Muscles Questions in English

(C) The $ATP$ binding site is located on the head of the myosin molecule. The myosin head also possesses an $ATPase$ enzyme activity,which hydrolyzes $ATP$ to provide the energy required for muscle contraction.

(B) During strenuous exercise,the oxygen supply to the skeletal muscles becomes insufficient for aerobic respiration.
As a result,the muscles undergo anaerobic respiration.
This process leads to the conversion of pyruvic acid into lactic acid via the enzyme lactate dehydrogenase.
The accumulation of lactic acid in the muscle fibers causes muscle fatigue and pain.

(D) The sarcomere is the functional unit of muscle contraction.
In the structure of a sarcomere,the $A-$band (anisotropic band) is located in the central region.
The $H-$zone is a lighter central region within the $A-$band where only thick filaments (myosin) are present.
The $M-$line is a thin,dark line that runs through the center of the $H-$zone,anchoring the thick filaments.
Therefore,all the given statements are correct.

(B) During muscle contraction,the $Ca^{2+}$ ions released into the sarcoplasm bind to the $TpC$ (Troponin $C$) subunit of the troponin complex.
This binding induces a conformational change in the troponin complex,which in turn moves the tropomyosin molecule away from the active sites on the actin filament.
This exposes the myosin-binding sites on actin,allowing the formation of cross-bridges between actin and myosin,which leads to muscle contraction.

(C) During the contraction of a skeletal muscle fibre,the sliding filament theory explains the process:
$1$. The $A-$band length remains constant because the length of myosin filaments does not change.
$2$. The $I-$band shortens as the actin filaments slide over the myosin filaments.
$3$. The $H-$zone shortens or may disappear completely during maximal contraction.
$4$. The $M-$line remains visible as the central attachment point for myosin filaments.
Therefore,none of the options $A, B, C, D$ are strictly correct as written,but based on standard textbook questions of this type,the $H-$zone shortens and the $I-$band shortens. If the question implies what happens to the $H-$zone,it shortens. Given the options provided,there is a common error in such questions; however,the $A-$band is the only structure that remains constant in length.

(B) $Mg^{2+}$ and $Ca^{2+}$ ions are essential for muscular contraction.
$1$. $Mg^{2+}$ ions are required for the polymerization of $G-\text{actin}$ into $F-\text{actin}$ filaments.
$2$. $Ca^{2+}$ ions bind to the $TpC$ subunit of troponin,which causes a conformational change that displaces the troponin-tropomyosin complex,thereby exposing the active myosin-binding sites on the actin filament.

(C) White muscle fibres,also known as fast-twitch fibres,contain less myoglobin and fewer mitochondria. They are adapted for rapid,powerful contractions but fatigue quickly.
They are primarily used for:
$1$. Moving of eye balls.
$2$. Fast and strenuous work for short duration.
$3$. Fast flights as in sparrows.
They are not used for sustained work at a slow rate for a prolonged duration,as this function is performed by red muscle fibres (slow-twitch fibres),which are rich in myoglobin and mitochondria and are resistant to fatigue.

(D) The backward bending of the shank (lower leg) at the knee joint is known as flexion.
This movement is primarily performed by the hamstring muscles located at the back of the thigh.
The gastrocnemius muscle also assists in this action by acting across the knee joint.
Therefore,the correct answer is $D$.

(C) Red muscle fibres are characterized by a smaller diameter,a higher number of mitochondria,and a greater density of blood capillaries compared to white muscle fibres.
They contain less sarcoplasmic reticulum because they rely more on aerobic respiration rather than rapid,short-term bursts of contraction that require quick calcium release.
Therefore,having 'more sarcoplasmic reticulum' is not a characteristic of red skeletal muscle.

(B) The blood supply to muscle tissues varies based on their metabolic activity and function.
Cardiac muscle requires a constant and high supply of oxygen and nutrients,thus it has a very rich blood supply.
Skeletal muscles also have a significant blood supply to support contraction and recovery.
Smooth muscles,which are found in the walls of hollow organs,generally have the least blood supply compared to cardiac and skeletal muscles,as their metabolic demands are lower and they often exhibit sustained,slow contractions.

(B) The muscles of the urinary bladder are composed of involuntary smooth muscle fibers,which are not under the voluntary control of the nervous system.
In contrast,the pharynx,the anterior end of the oesophagus,and the tongue are composed of skeletal muscles,which are under the voluntary control of the somatic nervous system.

(A) $Ca^{2+}$ ions play a crucial role in muscle contraction. When $Ca^{2+}$ is released into the sarcoplasm,it binds to the $TpC$ subunit of troponin. This binding induces a conformational change in the troponin complex,which in turn displaces tropomyosin from the active sites on the actin filament. This exposure allows the myosin heads to bind to the actin,forming an actomyosin complex,which initiates the contraction process.

(D) The 'All or None' principle states that a single muscle fibre contracts completely or not at all when stimulated by a threshold stimulus.
However,an entire muscle organ consists of many muscle fibres.
When a stimulus is applied to a whole muscle,the strength of contraction depends on the number of muscle fibres recruited.
Therefore,the whole muscle does not follow the 'All or None' principle; it shows graded responses.
Thus,the Assertion is incorrect,and the Reason is also incorrect because it incorrectly attributes the 'All or None' principle to the entire muscle rather than individual muscle fibres.

(C) : Tetanus in muscle physiology refers to a state of sustained contraction where the muscle fiber does not relax between stimuli because the frequency of nerve impulses is very high. This is a correct statement.
$R$: Many of our daily activities,such as holding a book,require the muscle to maintain a constant state of tension (tonus) rather than a full tetanic contraction. Tetanic contraction is a pathological or experimental state of maximal sustained contraction,whereas holding a book involves sustained muscle tone (tonus) maintained by asynchronous motor unit recruitment. Therefore,the Reason is incorrect.

(C) The $A$ is correct because the staircase effect (treppe) is a phenomenon where repeated stimulation of a muscle fibre at the onset of relaxation leads to progressively stronger contractions.
This occurs because of the increased availability of $Ca^{2+}$ ions in the sarcoplasm and the warming effect of the muscle,which improves enzyme efficiency.
However,the $R$ is incorrect because treppe is not caused by the summation of sub-threshold stimuli.
Sub-threshold stimuli are too weak to trigger an action potential,whereas treppe occurs with stimuli that are already at or above the threshold level.

(C) The latent period is the time interval between the application of a stimulus and the start of muscle contraction.
In skeletal muscle,the latent period is very short (approximately $0.01$ seconds).
In cardiac muscle,the latent period is significantly longer due to the prolonged refractory period,which prevents tetanic contraction.
Therefore,the statement that the latent period is minimum in cardiac muscle is incorrect.
Thus,Assertion is correct,but Reason is incorrect.

(A) According to the sliding filament theory of muscle contraction:
$1$. The $'H'$ zone disappears as actin filaments slide over myosin filaments.
$2$. The $'A'$ band (anisotropic band) retains its length because it represents the length of the myosin filaments,which do not shorten.
$3$. The $'I'$ band (isotropic band) reduces in width as actin filaments are pulled towards the center of the sarcomere.
$4$. Myosin heads hydrolyze $ATP$ to $ADP$ and $Pi$,providing energy for the power stroke.
$5$. The $Z$-lines attached to actin filaments are pulled inwards towards the center of the sarcomere,shortening the sarcomere.
Therefore,events $(a), (c), (d),$ and $(e)$ are correct.

(A) $1$. Biceps are voluntary muscles attached to bones,classified as skeletal muscles.
$2$. The iris contains smooth muscles (involuntary) that control the size of the pupil.
$3$. The heart is composed of cardiac muscles,which are specialized involuntary muscles.
Therefore,the correct sequence is: Biceps (Skeletal muscle),Iris (Smooth muscle),Heart (Cardiac muscle).

(D) Striations are characteristic features of muscles that contain organized sarcomeres.
$1$. Skeletal muscles are voluntary and striated.
$2$. Cardiac muscles are involuntary and also striated.
$3$. Smooth muscles are non-striated (unstriated) and involuntary.
Therefore,both skeletal and cardiac muscles show striations.

(D) Muscles are specialized tissues of mesodermal origin. They possess several unique properties that allow them to perform their functions effectively:
$1$. Excitability: Muscles respond to stimuli such as chemical,electrical,or mechanical signals.
$2$. Contractility: Muscles can shorten in length in response to stimulation,which is the primary mechanism for movement.
$3$. Extensibility: Muscles have the ability to be stretched beyond their resting length.
$4$. Elasticity: Muscles can return to their original shape and length after being stretched.
Since all the listed options (Excitability,Contractility,and Extensibility) are fundamental properties of muscle tissue,the correct answer is $D$.

(A) Skeletal muscles are primarily associated with the skeletal components of the body.
They are involved in locomotory actions and changes in body posture.
These muscles are under voluntary control and are attached to bones via tendons.
Smooth muscles and cardiac muscles are involuntary and are involved in internal organ functions,not in maintaining body posture.

(A) In the structure of a skeletal muscle,the muscle is organized into bundles of muscle fibers called fascicles $(P)$.
Each fascicle contains several muscle fibers $(Q)$.
These fibers are surrounded by a network of blood capillaries $(R)$ to supply oxygen and nutrients.

(C) In muscle cells, the $Sarcoplasmic$ $Reticulum$ $(SR)$ acts as a specialized storage site for calcium ions $(Ca^{2+})$.
During the resting state, calcium ions are sequestered within the $SR$ to prevent muscle contraction.
When an action potential reaches the muscle fiber, these calcium ions are released into the sarcoplasm, which triggers the binding of calcium to troponin, leading to muscle contraction.

(D) In the structure of a sarcomere,the $I$-band (Isotropic band) contains only thin actin filaments.
The $A$-band (Anisotropic band) contains both actin and myosin filaments.
The $H$-zone (Hensen's zone) is the central part of the $A$-band that contains only thick myosin filaments.
Therefore,the band containing only actin is the $I$-band,and the region containing only myosin is the $H$-zone.

(B) The functional unit of muscle contraction is the sarcomere.
In a sarcomere,the $A$-band (anisotropic band) is located in the center,containing both actin and myosin filaments.
The $M$-line is a thin dark line that runs through the center of the $A$-band.
The $I$-band (isotropic band) contains only actin filaments and is bisected by the $Z$-line.
When considering the structure between two successive $M$-lines,it spans across two adjacent sarcomeres.
Specifically,the region between two $M$-lines consists of one complete $A$-band (from the first sarcomere),two half $I$-bands (one from each side of the $Z$-line),and the $Z$-line itself.
However,in the context of standard structural definitions of the sarcomere,the region between two $Z$-lines is one sarcomere. The region between two $M$-lines effectively encompasses one full $A$-band and two half $I$-bands.

(B) $1$. Skeletal muscles are voluntary muscles attached to bones,such as the biceps $(P-I)$.
$2$. Smooth muscles are involuntary muscles found in internal organs like the iris of the eye $(Q-II)$.
$3$. Cardiac muscles are involuntary muscles found exclusively in the heart $(R-III)$.
Therefore,the correct matching is $(P-I), (Q-II), (R-III)$.

(B) Based on their location,appearance,and nature of regulation,muscles are classified into $3$ main types:
$1$. Skeletal muscles: These are closely associated with the skeletal components of the body and are striated.
$2$. Visceral (Smooth) muscles: These are located in the inner walls of hollow visceral organs and are non-striated.
$3$. Cardiac muscles: These are the muscles of the heart and are striated and involuntary.

(B) sarcomere is the functional unit of a striated muscle fiber.
It is defined as the portion of a myofibril located between two successive $Z$-lines (or $Z$-discs).
These $Z$-lines are dense protein structures that anchor the actin filaments.
Therefore,the correct option is $B$.

(D) In the structure of a sarcomere:
$P$ represents the $Z$-line (or $Z$-disc),which bisects the $I$-band.
$Q$ represents the $M$-line,which is the central line in the $A$-band.
$R$ represents the $H$-zone,which is the central part of the $A$-band containing only thick filaments.
$S$ represents the $I$-band (isotropic band),which contains only thin filaments and is bisected by the $Z$-line.
Therefore,the correct label for the $I$-band is $S$.

(A) In the structure of a sarcomere,the $M$-line is a thin fibrous membrane in the center of the $A$-band where thick filaments (myosin) are anchored.
Conversely,the $Z$-line (or $Z$-disc) is a dark fibrous membrane that bisects the $I$-band,where thin filaments (actin) are firmly attached.
Therefore,the protein/filament attached to the $M$-line is myosin $(P)$ and the protein/filament attached to the $Z$-line is actin $(Q)$.

(C) In the structure of a thin filament (actin filament),two filaments of $F$-actin are helically wound to each other. Two filaments of a protein called tropomyosin also run close to the $F$-actin throughout its length. $A$ complex protein called troponin is distributed at regular intervals on the tropomyosin. Troponin masks the active binding sites for myosin on the actin filaments in the resting state.

(B) The myosin filament is a polymerized protein consisting of many monomeric proteins called meromyosins. Each meromyosin has two parts: a globular head with a short arm and a tail. The globular head is an active ATPase enzyme and has binding sites for $ATP$ and active sites for actin. This head and short arm are collectively known as Heavy Meromyosin $(HMM)$,while the tail portion is known as Light Meromyosin $(LMM)$. Therefore,the $ATPase$ enzyme activity is located in the $HMM$ part of the myosin molecule.

(A) The thin filament (actin filament) is composed of two $F-actin$ polymers,which are helically wound to each other.
Each $F-actin$ is a polymer of monomeric $G-actin$ (globular actin) proteins.
Two filaments of another protein,$tropomyosin$,also run close to the $F-actin$ throughout its 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.
Therefore,the active binding sites for $myosin$ are present on the $actin$ (specifically $F-actin$) molecules.

(B) In the context of muscle structure,specifically regarding the myosin filament,$HMM$ stands for $Heavy$ $Meromyosin$.
Each myosin molecule consists of two parts: a globular head with a short arm and a tail.
The head and the short arm together are referred to as $Heavy$ $Meromyosin$ $(HMM)$,
while the tail portion is referred to as $Light$ $Meromyosin$ $(LMM)$.

(A) In the given diagram of the actin filament:
$P$ represents the troponin complex.
$Q$ represents the tropomyosin filament.
$R$ represents the $F$-actin (filamentous actin) strand.
During muscle contraction,$Ca^{++}$ ions bind to the troponin complex $(P)$,which causes a conformational change in the troponin-tropomyosin complex,thereby exposing the active binding sites on the actin filaments for the myosin heads to attach. Therefore,the correct answer is $P$.

(D) The diagram shows a myosin monomer. The head region labeled $P$ contains two important binding sites: one for actin and one for $ATP$.
During muscle contraction,the myosin head binds to actin to form a cross-bridge.
Additionally,the myosin head possesses $ATPase$ activity,which catalyzes the hydrolysis of $ATP$ to provide energy for the contraction process.
Therefore,both actin binding and $ATP$ hydrolysis occur at the myosin head $(P)$.

(C) According to the Sliding Filament Theory,during muscle contraction,the thin filaments (actin) slide over the thick filaments (myosin).
$1$. The $A$-band (anisotropic band) remains constant in length because the myosin filaments do not change their length.
$2$. The $I$-band (isotropic band) decreases in length as the actin filaments move deeper into the $A$-band.
$3$. The $Z$-lines move closer to each other,shortening the sarcomere.
$4$. The length of the individual filaments (actin and myosin) does not change; only their degree of overlap changes. Therefore,the statement that the length of filaments decreases is incorrect.

(A) neuromuscular junction $(NMJ)$ is a specialized chemical synapse between a motor neuron and a muscle fiber.
It allows the motor neuron to transmit a signal to the muscle fiber,causing muscle contraction.
The junction consists of the axon terminal of the motor neuron and the specialized region of the muscle fiber membrane known as the sarcolemma.
Therefore,the correct junction is between a motor neuron and the sarcolemma of a muscle fiber.

(A) During muscle contraction,the signal for contraction is sent by the central nervous system via a motor neuron.
When the action potential reaches the neuromuscular junction,it triggers the release of neurotransmitters,leading to an action potential in the sarcolemma.
This action potential spreads through the $T$-tubules and causes the release of $Ca^{2+}$ ions into the sarcoplasm from the sarcoplasmic reticulum.
These $Ca^{2+}$ ions bind to the $Troponin$ subunit on the actin filaments.
This binding causes a conformational change in the troponin-tropomyosin complex,exposing the active sites on the $F$-actin filaments for the myosin heads to bind,thereby initiating contraction.

(B) During muscle contraction,the cross-bridge formation between actin and myosin filaments occurs when $Ca^{2+}$ ions bind to troponin,exposing the active sites on actin. This step does not require $ATP$ hydrolysis directly.
However,the breaking of the cross-bridge (detachment of the myosin head from actin) requires the binding of a new $ATP$ molecule to the myosin head. The hydrolysis of this $ATP$ then provides the energy for the myosin head to return to its 'cocked' position.
Therefore,cross-bridge formation occurs in the presence of $Ca^{2+}$ (but does not require $ATP$ for the binding itself),while cross-bridge breaking strictly requires the presence of $ATP$.

(C) Red muscle fibers are characterized by the following features:
$1$. They contain a large number of mitochondria,which can utilize large amounts of $O_2$ stored in them for $ATP$ production.
$2$. They contain a high amount of myoglobin,which gives them a reddish appearance.
$3$. They have a less developed sarcoplasmic reticulum compared to white muscle fibers.
Based on the given statements:
- Statement $I$ (Many mitochondria) is correct.
- Statement $II$ (Use large amounts of $O_2$) is correct.
- Statement $III$ (Low amount of myoglobin) is incorrect,as red muscle fibers have high myoglobin content.
- Statement $IV$ (Less sarcoplasmic reticulum) is correct.
Therefore,the correct characteristics are $I, II,$ and $IV$.

(B) In the structure of a sarcomere:
$1$. The $H$-zone $(P)$ is the central part of the $A$-band where only thick myosin filaments are present.
$2$. The $I$-band $(Q)$ is the light band containing only thin actin filaments,bisected by the $Z$-line.
$3$. The $A$-band $(R)$ is the dark band that contains both actin and myosin filaments.
Therefore,the correct identification is $P = H$-band,$Q = I$-band,and $R = A$-band.

(B) The sliding filament theory states that muscle contraction occurs when thin actin filaments slide over thick myosin filaments.
During this process,the $A$-band remains constant in length because the length of the myosin filaments does not change.
The $I$-band shortens as the actin filaments are pulled toward the center of the sarcomere.
The $H$-zone also shortens or disappears.
Consequently,the overall length of the sarcomere decreases,leading to muscle contraction.

(C) Statement $A$ is incorrect because muscle bundles are held together by a collagenous connective tissue layer called fascia,not fascicle. The bundles themselves are called fascicles.
Statement $B$ is correct because the sarcoplasmic reticulum of muscle fibres is indeed a storehouse of calcium ions,which are essential for muscle contraction.
Statement $C$ is correct because the striated appearance of skeletal muscle fibres is due to the regular distribution pattern of actin and myosin proteins.
Statement $D$ is incorrect because the portion of the myofibril between two successive $Z$-lines is considered the functional unit of contraction,called a sarcomere,not the $M$-line.
Therefore,only statements $B$ and $C$ are correct.

(A) The mechanism of muscle contraction is explained by the sliding filament theory:
$1$. Muscle contraction is initiated by a signal sent by the $\text{CNS}$ via a motor neuron (not a sensory neuron),making statement $A$ incorrect.
$2$. The motor neuron releases a neurotransmitter (acetylcholine) which generates an action potential in the sarcolemma,making statement $B$ correct.
$3$. This action potential triggers the release of $Ca^{++}$ ions into the sarcoplasm. Increased $Ca^{++}$ levels lead to the binding of calcium with troponin on actin filaments,which exposes the active sites,making statement $C$ correct.
$4$. Masking of the active site prevents contraction,while unmasking leads to contraction,making statement $D$ incorrect.
$5$. The myosin head then utilizes the energy from $\text{ATP}$ hydrolysis to form a cross-bridge with the actin filament,making statement $E$ correct.
Therefore,statements $B, C,$ and $E$ are correct.

(C) The $ATP$ase enzyme activity in muscle fibers is associated with the head of the $Myosin$ filament.
During muscle contraction,the $Myosin$ head acts as an $ATP$ase,hydrolyzing $ATP$ into $ADP$ and inorganic phosphate $(Pi)$.
This energy release allows the $Myosin$ head to bind to the active sites on the $Actin$ filament,forming cross-bridges and facilitating muscle contraction.

(B) In the structure of a sarcomere,the thick filaments (myosin) are located in the center of the $A$-band.
During muscle contraction and relaxation,the thin filaments (actin) extend from the $Z$-lines towards the center of the sarcomere.
The central portion of the thick filament that is not overlapped by the thin filaments is known as the $H-$zone (Hensen's zone).
Therefore,the correct option is $B$.

(D) The thin filament (also known as the $I$-band) is primarily composed of three proteins: $Actin$,$Tropomyosin$,and $Troponin$.
$Actin$ forms the backbone of the thin filament.
$Tropomyosin$ runs along the $Actin$ filaments to regulate binding sites.
$Troponin$ is a complex of three proteins attached to $Tropomyosin$.
$Meromyosin$ is the structural unit of the thick filament $(Myosin)$,not the thin filament.

(A) In the structure of a thin filament (actin filament), the $F$-actin is composed of two helical strands of polymerized $G$-actin molecules.
Each $G$-actin monomer has a specific binding site for the myosin head.
In a resting state, these binding sites on the actin filaments are covered by the regulatory protein tropomyosin, which prevents the interaction between actin and myosin.
Therefore, the myosin binding site is located on the actin protein.

(A) The thin filament (actin) consists of two $F$-actin helically wound to each other. Each $F$-actin is a polymer of monomeric $G$-actin (globular actin) proteins. Two filaments of another protein,$Tropomyosin$,also run close to the $F$-actin throughout its 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.