Mechanism of Muscle Contraction: Have you ever wondered how we pull or push a gate or do any exercise? How do we bend or stretch our bodies? There are numerous muscles in our body. Muscle helps in performing various activities of our body. Thus, the contraction of muscles occurs when it gets stimulated. Several changes occur during the contraction of the muscle. Read this article in order to know more about the mechanism of muscle contraction, various events, and changes that occur during the contraction of the muscle.

Contractile Elements of Muscle

Skeletal muscle is made up of many long and thin cells called muscle fibres. Each muscle fibre is long, cylindrical, multinucleated, and is surrounded by a cell membrane called sarcolemma. The cytoplasm of the muscle fibre is called sarcoplasm. Muscle fibre contains several thousands of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils. Many muscle fibres are arranged to form muscle bundles or fascicles.

Cross striations are present in the skeletal muscles. Each myofibril consists of two alternating bands, which are called sections or segments, or disks. These bands are made up of muscle proteins. The two bands are:

1. Light band or ‘I’ band: It is called ‘I’ (isotropic) band because it is isotropic to polarized light.
2. Dark band or ‘A’ band: It is called ‘A’ (anisotropic) band because it is anisotropic to polarized light.

In an intact muscle fibre, the ‘I’ band and the ‘A’ band of the adjacent myofibrils are placed side-by-side. It gives the appearance of characteristic cross striations in the muscle fibre.

I band is divided into two portions with the help of a dark line called the ‘Z’ line. The portion of myofibril in between two ‘Z’ lines is known as a sarcomere.

Fig.: A. One Muscle Cell; B. One Myofibril

Contraction of muscle occurs with the help of contractile proteins. Actin and myosin are the contractile filaments present in the skeletal muscles, which helps in the contraction of a muscle. Myosin filaments are formed by myosin molecules. Actin filaments are formed by three types of proteins, namely actin, tropomyosin, and troponin. Hence, these four proteins are responsible for making the contractile proteins or the contractile elements of the muscle.

Fig: Sarcomere in Resting Muscle A. Contracted Muscle; B. During Contraction, Z Lines Come Close, H Zone and I Band are Reduced, and No Change in A Band.

Structure of Contractile Proteins

Muscle fibres are made up of four contractile proteins, namely: myosin, actin, troponin, and tropomyosin.
Each myosin filament consists of approximately 200 myosin molecules. Though there are about 18 classes of myosin, only myosin II is present in the sarcomere.
Each myosin molecule consists of two portions:
1. Tail portion: It consists of two heavy chains, which are arranged in the form of a double helix.
2. Head portion: At one end of the double helix, heavy chains twist in opposite directions and form the globular head portion. Therefore, the head portion consists of two parts where the head serves as a cross-bridge.
Myosin is split by enzyme trypsin into two fragments, namely: Heavy meromyosin (HMM) and light meromyosin (LMM).

Fig. Myosin Monomer

Site On Myosin Molecule

Each myosin head consists of two attachment sites. One site is for actin filament, and the other site is for one ATP molecule. Myosin head is absent in the centre part of myosin filament, which is in the ‘H’ zone.

Actin Molecule

Actin molecules are the main constituents of thin actin filaments. Each actin molecule is called F-actin. It is the polymer of a protein called G-actin. The actin molecules are arranged in the form of a double helix. On each F-actin molecule, an active site is present on which the myosin head is attached.

Fig. Actin Molecule

Tropomyosin

Approximately 40 to 60 tropomyosin molecules are situated along the double helix strand of the actin filament. In relaxed conditions, the tropomyosin molecules cover all the active sites of F-actin molecules.

Fig. Part of the Actin Filament. Troponin has Three Subunits, T, C, and I.

Troponin

It is formed by three subunits:

1. Troponin I: It is attached to F-actin
2. Troponin T: It is attached to tropomyosin
3. Troponin C: It is attached to calcium ions

Difference Between Actin and Myosin

Actin Filament	Myosin Filament
These are found in I-band and projects in A-band.	These are found in A-band only.
These are thin in nature.	These are thick in nature.
There are approximately 3000 actin filaments per myofibril.	There are approximately 1500 myosin filaments per myofibril.
These are lighter in weight.	These are heavy in weight.
Cross bridges are absent in them.	Cross bridges are present in them.

Physiological Changes during Contraction of Muscles

During resting as well as in active condition, various electrical events take place. Electrical potential during resting conditions is known as resting membrane potential. Electrical changes which occur in active conditions is known as the action potential.

Action potential

An action potential is defined as a series of electrical changes that occur in the membrane potential when the muscle or nerve gets stimulated.
An action potential occurs in two phases: Depolarization and Repolarization.

1. A specialized mechanism couples action potential with the release of calcium ions from the sarcoplasmic reticulum.
2. As a nerve impulse reaches the terminal end of the axon, small sacs known as synaptic vesicles fuse with the axon membrane leading to the release of a chemical transmitter called acetylcholine.
3. Acetylcholine diffuses across the synaptic cleft and binds to the receptor site of the motor endplate.
4. When depolarization of the motor endplate reaches a level, then action potential is created.
5. This action potential then passes from the motor endplate over the sarcolemma, then to the sarcoplasmic reticulum to stimulate the release of calcium ions into the sarcoplasm.

Molecular Basis of Muscle Contraction

In this mechanism, an actomyosin complex is formed that results in muscular contraction.
It includes three stages:

1. Excitation-Contraction Coupling
2. Role of Troponin and Tropomyosin
3. Sliding Mechanism

1. Excitation-Contraction Coupling
Excitation-contraction coupling is the process that takes place between the excitation and contraction of the muscle. In this process, a series of activities are involved which are responsible for the contraction of the excited muscle.

Stages of Excitation-Contraction Coupling
When a muscle gets stimulated by the impulses passing through the motor nerve and neuromuscular junction, an action potential is generated in the muscle fiber. Action potential spreads over the sarcolemma, and as soon as the action potential reaches the cisternae, these get excited. Calcium ions which are stored in the cisternae, get released into the sarcoplasm, which moves towards the actin filaments to produce the contraction.

2. Role of Troponin and Tropomyosin
The head of myosin molecules has a tendency to get attached to the active site of F-actin. But, in a relaxed condition, the active site of F-actin is covered by the tropomyosin, as a result of which the myosin head cannot combine with the actin molecule. A large number of calcium ions are released during the excitation of the muscle, which binds with troponin C. This causes a change in the position of the troponin molecule, which in turn pulls the tropomyosin molecule away from F-actin. Due to the movement of tropomyosin, the active site of F-actin gets exposed. As a result of this exposure, the head of the myosin gets attached to the actin.

Fig. Role of Calcium Ions in the Formation of Cross-Bridge

3. Sliding Mechanism and Formation of Actomyosin Complex – Sliding Theory
The sliding theory was proposed by H.E Huxley and J. Hanson and A. F. Huxley and R. Niedergerke in 1954. explains the concept of how the actin filaments slide over myosin filaments and thus, forms the actomyosin complex during muscular contraction. It is also known as ratchet theory or walk-along theory.
After binding with the active site of F-actin, the myosin head is tilted towards the arm so that the actin filament gets pulled with it. The tilting of the head is called a power stroke.
After this, the head breaks away from the active site and returns to its original position. Then it combines with a new active site on the actin molecule. And tilting movement occurs again. Thus, the head of cross-bridges bends back and pulls the actin filament towards the center of the sarcomere. Actin filaments of opposite sides get overlapped and form actomyosin complexes. This actomyosin complex results in the contraction of the muscle.

Changes in Sarcomere during Muscular Contraction

Changes that occur in sarcomere during muscular contraction are:

1. Length of sarcomeres decreases as the ‘Z’ lines come closer to each other.
2. Length of the ‘l’ band decreases as the actin filaments from opposite sides get overlapped.
3. ‘H’ zone decreases or disappears.
4. Length of ‘A’ band remains the same.

Fig: Sliding Mechanism

Fig: Stages in Cross-Bridge Formation

Energy for Muscular Contraction

Energy for movement of the myosin head is obtained by the breakdown of adenosine triphosphate into adenosine diphosphate and inorganic phosphate. The myosin head is the site for the attachment of ATP molecules. The energy released during this process is used in the contraction process.

Relaxation of the Muscle

Relaxation of the muscle occurs when the calcium ions are pumped back into the L tubules.

Fig: Sequence of Events during Relaxation of Muscle

Muscle Fatigue

The force of muscle contraction decreases when the muscle gets stimulated for a longer period of time. This is known as muscle fatigue. It occurs when a muscle gets stretched for a short period of time in the absence of oxygen. As a result of this, the muscle gets fatigued soon. Lactic acid gets accumulated in the muscles, which leads to pain in the muscle.

In order to get it resolved soon, one should breathe faster for a while to meet the requirement of extra oxygen in the body.

Oxygen Debt

Fatigued muscle needs extra oxygen to oxidize the accumulated lactic acid in the muscle in an aerobic environment. This is known as the oxygen debt of the muscle.

Red and White Muscle Fibres

There are two types of muscle fibres present in birds and mammals. These are :

1. Red muscle fibre
2. White muscle fibre

Difference between Red Muscle Fibres and White Muscle Fibres

Red Muscle Fibre	White Muscle Fibre
These are dark in color due to the presence of myoglobin.	These are lighter in color because myoglobin is present in very less amounts.
Mitochondria are more in number.	Mitochondria are less in number.
They carry out aerobic oxidation.	They depend upon anaerobic oxidation.
Fatigue does not occur in them.	They get fatigued soon.
They have a slow rate of contraction, for example, the extensor muscle of the back.	They have a fast rate of contraction, for example, eyeball muscle.

Summary

In the mechanism of muscular contraction, an actomyosin complex is formed that results in the contraction of muscles. Various events take place in this mechanism, like excitation-contraction coupling, the role of troponin and tropomyosin, and the sliding mechanism. In the sliding mechanism, the myosin head is tilted towards the actin arm after binding with F-actin.

As a result of this, the actin filament gets pulled. After this, the myosin head breaks away from the active site and returns to its original position. Then it combines with a new active site on the actin molecule, and tilting occurs again. Thus, the head of cross-bridges bends back and pulls the actin filament towards the center of the sarcomere. Actin filaments of opposite sides get overlapped, which forms the actomyosin complex. This complex results in the contraction of the muscle.

Frequently Asked Questions (FAQs) on the Mechanism of Muscle Contraction

Q.1. What are the contractile elements of muscle contraction?
Ans: Actin and myosin are the contractile filaments present in the skeletal muscles, which helps in the contraction of a muscle.

Q.2. Which type of myosin molecule is present in the sarcomere?
Ans: There are about 18 classes of myosin, but only myosin II is present in the sarcomere.

Q.3. What are the subunits of troponin?
Ans: There are three subunits, namely, Troponin I, Troponin T, and Troponin C.

Q.4. Define action potential.
Ans: The action potential is defined as a series of electrical changes that occur in the membrane potential when the muscle or nerve gets stimulated.

Q.5. What happens in the muscle during depolarization?
Ans: During depolarization, the inside of the muscle becomes positive, and the outside becomes negative. Thus, the polarised state is abolished.

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