Muscle

Q: What is the role of calcium ions in muscle contraction?

An action potential in the sarcolemma releases Ca2+ from the sarcoplasmic reticulum into the sarcoplasm. Calcium binds a subunit of troponin and removes the masking of the active sites on actin, so myosin heads can form cross bridges. When Ca2+ is pumped back, the sites are masked again and the muscle relaxes.

BiologyLocomotion and MovementFor NEET aspirants

Muscle is the specialised tissue that makes movement and locomotion possible. This page covers the three types of movement shown by human cells, the three types of muscle, and the structure of a skeletal muscle down to the sarcomere. It then explains the contractile proteins actin and myosin and the sliding filament theory of muscle contraction, step by step. It follows the NCERT Class 11 chapter Locomotion and Movement. NEET often asks the bands of the sarcomere, the role of calcium and troponin, and red versus white fibres.

On this page1Movement and locomotion2Types of movement3Types of muscle4Skeletal muscle structure5Contractile proteins6Mechanism of contraction7Red and white fibres8Exam essentials9Quick revision10Solved examples11Practice

Key Points at a Glance

★ Must learn Locomotion: a voluntary movement that changes place. All locomotions are movements, but all movements are not locomotions.
Human cells show three movements: amoeboid (macrophages, leucocytes), ciliary (trachea, female reproductive tract) and muscular.
Muscle is of mesodermal origin and forms 40-50% of adult body weight; properties: excitability, contractility, extensibility, elasticity.
★ Must learn Skeletal: striated, voluntary. Visceral: smooth, involuntary. Cardiac: striated, branched, involuntary.
Muscle → fascicles (held by fascia) → muscle fibres (syncytium) → myofibrils.
★ Must learn Sarcomere: the part of a myofibril between two successive Z lines; the functional unit of contraction.
I band (light, isotropic): actin. A band (dark, anisotropic): myosin. Z line bisects the I band; M line holds the thick filaments.
Thin filament: 2 F actins (polymers of G actin) + 2 tropomyosin filaments + troponin. Thick filament: many meromyosins.
★ Must learn Myosin head = ATPase, with binding sites for ATP and active sites for actin.
★ Must learn $C a^{2 +}$ from the sarcoplasmic reticulum binds troponin, unmasks actin, and myosin heads form cross bridges.
During contraction the I band and H zone shorten; the A band keeps its length.
Red fibres: much myoglobin, many mitochondria, aerobic. White fibres: little myoglobin, much sarcoplasmic reticulum, anaerobic.

1. Movement and Locomotion

Movement is one of the significant features of living beings. Both animals and plants show a wide range of movements.
Streaming of protoplasm in unicellular organisms such as Amoeba is a simple form of movement.
Many organisms show movement of cilia, flagella and tentacles; animals also move fins, limbs and wings.
Human beings can move their limbs, jaws, eyelids, tongue and other parts.

★ Very important Locomotion: voluntary movements that result in a change of place or location. Walking, running, climbing, flying and swimming are forms of locomotory movement.

Structures used for locomotion need not differ from those used for other movements:

Organism	Structure	Other movement	Also used for locomotion
Paramoecium	Cilia	Moves food through the cytopharynx	Yes
Hydra	Tentacles	Captures prey	Yes
Human beings	Limbs	Change of body posture	Yes

So movement and locomotion cannot be studied separately.
★ Exam imp All locomotions are movements, but all movements are not locomotions.
The method of locomotion varies with the animal's habitat and the demand of the situation.
Animals generally move in search of food, shelter, a mate, suitable breeding grounds and favourable climatic conditions, or to escape from enemies or predators.

Memory Trick

Why animals move: Few Smart Mice Build Cosy Exits = Food, Shelter, Mate, Breeding grounds, Climatic conditions, Escape from enemies or predators.

Key idea

Locomotion is a voluntary movement that changes place; the same structure often serves both locomotion and other movements.

2. Types of Movement in Human Cells

Cells of the human body show three main types of movement: amoeboid, ciliary and muscular.

Type	Shown by	How it works or what it does
Amoeboid	Specialised cells such as macrophages and leucocytes in blood	Pseudopodia formed by streaming of protoplasm (as in Amoeba); cytoskeletal elements such as microfilaments are also involved
Ciliary	Most internal tubular organs lined by ciliated epithelium	Coordinated beating of cilia in the trachea removes dust particles and foreign substances inhaled with air; cilia also help the passage of ova through the female reproductive tract
Muscular	Limbs, jaws, tongue and other body parts	The contractile property of muscles is used for locomotion and other movements by human beings and most multicellular organisms

★ Exam imp Locomotion requires a perfectly coordinated activity of the muscular, skeletal and neural systems.
Cilia and flagella are outgrowths of the cell membrane.
Flagellar movement helps in the swimming of spermatozoa and in the locomotion of protists such as Euglena.
Flagella also keep up the water current in the canal system of sponges.

Memory Trick

Human cells move in three ways, MAC: Muscular, Amoeboid, Ciliary.

Flagella examples: Sperm Swim Easily = Spermatozoa, Sponge canal system, Euglena.

Key idea

Free cells such as leucocytes move by pseudopodia, cilia of lining cells move materials over the surface, and body parts move by muscle contraction.

3. Muscle: Properties and Types

Muscle: a specialised tissue of mesodermal origin.
★ Exam imp Muscles contribute about 40-50 per cent of the body weight of a human adult.
Special properties of muscle: excitability, contractility, extensibility and elasticity.
Muscles are classified on the basis of their location, appearance and nature of regulation of their activities.
On the basis of location, there are three types: skeletal, visceral and cardiac.

Three types of muscle

Feature	Skeletal muscle	Visceral muscle	Cardiac muscle
Location	Closely associated with the skeletal components of the body	Inner walls of hollow visceral organs such as the alimentary canal and reproductive tract	Heart
Appearance	Striped under the microscope: striated	No striations: smooth (nonstriated)	Striated
Control	Voluntary: under control of the nervous system	Involuntary	Involuntary: the nervous system does not control them directly
Other names	Striated muscle, voluntary muscle	Smooth muscle, nonstriated muscle, involuntary muscle	Muscles of the heart
Role or feature	Locomotory actions and changes of body posture	Move food through the digestive tract and gametes through the genital tract	Many cardiac muscle cells assemble in a branching pattern

Memory Trick

Four properties: Every Contracting muscle Extends Elastically = Excitability, Contractility, Extensibility, Elasticity.

NEET Focus

Cardiac muscle is the odd one out: striated like skeletal muscle, yet involuntary like visceral muscle.
So the statements "all striated muscles are voluntary" and "all involuntary muscles are smooth" are both false.
Appearance (striated or smooth) and control (voluntary or involuntary) are separate criteria; check each one in a statement.

Key idea

Striation describes appearance and voluntary control describes regulation: cardiac muscle is striated but involuntary.

4. Structure of a Skeletal Muscle

4.1 From muscle to myofibril

Each organised skeletal muscle is made of a number of muscle bundles or fascicles.
The fascicles are held together by a common collagenous connective tissue layer called fascia.
Each muscle bundle contains a number of muscle fibres (muscle cells).

Test yourself: hide labels

Figure 1: Cross-sectional view of a muscle (diagrammatic). The muscle is made of fascicles (muscle bundles), each packed with muscle fibres; each fibre is covered by its sarcolemma, and blood capillaries run through the tissue between the bundles.

Each muscle fibre is lined by a plasma membrane called the sarcolemma, which encloses the sarcoplasm (the cytoplasm of the fibre).
★ Exam imp A muscle fibre is a syncytium: its sarcoplasm contains many nuclei.
The endoplasmic reticulum of a muscle fibre is the sarcoplasmic reticulum; it is the store house of calcium ions.
A characteristic feature of the fibre: a large number of parallelly arranged filaments in the sarcoplasm, called myofilaments or myofibrils.

Extra Depth: The two terms are often used together. Strictly, each myofibril is a long bundle made of many myofilaments, which are the thin (actin) and thick (myosin) filaments described below.

★ Very important Muscle fibre = anatomical unit of muscle. Sarcomere = functional unit of contraction. Sarcoplasmic reticulum = store house of

C a^{2 +}

4.2 Bands of a myofibril

Each myofibril shows alternate dark and light bands.
The striated appearance is due to the distribution pattern of two important proteins: actin and myosin.

I band

Light band
Isotropic band
Contains actin
Bisected by the Z line
Shortens during contraction

A band

Dark band
Anisotropic band
Contains myosin
M line in its middle
Keeps its length during contraction

Both proteins are rod-like structures, arranged parallel to each other and to the long axis of the myofibril.
Actin filaments are thinner than myosin filaments, so they are called thin filaments (actin) and thick filaments (myosin).
Z line: an elastic fibre in the centre of each I band that bisects it. The thin filaments are firmly attached to the Z line.
M line: a thin fibrous membrane in the middle of the A band that holds the thick filaments together.
The A and I bands are arranged alternately throughout the length of the myofibril.
★ Exam imp Sarcomere: the portion of the myofibril between two successive Z lines; the functional unit of contraction.
In the resting state, the edges of the thin filaments on either side partially overlap the free ends of the thick filaments, leaving their central part free.
H zone: the central part of the thick filaments not overlapped by thin filaments.
Each sarcomere has one central A band of thick myosin filaments.
It also has two half I bands of thin actin filaments, one on either side, ending at the Z lines.

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Figure 2: (a) Anatomy of a muscle fibre showing a sarcomere (b) a sarcomere (diagrammatic). One sarcomere runs from one Z line to the next: a central A band of thick filaments, half an I band on each side, and the H zone in the middle of the A band.

Memory Trick

Vowels match: I band is lIght and Isotropic; A band is dArk and Anisotropic.

M line = Middle of the Myosin (A) band; the Z line bisects the I band.

Tips and Tricks

Diagram-label order, from one end of a sarcomere: Z line → half I band → A band (H zone and M line at its centre) → half I band → Z line. If a label question shows a region with only thin filaments, it is the I band; only thick filaments, the H zone.

Quick Recall: tap to check

Which structure is the store house of calcium ions in a muscle fibre?

The sarcoplasmic reticulum.

Why is a muscle fibre called a syncytium?

Its sarcoplasm contains many nuclei.

Name the functional unit of contraction.

The sarcomere: the part of a myofibril between two successive Z lines.

In Figure 2, name the region of the A band not overlapped by thin filaments.

The H zone.

Key idea

A sarcomere runs from Z line to Z line: actin forms the light I band, myosin the dark A band, and the H zone is the unoverlapped centre.

5. Structure of Contractile Proteins

5.1 Actin (thin) filament

Each actin filament is made of two F (filamentous) actins helically wound to each other.
★ Exam imp Each F actin is a polymer of monomeric G (globular) actins.
Two filaments of another protein, tropomyosin, run close to the F actins throughout their length.
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 on the actin filaments.

5.2 Myosin (thick) filament

Each myosin filament is also a polymerised protein.
Many monomeric proteins called meromyosins make up one thick filament.
Each meromyosin has two important parts: a globular head with a short arm, and a tail.
The head with the short arm is the heavy meromyosin (HMM); the tail is the light meromyosin (LMM).
The HMM component projects outwards at regular distances and angles from the surface of the polymerised myosin filament. This projection is the cross arm.
Actin and myosin are both polymerised proteins with contractility.

★ Very important The globular head of myosin is an active ATPase enzyme. It has binding sites for ATP and active sites for actin.

Test yourself: hide labels

Figure 3: (a) An actin (thin) filament (b) myosin (meromyosin) with its globular heads. Two F actins twist around each other with tropomyosin and troponin on them; each head carries actin binding sites and ATP binding sites, and the head with its short arm forms the cross arm.

Actin and myosin compared

Feature	Actin (thin) filament	Myosin (thick) filament
Band	Light I band	Dark A band
Thickness	Thinner	Thicker
Made of	Two F actins, each a polymer of G actins	Many meromyosins (HMM + LMM)
Associated proteins	Two tropomyosin filaments and troponin	No other protein named
Held at	Z line	M line
Key sites	Active sites for myosin, masked by troponin at rest	Head: ATPase, ATP binding sites and active sites for actin

Memory Trick

Thin filament proteins, ATT: Actin, Tropomyosin, Troponin.

Heavy meromyosin carries the Head; Light meromyosin is the Long tail.

Quick Recall: tap to check

In Figure 3(a), which protein masks the active sites on actin at rest?

A subunit of troponin.

Name the monomer of F actin.

G (globular) actin.

Which part of meromyosin forms the cross arm?

The heavy meromyosin: the globular head with its short arm.

Which part of the myosin filament acts as an ATPase?

The globular head.

Key idea

Thin filaments carry the active sites for myosin, masked by troponin; thick filaments carry the ATPase heads that bind to them.

6. Mechanism of Muscle Contraction

★ Very important Sliding filament theory: contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.

6.1 The signal from the nerve

Muscle contraction is started by a signal sent by the central nervous system (CNS) through a motor neuron.
★ Exam imp Motor unit: a motor neuron along with the muscle fibres connected to it.
Neuromuscular junction or motor-end plate: the junction between a motor neuron and the sarcolemma of a muscle fibre.

6.2 Steps of contraction

A neural signal reaching the neuromuscular junction releases the neurotransmitter acetylcholine.
Acetylcholine generates an action potential in the sarcolemma.
The action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.
The rise in $C a^{2 +}$ level leads to binding of calcium with a subunit of troponin on the actin filaments. This removes the masking of the active sites for myosin.
Using energy from ATP hydrolysis, the myosin head binds to the exposed active sites on actin to form a cross bridge.
The cross bridge pulls the attached actin filaments towards the centre of the A band.
The Z lines attached to these actins are also pulled inwards. The sarcomere shortens: this is contraction.
The myosin releases ADP and $P_{i}$ and goes back to its relaxed state.
A new ATP binds to the myosin head and the cross bridge is broken.
The ATP is again hydrolysed by the myosin head. Cross bridge formation and breakage repeat, causing further sliding.

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Figure 4: Cross bridge cycle. The myosin head binds actin (cross bridge), rotates to pull the thin filament towards the centre of the A band, then lets go when a new ATP binds; hydrolysis of that ATP re-energises the head for the next cycle.

★ Exam imp During contraction the I bands get reduced, whereas the A bands retain their length. The H zone also narrows (Figure 5).

Test yourself: hide labels

Figure 5: Sliding filament theory. As the thin filaments slide over the thick filaments, the Z lines move closer; the I band and H zone shorten, but the A band (the length of the thick filaments) stays the same. At maximal contraction the H zone disappears.

What changes during contraction

Region	Change during contraction
A band	No change in length
I band	Reduced
H zone	Reduced; disappears at maximal contraction
Sarcomere (Z line to Z line)	Shortens, as the Z lines are pulled inwards

6.3 Relaxation

The cycle continues till the $C a^{2 +}$ ions are pumped back into the sarcoplasmic cisternae.
The actin filaments are masked again, and the Z lines return to their original position: this is relaxation.

Memory Trick

Order of events: Never Ask Any Calm Tiger Before Sunrise = Neural signal, Acetylcholine, Action potential, Calcium release, Troponin binds calcium, Bridge forms, Sliding.

NEET Focus

$C a^{2 +}$ has two roles: its release into the sarcoplasm starts contraction; its pumping back into the sarcoplasmic cisternae causes relaxation.
In the cross bridge cycle, ATP has two roles: its hydrolysis gives the energy for the head to bind actin; binding of a new ATP breaks the cross bridge.
Calcium binds troponin, not tropomyosin or myosin.

Tips and Tricks

Sliding filament check: a region made of one kind of filament only shrinks (I band: thin only; H zone: thick only). The A band equals the full length of the thick filaments, so it never changes.

Key idea

C a^{2 +}

unmasks actin, ATP-powered myosin heads pull the thin filaments inwards, and the I band and H zone shrink while the A band stays constant.

7. Fatigue and Red and White Fibres

The reaction time of the fibres can vary in different muscles.
★ Exam imp Repeated activation of muscles can cause accumulation of lactic acid, due to anaerobic breakdown of glycogen; this causes fatigue.
Muscle contains a red coloured, oxygen-storing pigment called myoglobin.
Muscles are classified as red fibres and white fibres mainly on the basis of their myoglobin content.

Red and white fibres

Feature	Red fibres	White fibres
Myoglobin	High content	Very little
Colour	Reddish	Pale or whitish
Mitochondria	Plenty; they use the large amount of oxygen stored by myoglobin for ATP production	Few
Sarcoplasmic reticulum	Comparatively less	High amount
Energy source	Aerobic; so also called aerobic muscles	Depend on anaerobic process

Memory Trick

Red is Rich: rich in myoglobin and mitochondria, runs on oxygen. White is Weak in myoglobin and works without oxygen, but has more sarcoplasmic reticulum.

Quick Recall: tap to check

In Figure 4, what breaks the cross bridge?

Binding of a new ATP to the myosin head.

Which band keeps its length during contraction?

The A band.

What causes muscle fatigue?

Accumulation of lactic acid from the anaerobic breakdown of glycogen.

Why are red fibres also called aerobic muscles?

Their plentiful mitochondria use the large amount of oxygen stored by myoglobin for ATP production.

Key idea

Red fibres are myoglobin-rich and aerobic; white fibres are myoglobin-poor, rich in sarcoplasmic reticulum and anaerobic.

8. Exam Essentials

Pairs to Match

Structure or term	Feature or role
Macrophages and leucocytes	Amoeboid movement
Ciliated epithelium of the trachea	Removes dust particles and inhaled foreign substances
Flagellar movement	Spermatozoa, canal system of sponges, Euglena
Fascia	Collagenous layer holding the fascicles together
Sarcolemma	Plasma membrane of a muscle fibre
Sarcoplasmic reticulum	Store house of calcium ions
I band	Isotropic; contains actin
A band	Anisotropic; contains myosin
Z line	Bisects the I band; thin filaments attached
M line	Holds the thick filaments together
Troponin	Masks active sites for myosin; binds calcium
Heavy meromyosin (HMM)	Globular head with short arm (cross arm)
Light meromyosin (LMM)	Tail
Acetylcholine	Neurotransmitter at the motor-end plate
Myoglobin	Red (aerobic) fibres

Exceptions

Cardiac muscle is striated but involuntary: not every striated muscle is voluntary.
Visceral muscle is the only one of the three types with no striations.
A muscle fibre is a single cell, yet it has many nuclei (syncytium).
During contraction the A band does not shorten; only the I band and the H zone shorten.
Calcium binds troponin, not tropomyosin or myosin.
The ATPase activity lies in the myosin head, not in actin.
White fibres have few mitochondria but a high amount of sarcoplasmic reticulum.
Cilia of Paramoecium and tentacles of Hydra serve locomotion and other functions.

Numbers to Remember

Main types of movement in human cells: 3 (amoeboid, ciliary, muscular).
Share of muscles in adult body weight: 40-50 per cent.
Properties of muscle: 4; types of muscle by location: 3.
Thin filament: 2 F actins and 2 tropomyosin filaments.
Parts of a meromyosin: 2 (HMM and LMM).
One sarcomere: 1 A band and 2 half I bands, between 2 successive Z lines.

9. Quick Revision

Locomotion: voluntary movement causing a change of place; all locomotions are movements, not the reverse.
Animals move for food, shelter, a mate, breeding grounds, favourable climate, or to escape enemies.
Human cells: amoeboid (macrophages, leucocytes), ciliary (trachea, female reproductive tract) and muscular movement.
Cilia and flagella are cell membrane outgrowths; flagella: spermatozoa, sponge canal system, Euglena.
Muscle: mesodermal, 40-50% of adult body weight; excitability, contractility, extensibility, elasticity.
Skeletal: striated, voluntary. Visceral: smooth, involuntary. Cardiac: striated, branched, involuntary.
Muscle → fascicles (fascia) → muscle fibres (sarcolemma, sarcoplasm, many nuclei) → myofibrils.
Sarcoplasmic reticulum stores $C a^{2 +}$ .
I band: light, isotropic, actin, Z line at its centre. A band: dark, anisotropic, myosin, M line at its centre.
Sarcomere: Z line to Z line, the functional unit; the muscle fibre is the anatomical unit.
Thin filament: 2 F actins (G actin polymers), tropomyosin, troponin. Thick filament: meromyosins (HMM head, LMM tail).
Contraction: motor neuron → acetylcholine → action potential → $C a^{2 +}$ release → troponin → cross bridge → sliding.
I band and H zone shorten; A band is unchanged. Relaxation follows when $C a^{2 +}$ is pumped back.
Fatigue: lactic acid from anaerobic glycogen breakdown. Red fibres aerobic; white fibres anaerobic.

10. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Amoeboid movement, B. Ciliary movement, C. Flagellar movement, D. Muscular movement
List II: I. Movement of jaws and tongue, II. Leucocytes in blood, III. Passage of ova through the female reproductive tract, IV. Water current in the canal system of sponges
Choose the correct answer:
(A) A-III, B-II, C-IV, D-I
(B) A-II, B-IV, C-III, D-I
(C) A-II, B-III, C-IV, D-I
(D) A-I, B-III, C-IV, D-II

Solution:

Answer: (C). Leucocytes move by pseudopodia (II); cilia move ova along the female tract (III); flagella keep up the water current in sponges (IV); jaws and tongue move by muscles (I).

Solved Example 2

Read the statements about a sarcomere.
A. The I band contains actin and is bisected by the Z line.
B. The M line lies in the middle of the I band.
C. The H zone is the central part of the thick filaments not overlapped by thin filaments.
D. A sarcomere is the portion of a myofibril between two successive M lines.
E. The A band is the anisotropic band.
Choose the correct answer:
(A) A, B and C only
(B) A, C and E only
(C) C, D and E only
(D) A, B, D and E only

Solution:

Answer: (B). B is wrong: the M line lies in the middle of the A band. D is wrong: a sarcomere lies between two successive Z lines. A, C and E are correct.

Solved Example 3

Arrange the events of muscle contraction in the correct order.
A. Release of calcium ions into the sarcoplasm
B. Release of acetylcholine at the neuromuscular junction
C. Calcium binds troponin and the active sites on actin are unmasked
D. An action potential is generated in the sarcolemma
E. The myosin head binds actin to form a cross bridge
Choose the correct answer:
(A) D, B, A, C, E
(B) B, A, D, C, E
(C) B, D, C, A, E
(D) B, D, A, C, E

Solution:

Answer: (D). Acetylcholine (B) generates an action potential (D), which releases calcium (A); calcium binds troponin and unmasks actin (C); then cross bridges form (E).

Solved Example 4

Which of the following does NOT happen when a skeletal muscle fibre contracts?
(A) The I bands get reduced
(B) The Z lines are pulled inwards
(C) The A bands get shorter
(D) The H zone narrows

Solution:

Answer: (C). The A band equals the length of the thick filaments, which only slide; it retains its length.

Solved Example 5

Besides two F actins, a thin filament of a myofibril contains:
(A) Tropomyosin and troponin
(B) Troponin and myosin
(C) Meromyosin and tropomyosin
(D) Myoglobin and troponin

Solution:

Answer: (A). Two tropomyosin filaments run along the F actins, and troponin sits on the tropomyosin at regular intervals.

Solved Example 6

Statement I: White fibres depend on anaerobic processes for energy.
Statement II: White fibres contain plenty of mitochondria and a large amount of myoglobin.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect

Solution:

Answer: (B). White fibres have very little myoglobin and few mitochondria, so they rely on anaerobic processes; they have a high amount of sarcoplasmic reticulum.

11. Practice Questions

Practice Questions

Match List I with List II.
List I: A. Sarcolemma, B. Sarcoplasmic reticulum, C. Fascia, D. Myoglobin
List II: I. Oxygen-storing red pigment, II. Plasma membrane of a muscle fibre, III. Store house of calcium ions, IV. Collagenous layer holding fascicles together
Choose the correct answer:
(A) A-II, B-IV, C-III, D-I
(B) A-III, B-II, C-IV, D-I
(C) A-II, B-III, C-I, D-IV
(D) A-II, B-III, C-IV, D-IAnswer: (D). Sarcolemma-II, sarcoplasmic reticulum-III, fascia-IV, myoglobin-I.
Read the statements about the types of muscle.
A. Skeletal muscles are under voluntary control.
B. Visceral muscles show striations.
C. Many cardiac muscle cells assemble in a branching pattern.
D. The nervous system directly controls the activity of cardiac muscles.
E. Visceral muscles help to transport gametes through the genital tract.
Choose the correct answer:
(A) A, B and C only
(B) B, D and E only
(C) A, C and E only
(D) A, C, D and E onlyAnswer: (C). Visceral muscles are smooth (B wrong); cardiac muscles are not directly controlled by the nervous system (D wrong).
Arrange the following from the largest to the smallest.
A. Myofibril
B. Muscle fibre
C. Fascicle
D. Sarcomere
E. Whole skeletal muscle
Choose the correct answer:
(A) E, B, C, A, D
(B) E, C, B, A, D
(C) C, E, B, D, A
(D) E, C, A, B, DAnswer: (B). A muscle has fascicles, a fascicle has fibres, a fibre has myofibrils, and a myofibril is a chain of sarcomeres.
Which of the following is NOT correct about the globular head of myosin?
(A) It is an active ATPase enzyme
(B) It has binding sites for ATP
(C) It has active sites for actin
(D) It forms the light meromyosinAnswer: (D). The head with its short arm is the heavy meromyosin; the light meromyosin is the tail.
Draw a labelled diagram of a sarcomere of skeletal muscle. Which regions must it show?Answer: Two Z lines at the ends; thin actin filaments attached to each Z line; thick myosin filaments in the middle, held at the M line; the A band (length of the thick filaments); half an I band on each side; and the H zone at the centre of the A band (Figure 2).
Define the sliding filament theory of muscle contraction.Answer: Contraction of a muscle fibre takes place by the sliding of the thin (actin) filaments over the thick (myosin) filaments.
Describe the important steps in muscle contraction.Answer: Signal from the CNS via a motor neuron; acetylcholine at the motor-end plate; action potential in the sarcolemma; $C a^{2 +}$ released into the sarcoplasm; $C a^{2 +}$ binds troponin and unmasks actin; myosin heads, energised by ATP hydrolysis, form cross bridges and pull actin towards the centre of the A band, so the sarcomere shortens; ADP and $P_{i}$ are released, a new ATP breaks the bridge and the cycle repeats; $C a^{2 +}$ is pumped back and the muscle relaxes.
(a) True or false: actin is present in the thin filament. (b) True or false: the H zone of a striated muscle fibre represents both thick and thin filaments. (c) Fill in the blanks: a thin filament contains 2 F actins and two other proteins, ____ and ____. (d) In a muscle fibre, calcium is stored in the ____.Answer: (a) True. (b) False: the H zone has only thick filaments, the part not overlapped by thin filaments. (c) Tropomyosin and troponin. (d) Sarcoplasmic reticulum.
Write the differences between (a) actin and myosin, and (b) red and white muscle fibres.Answer: (a) Actin: thin filament of the I band, two F actins with tropomyosin and troponin, attached to the Z line. Myosin: thick filament of the A band, made of meromyosins whose heads are ATPases, held at the M line. (b) Red: much myoglobin, many mitochondria, aerobic. White: little myoglobin, few mitochondria, much sarcoplasmic reticulum, anaerobic.
What are the different types of movement shown by the cells of the human body?Answer: Amoeboid (macrophages and leucocytes, by pseudopodia), ciliary (cilia of tubular organs such as the trachea and the female reproductive tract) and muscular (limbs, jaws, tongue).
How do you distinguish between a skeletal muscle and a cardiac muscle?Answer: Both are striated. Skeletal muscle is attached to the skeleton and is voluntary; cardiac muscle forms the heart, its cells branch, and it is involuntary.

Common Mistakes to Avoid

Watch out

Writing that the A band shortens during contraction. Correct: only the I band and the H zone shorten.
Calling cardiac muscle voluntary because it is striated. Correct: it is striated but involuntary.
Saying calcium binds tropomyosin or myosin. Correct: it binds a subunit of troponin.
Placing the M line in the I band. Correct: the M line is in the middle of the A band; the Z line bisects the I band.
Mixing up HMM and LMM. Correct: HMM is the head with the short arm; LMM is the tail.
Defining the sarcomere as the region between two M lines. Correct: it lies between two successive Z lines.
Saying white fibres have many mitochondria. Correct: they have few mitochondria but much sarcoplasmic reticulum.
Thinking the H zone contains thin filaments. Correct: it is the part of the thick filaments not overlapped by thin filaments.

Frequently Asked Questions

What is the difference between movement and locomotion?

Movement is any change in position shown by living beings or their parts, such as streaming of protoplasm, beating of cilia and flagella, or moving the jaws and eyelids. Locomotion is a voluntary movement that changes the place or location of the whole organism, such as walking or swimming. All locomotions are movements, but all movements are not locomotions.

What is a sarcomere?

A sarcomere is the portion of a myofibril between two successive Z lines. It has a central A band of thick myosin filaments and half an I band of thin actin filaments on either side. It is the functional unit of contraction, because contraction happens by the shortening of sarcomeres.

What is the sliding filament theory of muscle contraction?

The sliding filament theory states that a muscle fibre contracts by the sliding of thin actin filaments over thick myosin filaments. Myosin heads form cross bridges with actin and pull it towards the centre of the A band, so the Z lines move closer and the sarcomere shortens.

What is the role of calcium ions in muscle contraction?

An action potential in the sarcolemma releases $C a^{2 +}$ from the sarcoplasmic reticulum into the sarcoplasm. Calcium binds a subunit of troponin and removes the masking of the active sites on actin, so myosin heads can form cross bridges. When $C a^{2 +}$ is pumped back, the sites are masked again and the muscle relaxes.

Why does the A band not change in length during contraction?

The A band is the length of the thick myosin filaments. During contraction the filaments do not shorten; the thin filaments only slide over the thick ones. So the A band keeps its length, while the I band and the H zone, which depend on how far the filaments overlap, become shorter.

What is the difference between red and white muscle fibres?

Red fibres have a high content of the oxygen-storing pigment myoglobin and plenty of mitochondria, so they make ATP aerobically and are called aerobic muscles. White fibres have very little myoglobin and few mitochondria, but much sarcoplasmic reticulum, and they depend on anaerobic processes for energy.

Why is a muscle fibre called a syncytium?

A muscle fibre is called a syncytium because its sarcoplasm contains many nuclei within one continuous cell, bounded by a single plasma membrane, the sarcolemma. The fibre also contains the sarcoplasmic reticulum, which stores calcium ions, and many parallel myofibrils with alternate dark and light bands.

Which points from the muscle topic are most important for NEET?

NEET most often asks the bands and lines of the sarcomere, the proteins of the thin and thick filaments, the role of calcium and troponin, the order of events in contraction, and the changes in the I band, A band and H zone. Red versus white fibres and the three muscle types are also common.

Previous year questions on Muscle

4 questions from past papers, each with a step-by-step solution.

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