This MCQ module is based on: Contractile Proteins Muscle Contraction
TOPIC 77 OF 90
Contractile Proteins Muscle Contraction
🎓 Class 11
Biology
CBSE
Theory
Ch 17 – Locomotion and Movement
⏱ ~14 min
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This assessment will be based on: Contractile Proteins Muscle Contraction
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Contractile Proteins and the Sliding Filament Theory
In Part 1 we saw that a muscle fibre is packed with myofibrils, and that each myofibril is a chain of sarcomeres built from two kinds of filament — thin actin filaments anchored to the ‘Z’ line and thick myosin filaments held at the ‘M’ line. Those filaments are not passive threads. They are contractile proteins: machines that convert the chemical energy of ATP into mechanical pulling. This part opens them up, and then follows the entire contraction event from a nerve impulse to a shortened sarcomere and back to relaxation.
Why the structure matters: every step of muscle contraction is a direct consequence of molecular shape — a masking subunit that hides a binding site, a globular head that can swivel, an enzyme that splits ATP. Learn the architecture first and the mechanism becomes almost inevitable.
The Thin Filament — Actin, Tropomyosin and Troponin
Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound around each other. Each ‘F’ actin is itself a polymer of monomeric ‘G’ (globular) actins — so the thin filament is a double helix of beads, not a smooth rod.
Two filaments of another protein, tropomyosin, also run close to the ‘F’ actins throughout their length. A complex protein Troponin is distributed at regular intervals along the tropomyosin. This arrangement is the switch of the whole system: in the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments. The actin is ready to be pulled, but the handles are covered.
The Thick Filament — Meromyosin and the Cross Arm
Each myosin (thick) filament is also a polymerised protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts: a globular head with a short arm, and a tail. The head-plus-short-arm is the heavy meromyosin (HMM); the tail is the light meromyosin (LMM).
The HMM component — the head and short arm — projects outwards at regular distance and angle from each other from the surface of the polymerised myosin filament, and this projection is known as the cross arm. The globular head is the working end: it is an active ATPase enzyme, and it carries both binding sites for ATP and active sites for actin.
Read this carefully: the myosin head is simultaneously an enzyme (it hydrolyses ATP) and a gripper (it binds actin). Contraction needs both properties in the same molecule — that is the whole trick of muscle.
The Sliding Filament Theory
Mechanism of muscle contraction is best explained by the sliding filament theory, which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
Notice what the theory does not say. The filaments themselves do not shorten, coil or fold. They slide past one another, like two hands interlacing more deeply. Everything observable about a contracting muscle — the shortening, the band changes, the force — follows from that sliding.
A common misconception: students often picture actin and myosin contracting like elastic. They do not. Their lengths are fixed; only their overlap changes. That is why the ‘A’ band (the region occupied by thick filaments) keeps its length during contraction while the ‘I’ band shrinks.
From Nerve Signal to Cross Bridge — the Full Sequence
Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. A motor neuron along with the muscle fibres connected to it constitute a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate.
The sequence runs as follows:
| Step | Event | Key molecule / structure |
|---|---|---|
| 1 | CNS sends a signal along a motor neuron to its muscle fibres | Motor unit |
| 2 | Signal reaches the motor-end plate and releases a neurotransmitter | Acetyl choline |
| 3 | An action potential is generated in the sarcolemma and spreads through the fibre | Sarcolemma |
| 4 | Ca++ is released into the sarcoplasm | Sarcoplasmic reticulum |
| 5 | Ca++ binds a subunit of troponin, removing the masking of myosin active sites on actin | Troponin subunit |
| 6 | Using energy from ATP hydrolysis, the myosin head binds the exposed active site on actin | Cross bridge |
| 7 | The head rotates, pulling the actin filament towards the centre of the ‘A’ band; ‘Z’ lines are pulled inwards → sarcomere shortens | Power stroke |
| 8 | Myosin releases ADP and Pi and returns to its relaxed state; a new ATP binds and the cross bridge is broken | ATP |
| 9 | ATP is hydrolysed again; cross-bridge formation and breakage repeat, causing further sliding | Cycling |
| 10 | Ca++ is pumped back to the sarcoplasmic cisternae; actin is re-masked; ‘Z’ lines return to their original position → relaxation | Ca++ pump |
What changes and what does not, during contraction: the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length. The H zone also narrows as thin filaments advance towards the centre. Sarcomere length falls; filament length is unchanged.
📐 Activity — Model the Sliding Filaments with Your Hands
Hold your two hands in front of you, fingers spread, palms facing each other, with the fingertips of one hand just touching the fingertips of the other. Your fingers are the thin filaments; the gap between your palms is the region occupied by thick filaments.
Now slide the fingers of each hand past each other until the fingertips almost reach the opposite palm. Measure the distance between your wrists before and after.
Scenario: Your fingers did not shorten at any point. Yet the wrist-to-wrist distance fell sharply. Which band in the sarcomere corresponds to your wrists moving closer, and which measurement stayed constant?
The wrists are the ‘Z’ lines, so the falling wrist-to-wrist distance is the sarcomere length. The gap between your palms — the thick filament region — never changed, which is the ‘A’ band retaining its length. The stretch of finger that was not overlapping the other hand at the start, and is now overlapping, is the ‘I’ band, reduced. And the clear space in the middle of the palm gap that your fingertips have now invaded is the H zone, narrowed.
This is exactly why the theory is called sliding filament and not shortening filament.
Fatigue, Red Fibres and White Fibres
The reaction time of the fibres can vary in different muscles. Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.
Muscle contains a red coloured oxygen storing pigment called myoglobin. Its content is not the same in every muscle, and this single difference splits skeletal muscle into two functional classes.
| Feature | Red fibres | White fibres |
|---|---|---|
| Myoglobin content | High → reddish appearance | Very less → pale or whitish |
| Mitochondria | Plenty | Few |
| Sarcoplasmic reticulum | Comparatively less | High |
| Mode of energy supply | Aerobic — uses the stored O2 for ATP production | Anaerobic process |
| Also called | Aerobic muscles | — |
| Functional consequence | Sustained activity, slow to fatigue | Rapid powerful bursts, fatigue quickly |
Linking back to Chapter 12: the anaerobic breakdown of glycogen that fatigues a muscle is glycolysis followed by lactic acid fermentation. Red fibres, with their oxygen store and abundant mitochondria, can carry pyruvate onward into the Krebs cycle and the electron transport system — which is why they yield far more ATP per glucose and accumulate far less lactic acid.
🎯 Interactive: Trace a Contraction Step by Step
Select any stage of the contraction–relaxation cycle to see what is happening at the molecular level, and what would happen if that step failed.
🎯 Competency-Based Questions
Q1. A researcher treats an isolated muscle fibre with a drug that permanently blocks the calcium pump of the sarcoplasmic reticulum, while leaving everything else intact. The fibre is then stimulated once. Predict and explain the outcome.
The single stimulus will still release Ca++ into the sarcoplasm, troponin will be bound, actin will be unmasked and the fibre will contract normally. But relaxation requires that Ca++ be pumped back to the sarcoplasmic cisternae so that actin is re-masked. With the pump blocked, sarcoplasmic Ca++ stays high, troponin stays bound, active sites remain exposed, and cross-bridge cycling continues as long as ATP is available.
The fibre therefore remains contracted and cannot relax — a sustained contraction. This shows that relaxation is not simply the absence of contraction: it is an active, ATP-requiring process of calcium removal.
Q2. Two athletes are tested. Athlete A's calf muscle biopsy is deep red with abundant mitochondria; athlete B's is pale with few mitochondria but plentiful sarcoplasmic reticulum. Which athlete is likely a marathon runner and which a 100 m sprinter? Justify using fibre physiology.
Athlete A has predominantly red fibres — high myoglobin (hence the deep red) storing oxygen, and plenty of mitochondria to use that oxygen for aerobic ATP production. Such muscles sustain activity for long periods with little lactic acid accumulation, so athlete A suits the marathon.
Athlete B has predominantly white fibres — very little myoglobin (hence pale), few mitochondria, and a high amount of sarcoplasmic reticulum that can release calcium rapidly for very fast, powerful contractions. They depend on the anaerobic process, so they produce great force briefly and fatigue quickly as lactic acid accumulates. Athlete B suits the 100 m sprint.
Q3. Curare, an arrow poison, competitively blocks the receptors for acetyl choline on the sarcolemma. A person exposed to curare becomes paralysed although the brain keeps sending signals and the muscle proteins are undamaged. Explain precisely where the chain breaks.
The chain breaks at the neuromuscular junction (motor-end plate), at step 3 of the sequence. The CNS signal still travels down the motor neuron and acetyl choline is still released. But with its receptors blocked, the neurotransmitter cannot generate an action potential in the sarcolemma.
No action potential means no spread through the muscle fibre, no release of Ca++ into the sarcoplasm, no binding of Ca++ to troponin, no unmasking of actin active sites and therefore no cross bridges. The actin, myosin and ATP are all perfectly functional but never receive the trigger — so the muscle is flaccid and paralysed.
Q4. Under an electron microscope, a sarcomere in a contracting muscle is measured. The observer reports that the ‘A’ band length is unchanged but the ‘I’ band and H zone have both narrowed. A classmate concludes that the myosin filaments must have folded up. Evaluate this conclusion.
The conclusion is wrong, and the observer's own data disprove it. The ‘A’ band is defined by the extent of the thick (myosin) filaments. If the myosin filaments folded or shortened, the ‘A’ band length would have to decrease — but it was measured as unchanged.
The correct interpretation is the sliding filament theory: the thin filaments slide over the thick filaments towards the centre of the ‘A’ band. Because the thin filaments advance inwards, the non-overlapping region at the edges (‘I’ band) shrinks and the central thick-filament-only region (H zone) is invaded and narrows — exactly the pattern reported — with no change in either filament's own length.
Q5. A student argues: "ATP is needed only to produce the pulling force, so a muscle with no ATP would simply go limp." Critique this statement using at least two distinct ATP-requiring steps.
The statement is incomplete. ATP is used at more than one point:
(i) Energy for cross-bridge formation and the power stroke — the myosin head hydrolyses ATP and uses that energy to bind actin and rotate. This is the step the student has in mind.
(ii) Breaking the cross bridge — a new ATP must bind to the myosin head before the existing cross bridge can be broken. Without ATP the head stays attached to actin.
(iii) Pumping Ca++ back into the sarcoplasmic cisternae, which is required for re-masking of actin and hence for relaxation.
So a muscle with no ATP would not go limp; the existing cross bridges could neither be broken nor re-masked, leaving it stiff. Relaxation, no less than contraction, costs energy.
🧠 Assertion–Reason Questions
For each pair, decide whether both statements are true and whether the reason correctly explains the assertion.
Assertion (A): During contraction the ‘A’ band retains its length.
Reason (R): Contraction occurs by the sliding of thin filaments over thick filaments, not by any change in filament length.
Both A and R are true, and R is the correct explanation of A.
The ‘A’ band marks the extent of the thick filaments. Since the sliding filament theory holds that only the overlap between filaments changes, the thick filaments keep their length and so the ‘A’ band is unaltered, while the ‘I’ band (non-overlapping thin filament region) is reduced.
Assertion (A): In the resting state the myosin heads cannot attach to actin.
Reason (R): A subunit of troponin masks the active binding sites for myosin on the actin filaments.
Both A and R are true, and R is the correct explanation of A.
This masking is the molecular switch of muscle. Calcium binding to the troponin subunit removes the mask, which is why a rise in sarcoplasmic Ca++ is the immediate trigger for cross-bridge formation.
Assertion (A): Red fibres can sustain prolonged activity without rapid fatigue.
Reason (R): Red fibres possess a very high amount of sarcoplasmic reticulum.
A is true but R is false.
Red fibres do resist fatigue — but because of high myoglobin (an oxygen store) together with plenty of mitochondria, allowing aerobic ATP production with little lactic acid accumulation. It is the white fibres that have a high amount of sarcoplasmic reticulum, along with very little myoglobin and few mitochondria.
Frequently Asked Questions - Contractile Proteins and Muscle Contraction
What is the sliding filament theory of muscle contraction?
The sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin (actin) filaments over the thick (myosin) filaments. The filaments themselves do not shorten; only their overlap increases. This is why the 'I' band gets reduced and the H zone narrows during contraction while the 'A' band retains its length.
What is the structure of an actin filament?
Each actin (thin) filament is made of two 'F' (filamentous) actins helically wound to each other, and each 'F' actin is a polymer of monomeric 'G' (globular) actins. Two filaments of tropomyosin run close to the 'F' actins throughout their length, and a complex protein troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin.
What is the difference between heavy meromyosin and light meromyosin?
Each meromyosin monomer of a thick filament has a globular head with a short arm, and a tail. The head plus short arm is the heavy meromyosin (HMM) and the tail is the light meromyosin (LMM). The HMM projects outwards at regular distance and angle from the filament surface and is known as the cross arm. Its globular head is an active ATPase enzyme with binding sites for ATP and active sites for actin.
What is a motor unit and what is a neuromuscular junction?
A motor neuron along with the muscle fibres connected to it constitutes a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate. A neural signal reaching this junction releases the neurotransmitter acetyl choline, which generates an action potential in the sarcolemma.
What is the role of calcium ions in muscle contraction?
The action potential spreading through the muscle fibre causes the release of calcium ions into the sarcoplasm. The increase in Ca++ level leads to the binding of calcium with a subunit of troponin on the actin filaments, thereby removing the masking of the active sites for myosin. When Ca++ is pumped back to the sarcoplasmic cisternae the actin is re-masked and the muscle relaxes.
Why does a muscle become fatigued?
Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue. Muscles that rely more on the anaerobic process, such as white fibres, therefore fatigue faster than red fibres, which store oxygen in myoglobin and have plenty of mitochondria for aerobic ATP production.
What is the difference between red fibres and white fibres?
Red fibres have a high content of the red oxygen-storing pigment myoglobin, plenty of mitochondria and rely on aerobic metabolism, so they are also called aerobic muscles and resist fatigue. White fibres possess very little myoglobin and therefore appear pale or whitish, have few mitochondria but a high amount of sarcoplasmic reticulum, and depend on the anaerobic process for energy, giving fast powerful contractions that fatigue quickly.
Why is ATP needed for muscle relaxation and not only for contraction?
ATP is required at several points. Its hydrolysis provides energy for cross-bridge formation and the power stroke; a new ATP must bind to the myosin head before the existing cross bridge can be broken; and ATP powers the pumping of Ca++ back into the sarcoplasmic cisternae, which is necessary for re-masking of actin and hence relaxation. A muscle without ATP therefore becomes stiff rather than limp.
Which bands change in length during muscle contraction?
During contraction the 'I' bands get reduced and the H zone narrows, because the thin filaments slide inwards towards the centre of the 'A' band and pull the 'Z' lines closer together. The 'A' band retains its length throughout, since it corresponds to the thick filaments, whose length does not change.
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