What happens to the 'Z-lines' during a full muscle contraction?
- Pulled inwards
- Stay stationary
- Disappear
- Move apart
Explanation: As actin filaments slide over myosin, the Z-lines attached to them are pulled closer together, shortening the sarcomere.
The 'All-or-None' principle in the muscular system specifically applies to:
- The whole muscle
- A single fascicle
- The entire organ
- A single fiber
Explanation: An individual muscle fiber will either contract completely or not at all in response to a stimulus above the threshold.
The junction between a motor neuron and the sarcolemma of the muscle fiber is the:
- Neuromuscular junction
- Synaptic cleft
- Gap junction
- Desmosome
Explanation: Also called the motor end-plate, it is the site where the neural signal is transmitted to the muscle fiber to initiate contraction.
Which of the following is an autoimmune disorder that affects the neuromuscular junction?
- Muscular dystrophy
- Osteoporosis
- Myasthenia gravis
- Tetany
Explanation: Myasthenia gravis leads to fatigue, weakening, and paralysis of skeletal muscles by attacking acetylcholine receptors.
The neurotransmitter released at the neuromuscular junction to initiate contraction is:
- Adrenaline
- Dopamine
- Serotonin
- Acetylcholine
Explanation: Acetylcholine is released from the motor neuron, generating an action potential in the sarcolemma.
Which molecule acts as the immediate source of energy for the 'power stroke' during contraction?
- ATP
- Glucose
- Creatine phosphate
- Glycogen
Explanation: ATP hydrolysis by the myosin head provides the energy required for the formation of the cross-bridge and the sliding movement.
White muscle fibers are better suited for which of the following activities?
- Slow swimming
- Marathon running
- Short sprints
- Posture maintenance
Explanation: White fibers contract quickly and powerfully but fatigue easily, making them ideal for short bursts of intense activity.
The 'Sliding Filament Theory' of muscle contraction was primarily proposed by:
- Huxley and Hanson
- Darwin and Lamarck
- Watson and Crick
- Skevington and Miller
Explanation: Proposed in 1954, this theory states that muscle contraction occurs by the sliding of thin filaments over thick filaments.
The 'F-actin' filament is a polymer composed of monomeric subunits called:
- G-actin
- Fibrin
- Meromyosin
- Troponin
Explanation: Two 'F' (filamentous) actins are helically wound to each other; each F-actin is made of many globular 'G' actin monomers.
The 'Light Meromyosin' (LMM) component of a myosin molecule constitutes the:
- Active site
- Head
- Short arm
- Tail
Explanation: Myosin is made of Heavy Meromyosin (head/neck) and Light Meromyosin (tail).
Rigor mortis, the stiffening of muscles after death, is caused by the depletion of:
- ATP
- Acetylcholine
- Lactic acid
- Calcium
Explanation: Without ATP, the myosin heads cannot detach from the actin filaments, leaving the muscles in a permanently contracted state.
During the 'Power Stroke' of the sliding filament theory, the myosin head releases:
- Calcium ions
- ATP
- ADP and Pi
- Oxygen
Explanation: The release of ADP and inorganic phosphate (Pi) triggers the myosin head to pull the actin filament toward the M-line.
In a sarcomere, the 'I-band' (Isotropic band) contains which of the following proteins?
- Troponin only
- Myosin only
- Actin and Myosin
- Actin only
Explanation: The I-band is the light band of the myofibril and consists solely of thin filaments (actin).
The 'H-zone' in the skeletal muscle fiber is specifically defined as the:
- Central dark band
- Central light band
- Gap between Z-lines
- Central myosin-only part
Explanation: The H-zone is the central part of the thick filament (A-band) that is not overlapped by thin filaments (actin) during the relaxed state.
Which of the following ions is essential for triggering the process of muscle contraction?
- Sodium ions
- Magnesium ions
- Calcium ions
- Potassium ions
Explanation: Calcium ions (Ca²⁺) released from the sarcoplasmic reticulum bind to troponin, causing a conformational change that unmasks the actin binding sites.
Each 'Meromyosin' molecule has two important parts: a globular head and a:
- Fiber base
- Short arm
- Long tail
- Short neck
Explanation: A meromyosin consists of a Heavy Meromyosin (HMM - head and short arm) and a Light Meromyosin (LMM - tail).
From which embryonic germ layer is the majority of the muscular system derived?
- Mesoderm
- Endoderm
- Ectoderm
- Neural crest
Explanation: Approximately 40-50 percent of the body weight of a human adult is contributed by muscles, which are mesodermal in origin.
Cardiac muscles are unique because their cells are functionally linked by:
- Intercalated discs
- Synapses
- Tight junctions
- Pectinaceous fibers
Explanation: Intercalated discs contain communication junctions (gap junctions) that allow the heart to contract as a single unit.
The minimum strength of a stimulus required to initiate a muscle contraction is the:
- Latent period
- Peak value
- Action potential
- Threshold stimulus
Explanation: If a stimulus is below the threshold level, the muscle fiber will not respond or contract at all.
Which of the following layers directly surrounds a 'single' muscle fiber?
- Sarcolemma
- Epimysium
- Perimysium
- Endomysium
Explanation: Endomysium is the delicate connective tissue layer that wraps each individual muscle cell/fiber.
A contraction where muscle 'tension' increases but the 'length' remains the same is:
- Isotonic
- Auxotonic
- Eccentric
- Isometric
Explanation: In isometric contractions (e.g., pushing a wall), the muscle develops tension without changing its length or moving a joint.
The overall bundle of several fascicles into a functional muscle is held together by:
- Endomysium
- Ligaments
- Epimysium
- Perimysium
Explanation: The epimysium is the outermost dense connective tissue sheath that encases the entire muscle organ.
Rapid spasms or wild contractions in muscles due to low calcium in body fluids is:
- Gout
- Tetanus
- Tetany
- Dystrophy
Explanation: Hypocalcemia-induced tetany causes involuntary muscle contractions and increased neuronal excitability.
During muscle contraction, which of the following stays 'constant' in length?
- A-band
- I-band
- H-zone
- Sarcomere
Explanation: In the sliding filament mechanism, the A-band (myosin length) remains constant while the I-band and H-zone shorten.
The 'cross-bridge' is formed when the myosin head binds to an active site on:
- Meromyosin
- F-actin
- Tropomyosin
- Troponin
Explanation: Once the active site is unmasked, the energized myosin head binds to the actin (F-actin) to initiate the bridge.
Which of the following is a characteristic of 'Red' muscle fibers?
- Anaerobic respiration
- High myoglobin content
- Rapid fatigue
- Few mitochondria
Explanation: Red fibers contain high levels of myoglobin (oxygen-storing pigment) and many mitochondria, making them well-suited for aerobic respiration.
During the shortening of a sarcomere, which part 'completely' disappears at maximum contraction?
- I-band
- band
- H-zone
- Z-line
Explanation: As actin filaments from opposite sides meet or overlap in the center, the H-zone (the non-overlapping myosin part) disappears.
Which of the following is an example of an 'Extensor' muscle?
- Brachialis
- Biceps brachii
- Triceps brachii
- Gastrocnemius
Explanation: The triceps acts as an extensor by straightening the arm at the elbow joint.
What is the primary cause of fatigue in 'White' muscle fibers?
- High ATP
- Protein loss
- Excess oxygen
- Lactic acid
Explanation: White fibers rely on anaerobic glycolysis, which leads to the rapid buildup of lactic acid and earlier fatigue.
Muscles that act in 'opposition' to each other, such as the biceps and triceps, are called:
- Synergistic
- Fixators
- Antagonistic
- Agonists
Explanation: Antagonistic muscles work in pairs; when one contracts (agonist), the other relaxes (antagonist) to move a joint.
Progressive degeneration of skeletal muscle, mostly due to genetic disorders, is called:
- Tetany
- Arthritis
- Muscular dystrophy
- Myasthenia gravis
Explanation: Muscular dystrophy is a group of genetic diseases that cause progressive weakness and loss of muscle mass.
Ciliary movement is primarily used in the human body for which of the following?
- Pumping blood
- Digesting food
- Removing dust
- Moving limbs
Explanation: Ciliated epithelium in the trachea helps in removing dust particles and microbes inhaled along with atmospheric air.
The 'Summation' of muscle twitches occurs when a second stimulus arrives:
- At peak
- During latency
- After relaxation
- Before relaxation
Explanation: If a second stimulus is applied before the first contraction has finished relaxing, the forces combine to produce a stronger contraction.
The presence of which pigment gives 'Red muscle fibers' their characteristic color?
- Bilirubin
- Hemoglobin
- Myoglobin
- Melanin
Explanation: Myoglobin is a red-colored oxygen-storing pigment found in high concentrations in aerobic (red) muscle fibers.
The plasma membrane surrounding a muscle fiber is technically referred to as the:
- Sarcolemma
- Sarcoplasm
- Sarcosome
- Perimysium
Explanation: The sarcolemma is the specialized cell membrane of a muscle cell fiber that encloses the sarcoplasm.
The globular head of myosin contains an active site for binding with:
- Actin and ATP
- Calcium and Glucose
- ATP and Calcium
- Actin and Tropomyosin
Explanation: The myosin head is an active ATPase enzyme and has binding sites for both ATP and the actin filament.
The fatigue experienced in muscles during strenuous exercise is primarily due to the accumulation of:
- Pyruvic acid
- Carbon dioxide
- Lactic acid
- Uric acid
Explanation: Repeated activation can lead to anaerobic breakdown of glycogen, resulting in the accumulation of lactic acid, which causes muscle fatigue.
The connective tissue sheath that surrounds an individual 'Fascicle' is called:
- Epimysium
- Endomysium
- Perimysium
- Fascia
Explanation: A whole muscle is covered by epimysium; fascicles are covered by perimysium; and individual fibers are covered by endomysium.
Which type of cellular movement is exhibited by macrophages and leucocytes in the human body?
- Muscular movement
- Amoeboid movement
- Flagellar movement
- Ciliary movement
Explanation: Leucocytes move using pseudopodia, a process facilitated by the streaming of protoplasm and cytoskeletal elements like microfilaments.
Cardiac muscles are similar to skeletal muscles because they are both:
- Voluntary
- Striated
- Spindle shaped
- Unbranched
Explanation: Both cardiac and skeletal muscles have a banded (striated) appearance due to the organized arrangement of sarcomeres.
Tropomyosin is a long, fibrous protein that runs along the length of:
- The M-line
- The Sarcolemma
- Thick filaments
- Thin filaments
Explanation: Two filaments of tropomyosin wrap around the F-actin 'groove' in the thin filament structure.
During the development of a muscle fiber, G-actin monomers polymerize to form:
- Tropomyosin
- F-actin
- Meromyosin
- Sarcolemma
Explanation: Globular (G) actin units join together to form the long, filamentous (F) actin strands.
The 'M-line' in a sarcomere is a thin fibrous membrane that holds together which filaments?
- Myosin filaments
- Troponin complex
- Z-line fibers
- Actin filaments
Explanation: The M-line is the central line of the thick filament (A-band) that keeps the myosin filaments in alignment during contraction.
Which enzyme breaks down the neurotransmitter in the synaptic cleft to stop muscle contraction?
- ATPase
- Acetylcholinesterase
- Lipase
- Amylase
Explanation: This enzyme clears acetylcholine from the receptor sites, allowing the muscle to relax and prevent continuous stimulation.
Which of the following describes the histological nature of 'Skeletal' muscles?
- Striated and Voluntary
- Non-striated and Voluntary
- Striated and Involuntary
- Non-striated and Involuntary
Explanation: Skeletal muscles exhibit a striped (striated) appearance under the microscope and are under the conscious control of the nervous system.
The 'Dark band' of the myofibril is technically referred to as the:
- I-band
- Z-line
- M-line
- A-band
Explanation: The A-band (Anisotropic band) appears dark and contains the thick filaments (myosin) and parts of overlapping actin.
Which of the following is a 'Flexor' muscle that bends the arm at the elbow?
- Biceps
- Deltoid
- Triceps
- Trapezius
Explanation: The biceps brachii is the primary flexor of the forearm at the elbow joint.
Which storage house provides the majority of calcium ions for skeletal muscle contraction?
- Sarcoplasmic reticulum
- Mitochondria
- Nucleus
- Golgi body
Explanation: The sarcoplasmic reticulum is a specialized smooth ER that stores high concentrations of calcium ions.
The structural and functional unit of contraction in a skeletal muscle is the:
- Sarcomere
- Muscle fiber
- Myofibril
- Fascicle
Explanation: The sarcomere is the portion of a myofibril between two successive Z-lines and is considered the basic unit of muscle contraction.
Skeletal muscle fibers are described as 'syncytium' because they are:
- Enucleated
- Single nucleated
- Multinucleated
- Lacking nuclei
Explanation: During development, multiple cells fuse to form a single muscle fiber, resulting in a common sarcoplasm with many nuclei.
The enzymatic activity required for ATP hydrolysis during contraction is located on the:
- Myosin head
- Actin head
- Sarcolemma
- Troponin site
Explanation: The globular head of myosin acts as an ATPase, breaking down ATP into ADP and inorganic phosphate to power the cross-bridge.
Which specific subunit of 'Troponin' masks the binding site for myosin on the actin filament?
- Troponin I
- Troponin T
- Troponin M
- Troponin C
Explanation: Troponin consists of three subunits: TpC (binds calcium), TpT (binds tropomyosin), and TpI (inhibitory, masks the active site).
Visceral muscles, found in the inner walls of hollow visceral organs, are also known as:
- Skeletal muscles
- Cardiac muscles
- Striated muscles
- Smooth muscles
Explanation: Visceral muscles do not exhibit striations and are involuntary, earning them the name smooth or non-striated muscles.
Which protein makes up the 'Thick' filaments of the myofibril?
- Tropomyosin
- Myosin
- Troponin
- Actin
Explanation: Myosin is a polymerized protein; each myosin filament is made of many monomeric proteins called meromyosins.
The 'Sarcoplasm' of a muscle fiber contains an exceptionally high amount of:
- Sodium
- Potassium
- Calcium
- Glucose
Explanation: Like most intracellular fluids, sarcoplasm is rich in potassium, while the sarcoplasmic reticulum is rich in calcium.
A state of 'sustained' muscle contraction without relaxation is known as:
- Summation
- Fatigue
- Twitch
- Tetanus
Explanation: Physiological tetanus occurs when stimuli are delivered so frequently that the muscle cannot relax between them.
White muscle fibers are characterized by having which of the following?
- High sarcoplasmic reticulum
- High myoglobin
- High mitochondria
- Aerobic metabolism
Explanation: White fibers have low myoglobin and few mitochondria but possess a very high amount of sarcoplasmic reticulum for rapid contraction.
The 'Motor Unit' is physiologically defined as a single motor neuron plus:
- Sensory nerves
- The spinal cord
- One fiber
- All fibers
Explanation: A motor unit consists of one motor neuron and all the muscle fibers it innervates; all fibers in a unit contract simultaneously.
Which specific protein masks the active binding sites for myosin on the actin filaments?
- Meromyosin
- Actomyosin
- Tropomyosin
- Troponin
Explanation: In a resting state, a complex protein called tropomyosin, along with troponin, covers the myosin-binding sites on actin to prevent contraction.
Which specific protein is found distributed at regular intervals on the tropomyosin?
- Myosin
- Troponin
- Tititin
- Actin
Explanation: Troponin is a complex protein that sits on tropomyosin and binds with calcium to regulate contraction.