Muscle Contraction

Muscle contractility is the capacity of skeletal muscle tissue to contract powerfully. While pulling on its attachment sites, a muscle generates tension. If this tension is sufficient to overcome the resistance, the muscle shortens.

What is Muscle Contraction?

Muscle is a mesodermal tissue composed of specialised myocytes. These cells are capable of contracting and shortening and then returning to their original form. Thus, muscle contraction is the process of contracting of muscles, and then returning to their default relaxed state.

Chemical, neural, mechanical, thermal, or tactile stimulation generally triggers muscle excitation, which would lead to contractions. Stimuli received at one end of the muscle quickly travel to other regions and also to the neighbouring myocytes.

Sarcomere: The Functional Unit of Muscle

Sarcomeres are the repeating units inside myofibrils that give skeletal muscle its structure. They contain organized protein filaments that make muscle contraction possible. Here is more about the muscle fibres:

A skeletal muscle fibre contains numerous cylindrical, intracellular structures called myofibrils that extend the entire length of the muscle, constituting 80% of the muscle volume.
Each myofibril consists of a uniform arrangement of highly organised cytoskeletal elements- the thick filaments (special assemblies of the myosin protein) and the thin filaments (made primarily of actin, along with troponin and tropomyosin)
The sarcomere is the functional unit of skeletal muscle, and thus the smallest component of a muscle fibre that can contract.

Alt-text: Myofibril showing the demarcation of the sarcomere, thick filament, and thin filament.

Contractile Proteins in Muscle Fibres

There are two main contractile proteins, actin and myosin, described as follows:

Thick Filament

Myosin

Myosin consists of two identical subunits, and the tails are intertwined around each other with the two globular heads projecting out at one end. The two halves of each thick filament are mirror images made up of myosin molecules lying lengthwise with their tails oriented toward the centre of the filament and the heads protruding outward at regular intervals. These heads form the cross-bridges between the thick and thin filaments, and have two sites- an actin binding site and an ATP-splitting site.

Thin Filament

Actin

Actin is the primary structural protein of the thin filaments. These spherical protein molecules are joined to form two strands, which intertwine to form the backbone of the thin filament. Each actin molecule has a special binding site for attaching to a myosin cross-bridge.

Regulatory Proteins in Muscle Fibres

The following two regulatory proteins regulate the contraction that occurs through actin and myosin:

Tropomyosin

Tropomyosins are thread-like proteins lying end to end alongside the groove of the actin spiral. Tropomyosin covers the actin sites that bind with the cross-bridges, blocking the interaction that leads to muscle contraction.

Troponin

Troponin is a protein complex made of three subunits: one binds to tropomyosin, one binds to actin, and a third can bind to calcium (Ca²⁺)

Mechanism of Muscle Contraction

Here is how muscle contraction occurs:

The central nervous system (CNS) sends a neural signal via a motor neuron to initiate muscle contraction.
The neural signal after reaching the neuromuscular junction or motor-end plate (junction between a motor neuron and sarcolemma) releases the neurotransmitter acetylcholine to generate an action potential in the sarcolemma.
The action potential extends from the sarcolemma to activate the sarcoplasmic reticulum, causing calcium ions (Ca²⁺) to be released into the sarcoplasm.
A rise in the Ca²⁺ concentration in the sarcoplasm initiates filament sliding. Troponin changes its shape as a result of the Ca²⁺ released from the sarcoplasm. This change in shape shifts the troponin-tropomyosin complex away from the actin myosin-binding sites.

Alt-text: a) Default relaxed state

Alt-text: b) Excited state due to Ca²⁺

Myosin head functions as an ATPase and hydrolyses an ATP molecule. Utilising the energy from ATP hydrolysis, the myosin head binds to the exposed active sites on the actin filament to form a cross-bridge.
After the cross-bridge forms, the myosin head pulls the actin filament toward the centre of the sarcomere, i.e., the M-line. It causes the sliding of filaments and shortening of the muscle (covered in detail in the next section).
ATP binding to myosin then causes detachment of the cross-bridge. The ATP is hydrolysed, which re-cocks the myosin head into a high-energy state, ready for the next cycle. This cycle repeats until Ca²⁻ is pumped back into the sarcoplasmic cisternae, signalling relaxation.
The reaction time of the muscle fibres can vary in different muscles.

Alt-text: Cross-bridge Cycle

Effect of Contraction on Sarcomere Zones

According to the sliding-filament theory, muscle contraction is accomplished when the thin filaments from the opposite sides slide closer together between the thick filaments. Thus, neither the thick nor the thin filaments decreases in length to shorten the sarcomere. The changes in the width of the zones are as follows-

During contraction, the thin filaments on each side of a sarcomere slide inward over the stationary thick filaments toward the A band’s centre.
As they slide inward, the thin filaments pull the Z lines closer, shortening the sarcomere.
The H zone at the centre of the A band becomes smaller as thin filaments approach each other.
The I band, consisting of portions of the thin filaments, narrows during their inward slide.
Ca²⁺ is pumped back to the sarcoplasmic cisternae, and cross-bridge cycling stops.
The muscle fibres relax, and the ‘Z’ lines return to their original position.
The width of the A band remains unchanged during contraction because thick filaments do not change length.

Alt-text: Sarcomere in contracted and relaxed states

Frequently Asked Questions (FAQs)

Q1. What are the types of skeletal muscle contractions?

Skeletal muscle contractions can be classified into two types-

In an isometric (same measurement) contraction, the muscle does not shorten in length, even though muscle tension is considerably high.
In isotonic (same tension) contraction, the muscle decreases in length, but the muscle tension is relatively constant.

Q2 . How do red muscle fibres work for longer periods?

Red muscle fibres contain high amounts of myoglobin pigment for high oxygen storage. Moreover, these muscle fibres contain plenty of mitochondria that can utilise the stored oxygen to generate a large amount of energy via aerobic respiration through the complete glucose oxidation. Thus, red muscle fibres can work for longer periods.

Q3. What are the diseases associated with the muscle system?

The diseases associated with the muscle system include muscular dystrophy, myositis, myasthenia gravis, tendonitis, and others.