Muscles Questions in English

(N/A) $(1)$ Intercalated disc: These are specialized cell-to-cell junctions that connect adjacent cardiac muscle cells. They facilitate the rapid transmission of electrical impulses,allowing the heart muscle to contract as a single functional unit (syncytium).
$(2)$ Neuromuscular junction: This is the chemical synapse formed between a motor neuron and a muscle fiber. It is the site where the neurotransmitter (acetylcholine) is released to initiate muscle contraction.

(N/A) $(1)$ In a muscle fibre,the region between two consecutive $Z$-lines is termed as a sarcomere. Each sarcomere contains one $A$-band and two half $I$-bands. It is the structural and functional unit of a muscle fibre.
$(2)$ The scapula of the pectoral girdle is a large,flat,triangular bone. The flattened,expanded process present on the spine of the scapula is called the acromion process.

(N/A) $(1)$ Acetabulum: It is a cup-like cavity formed at the point of fusion of the ilium,ischium,and pubis bones of the pelvic girdle. The head of the femur bone of the thigh articulates with this cavity to form the hip joint.
$(2)$ Red muscles: These are muscle fibers that contain a high concentration of myoglobin,which gives them a reddish appearance. They are rich in mitochondria and are capable of aerobic respiration,allowing them to store more $O_2$ and synthesize $ATP$ efficiently for sustained activity,such as the flight muscles of birds.

(N/A) $(1)$ Location: In freely movable joints,a gap is found between the articulating surfaces of two bones,known as the synovial space.
Function: The synovial space is filled with viscous synovial fluid. It lubricates the joints of bones to facilitate easy movement.
$(2)$ Location: It is a small,complex globular protein distributed along with tropomyosin at definite intervals on the actin filaments.
Function: In the relaxed state,troponin masks the active binding sites for myosin on the actin filaments,preventing contraction until calcium ions are released.

(A) $(1)$ $A$-disc stands for Anisotropic disc. These are the dark bands present in the myofibrils of muscle fibers,which contain both actin and myosin filaments.
$(2)$ $I$-disc stands for Isotropic disc. These are the light bands present in the myofibrils of muscle fibers,which contain only actin filaments.

(C) $(1)$ The $Z-$ disc is also known as Krause's membrane or the intermediate disc (Zwischenscheibe).
$(2)$ The $H-$ zone (or $H-$ disc) is also known as Hensen's line or Hensen's zone.

(N/A) $(1)$ $HMM$ stands for Heavy Meromyosin,which is a component of the myosin filament in muscle fibers.
$(2)$ $ATP$ stands for Adenosine Triphosphate,which is the primary energy currency of the cell,required for muscle contraction.

(N/A) These terms refer to the muscle cell (muscle fiber).
$1$. Sarcolemma: The plasma membrane of the muscle fiber.
$2$. Sarcoplasm: The cytoplasm of the muscle fiber.
$3$. Sarcoplasmic reticulum: The endoplasmic reticulum of the muscle fiber,which serves as a storehouse for $Ca^{++}$ ions.

(N/A) The actin filament (thin filament) is composed of three main protein components:
$1$. $F$-actin: These are two filamentous actin helices that form the backbone of the filament.
$2$. Tropomyosin: These are two strands of tropomyosin protein that run close to the $F$-actin throughout its length.
$3$. Troponin: These are complex proteins distributed at regular intervals on the tropomyosin.

(A) Generally,muscles are of two types: $(1)$ Red muscle,$(2)$ White muscle.
Red muscles can work for longer periods because:
$(i)$ They contain a red-coloured oxygen-binding protein called myoglobin. Myoglobin stores oxygen in the form of oxymyoglobin,which releases $O_{2}$ during muscle contraction.
$(ii)$ They have a high density of mitochondria,which allows for sustained aerobic respiration.
$(iii)$ They have a less developed sarcoplasmic reticulum compared to white muscles.
$(iv)$ They perform aerobic respiration for longer periods without accumulating significant amounts of lactic acid,thus delaying fatigue.
$(v)$ The rate of contraction is slower,allowing for sustained activity,for example,the back muscles in humans.

(B) The correct answer is $B$. Regular exercise,such as weight training,leads to hypertrophy of muscle fibers.
When an individual exercises regularly,the volume and size of the muscles increase due to an increase in the amount of sarcoplasm,the number of myofibrils,and the density of mitochondria within the muscle cells.
While fat mass is typically reduced through consistent physical activity,the overall body weight may increase because muscle tissue is denser and heavier than adipose (fat) tissue.
Therefore,the weight gain observed in a person who exercises regularly is primarily due to the development of lean muscle mass rather than fat accumulation.

(B) $ATP$ is the source of energy for muscle contraction.
In each myosin molecule,the $ATPase$ enzyme is present at the head region.
In the presence of this enzyme,along with $Ca^{2+}$ and $Mg^{2+}$ ions,the $ATP$ molecule dissociates into $ADP + Pi$.
Energy is released from the head of the myosin. With the energy obtained from $ATP$,the excited myosin forms a cross-bridge with actin,and muscle contraction is initiated.

(N/A) The mechanism of muscle contraction is best explained by the sliding filament theory,which states that the 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 $J$. Hanson.
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.
An increase in $Ca^{++}$ level leads to the binding of $Ca^{++}$ with a subunit of troponin on actin filaments,thereby removing 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 is also pulled inwards,thereby causing a shortening of the sarcomere,i.e.,contraction.
It is clear from the above steps that during the 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 until 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 the anaerobic breakdown of glycogen in them,causing fatigue.

(N/A) neural signal reaching the neuromuscular 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,thereby removing 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 the 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 until 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).

(N/A) Calcium ions $(Ca^{2+})$ play a crucial regulatory role in muscle contraction.
$1$. Upon stimulation,$Ca^{2+}$ ions are released from the sarcoplasmic reticulum into the sarcoplasm.
$2$. These $Ca^{2+}$ ions bind to the troponin complex on the thin filaments ($F$-actin).
$3$. This binding induces a conformational change in the troponin-tropomyosin complex,which shifts the position of tropomyosin,thereby exposing the active binding sites on the actin filaments.
$4$. The myosin heads then bind to these exposed active sites on actin to form cross-bridges.
$5$. The myosin head,which possesses $ATPase$ activity,hydrolyzes $ATP$ into $ADP$ and inorganic phosphate $(Pi)$. The energy released from this hydrolysis allows the myosin head to pivot,pulling the actin filament towards the center of the sarcomere (power stroke),resulting in muscle contraction.
$6$. During relaxation,$Ca^{2+}$ ions are actively pumped back into the sarcoplasmic reticulum,causing the troponin-tropomyosin complex to return to its original position,blocking the active sites on actin and preventing further cross-bridge formation.

(A) $(1)$ White muscle fibers contain fewer mitochondria and a more extensive sarcoplasmic reticulum compared to red muscle fibers,which allows for faster contraction and anaerobic metabolism.
$(2)$ When a neural signal reaches the neuromuscular junction,it triggers the release of the neurotransmitter acetylcholine into the synaptic cleft,which initiates an action potential in the muscle fiber membrane.

(A) $(1)$ The functional unit of muscle contraction is the sarcomere,which is defined as the segment of a myofibril between two successive $Z$-lines.
$(2)$ The thick filaments (myosin) in the center of the $A$-band are connected by a thin fibrous membrane known as the $M$-line.

(B) Visceral muscles,also known as smooth muscles,are involuntary muscles found in the walls of internal organs. They are involved in functions such as the transportation of food through the digestive tract and the movement of gametes through the genital tract.

(N/A) The sarcolemma is the specialized plasma membrane that surrounds each muscle fibre.
It encloses the sarcoplasm,which is the cytoplasm of the muscle cell.
Because the muscle fibre contains multiple nuclei,it is considered a syncytium.

(B) In muscle cells,the specialized form of the endoplasmic reticulum is known as the sarcoplasmic reticulum.
It acts as the primary storage site for calcium ions $(Ca^{2+})$.
During muscle contraction,these calcium ions are released into the sarcoplasm to initiate the interaction between actin and myosin filaments.

(B) The portion of the myofibril between two successive $Z$-lines is considered as the functional unit of contraction and is called a sarcomere. It is the basic contractile unit of the striated muscle fiber.

(B) $G$-actin (Globular actin) is a monomeric protein.
In the presence of $Mg^{++}$ ions and $ATP$,$G$-actin molecules undergo polymerization to form long,helical,polymeric filamentous structures known as $F$-actin (Filamentous actin).
This process is essential for the formation of thin filaments in muscle fibers.

(N/A) Troponin is a regulatory protein complex associated with the actin filament in muscle fibers. It consists of three distinct subunits:
$(a)$ Troponin $I$ $(T_pI)$: This subunit inhibits the interaction between actin and myosin.
$(b)$ Troponin $T$ $(T_pT)$: This subunit binds to tropomyosin,anchoring the troponin complex to the tropomyosin strand.
$(c)$ Troponin $C$ $(T_pC)$: This subunit acts as a binding site for calcium ions $(Ca^{2+})$,which triggers the contraction process.

(B) Each myosin filament is a polymerised protein.
Many monomeric proteins called meromyosins constitute one thick filament.
Therefore,a thick filament (myosin) is formed by the polymerization of many meromyosin units.

(N/A) Each meromyosin molecule consists of two important parts: a globular head with a short arm and a tail. The globular head with the short arm is called the heavy meromyosin $(HMM)$,while the tail portion is called the light meromyosin $(LMM)$.

(C) The sliding filament theory was proposed by Andrew $F$. Huxley and Jean Hanson,along with Hugh $E$. Huxley and Rolf Niedergerke,in $1954$.
It explains the mechanism of muscle contraction where thin filaments slide over thick filaments,shortening the sarcomere without changing the length of the filaments themselves.

(A) Muscle fatigue is a condition where muscles lose their ability to contract effectively after prolonged or intense activity.
Repeated activation of the muscles leads to the accumulation of lactic acid due to the anaerobic breakdown of glycogen in the muscle fibres.
This accumulation of lactic acid causes the sensation of fatigue and muscle soreness.

(SKIN, CARDIAC MUSCLE) $(i)$ Skin: Frogs respire through their skin (cutaneous respiration) both in water and on land,making it the most important respiratory organ.
(ii) Cardiac muscle: Cardiac muscles are involuntary and exhibit rhythmic,spontaneous contractions throughout life.

(D) During the muscle contraction cycle,the myosin head remains bound to the actin filament in a cross-bridge state. When a new molecule of $ATP$ binds to the myosin head,the affinity of the myosin head for actin decreases significantly. Consequently,the myosin head detaches from the actin filament. Therefore,the separation occurs when $ATP$ attaches to the myosin head.

(B) Skeletal muscles (striated or striped muscles) are voluntary muscles,meaning they are under conscious control,$e.g.$,movement of arms and legs.
Smooth muscles (unstriped or non-striated muscles) are involuntary muscles,meaning they are not under conscious control,$e.g.$,muscles in the posterior part of the oesophagus,stomach,intestine,and blood vessels.
Cardiac muscles are specialized involuntary muscles found in the walls of the heart.
Therefore,both smooth muscles $(b)$ and cardiac muscles $(c)$ act involuntarily.

(C) Skeletal or striated muscle fibers are long,cylindrical,and unbranched structures. These fibers are multinucleated,meaning they contain many nuclei. $A$ characteristic feature of these muscle fibers is that their nuclei are pushed towards the periphery,just beneath the sarcolemma.

(A) Cardiac muscle fibres exhibit characteristic transverse dark and light bands,which alternate with each other,giving them a striated appearance. Furthermore,these muscles are controlled by the autonomic nervous system and are not under conscious control,making them involuntary.

(D) Muscles possess a contractile nature.
The contractile structures within muscle cells are known as myofibrils.
Myofibrils are composed of myofilaments.
Myofilaments are of two primary types: myosin and actin.
Muscle contraction occurs due to the sliding of actin filaments over myosin filaments.

(C) The biceps muscle is a type of skeletal muscle,which is also known as striated muscle.
Skeletal muscles are voluntary,meaning we can control their contraction through conscious thought.
They exhibit alternate light and dark bands (striations) due to the arrangement of actin and myosin filaments.
Their muscle fibres are bundled together in a parallel fashion.
In contrast,smooth muscle fibres are spindle-shaped (fusiform) and taper at both ends,which is not a characteristic of skeletal muscles like the biceps.

(A) sarcomere is the functional unit of a muscle contraction,defined as the segment between two successive $Z-$lines.
It consists of an arrangement of thick filaments (myosin) and thin filaments (actin).
In the resting state,the thin filaments partially overlap the thick filaments,which is the structural basis for the sliding filament theory.
Therefore,a sarcomere is characterized by the overlapping of actin and myosin filaments.

(C) Troponin is a regulatory protein complex associated with the thin filament (actin). It consists of three subunits: $TnC$ (binds to $Ca^{2+}$),$TnI$ (binds to actin),and $TnT$ (binds to tropomyosin). During muscle contraction,troponin is found distributed at regular intervals on the tropomyosin protein,which in turn lies along the actin filament. Therefore,it is closely associated with tropomyosin.

(B) The $Gluteus$ $maximus$ (buttock muscle) is the largest and most powerful muscle in the human body. It is primarily responsible for the movement of the hip and thigh.

(A) Tendons are dense regular connective tissues that primarily consist of collagen fibers. They serve the specific function of attaching skeletal muscles to bones,allowing for the transmission of force generated by muscle contraction to the skeletal system,which facilitates movement.

(C) The mechanism of muscle contraction is best explained by the $Sliding$ $Filament$ $Theory$.
This theory states that the contraction of a muscle fiber occurs due to the sliding of thin actin filaments over thick myosin filaments,which shortens the sarcomere without changing the length of the filaments themselves.

(B) Both proteins,$i.e.$,actin and myosin,are arranged as rod-like structures,parallel to each other and also to the longitudinal axis of the myofibrils.
Actin filaments are thinner compared to myosin filaments; hence,they are commonly called thin and thick filaments,respectively.

(C) The action potential in the sarcolemma travels through the $T$-tubules and triggers the release of $Ca^{2+}$ ions from the sarcoplasmic reticulum into the sarcoplasm.
An increase in the $Ca^{2+}$ concentration in the sarcoplasm leads to the binding of $Ca^{2+}$ with a subunit of troponin on the actin filaments.
This binding causes a conformational change in the troponin-tropomyosin complex,which removes the masking of the active sites on actin filaments for myosin heads,thereby initiating muscle contraction.

(B) Sarcolemma is the fine,transparent,and elastic plasma membrane that covers the muscle fiber (myocyte).
It encloses the sarcoplasm,which contains myofibrils,mitochondria,and other organelles.
Therefore,the correct answer is muscle fiber.

(D) Each myofibril contains alternate dark and light bands. The light bands contain actin and are called $I$-bands or isotropic bands,whereas the dark bands are called $A$-bands or anisotropic bands,which contain myosin.

(A) Skeletal muscles are responsible for voluntary movements under the conscious control of the brain,hence they are called voluntary muscles.
Each muscle fiber contains many parallel filaments called myofibrils.
The segment of a myofibril located between two adjacent $Z$-lines (or $Z$-bands) is known as a sarcomere.
The sarcomere is considered the functional unit of contraction in skeletal muscle because it contains the contractile proteins,actin and myosin,arranged in a specific pattern that allows for muscle shortening.

(A) The sarcomere is the functional and structural unit of a muscle fiber.
It is defined as the region of a myofibril located between two successive $Z-$lines (or $Z-$discs).
These $Z-$lines anchor the actin filaments,and the contraction of the muscle occurs due to the shortening of the sarcomere.

(C) The structural hierarchy of muscle tissue is as follows: $Actin$ and $Myosin$ filaments are the smallest contractile proteins.
$Actin$ and $Myosin$ filaments organize into $Sarcomeres$, which are the functional units of a muscle.
Many $Sarcomeres$ arranged in series form a $Myofibril$.
Many $Myofibrils$ bundled together form a $Muscle \text{ } fibre$ (muscle cell).
Therefore, among the given options, $Actin$ is the smallest component.

(A) Muscle contraction requires energy to facilitate the sliding of actin and myosin filaments.
$ATP$ (Adenosine Triphosphate) acts as the immediate source of chemical energy for this process.
When $ATP$ is hydrolyzed by the enzyme myosin ATPase,it releases the energy required for the power stroke of the myosin head.

(C) The $A$-band (anisotropic band),which is composed of thick filaments (myosin),remains unchanged in length during skeletal muscle contraction.
During contraction,the thin filaments (actin) slide over the thick filaments,which causes the $I$-band to shorten and the $H$-zone to disappear or narrow,but the $A$-band length remains constant.

(C) motor unit consists of a motor neuron and the muscle fibres connected to it. Muscle contraction is initiated by a signal sent by the Central Nervous System $(CNS)$ via motor neurons. $A$ single motor neuron,along with all the muscle fibres it innervates,constitutes a motor unit.

(A) The thin filaments of a muscle fiber are primarily composed of two $F$-actin filaments, along with two regulatory proteins, $troponin$ and $tropomyosin$.