Muscles Questions in English

(C) Muscle fibers are classified based on their contraction speed and metabolic pathways.
$1$. $Slow$ $oxidative$ $fibers$ ($Type$ $I$) are rich in myoglobin and mitochondria, making them highly resistant to fatigue.
$2$. $Fast$ $oxidative-glycolytic$ $fibers$ ($Type$ $IIa$) have intermediate properties.
$3$. $Fast$ $glycolytic$ $fibers$ ($Type$ $IIb$) rely primarily on anaerobic glycolysis for energy. They have fewer mitochondria and less myoglobin, which leads to a rapid accumulation of lactic acid and depletion of glycogen stores, causing them to fatigue very quickly compared to other types.

(C) According to the Sliding Filament Theory,muscle contraction occurs when thin filaments (actin) slide over thick filaments (myosin).
$1$. The length of the individual actin and myosin filaments does not change.
$2$. The $A$ band,which represents the length of the myosin filaments,remains constant in length.
$3$. The $I$ band and the $H$ zone shorten as the actin filaments slide toward the center of the sarcomere.
$4$. The sarcomere itself shortens as the $Z$ lines are pulled closer together.
Therefore,the length of the $A$ band remains the same during contraction.

(B) The correct answer is $B$ (Renin).
$1$. Actin,Tropomyosin,and Myosin are the primary contractile proteins found in muscle fibers.
$2$. Actin and Tropomyosin are thin filament proteins,while Myosin is a thick filament protein.
$3$. Renin is an enzyme secreted by the juxtaglomerular cells of the kidney,which plays a role in the renin-angiotensin-aldosterone system $(RAAS)$ to regulate blood pressure.
$4$. Therefore,Renin is the odd one out as it is not a muscle protein.

(B) The sliding filament theory states that muscle contraction occurs due to the sliding of thin filaments (actin) over thick filaments (myosin).
During this process,the length of the individual filaments (actin and myosin) remains constant; they do not shorten.
Instead,the filaments slide past each other,which increases the overlap between them,thereby shortening the sarcomere and causing muscle contraction.

(C) The sarcoplasmic reticulum is a specialized type of smooth endoplasmic reticulum found in smooth and striated muscle fibers.
Its primary function is to store and release calcium ions $(Ca^{2+})$.
When a muscle fiber is stimulated,calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm,which triggers the process of muscle contraction.
Therefore,the correct answer is $C$.

(D) Endomysium is the innermost layer of connective tissue that surrounds individual muscle fibers.
It is a characteristic feature of all three types of muscle tissues: skeletal,cardiac,and smooth muscles.
In skeletal muscle,it surrounds each muscle fiber.
In cardiac muscle,it surrounds individual cardiomyocytes.
In smooth muscle,it surrounds individual smooth muscle cells.
Therefore,the correct answer is that it is present in all of these muscle types.

(B) During muscle contraction,an action potential travels along the sarcolemma and reaches the $T$-tubules,triggering the release of $Ca^{+2}$ ions from the sarcoplasmic reticulum into the sarcoplasm.
These $Ca^{+2}$ ions bind to the troponin complex on the thin filaments (actin).
This binding causes a conformational change in the troponin-tropomyosin complex,which shifts the tropomyosin away from the active sites on the actin filament.
This exposure of active sites allows the myosin heads to bind to actin,forming cross-bridges and initiating muscle contraction.

(C) Each myosin filament is a polymerized protein consisting of many monomeric proteins called $Meromyosin$. Each $Meromyosin$ has two important 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 $Actin$.

(A) In muscle physiology,when a muscle is stimulated repeatedly at a high frequency,it does not get enough time to relax between successive stimuli.
This leads to a state of sustained contraction known as $Tetanus$.
$Paralysis$ refers to the loss of muscle function.
$Osteoporosis$ is a bone disorder characterized by decreased bone mass.
$Fatigue$ is the decline in the ability of a muscle to generate force after prolonged activity.

(D) Statement $(I)$ is correct: Skeletal muscles are composed of many muscle bundles (fascicles).
Statement $(II)$ is correct: Each muscle bundle contains many muscle fibers,and each muscle fiber is enclosed by a plasma membrane called the sarcolemma.
Statement $(III)$ is incorrect: Muscle fibers are syncytial,meaning the sarcoplasm contains many nuclei (multinucleated).
Statement $(IV)$ is correct: The sarcoplasmic reticulum of the muscle fiber is the storehouse of calcium ions,which are essential for muscle contraction.
Therefore,statements $(I), (II),$ and $(IV)$ are correct.

(D) Visceral muscles (smooth muscles) are located in the inner walls of hollow visceral organs like the alimentary canal and reproductive tract.
They do not exhibit striations under a microscope,hence they are called non-striated or smooth muscles.
Their activities are not under the voluntary control of the nervous system; they are involuntary muscles.
Therefore,statement $A$ is true,but statement $R$ is false.

(A) In the structure of a sarcomere,the $A$-band contains both actin and myosin filaments.
Within the $A$-band,there is a central region where only thick myosin filaments are present,known as the $H$-zone.
The $H$-zone is bisected by a thin,dark fibrous membrane called the $M$-line.
Therefore,the dark line lying in the middle of the $H$-zone is the $M$-line.

(C) According to the sliding filament theory of muscle contraction:
$1$. The length of the thin and thick filaments remains constant; they slide past each other,which is a correct statement.
$2$. During contraction,the $Z$-lines defining the sarcomere are pulled closer together,which is a correct statement.
$3$. During contraction,the light band ($I$-band) shortens because the actin filaments slide into the $A$-band. Therefore,the statement that there is no change in the light band is incorrect.
Since the question asks for the statement that is not true,option $C$ is the correct answer.

(C) Assertion $(A)$ is correct: Calcium ions $(Ca^{2+})$ bind to the troponin subunit on actin filaments,which causes a conformational change that exposes the active sites for myosin on the actin filament,thereby activating the interaction.
Reason $(R)$ is incorrect: While the formation of the cross-bridge requires $ATP$ hydrolysis,the breaking or detachment of the cross-bridge specifically requires the binding of a new $ATP$ molecule to the myosin head. The statement claims breaking occurs in the absence of $ATP,$ which is false; in the absence of $ATP,$ the cross-bridge remains locked (a state known as rigor mortis).
Therefore,$A$ is correct and $R$ is incorrect.

(D) Assertion $(A)$ is incorrect because $G$-actin (Globular actin) is the monomeric form,while $F$-actin (Filamentous actin) is the polymeric form. The statement incorrectly swapped these terms.
Reason $(R)$ is correct. Tropomyosin is indeed a rod-shaped fibrous protein that forms two helical strands,which run along the length of the $F$-actin filaments to regulate muscle contraction.

(D) In the resting state,the protein complex troponin is distributed at regular intervals on the tropomyosin filament.
Specifically,a subunit of troponin masks the active binding sites for myosin heads on the actin filaments.
This prevents the interaction between actin and myosin,thereby keeping the muscle in a relaxed state.

(D) Cardiac muscles are involuntary,striated muscles found in the heart.
$1$. They possess intercalated discs,which are specialized junctions that allow rapid communication between cells.
$2$. They exhibit light and dark bands (striations) due to the arrangement of actin and myosin filaments.
$3$. They contract as a unit because the intercalated discs allow the impulse to spread quickly,ensuring the heart beats in a synchronized manner.
$4$. Cardiac muscle cells are typically uninucleated (or sometimes binucleated),whereas skeletal muscle cells are multinucleated. Therefore,being a multinucleated cell is not a characteristic of cardiac muscle.

(B) The functional unit of contraction in striated muscle is the $Sarcomere$.
It is defined as the segment of a myofibril between two successive $Z$-lines.
Each $Sarcomere$ contains the contractile proteins,actin and myosin,arranged in a specific pattern that allows for muscle shortening during contraction.

(D) Involuntary muscles (smooth muscles) are found in the walls of internal organs such as blood vessels,glandular ducts,and the iris. The urethra,however,contains both involuntary smooth muscles (internal urethral sphincter) and voluntary skeletal muscles (external urethral sphincter) to control micturition. Therefore,the urethra does not consist solely of involuntary muscles.

(A) Muscle contraction is initiated by the release of calcium ions $(Ca^{2+})$ from the sarcoplasmic reticulum into the sarcoplasm.
These $Ca^{2+}$ ions bind to the troponin complex on the actin filaments,which causes a conformational change that exposes the myosin-binding sites on the actin.
This allows the myosin heads to bind to actin,forming cross-bridges and leading to muscle contraction via the sliding filament mechanism.

(A) Muscle contraction is primarily driven by the interaction between two key contractile proteins: $Actin$ and $Myosin$.
$Actin$ forms the thin filaments,while $Myosin$ forms the thick filaments.
These proteins slide past each other during the contraction process,known as the sliding filament theory.
$Tubulin$ is a structural protein involved in the formation of microtubules,not muscle contraction.
Therefore,both $Actin$ and $Myosin$ are contractile proteins,making the correct choice 'All of the above'.

(D) sarcomere is the functional unit of a striated muscle fiber.
It is defined as the segment of a myofibril located between two successive $Z$-lines (or $Z$-discs).
These $Z$-lines are dense protein structures that bisect the $I$-bands and serve as the anchor points for actin filaments.
Therefore,the correct option is $D$.

(B) According to the Sliding Filament Theory of muscle contraction:
$1$. During contraction,the thin filaments slide over the thick filaments,which pulls the $Z$-lines closer together,thereby shortening the sarcomere.
$2$. The $A$-band (Anisotropic band) contains myosin filaments and its length remains constant throughout the contraction process.
$3$. The $I$-band (Isotropic band) and the $H$-zone shorten as the actin filaments move deeper into the $A$-band.
$4$. Therefore,the statement that the length of the $A$-band remains constant is correct.

(C) The $ATPase$ enzyme is located on the head of the myosin filament.
During muscle contraction,the myosin head binds to the active site on actin to form a cross-bridge.
The $ATPase$ activity of the myosin head hydrolyzes $ATP$ into $ADP$ and inorganic phosphate $(Pi)$,which provides the energy required for the power stroke and muscle contraction.

(A) The contractile proteins in skeletal muscle include actin and myosin.
Myosin is a motor protein that forms the thick filaments.
The head of the myosin molecule contains an $ATPase$ enzyme site,which hydrolyzes $ATP$ to provide the energy required for muscle contraction.
Therefore,myosin is the protein associated with $ATPase$ activity.

(A) In a skeletal muscle fiber,the $A$-band contains both actin and myosin filaments.
However,the central part of the thick filament (myosin) is not overlapped by thin filaments (actin).
This central,lighter region within the $A$-band is known as the $H$-zone.
Therefore,the $H$-zone represents the portion of the myosin filament that does not overlap with actin filaments.

(C) 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 triggers the release of $Ca^{2+}$ ions from the sarcoplasmic reticulum into the sarcoplasm.
These $Ca^{2+}$ ions bind to the troponin subunit on the actin filaments.
This binding causes a conformational change that moves tropomyosin away from the active sites on the actin filament,thereby 'unmasking' them.
Once the active sites are exposed,the myosin heads can bind to them to form cross-bridges,initiating the contraction process.
Therefore,$Ca^{2+}$ ions are responsible for this process.

(B) When a muscle is stimulated by repeated,rapid stimuli,it does not get enough time to relax between the stimuli. This results in a state of sustained,continuous contraction known as $Tetanus$. This is different from the disease tetanus,which is caused by the bacterium $Clostridium$ $tetani$.

(A) The stimulation of muscle fibers by a motor neuron occurs at the neuromuscular junction (also known as the motor end plate).
This is a chemical synapse formed between a motor neuron and a muscle fiber.
When an action potential reaches the axon terminal of the motor neuron,it triggers the release of the neurotransmitter acetylcholine into the synaptic cleft.
This neurotransmitter binds to receptors on the sarcolemma of the muscle fiber,initiating an action potential that leads to muscle contraction.

(B) During skeletal muscle contraction,the binding of $Ca^{2+}$ ions to the $Troponin$ complex is a critical step.
In a relaxed state,the active sites on the actin filament are masked by the $Tropomyosin$ protein.
When $Ca^{2+}$ ions are released from the sarcoplasmic reticulum,they bind to $Troponin$.
This binding causes a conformational change in the $Troponin-Tropomyosin$ complex,which shifts the $Tropomyosin$ away from the active sites on the actin filament.
This exposure allows the myosin heads to bind to the actin,forming cross-bridges and initiating muscle contraction.

(C) Sphincters are specialized circular muscles that control the passage of substances through body openings.
$(i)$ Sphincters are mesodermal in origin,as they are composed of muscle tissue.
$(ii)$ They contain stretch receptors that help in regulating their tone and contraction based on the presence of contents.
$(iii)$ Rhythmic contractions are characteristic of cardiac muscle or peristaltic movements,not typical sphincters,which generally maintain a tonic state of contraction.
$(iv)$ Sphincters do not fatigue during the life of the animal because they are designed to maintain a constant state of contraction to prevent leakage.
Therefore,the characteristics $(i), (ii),$ and $(iv)$ are associated with sphincters.

(D) During prolonged strenuous physical work,the demand for $ATP$ in muscle cells increases significantly. When the oxygen supply is insufficient to meet this demand through aerobic respiration,muscle cells switch to anaerobic respiration. In this process,glucose is broken down into lactic acid. The accumulation of lactic acid in the muscle tissue leads to a decrease in $pH$ and interferes with muscle contraction,resulting in the sensation of fatigue.

(B) Step $A$: Attachment of myosin head to actin forming a cross-bridge.
Step $B$: Release of phosphate. Myosin changes shape to pull actin.
Step $C$: Attachment of new $ATP$ to myosin head. The cross-bridge detaches.
Step $D$: Splitting of $ATP$ into $ADP$ and $Pi$. Myosin cocks into its high-energy conformation.

(A) Muscle fatigue is defined as the inability of a muscle to contract or relax effectively after prolonged or repeated stimulation.
This condition is primarily caused by the accumulation of lactic acid in the muscle tissue,which occurs due to anaerobic respiration during intense or repeated muscle contractions.
Therefore,both the Assertion and the Reason are correct,and the Reason provides a valid explanation for the phenomenon of muscle fatigue.

(C) The assertion is correct because muscle contraction involves the formation and breaking of cross-bridges between myosin heads and actin filaments,known as the sliding filament theory.
The reason is incorrect because muscle contraction is initiated by a signal sent by the $Central \ Nervous \ System$ $(CNS)$,not the peripheral nervous system,via a motor neuron.
Therefore,the assertion is correct,but the reason is incorrect.

(N/A) sarcomere is the functional unit of a skeletal muscle fiber. It is defined as the segment of a myofibril between two successive $Z$-lines.
Key components include:
$1$. $Z$-line: $A$ dark,fibrous protein band that bisects the $I$-band.
$2$. $I$-band (Isotropic band): Contains only thin actin filaments.
$3$. $A$-band (Anisotropic band): Contains both thick myosin filaments and overlapping thin actin filaments.
$4$. $M$-line: $A$ thin fibrous membrane in the middle of the $A$-band.
$5$. $H$-zone: The central part of the $A$-band where only thick filaments are present.

(N/A) The sliding filament theory explains the process of muscle contraction during which the thin filaments slide over the thick filaments,which shortens the myofibril.
Each muscle fibre has alternate light and dark bands,which contain special contractile proteins called actin and myosin,respectively.
Actin is a thin contractile protein present in the light band,known as the $I$-band,whereas myosin is a thick contractile protein present in the dark band,known as the $A$-band.
There is an elastic fibre called the $Z$-line that bisects each $I$-band. The thin filament is firmly anchored to the $Z$-line.
The central part of the thick filament that is not overlapped by the thin filament is known as the $H$-zone.
During muscle contraction,the myosin heads or cross-bridges come in close contact with the thin filaments.
As a result,the thin filaments are pulled towards the middle of the sarcomere.
The $Z$-line attached to the actin filaments is also pulled,leading to the shortening of the sarcomere.
Hence,the length of the $A$-band remains constant,while the $I$-band shortens and the $H$-zone disappears.

(N/A) During skeletal muscle contraction,the thick filament slides over the thin filament by a repeated binding and release of myosin along the filament. This whole process occurs in a sequential manner.
Step $1$: Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. The neuromuscular junction is a junction between a neuron and the sarcolemma of the muscle fibre. As a result,Acetylcholine (a neurotransmitter) is released into the synaptic cleft,generating an action potential in the sarcolemma.
Step $2$: The generation of this action potential releases calcium ions $(Ca^{2+})$ from the sarcoplasmic reticulum into the sarcoplasm.
Step $3$: The increased level of calcium ions in the sarcoplasm leads to the activation of actin sites. Calcium ions bind to the troponin on actin filaments and remove the tropomyosin wrapped around actin filaments. Hence,active actin sites are exposed,allowing myosin heads to attach to these sites.
Step $4$: In this stage,the myosin head attaches to the exposed site of actin and forms cross-bridges by utilizing energy from $ATP$ hydrolysis. The actin filaments are pulled toward the center of the $A$-band. As a result,the $H$-zone reduces. This is the stage where the actual contraction of the muscle occurs.
Step $5$: After muscle contraction,the myosin head pulls the actin filament and releases $ADP$ along with inorganic phosphate. New $ATP$ molecules bind to the myosin head,causing it to detach and the cross-bridges to break.
Step $6$: This process of formation and breaking down of cross-bridges continues until the stimulus stops. When the stimulus ceases,calcium ions are pumped back into the sarcoplasmic reticulum. As a result,the concentration of calcium ions in the sarcoplasm decreases,thereby masking the actin filaments again and leading to muscle relaxation.

(N/A) $-\quad$ Muscle is a specialised tissue of mesodermal origin.
$\Rightarrow$ It contributes to $40-50 \%$ of the body weight of a human adult.
- Muscles possess special properties like excitability,contractility,extensibility,and elasticity.
- Muscles are classified based on different criteria,such as location,appearance,and nature of regulation.
- Based on their location,there are three types of muscles: $(i)$ Skeletal muscles,$(ii)$ Visceral muscles,and $(iii)$ Cardiac muscles.
$(i)$ Skeletal muscles: These are closely associated with the skeletal components of the body (e.g.,limbs,trunk).
- Under the microscope,they show dark and light stripes,hence they are called striated muscles.
- They are under the control of the voluntary nervous system,hence they are known as voluntary muscles.
- These muscles are involved in locomotory actions and changes in body posture.
$(ii)$ Visceral muscles (Non-striated): These are located in the inner walls of hollow visceral organs of the body,such as the alimentary canal,reproductive tract,and trachea.
- They do not show striations and appear smooth,hence they are called smooth muscles (non-striated muscles).
- Their activities are not under voluntary control,hence they are known as involuntary muscles.
- They assist in functions like the transportation of food through the digestive tract and gametes through the genital tract.
$(iii)$ Cardiac muscles: These are the muscles of the heart. Many cardiac muscle cells assemble in a branching pattern. Cardiac muscles are short,cylindrical,and connected to each other by branching processes.
- These muscles are innervated by the autonomic nervous system,so they are involuntary in nature.
- Contraction of cardiac muscles is rhythmic,rapid,and continuous (they do not get fatigued). They are connected by intercalated discs and receive a rich blood supply.

(N/A) Each organised skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia. Each muscle bundle contains a number of muscle fibres.
- Each muscle fibre is lined by the plasma membrane called sarcolemma enclosing sarcoplasm.
- Muscle fibre is a syncytium as the sarcoplasm contains many nuclei.
- The endoplasmic reticulum of the muscle fibres (sarcoplasmic reticulum) is the storehouse of calcium ions.
- $A$ characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.
- Each myofibril has alternate dark and light bands on it. It is due to the distribution pattern of two important proteins - Actin and Myosin.
- The light bands contain actin and is called $I$-band (Isotropic band).
- The dark band is called $A$-band (Anisotropic band) and contains myosin protein.
- Both the proteins are arranged as rod-like structures,parallel to each other and to the longitudinal axis of the myofibrils.
- Actin filaments are thinner as compared to the myosin filaments.
- In the centre of each $I$-band is an elastic fibre called $Z$-line which bisects it. The thin filaments are firmly attached to the $Z$-line.
- The thick filaments in the $A$-band are also held together in the middle of this band by a thin fibrous membrane called $M$-line.
- The $A$ and $I$ bands are arranged alternately throughout the length of the myofibrils.
- The portion of the myofibril between two successive $Z$-lines is considered as the functional unit of contraction and is called a sarcomere.
- In a resting state,the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of the thick filament,not overlapped by thin filaments,is called $H$-zone.

(N/A) Contractile proteins consist of actin (thin) filaments and myosin (thick) filaments.
$1$. Actin Filaments:
- Each actin filament is composed of two '$F$' (filamentous) actins helically wound to each other.
- Each '$F$' actin is a polymer of monomeric '$G$' (globular) actins.
- 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 on the tropomyosin.
- In the resting state,a subunit of troponin masks the active binding sites for myosin on the actin filaments.
$2$. Myosin Filaments:
- Each myosin filament is a polymerised protein consisting of many monomeric proteins called meromyosins.
- Each meromyosin has two important parts: a globular head with a short arm and a tail.
- The head and short arm are called heavy meromyosin $(HMM)$,and the tail is called light meromyosin $(LMM)$.
- The $HMM$ component projects outwards at regular distances and angles from the surface of a polymerised myosin filament and is known as the cross-arm.
- The globular head is an active $ATPase$ enzyme and has binding sites for $ATP$ and active sites for actin.

(N/A) The 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.
This theory was postulated by $A$.$F$. Huxley and $J$. Jensen.
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 constitutes a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction.
$A$ neural signal reaching this junction releases a neurotransmitter (Acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of $Ca^{++}$ (calcium ions) into the sarcoplasm.
Increase in $Ca^{++}$ level leads to the binding of $Ca^{++}$ with a subunit of troponin on actin filaments,and thereby removes the masking of active sites for myosin.
Utilising the energy from $ATP$ hydrolysis,the myosin head now binds to exposed active sites on actin to form a cross-bridge.
This pulls the attached actin filaments towards the centre of the '$A$' band. The '$Z$' line attached to these actins are also pulled inwards,thereby causing a shortening of the sarcomere,i.e.,contraction.
It is clear from the above steps that during shortening of the muscle (contraction),the '$I$' bands get reduced,whereas the '$A$' band retains its length.
The myosin,releasing the $ADP$ and $P_i$,goes back to its relaxed state. $A$ new $ATP$ binds and the cross-bridge is broken.
The $ATP$ is again hydrolysed by the myosin head and the cycle of cross-bridge formation and breakage is repeated,causing further sliding. The process continues till the $Ca^{++}$ ions are pumped back to the sarcoplasmic cisternae,resulting in the masking of actin filaments. This causes the return of '$Z$' lines back to their original position (i.e.,relaxation).
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.

(N/A) Muscles contain a red-colored $O_{2}$-storing pigment called myoglobin.
Myoglobin content is high in some muscles,which gives them a reddish appearance; these are called red fibers.
Red fibers contain a large number of mitochondria,which utilize the stored oxygen for $ATP$ production,making them aerobic muscles (e.g.,flight muscles in $Aves$).
Conversely,some muscles possess very little myoglobin and appear pale or whitish; these are called white fibers.
White fibers have fewer mitochondria but a high amount of sarcoplasmic reticulum,and they rely on anaerobic processes for energy (e.g.,human eyeball muscles).

(B) Skeletal muscles are primarily attached to the bones of the skeleton.
They are under the control of the voluntary nervous system,which is why they are also known as voluntary muscles.
Under a microscope,they exhibit a striped or striated appearance due to the arrangement of actin and myosin filaments,hence they are called striated muscles.
These muscles are primarily involved in locomotory actions and changes in body postures.

(A) Relaxation occurs when $Ca^{++}$ ions are pumped back into the sarcoplasmic reticulum,resulting in the masking of actin filaments.
As the $Ca^{++}$ concentration decreases,the troponin-tropomyosin complex returns to its original position,covering the active sites on the actin filament.
This prevents the formation of cross-bridges,allowing the muscle fiber to return to its original state (relaxation) and the '$Z$' lines to return to their original position.

(N/A)

Striated muscle fiber	Visceral muscle fiber
$(1)$ These are closely associated with the skeleton.	$(1)$ These are found in the walls of internal organs.
$(2)$ Contraction and relaxation are very fast.	$(2)$ Contraction and relaxation are very slow.
$(3)$ They get fatigued easily.	$(3)$ They do not fatigue easily.
$(4)$ Light and dark striations are observed.	$(4)$ No dark and light striations are observed.
$(5)$ They are innervated by voluntary nerves.	$(5)$ They are innervated by involuntary nerves.

(N/A)

Red muscles	White muscles
$(1)$ Presence of abundant myoglobin gives red colour to muscles.	$(1)$ Myoglobin is scarce,so they appear pale or white in colour.
$(2)$ They contain a higher number of mitochondria.	$(2)$ Mitochondria are fewer in number.
$(3)$ They store more $O_2$ and synthesize more $ATP$ through aerobic respiration.	$(3)$ They store less $O_2$ and rely more on anaerobic processes,producing less $ATP$.
$(4)$ Sarcoplasmic reticulum is comparatively less developed.	$(4)$ Sarcoplasmic reticulum is abundantly observed.
$(5)$ Example: Flight muscles of pigeon.	$(5)$ Example: Eyeball muscles.

(N/A) Striated muscles undergo rapid contraction and relaxation.
$\Rightarrow$ During intense activity,glycogen is broken down anaerobically,which leads to the accumulation of lactic acid.
- Due to the accumulation of lactic acid and the depletion of energy reserves,the muscles become unresponsive to further stimulation.
$=$ This state of temporary inability to contract is known as muscle fatigue.

(N/A) Birds do not fatigue easily during flight because their flight muscles are primarily composed of red muscle fibers.
- Red muscle fibers contain a high concentration of myoglobin,which acts as an oxygen-storing pigment.
- These muscles are rich in mitochondria,which facilitate aerobic respiration to produce a large amount of $ATP$.
- Due to the efficient and continuous production of $ATP$ through aerobic pathways,these muscles do not accumulate lactic acid and thus do not fatigue easily,allowing birds to fly for long distances.

(N/A) $(1)$ Skeletal muscles: These muscles are closely associated with the skeletal components of the body,such as the head,trunk,and limbs. Due to this association,they are termed skeletal muscles. They are typically striated and under voluntary control.
$(2)$ Visceral muscles: These muscles are located in the inner walls of hollow visceral organs of the body,such as the alimentary canal (digestive system),respiratory tract,and reproductive tract. They are smooth in appearance and are under involuntary control.