The main function of the myofibrils is to perform muscle contraction.