Muscle Contraction Practice Questions
40 free Muscle Contraction practice questions for the Zoology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following describes the function of tropomyosin in a relaxed muscle fiber?
- A It anchors the thick filaments to the Z-disc
- B It blocks the myosin-binding sites on the actin filaments
- C It acts as an ATPase to provide energy for the power stroke
- D It binds to calcium ions to initiate contraction
Correct answer: It blocks the myosin-binding sites on the actin filaments
In a resting muscle, tropomyosin is positioned over the active sites of the G-actin subunits. This prevents the myosin heads from forming cross-bridges until a calcium signal shifts the tropomyosin-troponin complex away.
What occurs during the 'power stroke' phase of the sliding filament mechanism?
- A ATP binds to the myosin head, causing it to detach from actin
- B The myosin head hydrolyzes ATP to ADP and inorganic phosphate
- C The myosin head pivots after releasing ADP and phosphate, pulling the actin filament
- D Calcium is actively pumped back into the sarcoplasmic reticulum
Correct answer: The myosin head pivots after releasing ADP and phosphate, pulling the actin filament
After a cross-bridge forms, the release of stored energy from the previous ATP hydrolysis causes the myosin head to bend toward the M-line. This movement slides the thin filament over the thick filament, shortening the sarcomere.
According to the sliding filament theory, which region of the sarcomere remains constant in length during contraction?
- A The I-band, which contains only thin filaments
- B The H-zone, which lies in the center of the A-band
- C The A-band, which spans the full length of thick filaments
- D The distance between two adjacent Z-discs
Correct answer: The A-band, which spans the full length of thick filaments
The A-band represents the total length of the thick (myosin) filaments. While the I-bands and H-zones shorten as filaments overlap more, the length of the thick filaments themselves does not change.
Which protein acts as a calcium sensor, undergoing a conformational change when $Ca^{2+}$ binds to it?
- A Actin thin filament
- B Myosin thick filament
- C Troponin complex
- D Titin elastic protein
Correct answer: Troponin complex
Troponin is a three-polypeptide complex; one part (Troponin C) has a high affinity for calcium ions. Upon binding, it pulls tropomyosin off the binding sites, allowing contraction to proceed.
In skeletal muscle, what is the specific role of the T-tubules (transverse tubules)?
- A Storage of glycogen reserves for anaerobic energy metabolism
- B Synthesis of contractile proteins such as actin and myosin
- C Conducting action potentials deep into the muscle fiber interior
- D Anchoring the muscle fiber mechanically to the surrounding tendon
Correct answer: Conducting action potentials deep into the muscle fiber interior
T-tubules are invaginations of the sarcolemma that reach deep into the cell. They ensure that an electrical impulse reaches all the myofibrils nearly simultaneously, triggering uniform calcium release from the sarcoplasmic reticulum.
What is the primary cause of 'Rigor Mortis' after death?
- A Excessive release of acetylcholine at the neuromuscular junction
- B Absence of ATP, which is required for myosin heads to detach from actin
- C Rapid breakdown of troponin by lysosomal enzymes after death
- D Build-up of lactic acid causing muscle membrane depolarization
Correct answer: Absence of ATP, which is required for myosin heads to detach from actin
ATP binding is necessary to break the cross-bridge between myosin and actin. When ATP production ceases after death, the myosin heads remain 'locked' to the actin, causing permanent muscle stiffness until tissue decay begins.
Which enzyme is responsible for the hydrolysis of the neurotransmitter at the neuromuscular junction to allow muscle relaxation?
- A Creatine kinase
- B Myosin ATPase
- C Acetylcholinesterase
- D Phosphofructokinase
Correct answer: Acetylcholinesterase
Acetylcholinesterase breaks down acetylcholine (ACh) in the synaptic cleft. This stops the continuous stimulation of the muscle fiber, allowing the sarcolemma to repolarize and calcium to be pumped back into storage.
During muscle contraction, the sarcoplasmic reticulum (SR) releases calcium through which specific channels?
- A Ryanodine receptors (RyR)
- B Dihydropyridine receptors (DHPR)
- C SERCA transport pumps
- D Inositol trisphosphate receptors (IP3R)
Correct answer: Ryanodine receptors (RyR)
The RyR are calcium-release channels located in the SR membrane. They are opened when they receive a signal from the DHPR (voltage sensors) in the T-tubules, allowing calcium to flood the sarcoplasm and initiate contraction. DHPR senses the voltage change but does not itself release SR calcium.
The resting membrane potential of a skeletal muscle fiber is primarily maintained by the unequal distribution of which ions?
- A Ca2+ and Mg2+
- B Na+ and K+
- C Cl- and HCO3-
- D Fe2+ and PO43-
Correct answer: Na+ and K+
Like neurons, muscle fibers utilize the Sodium-Potassium pump to maintain a high concentration of K+ inside and Na+ outside. This creates an electrochemical gradient essential for excitability.
What is the function of the protein 'Titin' in the sarcomere?
- A It binds to actin at Z-discs to prevent over-stretching
- B It provides elasticity and keeps the thick filaments centered
- C It hydrolyzes ATP to power myosin head movement
- D It forms the structural boundary of the H-zone
Correct answer: It provides elasticity and keeps the thick filaments centered
Titin is a giant elastic protein that extends from the Z-disc to the M-line. It acts like a spring, allowing the sarcomere to recoil after being stretched and keeping the thick filaments aligned in the center.
Which of the following is the 'rate-limiting step' for muscle relaxation?
- A Dissociation of ACh from the motor end-plate receptor
- B The passive sliding of filaments back to the resting position
- C Active transport of Ca2+ back into the sarcoplasmic reticulum by SERCA pumps
- D The closing of voltage-gated sodium channels in the sarcolemma
Correct answer: Active transport of Ca2+ back into the sarcoplasmic reticulum by SERCA pumps
Relaxation cannot occur as long as calcium is present in the sarcoplasm. The SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase) pumps must use ATP to move calcium against its gradient back into the SR.
In the cross-bridge cycle, what must happen before the myosin head can return to its 'high-energy' (cocked) position?
- A Calcium must bind to the myosin tail region first
- B ATP must be hydrolyzed into ADP and Pi
- C The actin filament must rotate around its axis
- D The Z-discs must approximate toward the M-line
Correct answer: ATP must be hydrolyzed into ADP and Pi
Hydrolysis of ATP by the myosin head provides the energy necessary to 'cock' the head into its high-energy state. This energy is later used to drive the power stroke when the head binds to actin.
What is a 'Motor Unit'?
- A All the muscles contained within a single limb segment
- B A single motor neuron and all the muscle fibers it innervates
- C The full set of mitochondria found within one muscle cell
- D A bundle of myofibrils grouped inside a single muscle fiber
Correct answer: A single motor neuron and all the muscle fibers it innervates
A motor unit is the functional unit of muscle contraction. When the neuron fires, all fibers within that specific unit contract simultaneously; smaller units are used for fine control, while larger ones are for strength.
The 'latent period' of a muscle twitch represents the time required for which of the following?
- A The time required for synthesis of new ATP molecules
- B The interval needed for oxygen to diffuse into the fiber
- C The time for excitation-contraction coupling and calcium release
- D The period needed for lactic acid to accumulate in the sarcoplasm
Correct answer: The time for excitation-contraction coupling and calcium release
The latent period is the delay between the arrival of the action potential and the start of tension development. This time is spent on the electrical signal traveling down T-tubules and the subsequent release and binding of calcium.
Which type of muscle contraction occurs when the muscle develops tension but does not change in length?
- A Isotonic concentric
- B Isotonic eccentric
- C Isometric static
- D Isokinetic dynamic
Correct answer: Isometric static
In an isometric contraction, the load is greater than the force produced by the muscle. The cross-bridges form and generate tension, but the thin filaments do not slide enough to shorten the overall muscle.
What is 'Treppe' (the staircase effect) in muscle physiology?
- A A decrease in contraction strength caused by accumulated fatigue products
- B A gradual rise in contraction strength with repeated stimulation after rest
- C The merging of individual twitches into a smooth, sustained contraction
- D The involuntary contraction of a muscle triggered by passive stretching
Correct answer: A gradual rise in contraction strength with repeated stimulation after rest
Treppe occurs because early contractions increase the availability of calcium in the sarcoplasm and raise the internal temperature of the muscle. This makes subsequent contractions more efficient and stronger until a plateau is reached.
Which molecule provides a rapid, short-term reserve of high-energy phosphates to regenerate ATP during the first few seconds of muscle activity?
- A Lactic acid (anaerobic byproduct)
- B Creatine phosphate
- C Blood glucose (from circulation)
- D Myoglobin (oxygen-storage protein)
Correct answer: Creatine phosphate
Creatine phosphate can quickly transfer its phosphate group to ADP to form ATP via the enzyme creatine kinase. This provides enough energy for about 10–15 seconds of maximal exertion.
What is the primary difference in the contraction mechanism of smooth muscle compared to skeletal muscle?
- A Smooth muscle lacks actin and myosin filaments entirely
- B Smooth muscle uses calmodulin instead of troponin to bind calcium
- C Smooth muscle contraction proceeds without any ATP input
- D Smooth muscle relies on T-tubules and a well-developed SR for activation
Correct answer: Smooth muscle uses calmodulin instead of troponin to bind calcium
Smooth muscle does not have the troponin-tropomyosin complex. Instead, calcium binds to calmodulin, which then activates Myosin Light Chain Kinase (MLCK) to phosphorylate the myosin heads and initiate contraction.
The 'Length-Tension Relationship' suggests that maximal force is generated when:
- A The muscle is extremely stretched beyond its normal resting length
- B The sarcomeres are maximally compressed and fully overlapped internally
- C There is an optimal overlap between thick and thin filaments
- D The Z-discs are as far apart as structurally possible in each sarcomere
Correct answer: There is an optimal overlap between thick and thin filaments
At optimal resting length, the maximum number of cross-bridges can form. If the muscle is too stretched, there is little overlap for binding; if it is too compressed, the filaments interfere with each other, reducing force.
In 'Tetanus,' why is the muscle able to maintain a sustained contraction?
- A Calcium remains high because the SR cannot reabsorb it between rapid stimuli
- B The motor neuron ceases firing but chemical residue keeps the muscle active
- C ATP is fully depleted, locking myosin heads in the attached cross-bridge state
- D The actin filaments reach their fully extended length and cannot slide further
Correct answer: Calcium remains high because the SR cannot reabsorb it between rapid stimuli
When stimuli arrive fast enough, the SR doesn't have time to reabsorb the calcium between shocks. This constant high calcium concentration keeps the troponin-tropomyosin complex out of the way, allowing for continuous cross-bridge activity.
Which of the following occurs within the sarcomere during the 'Power Stroke' of muscle contraction?
- A ADP and inorganic phosphate are released, and the myosin head pivots
- B ATP binds to the myosin head, causing the cross-bridge to detach
- C Calcium ions are pumped back into the sarcoplasmic reticulum by SERCA
- D Acetylcholine is broken down by acetylcholinesterase in the synaptic cleft
Correct answer: ADP and inorganic phosphate are released, and the myosin head pivots
During the power stroke, the myosin head releases the bound phosphate and ADP, which triggers the head to pivot and pull the actin filament toward the M-line of the sarcomere.
In the 'Sliding Filament Theory', which zone or band of the sarcomere disappears or significantly narrows during a full contraction?
- A A-band
- B Z-disc
- C H-zone
- D M-line
Correct answer: H-zone
The H-zone contains only thick filaments. As actin filaments are pulled toward the center during contraction, they overlap the thick filaments, causing the H-zone to narrow or disappear.
What is the specific role of 'Calmodulin' in the contraction of smooth muscle?
- A It binds calcium and activates myosin light-chain kinase (MLCK)
- B It binds calcium ions and directly moves the tropomyosin molecule
- C It hydrolyzes ATP to help cock the myosin head for the next cycle
- D It anchors the thin filaments to the dense bodies in the cytoplasm
Correct answer: It binds calcium and activates myosin light-chain kinase (MLCK)
Smooth muscle lacks troponin. Instead, calcium binds to calmodulin, which then activates MLCK to phosphorylate myosin heads, enabling cross-bridge formation.
Which protein is responsible for anchoring the thick filaments to the Z-discs and providing elastic recoil to the sarcomere?
- A Actin
- B Dystrophin
- C Titin
- D Actinin
Correct answer: Titin
Titin is a giant protein that functions as a molecular spring, maintaining the structural integrity of the sarcomere and assisting in passive tension during relaxation.
The 'Triad' in skeletal muscle fibers consists of which structures?
- A One T-tubule and two terminal cisternae of the SR
- B Two T-tubules and one terminal cisterna
- C Actin, myosin, and titin
- D The motor neuron, synaptic cleft, and motor end plate
Correct answer: One T-tubule and two terminal cisternae of the SR
A triad is the functional unit of excitation-contraction coupling, composed of a central T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.
Which molecule must bind to the myosin head to initiate 'Dissociation' (detachment) from the actin filament?
- A Calcium
- B ADP
- C Inorganic Phosphate
- D ATP
Correct answer: ATP
The binding of a new ATP molecule to the myosin head reduces its affinity for actin, causing the cross-bridge to detach. This is why a lack of ATP leads to rigor mortis.
What is the function of the 'Dihydropyridine (DHP) receptors' in skeletal muscle?
- A They act as voltage sensors in the T-tubule membrane
- B They are calcium-release channels in the SR membrane
- C They pump calcium back into the sarcoplasmic reticulum
- D They bind acetylcholine at the neuromuscular junction
Correct answer: They act as voltage sensors in the T-tubule membrane
DHP receptors are L-type voltage-gated calcium channels in the T-tubule that sense depolarization and mechanically pull open the ryanodine receptors (RyR) on the SR.
During 'Isometric Contraction', which of the following remains constant?
- A Muscle length
- B Muscle tension
- C ATP consumption
- D Calcium concentration
Correct answer: Muscle length
In an isometric contraction, the muscle develops tension but does not shorten because the load is equal to or greater than the tension generated.
Which of the following is the primary source of ATP for a marathon runner during the middle of a race?
- A Creatine phosphate
- B Anaerobic glycolysis
- C Aerobic respiration
- D Stored ATP in the sarcoplasm
Correct answer: Aerobic respiration
Aerobic respiration in the mitochondria provides a steady, long-term supply of ATP using oxygen and glucose or fatty acids for sustained endurance activities.
The 'Refractory Period' of cardiac muscle is significantly longer than that of skeletal muscle. This prevents:
- A Tetanus
- B Fatigue
- C High blood pressure
- D Oxygen debt
Correct answer: Tetanus
The long absolute refractory period in cardiac muscle ensures the heart relaxes and fills with blood between beats, preventing lethal sustained contractions (tetanus).
Which component of the Troponin complex is responsible for binding Calcium ions?
- A Troponin I
- B Troponin T
- C Troponin C
- D Troponin M
Correct answer: Troponin C
Troponin C (TnC) is the specific subunit that binds Ca2+, triggering a conformational change that moves Troponin T and Troponin I to shift tropomyosin.
What happens to the 'I-band' during muscle contraction according to the sliding filament model?
- A It stays the same length
- B It lengthens
- C It rotates 90 degrees
- D It shortens or disappears
Correct answer: It shortens or disappears
The I-band consists of thin filaments only. As thin filaments slide over thick filaments toward the center, the distance between the ends of thick filaments and the Z-discs (the I-band) decreases.
Which enzyme is used to quickly transfer a phosphate group from creatine phosphate to ADP?
- A Myosin ATPase
- B Creatine kinase
- C Lactate dehydrogenase
- D Acetylcholinesterase
Correct answer: Creatine kinase
Creatine kinase (CK) catalyzes the reversible reaction that generates ATP and creatine from ADP and creatine phosphate during the onset of muscle activity.
In smooth muscle, 'Dense Bodies' are functionally analogous to which structure in skeletal muscle?
- A T-tubules
- B Z-discs
- C Sarcoplasmic reticulum
- D M-line
Correct answer: Z-discs
Dense bodies serve as the attachment points for thin filaments in smooth muscle cells, helping to transmit the force of contraction to the cell membrane, similar to Z-discs.
Which process is responsible for the 'Latent Period' seen on a myogram of a muscle twitch?
- A The time it takes for ATP to be synthesized
- B The duration of the actual power stroke
- C The time for excitation-contraction coupling
- D The time for the muscle to return to resting length
Correct answer: The time for excitation-contraction coupling
The latent period is the delay between the stimulus and the start of tension, representing the time for action potential propagation, Ca2+ release, and cross-bridge initiation.
What occurs during the 'Recovery Period' to pay back 'Oxygen Debt'?
- A Lactic acid converts into pyruvic acid or glucose
- B Creatine phosphate stores become fully depleted
- C The heart rate immediately and completely drops to zero
- D Muscle proteins are broken down for extra energy needs
Correct answer: Lactic acid converts into pyruvic acid or glucose
Oxygen debt (EPOC) is the extra oxygen required after exercise to restore metabolic conditions, including the conversion of accumulated lactate back into glucose in the liver (Cori cycle).
The 'Fenn Effect' in muscle physiology refers to:
- A The increase in heat production observed during active muscle shortening
- B The decrease in tension observed as the muscle length increases
- C Extra energy is released when a muscle does work while shortening
- D The complete failure of a muscle to relax due to a lack of calcium
Correct answer: Extra energy is released when a muscle does work while shortening
The Fenn effect states that a muscle generates more total energy (heat + work) when it is allowed to shorten and perform work than when it remains isometric.
In a skeletal muscle fiber, where is the highest concentration of $Ca^{2+}$ found when the muscle is at rest?
- A The general sarcoplasm surrounding the myofibrils
- B The nucleus of the muscle fiber
- C Terminal cisternae of the sarcoplasmic reticulum
- D The synaptic cleft at the neuromuscular junction
Correct answer: Terminal cisternae of the sarcoplasmic reticulum
The SR terminal cisternae act as the primary storage site for calcium in a resting muscle, sequestered by the protein calsequestrin.
Which of the following describes 'Multi-unit' smooth muscle?
- A Cells are linked together by gap junctions and contract as a single unified sheet
- B Cells operate independently and are often innervated by a single nerve ending
- C It is found primarily in the walls of the digestive tract and other organs
- D It exhibits rhythmic, spontaneous waves of contraction without any nerve input
Correct answer: Cells operate independently and are often innervated by a single nerve ending
Multi-unit smooth muscle (like in the iris of the eye) consists of discrete fibers that contract independently, allowing for fine, graded control, unlike single-unit (visceral) smooth muscle.
What is the primary function of the 'SERCA' pump in muscle physiology?
- A To release calcium ions from the terminal cisternae of the tubules
- B To actively transport calcium from the sarcoplasm back into the SR
- C To bind acetylcholine molecules at the neuromuscular motor end plate
- D To phosphorylate the myosin light chain during smooth muscle contraction
Correct answer: To actively transport calcium from the sarcoplasm back into the SR
SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase) uses ATP to pump calcium back into the SR, which is the essential step for muscle relaxation.