Muscles Questions in English

(C) The mechanism of muscle contraction is explained by the Sliding Filament Theory.
According to this theory,muscle contraction occurs when thin filaments (actin) slide over the thick filaments (myosin).
This process is initiated by a signal sent by the central nervous system via a motor neuron.
As the thin filaments slide over the thick filaments,the $Z$-lines attached to the actin filaments are pulled inwards,causing the shortening of the sarcomere,which results in muscle contraction.

(A) Statement-$I$ is true: In the human body,each organised skeletal muscle is composed of many muscle bundles or fascicles. These fascicles are held together by a common connective tissue layer called fascia.
Statement-$II$ is true: The portion of the myofibril between two successive '$Z$' lines is called a sarcomere. It is the functional unit of muscle contraction.
Therefore,both statements are correct.

(B) Statement $(a)$ is correct: Skeletal muscle fibres are syncytial because the sarcoplasm contains many nuclei.
Statement $(b)$ is incorrect: In the centre of each $I$-band is an elastic fibre called the $Z$-line which bisects it,not the $H$-line.
Statement $(c)$ is correct: In the resting state,a subunit of troponin masks the active binding sites for myosin on the actin filaments.
Statement $(d)$ is incorrect: The contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments,not the other way around.
Therefore,the incorrect statements are $(b)$ and $(d)$.

(D) In the structure of a muscle fiber (sarcomere),the '$A$' band contains thick filaments (myosin).
These thick filaments are held together in the central part of the '$A$' band by a thin fibrous membrane known as the '$M$-line' (or $M$-disk).
The '$H$-zone' is the central part of the '$A$' band where only thick filaments are present,while the '$M$-line' runs through the center of this '$H$-zone'.

(A) The sarcoplasmic reticulum $(SR)$ 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 $(Ca^{2+})$,which are essential for muscle contraction. Therefore,it is a characteristic structure of muscle cells.

(C) According to the sliding filament theory of muscle contraction:
$(i)$ The $A$-bands (Anisotropic bands) retain their length because the length of myosin filaments remains constant.
$(ii)$ The $I$-bands (Isotropic bands) and the $H$-zone get reduced in length as the actin filaments slide over the myosin filaments towards the center of the sarcomere.
$(iii)$ Therefore,$(P)$ refers to $I$-bands (or $H$-zones) and $(Q)$ refers to $A$-bands. Given the options,$P$ is $I$-bands and $Q$ is $A$-bands.

(C) The enzyme $\text{ATP}$ase is present in the head of the myosin filament.
During muscle contraction,the myosin head binds to the active site on actin to form a cross-bridge.
This process requires energy,which is provided by the hydrolysis of $\text{ATP}$ catalyzed by the $\text{ATP}$ase enzyme located on the myosin head.

(D) $Statement-I$ is incorrect because plasma creatinine is produced by the non-enzymatic dehydration of creatine phosphate (or creatine) in muscles, not specifically during the catabolism of creatinine phosphate during contraction. Creatinine is a waste product.
$Statement-II$ is correct because creatine phosphate (phosphocreatine) acts as a rapid energy reserve in skeletal muscle, donating a phosphate group to $ADP$ to regenerate $ATP$ during intense muscle activity.

(B) Proprioceptors are specialized sensory receptors that provide information about the position,movement,and orientation of the body parts. They are primarily located in muscles,tendons,and joints,allowing the brain to sense the body's position in space (kinesthesia).

Receptors	Location
Photoreceptors	Retina of Eye
Statoreceptors	Internal Ear (semicircular canals)
Chemoreceptors	Taste buds of tongue,Olfactory Epithelium of Nose
Baroreceptors	Wall of venae cavae and carotid sinus
Proprioceptors	Joints,muscles and tendons

(D) According to the sliding filament theory of muscle contraction,the thin filaments slide over the thick filaments.
During this process,the $I$ bands (isotropic bands) get reduced in length as the actin filaments move deeper into the $A$ bands.
Conversely,the $A$ bands (anisotropic bands) retain their original length because the myosin filaments do not change in size.
Therefore,the correct answer is $I$ bands,$A$ bands.

(A) The correct answer is $A$. The functional unit of muscle contraction is known as the $sarcomere$.
It is defined as the portion of a myofibril that lies between two successive $Z$-lines.
During muscle contraction,these $sarcomeres$ shorten as the actin and myosin filaments slide past each other.

(A) The correct answer is $A$.
$A$ sarcomere is defined as the functional unit of muscle contraction.
Structurally,it is the segment of a myofibril located between two successive '$Z$' lines (also known as '$Z$' discs).
These '$Z$' lines are elastic fibres that bisect the '$I$' bands and serve as the anchor points for actin filaments.

(A) Based on the structure of the myosin monomer (meromyosin):
$A$ represents the Actin binding sites located on the head.
$B$ represents the Head (globular head).
$C$ represents the $ATP$ binding site located on the head.
$D$ represents the Cross arm (short arm connecting the head to the tail).
Therefore,the correct labeling is: $A$ = Actin binding site,$B$ = Head,$C$ = $ATP$ binding site,$D$ = Cross arm.

(C) The correct answer is $C$.
In a striated muscle fiber,the parts are labeled as follows:
$A$ represents the Light band ($I$-band).
$B$ represents the Sarcoplasm (cytoplasm of the muscle cell).
$C$ represents the Myofibril (contractile unit).
$D$ represents the Sarcolemma (plasma membrane of the muscle fiber).
$E$ represents the Nucleus (located at the periphery).
$F$ represents the Dark band ($A$-band).

(D) The correct statements are $A, B, D,$ and $E$.
Statement $A$ is correct: $A$ motor neuron transmits a signal from the $CNS$ to the sarcolemma.
Statement $B$ is correct: The neural signal triggers an action potential,leading to the release of $Ca^{++}$ ions into the sarcoplasm.
Statement $C$ is incorrect: $Ca^{++}$ binds to troponin on the actin filament,which exposes the active sites on actin,thereby facilitating (not inactivating) the formation of cross-bridges.
Statement $D$ is correct: The exposed active sites on actin bind to the myosin head to form a cross-bridge.
Statement $E$ is correct: The myosin heads pull the actin filaments towards the center of the '$A$' band,causing the sarcomere to shorten.
Therefore,the correct sequence of events leads to the conclusion that $A, B, D,$ and $E$ are correct.