Muscle Contraction And Contractile Proteins: Definition, Explanation, Function

Definition of Muscle Contraction

Muscle contraction is a process whereby muscle fibres develop tension and obtain shortening or lengthening to enable movement and generation of forces through the body. It is a foremost vital process in various biological functions, starting from voluntary movements like locomotion to continuing posture producing heat, and protecting viscera. This means that the said organ takes part in the performance and functioning of the heart and the process of moving substances by the digestive and vascular systems.

What are Contractile Proteins?

Contractile proteins are a special kind of protein witnessed in muscle cells. Two key contractile proteins, actin and myosin, are very vital in muscle contractility and movement.

Actin

Actin is a thin filament, and myosin is a thick filament with little projections binding to actin to cause force.

Myosin

During muscle contraction, myosin heads form a bond with the actin filament and swing in what is referred to as a power stroke or stroke of energy that drags the actin filaments in, consequently shortening the muscle fibre.

Troponin and Tropomyosin

This interaction is controlled by other proteins, such as troponin and tropomyosin, which regulate the bond of actin and myosin in response to the concentration of calcium ions.

Types of Muscle Tissue

The different types of muscle tissue are:

Skeletal Muscle

It is striated. It comes under voluntary control. It is attached to the bones using the tendons and so is in charge of movement and posture. The muscle fibres are long, cylindrical cells with multiple nuclei peripherally placed.

Cardiac Muscle

Present only in the heart, cardiac muscle is striated and involuntary. It is composed of very short cells, branching and interconnected to other cells by intercalated discs. The presence of these discs enables the cells to contract in a coordinated fashion, responsible for the pumping action of the heart.

Smooth Muscle

In this class of muscles, one does not find striations, and functioning is unconscious. It exists lining the inner walls of internal organs, like the digestive tract and blood vessels. The muscle fibres usually have a spindle shape with a single centrally located nucleus and carry out movements like peristalsis and vasoconstriction.

Structure of Skeletal Muscle

The structure of muscle fibre includes:

Muscle Fibre

The fibre is the single cellular unit of skeletal muscle tissue. It presents as a long structure with a cylindrical shape. It is enveloped by a plasma membrane, referred to as a sarcolemma. Each fibre is multinucleated and contains many myofibrils, and each is capable of contraction.

Myofibrils

These are individual subunits of the muscle fibre, where the contractile proteins—actin and myosin—are housed. Normally, they happen to line up parallel to each other and in orientation within the fibre. They are grouped into units called sarcomeres, which are the functioning units for contraction in muscles.

Sarcomere

A sarcomere is the simplest contractual entity of a muscle fibre, bounded on both ends by Z-lines. It overlaps the thick (myosin) and thin (actin) filaments of the muscle fibres, whose interactions bring about muscle con­traction—shortening of the sarcomere.

Types of Contractile Proteins

The contractile proteins are discussed below-

Actin–Myosin Interaction

Actin is a globular protein that polymerises to form thin filaments in muscle fibres. Each actin filament is a double helix of actin subunits with sites available for binding with myosin heads. The actin filaments provide the track along which the myosin heads move during muscle contraction.

Interaction with Myosin

The actin and myosin interact through the process of muscle contraction by making cross-bridges. Myosin becomes attached to pinpoint bindings on the actin filament so that the myosin can drag the actin filaments inwards resulting in the shortening of the sarcomere and thus in muscle contraction.

Myosin Head & Power Stroke

One of the thick filament proteins, myosin is typical, having a long, fibrous tail and a globular head. Into the head portion, myosin molecules possess ATPase activity, which is very important during the production of energy for contraction. Its primary function is the interaction with actin to create force and motion.

Myosin Head and Power Stroke

The myosin head binds to the actin molecule to form a cross-bridge. During the power stroke, the myosin head swivels, which pulls the actin filament toward the centre of the sarcomere. This results in the shortening of the sarcomere and the creation of tension within the muscle. Once ATP is attached to the myosin, the myosin head releases from the actin and reattaches for the next cycle.

Regulation of Muscle Contraction

Tropomyosin and troponin are proteins that act as regulators for the action of actin with myosin. In resting muscle, tropomyosin masks the sites on the actin filament to which myosin does not bind. Muscle contraction occurs because of a conformational change in the troponin as a result of the binding of tropomyosin to calcium ions that move tropomyosin out of blocking those sites.

Interaction with Actin and Myosin

Attached to the actin filament is tropomyosin, and attached to tropomyosin is troponin. Calcium binding to troponin causes a conformation change; this change causes movement in tropomyosin quickly enough to expose sites where myosin will attach to the actin molecule. There myosin heads attach to the actin, and contraction begins.

The Mechanism of Muscle Contraction

The mechanism of muscle contraction is discussed below-

Sliding Filament Theory

Thus, in the sarcomere, this happens through the sliding motion of the actin and myosin filaments. During contraction, myosin heads bind to the actin and pull it toward the centre of the sarcomere using the power stroke.

This results in the shortening of the sarcomere, which leads to a contraction of the muscular tissue. Here, the theory extends to the dynamic aspect of the movement of filaments, whereby the constant filament length changes in their overlap lead to short muscles.

Stepwise Mechanism

Calcium ions usually play a very fundamental role in the muscle contraction process, since they orchestrate in the actin-myosin interaction. After a muscle fibre has been stimulated, calcium ions flow from the sarcoplasmic reticulum into the sarcoplasm.

Calcium is bound to troponin, which creates a conformational change that rolls the tropomyosin away from myosin-binding sites on actin. This exposure finally makes myosin heads possible to bind and initiate contraction. Removal of the calcium ions stops the reaction, and thus the muscle will finally relax.

ATP and Muscle Contraction

ATP is the energy source that drives muscle contraction. It does this by energizing the myosin head, so that it may bind to actin, perform the power stroke of contraction, and then be unbound from the actin filament.

It is also used in the restoration of the myosin heads and in the active transport mechanisms that pump Ca2+ back into the sarcoplasmic reticulum after a contraction has ceased. If ATP were not continuously available, the muscle would pause in its activity, since its contraction and recovery require energy obtained only through the hydrolysis of ATP.

The Neuromuscular Junction

The neuromuscular junction is discussed below:

Structure

The NMJ is a specialised synapse by which a motor neuron communicates with a muscle fibre; thus, the NMJ consists of the axon terminal of the motor neuron, the synaptic cleft, and the motor end plate on the muscle fibre. The junction serves to transmit the nerve impulse for pertaining muscle contraction.

Neurotransmitters

The neurotransmitter at the NMJ, from the motor neuron, is acetylcholine. The ACh binds onto the motor end plate of the muscle fibre and influxes sodium ions, generating an action potential. The action potential is propagated along the muscle fibre. The action potential results in muscle contraction through the release of calcium ions from the sarcoplasmic reticulum.

Muscle Contraction & Proteins NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Anatomy of muscle tissue

• Structure of skeletal muscle

• Mechanism of muscle contraction

Practice Questions for NEET

Q1. Name the ion responsible for unmasking active sites for myosin for cross-bridge activity during muscle contraction.

1. Calcium

2. Magnesium

3. Sodium

4. Potassium

Correct answer: 1) Calcium

Explanation:

The mechanism of muscle contraction is best explained by the sliding filament theory which states that the contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. Calcium is the ion released into the sarcoplasm from the sarcoplasmic reticulum during the polarisation.

Ca++ attaches to the Troponin-C. This brings a conformational change in the tropomyosin. As a result, unmasking of active sites on actin for myosin takes place. Cross bridges are formed between actin and myosin. This results in muscle contraction. Magnesium is used in phosphorylation reactions involving ATP. Sodium and potassium help in maintaining the membrane potential.

Hence, the correct answer is option 1) Calcium.

Q2. Consider the following options related to ATP's role in providing energy for muscle contraction:

1. Facilitating the generation of an action potential in the muscle cell.

2. Enabling the attachment of Myosin cross-bridges to Actin.

3. Enabling the detachment of Myosin cross-bridges from Actin.

4. Triggering the release of Ca2+ from the sarcoplasmic reticulum.

Correct answer: 3) Enabling the detachment of Myosin cross-bridges from Actin

Explanation:

Option 3 correctly states that ATP enables the detachment of Myosin cross-bridges from Actin during muscle contraction. ATP provides the energy required for Myosin to detach from Actin, allowing for the relaxation phase of the muscle. This process is crucial for the repeated contraction and relaxation cycles that occur during muscle activity.

Hence, the correct answer is option 3) Enabling the detachment of Myosin cross-bridges from Actin.

Q3. The muscle fibers that contract slowly are

1. Red muscle fibres

2. White muscle fibres

3. Both a and b

4. None of these

Correct answer: 1) Red muscle fibres

Explanation:

Red muscle fibers, also known as slow-twitch fibers, contract slowly but are highly resistant to fatigue. They are rich in myoglobin, mitochondria, and blood supply, which enable sustained aerobic energy production. These fibers are well-suited for endurance activities like walking, running long distances, or maintaining posture, as they rely on oxidative metabolism for energy.

Hence, the correct answer is option 1) Red muscle fibers.

Also Read:

• MCQs on Mechanism of Muscle Contraction

• Axial Skeleton System

• Synovial Joints

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