Muscles Questions in English

(D) The correct answer is $D$. The Achilles tendon,also known as the calcaneal tendon,is the strongest and thickest tendon in the human body.
It is formed by the union of the tendons of the gastrocnemius and soleus muscles.
This tendon is inserted into the calcaneus (heel bone),which is also referred to as the oscalcis.

(A) The $ATPase$ enzyme is located in the head of the $Myosin$ filament.
During muscle contraction,the $Myosin$ head binds to the active sites on the $Actin$ filament to form a cross-bridge.
The $Myosin$ head contains an $ATPase$ enzyme that hydrolyzes $ATP$ into $ADP$ and inorganic phosphate $(Pi)$,releasing the energy required for the power stroke,which causes the sliding of $Actin$ filaments over $Myosin$ filaments.

(B) During muscle contraction,the binding of cross-bridges between actin and myosin filaments is regulated by calcium ions $(Ca^{++})$.
When an action potential reaches the sarcolemma,it triggers the release of $Ca^{++}$ from the sarcoplasmic reticulum into the sarcoplasm.
These $Ca^{++}$ ions bind to the troponin complex on the actin filaments,which causes a conformational change in the tropomyosin.
This shift exposes the active binding sites on the actin filament,allowing the myosin heads (cross-bridges) to attach to actin and initiate the contraction cycle.

(C) The $Sarcolemma$ is the plasma membrane of a muscle cell.
Specifically,it is the fine,transparent,and elastic membrane that encloses the $Skeletal$ $muscle$ $fibre$.
It plays a crucial role in the transmission of action potentials along the muscle fibre,which leads to muscle contraction.

(D) The strongest muscle in the human body based on its ability to exert force is the masseter muscle, which is located in the jaw. It is responsible for the process of mastication (chewing) and can exert a force of up to $890 \text{ N}$ $(200 \text{ lbs})$ on the molars.

(C) Muscle fatigue is the physiological phenomenon where a muscle loses its ability to contract effectively after prolonged or repetitive stimulation. This occurs primarily due to the accumulation of lactic acid and the depletion of $ATP$ and glycogen reserves within the muscle fibers.

(B) Muscle contraction requires $ATP$ as the primary energy source.
Glycogen stored in the muscle fibers is broken down into glucose through the process of glycogenolysis.
This glucose then undergoes glycolysis and cellular respiration to produce $ATP$,which provides the necessary energy for the sliding filament mechanism of muscle contraction.

(D) The sliding filament theory,which explains the mechanism of muscle contraction,was proposed by $A.F. Huxley$,$H.E. Huxley$,and $J. Hansen$ in $1954$.
This theory states that muscle contraction occurs due to the sliding of thin actin filaments over thick myosin filaments,which shortens the sarcomere without the filaments themselves changing in length.

(D) The muscles of the eyelids exhibit the shortest duration of contraction among the given options.
These muscles are responsible for the rapid blinking reflex,which is an involuntary,quick movement designed to protect the eye from foreign particles or dryness.
In contrast,cardiac muscles (heart) have a longer refractory period,intestinal muscles exhibit slow,rhythmic peristaltic contractions,and jaw muscles are involved in sustained chewing movements.

(C) The $Gastrocnemius$ is the principal calf muscle located in the shank (lower leg) of a tetrapod.
It arises by two heads connected to the condyles of the femur.
At its lower end,it possesses a strong tendon which joins with the tendon of the $Soleus$ muscle to form the $Achilles$ tendon.
Its primary function is to provide the propelling force required for walking and running.

(C) Each organized skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common connective tissue layer called fascia.
Each muscle bundle contains a number of muscle fibres.
Each muscle fibre is lined by the plasma membrane called the sarcolemma enclosing the sarcoplasm.
- $Epimysium$: The connective tissue sheath that surrounds the entire muscle.
- $Perimysium$: The connective tissue sheath that surrounds the muscle bundles (fascicles).
- $Endomysium$: The connective tissue sheath that surrounds individual muscle fibres.
Therefore,the connective tissue sheath surrounding the muscle bundles is the $Perimysium$.

(A) The red color of certain muscle fibers,known as red muscle fibers,is primarily due to the presence of a high concentration of $Myoglobin$.
$Myoglobin$ is an iron-containing pigment similar to $Haemoglobin$,which stores oxygen in the muscle cells.
Additionally,these muscle fibers contain a large number of $Mitochondria$,which utilize the stored oxygen for aerobic respiration to produce energy.
Therefore,the combination of $Myoglobin$ and $Mitochondria$ gives these muscles their characteristic red appearance.

(B) Each myofibril consists of alternating dark and light bands.
The dark bands are known as $A$-bands or anisotropic bands,which contain both actin and myosin filaments.
The light bands are known as $I$-bands or isotropic bands,which contain only actin filaments.

(C) Muscle fatigue occurs primarily due to the accumulation of lactic acid in the muscle fibers.
During intense exercise,when the oxygen supply is insufficient for aerobic respiration,muscles switch to anaerobic respiration.
This process produces lactic acid as a byproduct.
The accumulation of lactic acid lowers the $pH$ of the cytoplasm,making it acidic.
This acidic environment inhibits the activity of various enzymes involved in muscle contraction and energy production,leading to a cessation of muscular contraction and the sensation of fatigue.

(D) The part of the myofibril between two successive $Z$-lines is called a sarcomere.
$A$ sarcomere is considered the functional unit of the contractile system in striated (striped) muscle because it contains the necessary protein filaments ($actin$ and $myosin$) required for muscle contraction.

(C) Visceral muscles are also known as smooth muscles. The longest smooth muscle cells are found in the wall of the uterus of a pregnant female. During pregnancy,these cells undergo significant hypertrophy (increase in size) to accommodate the growing fetus.

(D) The human body contains approximately $639$ muscles.
These muscles collectively account for about $40-50\%$ of the total body weight.
Therefore,the correct option is $D$.

(D) Skeletal muscles are named based on several criteria to describe their functional and anatomical characteristics.
These criteria include:
$1$. Location: e.g.,$Tibialis$ $anterior$ (located near the tibia).
$2$. Size: e.g.,$Gluteus$ $maximus$ (large muscle).
$3$. Number of origins: e.g.,$Biceps$ $brachii$ (two origins).
$4$. Direction of fibres: e.g.,$Rectus$ $abdominis$ (straight fibres).
$5$. Origin and insertion: e.g.,$Sternocleidomastoid$ (originates from sternum and clavicle,inserts on mastoid process).
$6$. Action: e.g.,$Flexor$ $carpi$ $radialis$ (flexes the wrist).
Therefore,all the mentioned factors are used for naming skeletal muscles.

(C) The human back contains the largest number of muscles. This is because the back muscles are responsible for maintaining posture,supporting the vertebral column,and facilitating complex movements of the trunk and limbs. The back musculature is organized into several layers,including superficial,intermediate,and deep muscles (such as the erector spinae group),which collectively account for the highest count of individual muscle units in the body.

(D) During muscle contraction,myosin heads bind to actin filaments to form cross-bridges.
This interaction results in the formation of an actomyosin complex,which facilitates the sliding of actin filaments over myosin filaments,leading to muscle contraction.

(A) The biceps and triceps are muscles located in the upper arm (forearm region is distal to the elbow,but these muscles act on the elbow joint).
Specifically,the biceps brachii is a flexor muscle attached to the humerus and radius,responsible for flexing the arm at the elbow.
The triceps brachii is an extensor muscle located on the posterior side of the humerus,responsible for extending the arm at the elbow.
Therefore,these muscles are associated with the movement of the arm.

(B) Skeletal muscle fibers contain alternating light and dark bands.
$1$. The light bands contain actin and are called $I$-bands or Isotropic bands.
$2$. The dark bands contain myosin and are called $A$-bands or Anisotropic bands.
$3$. Therefore,the dark bands are known as Anisotropic bands.

(B) During muscle contraction,the chemical energy stored in $ATP$ (Adenosine Triphosphate) molecules is utilized by the muscle fibers to perform work.
This process involves the sliding of actin filaments over myosin filaments,which results in the shortening of the sarcomere.
Consequently,the chemical energy is converted into mechanical energy,which is responsible for the movement of the body parts.

(D) . The $Latissimus$ $dorsi$ is a large, flat muscle on the back that stretches to the sides, behind the arm. It is primarily responsible for the adduction, extension, and medial rotation of the shoulder joint. These movements are essential for activities such as swimming, rowing, climbing, pulling, folding the arm behind the back, and scratching the opposite scapula. Additionally, it assists in forceful expiratory efforts such as coughing and sneezing.

(B) According to the sliding filament theory,during muscle contraction,the thin filaments slide over the thick filaments.
$1$. The $A$-band (containing myosin) maintains its length because the thick filaments do not change size.
$2$. The $I$-band (containing actin) shortens as the filaments overlap more.
$3$. The $H$-zone (the central part of the $A$-band where only myosin is present) narrows or disappears as actin filaments move into it.
$4$. Therefore,both statements $A$ and $B$ are technically correct observations of muscle contraction. However,in most standard competitive biology contexts,the reduction of the $H$-zone is a primary characteristic feature of the sliding mechanism.

(A) In a third-order lever,the effort (force) is applied between the fulcrum and the load (resistance).
When the biceps muscle flexes the arm at the elbow,the elbow joint acts as the fulcrum,the weight of the forearm and hand acts as the load,and the biceps muscle attaches to the radius bone between the elbow and the hand,applying the effort.
Therefore,this arrangement represents a third-order lever.

(B) The skeletal muscle fibre contains two types of filaments: thick and thin.
$(b)$ The thick filaments are composed primarily of the protein myosin.
These filaments are confined to the $A$-bands of the sarcomere.
In contrast,actin,tropomyosin,and troponin are components of the thin filaments.

(C) The $Sartorius$ muscle is the longest muscle in the human body. It originates from the anterior superior iliac spine and inserts into the medial surface of the tibia. Its primary functions include the flexion,abduction,and lateral rotation of the hip joint,as well as the flexion of the knee joint. Therefore,it is often referred to as the 'tailor's muscle' because it helps in assuming the cross-legged sitting position.

(A) In muscle physiology, a muscle twitch is the response of a muscle to a single stimulus.
$1$. The $Latent \text{ period}$ is the short delay between the application of the stimulus and the beginning of the contraction.
$2$. The $Contraction \text{ time}$ is the interval from the beginning of the contraction (or the end of the latent period) to the peak of tension development.
$3$. The $Relaxation \text{ time}$ is the period from the peak of tension back to the resting state.
Therefore, the time period between the beginning of the electrical response (which triggers the contraction) and the peak of tension is defined as the contraction time.

(B) In the structure of skeletal muscle,the myofibrils contain two types of filaments: thick filaments and thin filaments.
Thick filaments are primarily composed of the protein myosin.
Thin filaments are primarily composed of the protein actin,along with regulatory proteins like troponin and tropomyosin.
The $I$-bands (isotropic bands) contain only thin filaments,while the $A$-bands (anisotropic bands) contain both thick and thin filaments.
Therefore,the contractile protein actin is specifically a major component of the thin filaments found in the $I$-bands.

(B) The contraction of a muscle fibril occurs via the sliding filament theory,where actin filaments slide over myosin filaments.
This process is initiated by the binding of $Ca^{2+}$ ions to troponin,which exposes the active sites on the actin filament.
However,the actual mechanical power stroke that drives the sliding process is directly powered by the hydrolysis of $ATP$ bound to the myosin head.
The splitting (hydrolysis) of the $ATP$-myosin complex provides the energy required for the myosin head to undergo a conformational change,pulling the actin filament inward.
Therefore,the splitting of the $ATP$-myosin complex is the event most closely related to the actual sliding mechanism.

(D) According to the sliding filament theory,during muscular contraction,the thin filaments slide over the thick filaments.
As a result,the $I$-band and the $H$-zone decrease in length.
However,the $A$-band remains constant in length.
Since both $A$ and $D$ are technically correct in many contexts,the $H$-zone is the most specific region that disappears or shortens significantly during contraction.
Therefore,the $H$-zone decreases in length.

(D) The superior rectus is one of the six extraocular muscles that control the movement of the eyeball. Its primary function is to rotate the eyeball upward (elevation). Therefore,it rolls the eyeball upward.

(C) The resting membrane potential of a muscle fibre is typically around $-90 \, mV$.
When a muscle fibre is stimulated, the membrane potential undergoes a rapid change, known as an action potential.
The peak value of the action potential in a skeletal muscle fibre typically reaches approximately $+45 \, mV$ to $+50 \, mV$ due to the rapid influx of $Na^+$ ions.

(A) Red muscle fibres contain a high concentration of myoglobin,which is an iron-containing pigment protein.
Myoglobin has a high affinity for oxygen and acts as an oxygen storage molecule.
It combines with oxygen to form oxymyoglobin,which facilitates the supply of oxygen to the mitochondria during sustained muscle contraction.
White muscle fibres have a very low content of myoglobin and are adapted for short bursts of anaerobic activity.

(A) The $refractory$ $period$ in muscle tissue is a brief interval following an initial stimulus during which the muscle fiber is unresponsive to a second stimulus.
During this time,the membrane potential is being restored (repolarization),and the ion channels are resetting,meaning the muscle cannot be stimulated to contract again until this period has passed.
Therefore,it is a period when stimulation does not lead to contraction.

(A) In an $Isotonic$ contraction,the muscle changes its length while the tension or force generated by the muscle remains constant. Because the muscle shortens,it is capable of moving a load,which means mechanical work is performed. In contrast,an $Isometric$ contraction involves tension development without a change in muscle length,resulting in no mechanical work.

(A) The contraction of striated muscles is explained by the $Sliding \text{ } Filament \text{ } Theory$.
According to this theory, muscle contraction occurs when thin actin filaments slide over thick myosin filaments.
This process is facilitated by the formation of cross-bridges between the myosin heads and the actin binding sites, which pull the actin filaments towards the center of the $A$-band (the $M$-line), thereby shortening the sarcomere.

(A) The functional unit of contraction in a skeletal muscle is known as the $Sarcomere$.
It is defined as the segment of a myofibril located between two successive $Z$-lines (or $Z$-bands).
These $Z$-lines are dense fibrous connective tissue structures that anchor the actin filaments.
Therefore,the distance between two adjacent $Z$-bands is called a $Sarcomere$.

(A) The muscle fiber consists of alternating dark and light bands. The dark band is known as the $A$-band (Anisotropic band),which contains both actin and myosin filaments. In the center of the $A$-band,there is a relatively lighter region called the $H$-zone,which consists only of thick myosin filaments. Therefore,the $H$-zone is located in the center of the $A$-band.

(C) In a myofibril,the dark bands are known as $A$-bands (Anisotropic bands).
These bands contain both thick filaments (myosin) and thin filaments (actin) that overlap in the region of the $A$-band.
The central part of the $A$-band,where only thick filaments are present and no overlap occurs,is called the $H$-zone.

(A) The muscle responsible for rotating the forearm to turn the palm downwards (or posteriorly) is known as the $Pronator$ muscle.
Conversely,the $Supinator$ muscle rotates the forearm to turn the palm upwards (or anteriorly).
$Adductor$ muscles move a limb towards the midline of the body,while $Abductor$ muscles move a limb away from the midline.

(C) According to the sliding filament theory,when a skeletal muscle contracts,the actin filaments slide over the myosin filaments.
$1$. The $I$-band (Light band) shortens as the actin filaments are pulled towards the center of the sarcomere.
$2$. The $A$-band (Dark band) remains unchanged in length because the length of the myosin filaments does not change.
$3$. The $H$-zone shortens and eventually disappears as the actin filaments overlap completely in the center of the sarcomere.
Therefore,the correct sequence is: $I$-band shortens,$A$-band remains unchanged,and $H$-zone disappears.

(B) When we lift a heavy object,the movement involves the flexion of the elbow joint.
During flexion,the biceps brachii muscle (the agonist) contracts,becoming shorter and thicker,which pulls the radius and ulna towards the humerus.
Simultaneously,the triceps brachii muscle (the antagonist) relaxes to allow this movement to occur.
Therefore,the correct mechanism is that the biceps contracts and the triceps relaxes.

(A) Muscular hypertrophy refers to the increase in the size of muscle cells (muscle fibers).
This process occurs due to an increase in the synthesis of contractile proteins,which leads to the formation of more myofibrils within the existing muscle fibers.
It does not involve an increase in the number of cells (hyperplasia) or a decrease in cell size (atrophy).

(D) During muscle contraction, $ATP$ is rapidly consumed. The immediate regeneration of $ATP$ is facilitated by $Creatine$ phosphate (also known as phosphocreatine).
In resting muscle, excess $ATP$ is used to synthesize $Creatine$ phosphate, which acts as an energy reservoir.
When muscle contraction begins, the enzyme creatine kinase transfers a phosphate group from $Creatine$ phosphate back to $ADP$ to quickly reform $ATP$.

(B) The $Quadriceps$ $femoris$ is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the great extensor muscle of the knee.
The $Gastrocnemius$ is a muscle located on the back portion of the lower leg,forming the main part of the calf muscle.
Both of these muscles are primary components of the human leg anatomy.

(D) . The $Stapedius$ muscle is the smallest skeletal muscle in the human body. It is located in the middle ear and is innervated by the facial nerve (cranial nerve $VII$). Its primary function is to stabilize the stapes bone.

(C) The correct answer is $(C)$.
Smooth muscles are involuntary muscles found in the walls of hollow visceral organs.
During pregnancy,the uterus undergoes significant hypertrophy and hyperplasia,leading to a massive increase in the size of its smooth muscle fibers (myometrium) to accommodate the growing fetus and facilitate labor contractions.
Therefore,the largest smooth muscle cells in the human body are found in the uterus of a pregnant woman.

(C) Muscle fatigue is a condition where a muscle is unable to contract or respond to stimulation effectively due to prolonged or intense activity.
During strenuous exercise,the demand for $O_2$ exceeds the supply,leading to anaerobic respiration in muscle cells.
This process results in the accumulation of lactic acid within the muscle tissue.
The buildup of lactic acid lowers the $pH$ of the muscle environment,which interferes with the contractile mechanism and leads to the sensation of fatigue.