Introduction

Skeletal muscles are the most abundant tissues found in animals. They make up more than 40% of the total weight of an average human. These muscles are responsible for all kinds of movements exhibited by a human being. They are under voluntary control meaning that we can consciously control their contractions.

Skeletal muscles are made up of parallel fibres called skeletal muscle fibres. Each fibre is a multinuclear cell having hundreds to thousands of myofibrils. Myofibrils are the thin fibres made up of acti9n and myosin filaments and are present among the cytoplasm of skeletal muscle fibres. They are the contractile units that are responsible for the contraction of skeletal muscles. Each skeletal muscle fibre is innervated by a separate axon and can undergo contraction independent of other cells.

In this article, we will discuss the general structure of skeletal muscle fibre, the structure of myofibrils, their major components, the mechanism of contraction, and some major diseases associated with them. So, keep reading.

General Structure of a Skeletal Muscle Fiber

Skeletal muscle fibres are cylindrical cells with some special modification to help them undergo contraction. Like other animal cells, they have a plasma membrane, and other organelles like ribosomes, mitochondria, endoplasmic reticulum, etc. They have multiple nuclei within one cell. Some of the important features of these cells are as follows.

Sarcoplasm

The cytoplasm of skeletal muscle cells is called sarcoplasm. It is surrounded on the outside by a plasma membrane called sarcolemma. Sarcoplasm has abundant mitochondria that provide energy in the form of ATP needed for muscle contractions. They have multiple nuclei that lie flat in the periphery of the fibre. Nuclei are present adjacent to the sarcolemma of cells.

Myofibrils are arranged in a parallel fashion in the skeletal muscle fibres separated by the sarcoplasm. Mitochondria lie close to the myofibrils so that energy can be provided to these contractile fibres.

Sarcoplasmic Reticulum

It is a network of a rich network of endoplasmic reticulum found only in the skeletal muscle fibres. Its function is to store calcium ions and release them upon excitation for the contraction of skeletal muscles.

It appears as a complex network of tubules present among the myofibrils. These tubules have terminal dilations or sacs called terminal cisternae. The invaginations of the sarcolemma are present in between these cisternae. The action potential is carried by these invaginations to the sarcoplasmic reticulum so that they are excited to release calcium ions.

T-tubules

These are the invaginations of sarcolemma having the same concentration of ions as is present in the extracellular fluid. An action potential generated at any point along the length of the sarcolemma is carried by these tubules to the core of muscle fibres. They are connected to the terminal cisternae of the sarcoplasmic reticulum and are responsible for releasing calcium ions and coupling the contraction of skeletal muscles with excitation. 

Structure of Myofibrils

After understanding the general structure of a skeletal muscle fibre, let us now study the structure of myofibrils. As mentioned earlier, each skeletal muscle fibre consists of hundreds to thousands of myofibrils extending throughout the length of the fibre. They are the contractile structures in skeletal muscles. The contraction of these myofibrils causes the contraction of skeletal muscle.

Some important features of myofibrils are described below.

Composition

Each myofibril is composed of two types of contractile filaments;

• Thin or Light filaments
• Thick or Dark filaments

The number of these filaments varies in individual myofibrils. Each myofibril contains around 3000 thin filaments and 1500 thick filaments. These light and dark filaments are arranged within a myofibril in such a way that they interdigitate and give a striated appearance to it. These striations of myofibrils are responsible for the overall striated appearance of skeletal muscles.

Thin filaments

The thin filaments are also called actin filaments. They are made up of three types of protein;

• Actin
• Troponin
• Tropomyosin

As myofibrils are made up of interdigitating thick and thin filaments, the parts containing only thin filaments appear as light bands called the I bands. They are named I bands because they are isotropic to polarized light.

Actin

Actin is a contractile filamentous protein that makes the backbone of actin filaments. This filamentous actin protein is made up of F-actin filaments, each of which consists of polymerized G-actin molecules. Two F-actin filaments are wrapped around each other to form a helix in thin filaments.

A molecule of ADP is associated with each G-actin molecule. These ADP molecules act as active sites for the formation of cross-bridges (formed by the association of thick and thin filaments).

Tropomyosin

Tropomyosin is another filamentous protein present in the thin filaments of myofibrils. Two tropomyosin filaments are wrapped around the double helix formed by F-actin. The tropomyosin filaments are wrapped in such a way that they lie in the grooves of the F-actin double helix.

The tropomyosin filaments cover the active sites on F-actin molecules and prevent the formation of actin-myosin cross-bridges in the resting state.

Troponin

Troponin is a non-filamentous protein associated with thin filaments. It is found attached to the tropomyosin molecules at periodic sites along the length of a myofibril. Troponin protein functions to attach tropomyosin with F-actin in thin or actin filaments.

Each troponin molecule is made up of three subunits;

• Troponin I, having high affinity for F-actin filaments
• Troponin T, having an affinity for tropomyosin filaments
• Troponin C, having a high affinity for calcium ion

Troponin also serves to initiate the process of contraction by dispacing tropomyosin and exposing myosin-binding sites on actin filaments. Its function will be discussed in detail under the section of contraction.

Thick filaments

These are also called myosin filaments as they are made up of myosin molecules. Myosin is a filamentous protein that polymerizes to form thick filaments in myofibrils. There are small projections at the ends of thick filaments known as myosin cross-bridges. The contraction of skeletal muscle results from an interaction between the actin filaments and cross-bridges of myosin filaments.

One thick filament or myosin filament is made up of around 200 myosin molecules that are spirally arranged around each other. Each of these myosin molecules is composed of six polypeptide chains, two heavy chains, and four light chains.

The two heavy chains of myosin molecules are coiled around each other to form a double helix forming the tail of myosin thick filaments. Each end of these heavy chains is folded to form a globular structure. This structure incorporates two light chains to form myosin heads. Two myosin heads are present at the end of each tail.

The myosin cross-bridges of thick filaments are made up of myosin heads along with the arms by which they are attached to the thick filament. A pair of cross-bridges are present together as they project from the main filament. The successive bridges are arranged in such a way that there is an angle of 120 degrees between two pairs.

The areas of myofibrils that contain myosin filaments along with the ends of the actin or thin filaments have a dark appearance. Therefore, they are known as dark bands. They are also called A bands as they are anisotropic when exposed to polarized light.

Z-Disk

These are the filamentous structures to which the ends of thin filaments are attached in myofibrils. The actin or thin filaments extend from these Z-disks and interdigitate with the myosin filaments.

The Z-disks are also made up of a filamentous protein that is non-contractile. Because of its non-contractile nature, this protein has a different structure as compared to actin or myosin proteins.

The Z-disks not only bind the thin filaments but also function to connect myofibrils as they extend from one myofibril to the next one. Alternate light, dark and light bands are present in between two successive Z-disks. The same pattern is seen in all the myofibrils giving them a striated appearance. Because of this striated appearance, the skeletal muscles are called striated muscles.  

Sarcomere

Sarcomeres are the contractile units of myofibrils present between two successive Z-disks. Each sarcomere is composed of two I bands separated by an A band present in the centre. All the thick and thin filaments in myofibrils are organized in the form of sarcomeres.

A slightly light is present in the centre of the sarcomere called the H zone. It appears slightly light as compared to the surrounding dark band as it has only thick filaments. The rest of the dark band on both sides of the H zone has interdigitating thick and thin filaments and appears darker. The H zone is bisected by a line called the M line. It divides the sarcomere into two halves.

The length of the sarcomeres does not remain constant as it changes when muscle contracts or relaxes. Its length decreases upon contraction and increases upon relaxation. In a fully contracted state, each sarcomere in skeletal muscle fibres has a minimum length of 2 micrometres. In this state, the actin filaments completely overlap the myosin filaments, and the H zone disappears. When the muscle relaxes, the H zone begins to appear again as the interdigitation of fibres decreases

Titin filaments

Titin is another filamentous protein that serves to keep the actin and myosin filaments in place. Its filaments are highly elastic and springy.

One end of the titin filament is attached to the Z-disk while the other end is attached to the myosin filaments present in the centre of the sarcomere. The end attached to the Z-disk is very elastic and springy. It acts as a spring that can undergo a change in its length during cycles of contraction and relaxation. This helps in keeping the thick and thin filaments in place after each cycle of contraction.

Read more about Muscles and Contraction

Excitation

Recall that the contraction of a muscle begins only when it is excited by a motor neuron. The contraction of the skeletal muscle results from the contraction of sarcomeres. They are not directly stimulated by the motor neurons. So how do the sarcomeres in myofibrils get to know when to begin contraction? The answer to this question lies is discussed in this heading.

When a skeletal muscle fibre is excited, the action potential is carried by the T-tubules to the sarcoplasmic reticulum. The activation of DHP receptors at the sarcoplasmic reticulum releases calcium ions into the sarcoplasm of the muscle cells.

These calcium ions bind to the troponin-C subunit and cause a change in its configuration. As a result, it changes the positioning of actin and tropomyosin proteins in the thin filaments. The myosin-binding sites on the actin filaments are exposed and cross-bridges can be formed to cause the contraction.

Thus, the rise in intracellular calcium ions provides a signal to begin the contraction process. This rise in calcium ions is responsible for exposing the myosin-binding sites, the only method by which the myofibrils know it is the time to undergo contraction.

Contraction

The contraction of myofibrils is responsible for the contraction of skeletal muscles. It takes place via the sliding filament model and involves the following steps;

• A molecule of ATP is attached to the myosin heads. This ATP molecule is hydrolyzed to charge the myosin cross-bridges. The energy is stored in the myosin heads as ADP and inorganic phosphate remain attached to them.
• These charged myosin cross-bridges attach to the myosin binding-sites on the actin filaments resulting in the formation of actin-myosin cross-bridges.
• The next step is called the power stroke. During this phase, myosin heads bend towards the center of the sarcomere generating a pulling force. As a result, the actin filaments are pulled towards the center of the sarcomere and its length decreases. This causes the contraction of muscle.
• When the muscle is about to relax or it is to begin another cycle of contraction, another ATP molecule is attached to the myosin heads. It causes the detachment of myosin heads from the actin filaments.
• The hydrolysis of this ATP again charges the myosin heads and the cycle continues. 

The ATP molecules needed for the contraction of myofibrils are provided by the abundant mitochondria present in the neighbouring regions of the sarcoplasm.

Clinical Conditions

After understanding the structure and mechanism of the contraction of myofibrils, let us now discuss some diseases associated with them.

Myofibrillar Myopathies

It is a group of disorders associated with the myofibrils of the skeletal muscles. They are a part of a more diverse group of muscular disorders called muscular dystrophies.

Myofibrillar myopathies can arise from mutations in various genes that encode the contractile and non-contractile proteins found in the myofibrils. Six genes have been discovered to be associated with these disorders. Mutation in any of these genes can cause abnormal structure and function of myofibrils.

Muscle weakness is the major sign of these disorders. It is mostly encountered in middle age, but can also occur in other phases of life. Muscle weakness begins in the hands and feet and then progresses to the higher areas of the body. The weakness of facial muscles can cause difficulties in the swallowing process. It also gets worse with time.

The disease can be diagnosed by various genetic tests performed at different clinical laboratories.

Read more about Skeletal Muscles

Summary

Skeletal muscles are the most abundant tissue found in the human body. They are responsible for all types of movements.

Skeletal muscles consist of multinucleated skeletal muscle fibres that are arranged parallel to each other throughout the length of the muscle. The contractile proteins inside the muscle fibres are arranged in the form of myofibrils.

The myofibrils inside the cell are surrounded by a cytoplasm called sarcoplasm.

• The energy for the contraction of myofibrils is provided by the abundant mitochondria present in the surrounding sarcoplasm.
• The calcium ions to begin contraction are stored and provided by sarcoplasmic reticulum.
• The action potential is carried deep to the sarcoplasmic reticulum by the T-tubules.

Myofibrils are dispersed in the sarcoplasm and extend throughout the length of the muscle fibres. They are made up of two types of filaments; actin filaments and myosin filaments.

Actin filaments are made up of three types of proteins;

• Two chains of F-actin molecules wind to form a double helix that acts as the backbone of actin filaments. The f-actin molecules have binding sites for myosin heads.
• Two chains of tropomyosin wind around the F-actin double helix and fit in the grooves so that all the myosin-binding sites are covered.
• Troponin keeps the actin and tropomyosin bound to each other.

Myosin filaments are made up of two heavy chains and four light chains of myosin molecules.

• Heavy chains form a double helix that acts as the tail of the filament
• Ends of the heavy chains along with light chains make myosin heads

Sarcomeres are the basic contractile units that are formed by the characteristic arrangement of actin and myosin molecules between two successive z-disks.

Excitation of myofibrils occurs when calcium ions bind to troponin and tropomyosin is displaced, exposing the binding sites for myosin heads.

Contraction takes place via the sliding filament model where the actin-myosin cross-bridges are formed and actin filaments are pulled towards the centre of the sarcomere.

Myofibrillar myopathies are a group of disorders associated with abnormal proteins in myofibrils. They are characterized by muscle weakness and are due to mutations in some specific genes.

Frequently Asked Questions

What are myofibrils?

Myofibrils are long rod-like organelles found in the skeletal muscles that are made up of sarcomeres. The contraction of these myofibrils causes the contraction of muscle fibres. 

What is the composition of myofibrils?

Myofibrils consist of the contractile system of the skeletal muscles. They are made up of four types of contractile proteins. These include actin, myosin, tropomyosin and troponin. These proteins are arranged in a framework known as a sarcomere.

What is the function of myofibrils?

All the myofibrils present in a muscle fibre contract together to cause its contraction. The contraction of myofibril happens via interaction among the contractile proteins present in it.

What is the difference between sarcomere and myofibrils?

Sarcomeres are the building blocks of myofibrils. One sarcomere is a space between two consecutive Z lines. On the other hand, one myofibril contains multiple sarcomeres arranged together.

References

1. McCracken, Thomas (1999). New Atlas of Human Anatomy. China: Metro Books. pp. 1–120. ISBN 1-5866-3097-0.
2. Muscle Physiology – Myofilament Structure
3. Marieb, E. N., Hoehn, K., & Hoehn, F. (2007). Human Anatomy & Physiology. (7th ed., pp. 284–87). San Francisco, California: Benjamin-Cummings Pub Co