Loco – Physiology

The Z disc (Z line) is a structural protein region in the sarcomere where thin filaments (actin) are anchored.

• α-Actinin: The main protein of the Z disc; it cross-links actin filaments from adjacent sarcomeres, maintaining alignment.
• Nebulin: Associated with thin filaments; acts as a molecular ruler for actin length, but not the Z disc itself.
• Myomesin: Located in the M line, not Z disc.
• Actin: Structural component of thin filaments, but it is anchored to the Z disc, not the disc itself.
• C-protein: Found in the M line and with thick filaments, stabilizing myosin.

Key Point to Remember

• Z disc → α-Actinin
• M line → Myomesin, C-protein
• Thin filament stabilization → Nebulin

• Titin is a giant elastic protein that spans from the Z-disc to the M-line.
• It works like a spring, allowing the sarcomere to stretch and recoil, which maintains structural integrity and prevents overstretching.

Why not the others?

• Stabilizes the myosin filaments → Partly true, but its primary role is elasticity.
• Acts as an ATPase enzyme → That’s myosin head, not titin.
• Anchors the actin filaments to the Z-lines → That’s α-actinin.
• Facilitates calcium release from SR → That’s ryanodine receptors / calsequestrin system, not titin.

📌 Key point: Titin = the “molecular spring” of muscle.

• Titin is the largest known protein in the human body and extends from the Z-line to the M-line in the sarcomere.

• Its primary role is to act like a molecular spring that:

  • Provides elasticity to skeletal muscle.

  • Allows sarcomeres to recoil back to resting length after being stretched.

  • Prevents overstretching of muscle fibers.

• This property helps maintain sarcomere alignment and contributes to the passive tension of muscles.

❌ Why the other options are incorrect:

• Stabilizes the myosin filaments: Partly true (titin helps position myosin), but its main function is elasticity and recoil, not stabilization alone.

• Acts as an ATPase enzyme: That’s the function of myosin heads, not titin.

• Anchors the actin filaments to the Z-lines: This is the role of α-actinin, not titin.

• Facilitates the release of calcium ions from the sarcoplasmic reticulum: That’s the function of ryanodine receptors (RyR) and Ca²⁺ channels, not titin.

• A sarcomere extends from one Z line to the next.

• Actin (thin) filaments are firmly anchored to the Z line at both ends.

• Therefore, the distance between one actin filament and the next actin filament on the same sarcomere corresponds to the Z line, since it is the structural landmark that defines the boundary of each actin filament array.

❌ Why the other options are incorrect:

• H zone: Contains only thick filaments (myosin), no actin.

• A band: Represents the length of thick filaments; includes overlap, not the actin–actin distance.

• M line: Central anchoring site for myosin, unrelated to actin positioning.

• I band: Contains only thin filaments, but it is a region—not the structural anchor—so it does not mark the actin-to-actin spacing.

• The force of skeletal muscle contraction depends on the length–tension relationship.

• At rest, skeletal muscle sarcomeres are typically around 2.0–2.2 µm long.

• At 2.2 µm, there is optimal overlap between actin and myosin:

  • Enough cross-bridges can form.

  • Neither actin filaments overlap excessively nor pull past the myosin filaments.

• If sarcomeres are shorter (<1.8 µm), actin filaments begin to interfere with each other and push against Z-lines.

• If sarcomeres are too long (>2.5–3.0 µm), the overlap between actin and myosin decreases, reducing cross-bridge formation and force.

❌ Why the other options are incorrect:

• 1.8 mm: Wrong unit (mm instead of µm) and far too large to be a sarcomere length.

• 2.5 µm: Slightly stretched sarcomere → reduced overlap, less than maximum tension.

• 1.65 µm: Too short → actin overlaps excessively, cross-bridge interference reduces force.

• 3.0 µm: Too long → actin and myosin barely overlap, minimal contraction force.

• In muscle physiology, when a muscle contracts and does more work (like lifting a heavier load), it consumes more ATP.

• This direct proportionality between mechanical work and ATP cleavage to ADP is called the Fenn effect, discovered by Wallace O. Fenn in 1923.

• It emphasizes that ATP utilization is not wasted but tightly matched to the mechanical energy requirement.

❌ Why the other options are incorrect:

• Ratchet theory of contraction: Outdated explanation of cross-bridge action; not about ATP-work relationship.

• Power stroke: Refers to the bending movement of the myosin head pulling actin during contraction, not ATP proportionality.

• Actin power-stroke theory: Just another term describing the mechanism of actin-myosin interaction, not ATP usage.

• Cross-bridge theory: Explains how myosin heads attach/detach to actin for contraction, but doesn’t describe ATP consumption relative to work.

• Skeletal muscle contraction:

  • Calcium binds to troponin C on the thin filament.

  • This causes a conformational change that moves tropomyosin, uncovering the myosin-binding sites on actin.

  • Cross-bridge cycling begins.

  • So, skeletal muscle contraction is troponin-dependent.

• Smooth muscle contraction:

  • Smooth muscle does not have troponin.

  • Instead, calcium binds to calmodulin, forming a Ca²⁺-calmodulin complex.

  • This activates myosin light-chain kinase (MLCK), which phosphorylates myosin and initiates contraction.

  • Thus, contraction is calmodulin- and MLCK-dependent.

❌ Why the other options are incorrect:

• Myosin: Present and required in both types.

• Myosin light-chain kinase (MLCK): Required in smooth muscle, not in skeletal.

• Actin: Present in both; essential for contraction.

• Calmodulin: Required in smooth muscle, not skeletal.

Bone resorption is mainly mediated by osteoclasts. Certain hormones regulate this process:

• Calcitonin → Inhibits osteoclast activity, thereby reducing bone resorption and lowering calcium and phosphate levels in the blood.

• Parathyroid hormone (PTH) → Opposite effect: stimulates osteoclast activity (indirectly through osteoblasts), increasing calcium mobilization from bone.

• Insulin, Testosterone, Thyroxine → These have effects on bone metabolism, but none primarily inhibit osteoclasts like calcitonin.

❌ Why the other options are incorrect:

• Insulin: Promotes bone formation but not the main inhibitor of osteoclasts.

• Testosterone: Supports bone growth by stimulating osteoblasts.

• Parathyroid hormone (PTH): Increases bone resorption, not inhibition.

• Thyroxine: Excess thyroid hormone causes bone resorption and osteoporosis.

• The strength of a muscle contraction depends primarily on how many motor units are activated.

• Motor unit recruitment: Each motor unit contains a motor neuron and all the muscle fibers it innervates.

• More fibers recruited → greater total force because the combined tension of all active fibers determines overall contraction strength.

• Other factors, like muscle fiber type or resting length, can modify contraction properties, but the primary determinant of force is recruitment.

❌ Why Other Options Are Incorrect:

• The length of muscle at rest → Affects tension-length relationship, but not the primary determinant under normal recruitment.

• The type of muscle fibers → Determines speed and fatigue resistance (fast vs slow), not total contraction strength.

• The presence of acetylcholine receptors → Necessary for activation but doesn’t determine overall strength.

• The thickness of sarcolemma → Not a factor in determining contraction force.

• A twitch contraction is a single, brief contraction following one action potential.

• Tetanic contraction occurs when a muscle is stimulated repeatedly at a high frequency, preventing relaxation between stimuli.

• The strength of tetanic contraction is greater because:

  • Repeated stimulation → more calcium released from the sarcoplasmic reticulum → sustained high intracellular calcium → more actin-myosin cross-bridges formed → greater force.

• This process is called temporal summation, and it allows muscles to generate a much stronger contraction than a single twitch.

❌ Why Other Options Are Incorrect:

• Longer duration of acetylcholine receptor at the neuromuscular junction → Not the primary factor; receptor kinetics don’t explain increased force in tetanus.

• Increased release of acetylcholine → Repetitive stimulation does not necessarily increase ACh release significantly; force increase is mainly due to calcium dynamics.

• Rapid action potentials along T-tubules → Necessary for repeated stimulation but not the direct cause of increased contraction force.

• Decreased potassium in extracellular fluid → May affect excitability but not the mechanism for tetanic contraction.

• During muscle contraction, myosin heads form cross-bridges with actin.

• ATP is required for:

  1. Detachment of myosin from actin after the power stroke.

  2. Re-cocking the myosin head for the next contraction.

• If ATP is insufficient, myosin heads cannot detach from actin, resulting in a prolonged, rigid contraction.

• This is the mechanism underlying rigor mortis after death, when ATP production ceases.

❌ Why Other Options Are Incorrect:

• The muscle fiber remains contracted indefinitely → Exaggerated; contraction is limited by other factors, but ATP shortage does cause temporary rigor.

• Troponin binds to myosin → Incorrect; troponin regulates actin binding, not myosin detachment.

• Calcium ions are released from sarcoplasmic reticulum → Calcium release is upstream; ATP shortage affects cross-bridge cycling, not calcium release.

• The muscle fiber relaxes immediately → Opposite of what happens; relaxation requires ATP for cross-bridge detachment.

Explanation:

• The resting membrane potential (RMP) of neurons is typically about –70 mV.
• The primary reason is the selective permeability of the membrane to potassium (K⁺).
• At rest:
  • The neuronal membrane is much more permeable to K⁺ (due to leak channels) than to Na⁺ or Cl⁻.
  • K⁺ tends to diffuse out of the cell (down its concentration gradient), leaving behind negatively charged proteins → creating a negative inside.
  • The exact RMP is determined largely by the K⁺ equilibrium potential (E_K), modified slightly by Na⁺ leak and the Na⁺/K⁺ pump.
• Thus, while the sodium-potassium pump is important for maintaining gradients, the immediate and primary cause of the resting potential is K⁺ permeability.

❌ Chloride ion distribution

• Cl⁻ does contribute to overall potential in some neurons, but it is not the main determinant of the resting potential.

❌ Equilibrium of sodium and potassium ions

• Resting potential is not a simple equilibrium between Na⁺ and K⁺.
• Instead, it is closer to the K⁺ equilibrium potential because the membrane is more permeable to K⁺.

❌ Neurotransmitter release

• Neurotransmitters affect postsynaptic potentials (EPSPs/IPSPs), not the intrinsic resting potential of a neuron.

❌ Sodium-potassium pump activity

• The pump (3 Na⁺ out, 2 K⁺ in) is essential to maintain ion gradients.
• Without it, the gradients would collapse over time, but the pump is not the direct primary cause of the –70 mV RMP.
• It contributes a small electrogenic effect (~ –5 mV).

Explanation:

• Titin is a giant elastic protein that extends from the Z-line to the M-line within the sarcomere.
• It plays two critical roles:
  • Structural support → keeps thick (myosin) filaments properly aligned within the sarcomere.
  • Elasticity → acts like a spring, allowing the sarcomere to return to resting length after being stretched.
• Without titin, sarcomeres would lose stability, and muscle fibers would not recoil efficiently after stretch.

❌ Anchoring of actin filaments to the Z-line

• This is the job of α-actinin, not titin.

❌ Anchoring of myosin filaments to the Z-line

• Myosin is centered at the M-line.
• Titin spans between Z-line and M-line, but its role is elasticity and support, not just “anchoring.”

❌ Initiating the power stroke of myosin heads

• The power stroke comes from ATP hydrolysis by the myosin head, not titin.

❌ Regulating the contraction speed of myosin

• Contraction speed is determined by the myosin ATPase isoform (fast vs slow fibers), not titin.

✅ Correct Answer:

A-alpha fibres

Explanation:

• Aα fibres are the largest diameter, heavily myelinated fibres in the peripheral nervous system.
• They include:
  • Ia fibres → from muscle spindles (sense muscle length → proprioception).
  • Ib fibres → from Golgi tendon organs (sense muscle tension → proprioception).
  • Alpha motor neurons → innervate extrafusal skeletal muscle fibers (motor control).
• Their fast conduction velocity (~80–120 m/s) makes them ideal for quick reflexes and coordinated movement.

❌ A-beta fibres

• Function: touch, vibration, pressure.
• They do not carry motor commands.

❌ A-delta fibres

• Function: fast pain and temperature.
• Thinly myelinated, slower than Aα and Aβ.

❌ B fibres

• Small, myelinated autonomic preganglionic fibers.
• Not related to skeletal muscle or proprioception.

❌ C fibres

• Function: slow pain, crude touch, temperature.
• Unmyelinated, slowest conduction.

✅ Correct Answer:

A-alpha fibres

Explanation:

• Aα fibres are the largest diameter, heavily myelinated fibres in the peripheral nervous system.
• They include:
  • Ia fibres → from muscle spindles (sense muscle length → proprioception).
  • Ib fibres → from Golgi tendon organs (sense muscle tension → proprioception).
  • Alpha motor neurons → innervate extrafusal skeletal muscle fibers (motor control).
• Their fast conduction velocity (~80–120 m/s) makes them ideal for quick reflexes and coordinated movement.

❌ A-beta fibres

• Function: touch, vibration, pressure.
• They do not carry motor commands.

❌ A-delta fibres

• Function: fast pain and temperature.
• Thinly myelinated, slower than Aα and Aβ.

❌ B fibres

• Small, myelinated autonomic preganglionic fibers.
• Not related to skeletal muscle or proprioception.

❌ C fibres

• Function: slow pain, crude touch, temperature.
• Unmyelinated, slowest conduction.

✅ Correct Answer:

Creatine phosphate system

Why it’s correct:

For efforts lasting ~0–10 seconds at maximal intensity, ATP is regenerated primarily by the phosphagen (ATP-PCr) system. Creatine phosphate (phosphocreatine, PCr) rapidly donates its phosphate to ADP via creatine kinase, restoring ATP almost instantaneously to fuel explosive contractions at the start and throughout a 100 m sprint.

❌ Fatty acid

Too slow (requires aerobic β-oxidation and mitochondrial processing). Dominates during prolonged, lower-intensity exercise—not a 10-second sprint.

❌ Oxidative phosphorylation

High ATP yield but slow activation kinetics (needs sufficient oxygen delivery and mitochondrial steps). Insufficient to meet the immediate, maximal power demand of a 100 m dash.

❌ Glycogen pathway

Refers to anaerobic glycolysis from muscle glycogen—important for ~10–60 seconds bouts (e.g., 200–400 m), but it lags the phosphagen system and is not the primary source in the first ~10 seconds.

❌ Blood glucose

Contribution is minimal in the first seconds; uptake/processing is too slow relative to the instantaneous ATP demand.

✅ Correct Answer:

Creatine phosphate system

Why it’s correct:

For efforts lasting ~0–10 seconds at maximal intensity, ATP is regenerated primarily by the phosphagen (ATP-PCr) system. Creatine phosphate (phosphocreatine, PCr) rapidly donates its phosphate to ADP via creatine kinase, restoring ATP almost instantaneously to fuel explosive contractions at the start and throughout a 100 m sprint.

❌ Fatty acid

Too slow (requires aerobic β-oxidation and mitochondrial processing). Dominates during prolonged, lower-intensity exercise—not a 10-second sprint.

❌ Oxidative phosphorylation

High ATP yield but slow activation kinetics (needs sufficient oxygen delivery and mitochondrial steps). Insufficient to meet the immediate, maximal power demand of a 100 m dash.

❌ Glycogen pathway

Refers to anaerobic glycolysis from muscle glycogen—important for ~10–60 seconds bouts (e.g., 200–400 m), but it lags the phosphagen system and is not the primary source in the first ~10 seconds.

❌ Blood glucose

Contribution is minimal in the first seconds; uptake/processing is too slow relative to the instantaneous ATP demand.

✅ Correct Answer:

Creatine phosphate system

Why it’s correct:

For efforts lasting ~0–10 seconds at maximal intensity, ATP is regenerated primarily by the phosphagen (ATP-PCr) system. Creatine phosphate (phosphocreatine, PCr) rapidly donates its phosphate to ADP via creatine kinase, restoring ATP almost instantaneously to fuel explosive contractions at the start and throughout a 100 m sprint.

❌ Fatty acid

Too slow (requires aerobic β-oxidation and mitochondrial processing). Dominates during prolonged, lower-intensity exercise—not a 10-second sprint.

❌ Oxidative phosphorylation

High ATP yield but slow activation kinetics (needs sufficient oxygen delivery and mitochondrial steps). Insufficient to meet the immediate, maximal power demand of a 100 m dash.

❌ Glycogen pathway

Refers to anaerobic glycolysis from muscle glycogen—important for ~10–60 seconds bouts (e.g., 200–400 m), but it lags the phosphagen system and is not the primary source in the first ~10 seconds.

❌ Blood glucose

Contribution is minimal in the first seconds; uptake/processing is too slow relative to the instantaneous ATP demand.

✅ Correct Answer:

Fatty acids

Why it’s correct:

After 2–3+ hours of steady endurance work, muscle and liver glycogen are largely depleted, and skeletal muscle relies mainly on aerobic oxidation of fatty acids for ATP. Fat metabolism is slower but sustainable, making it the dominant fuel late in a marathon.

❌ Amino acids

Minor contribution (~5–10%) during prolonged exercise; primarily a backup when glycogen is low, not the main fuel.

❌ Blood glucose

Still used (from hepatic output), but limited supply; cannot meet most of the energy demand compared with fatty acid oxidation this late.

❌ Creatine phosphate

Supports only the first 5–10 seconds of maximal effort; irrelevant during steady-state hours into a race.

❌ Muscle glycogen

Key early fuel; by 20 miles much is depleted (“hitting the wall”), so it’s no longer predominant.

✅ Correct Answer:

Eccentric contraction

Why it’s correct:

In an eccentric contraction, the muscle generates force while lengthening.

Here, the quadriceps are active but lengthening to slow the body against gravity during downhill running. This controlled lengthening protects joints and regulates movement, though it can cause delayed-onset muscle soreness (DOMS).

❌ Concentric contraction

Force is produced while shortening the muscle (e.g., quadriceps contracting to stand up from a squat). Not happening here since the muscle is lengthening, not shortening.

❌ Isometric contraction

Force is generated without a change in length (e.g., holding a squat position). Not the case here, as the muscle length is changing.

❌ Isotonic contraction

A general term for muscle contraction that changes muscle length under constant tension. It includes both concentric and eccentric contractions. The question asks for the specific type.

❌ Isokinetic contraction

Muscle contracts at a constant speed (usually measured with special equipment in labs/rehab). Not relevant in natural running conditions.

✅ Correct Answer:

Lack of ATP prevents myosin detachment from actin

Why it’s correct:

• Normally, after the power stroke, ATP must bind to the myosin head to release it from actin and allow a new contraction cycle.
• After death, ATP production stops. Without ATP, myosin heads remain bound to actin filaments → cross-bridges stay locked → muscles stiffen = rigor mortis.

❌ Excessive calcium causes continuous contraction

Calcium does leak from the sarcoplasmic reticulum after death, exposing actin binding sites. But this alone doesn’t cause rigor mortis. Without ATP, myosin heads cannot detach — that’s the direct mechanism.

❌ Increased ATP leads to sustained contraction

This is the opposite of what happens. ATP levels fall, they don’t rise. Without ATP, cross-bridge cycling stops.

❌ Lactic acid accumulation locks muscle fibers

Lactic acid buildup lowers pH but does not cause the actin–myosin cross-bridge locking seen in rigor mortis.

❌ Troponin-tropomyosin breakdown freezes muscles

These proteins regulate actin–myosin binding, but rigor mortis is not due to their breakdown. It’s due to lack of ATP.

✅ Correct Answer:

. Complete overlap of actin filaments

🔎 Explanation

• Active tension in muscle is described by the length–tension relationship.
• Optimal sarcomere length for maximum force: ~2.0–2.2 μm (enough actin–myosin overlap without interference).
• If sarcomere length shortens below ~1.6 μm:
  • Actin filaments from opposite Z-discs overlap each other.
  • This blocks myosin binding sites and interferes with cross-bridge formation.
  • Result → active tension falls to zero even though ATP and calcium may still be present.

❌ Why not the other options?

• . Crumpling of myosin filaments → Myosin doesn’t crumple; the issue is actin–actin interference.
• . Lack of ATP availability → That causes rigor mortis, not this physiological length–tension effect.
• . Low calcium release → Would reduce contraction in general, but not specifically when sarcomere <1.6 μm.
•  Maximum overlap of myosin cross bridges → This is what gives maximum tension (at optimal length), not zero.

✅ Correct Answer:

Tetanization

🔎 Explanation

• A muscle twitch = single contraction–relaxation cycle from one action potential.
• If another stimulus arrives before full relaxation, the effects add together → summation.
• With rapid repeated stimuli:
  • The muscle doesn’t relax completely.
  • The force of contraction keeps adding up.
  • Eventually, it reaches a sustained, smooth contraction = tetanus (tetanization).

This is the additive effect of twitches → increased strength.

❌ Why not the other options?

•  Treppe (“staircase phenomenon”) → Strength increases gradually with repeated twitches due to improved Ca²⁺ handling, but it’s stepwise, not continuous summation.
•  Muscle tone → Baseline partial contraction for posture, unrelated to twitch addition.
•  Muscle fatigue → Decline in force after prolonged activity, not increase.
•  Isotonic contraction → Muscle changes length (shortens/lengthens) under constant load; not about twitch summation.

✅ Correct Answer: High myoglobin content and oxidative metabolism

• Slow muscle fibers (Type I fibers) are designed for endurance and sustained contractions.
• They have:
  • High myoglobin (which gives them a red color, hence also called “red fibers”)
  • Numerous mitochondria → supports oxidative phosphorylation
  • Rich capillary supply → ensures continuous oxygen delivery
• These features allow them to resist fatigue and sustain long-duration activities (like marathon running, posture maintenance).

❌ Extensive sarcoplasmic reticulum

• This is more characteristic of fast muscle fibers (Type II), which require rapid calcium release and reuptake for quick, powerful contractions.

❌ Largest size and glycolytic enzymes

• Again, this is a property of fast glycolytic fibers (Type IIb), which are thick, pale, and rely heavily on glycolysis for short bursts of power.

❌ Rapid contraction and fewer mitochondria

• This describes fast fibers (low mitochondria → less oxidative capacity), not slow fibers.

❌ Small nerve fibers and rapid excitation

• Slow fibers are indeed innervated by small motor neurons, but their excitation is slower (not rapid). The rapid part here makes it incorrect.

✅ Correct Answer:

Fenn Effect

• Described by Wallace O. Fenn (1923).
• States that the amount of energy liberated (ATP hydrolyzed) during muscle contraction is proportional to the work performed.
• If the muscle shortens and does work → more ATP is consumed.
• If it contracts isometrically (no shortening) → less ATP is consumed.

❌ Why not the others?

• Actin PowerStroke theory → Describes the physical movement of actin by myosin heads, not the proportionality of ATP use to work.
•  Crossbridge theory → Explains how myosin heads bind, pivot, and detach, fueled by ATP. It’s the mechanism, not the energy–work correlation.
•  Power stroke → The actual movement of myosin head pulling actin.
•  Ratchet theory of contraction → Old explanation for repeated crossbridge cycling, not about ATP-work proportionality.

• Length of muscle at rest: This matters because of the length–tension relationship. There is an optimal sarcomere length (~2.0–2.2 μm) that gives maximal overlap of actin and myosin. However, this is not the primary determinant of whole muscle contraction strength.
• Number of muscle fibres recruited: The more motor units (each motor neuron + the fibres it innervates) that are activated, the greater the overall force. This is the primary determinant of contraction strength in vivo. ✅
• Number of ACh receptors: Important for neuromuscular transmission efficiency (e.g., reduced in myasthenia gravis), but not the main determinant under normal physiology.
• Thickness of Sarcolemma: This does not play a major role in determining contraction strength.
• The type of muscle fibre: This influences endurance vs strength (slow oxidative vs fast glycolytic), but recruitment of fibres is still the dominant factor for acute contraction strength.

✅ Correct Answer: Number of muscle fibres recruited

• • Actin (thin) filaments are anchored to the Z line via proteins like α-actinin.

  • The lateral spacing (distance) between adjacent actin filaments within the same sarcomere is organized and fixed at the Z line.

  • This spacing determines the hexagonal lattice arrangement of thin filaments.

  So when asked:

  “Distance between one actin filament and the next actin filament on the same sarcomere”

  It refers to the structural anchoring and spacing point, which is the Z line.

  ---

  Why the other options are incorrect ❌

  • M line ❌
    → Anchors myosin (thick) filaments, not actin.

  • I band ❌
    → Region containing actin only, but does not define spacing.

  • A band ❌
    → Region of overlap of actin and myosin; spacing is not set here.

  • H zone ❌
    → Contains only myosin; no actin filaments.

• Inside the sarcoplasmic reticulum (SR) of skeletal muscle, calcium must be stored efficiently until it is released during contraction.

• The storage protein responsible for this is calsequestrin, which binds large amounts of calcium ions while keeping the free concentration low.

• This helps maintain a steep calcium gradient between the SR and cytosol, ensuring rapid calcium release during excitation–contraction coupling.

❌ Why Other Options Are Incorrect:

• Calbindin → Calcium-binding protein found mainly in intestine and kidney for Ca²⁺ transport.

• Calcineurin → Calcium/calmodulin-dependent phosphatase, involved in T-cell activation and muscle remodeling, not SR storage.

• Calexcitin → A neuronal calcium-binding protein involved in learning and memory, not skeletal muscle SR.

• Calmodulin → Cytosolic calcium-binding protein that activates enzymes (e.g., MLCK), not for SR calcium storage.

• Dystrophin is a large cytoskeletal protein that connects actin filaments of the muscle cytoskeleton to the extracellular matrix via the dystrophin–glycoprotein complex.

• Its role is crucial in stabilizing the sarcolemma during muscle contraction and relaxation.

• Mutations or absence of dystrophin lead to muscular dystrophies such as Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD).

• Without dystrophin, the sarcolemma becomes fragile, leading to repeated injury, degeneration, and progressive weakness.

❌ Why Other Options Are Incorrect:

•  Actin → Actin is a cytoskeletal protein involved in contraction (thin filaments), but it does not anchor the cytoskeleton to the extracellular matrix.

• Myosin → Thick filament motor protein that interacts with actin for contraction, not for attachment to ECM.

• Titin → A giant protein that provides elasticity and stabilizes myosin, but does not link cytoskeleton to ECM.

• Troponin→ Regulatory protein in contraction, binds calcium to initiate actin-myosin interaction, not involved in cytoskeletal attachment.

Dystrophin is a key structural protein that links the cytoskeleton of muscle fibers (actin filaments) to the extracellular matrix via the dystrophin–glycoprotein complex. This provides mechanical stability during contraction and relaxation.

• When dystrophin is absent or defective (as in Duchenne and Becker muscular dystrophies), the sarcolemma (muscle cell membrane) becomes fragile.

• Contraction-induced stress leads to microtears in the sarcolemma → leakage of intracellular enzymes like creatine kinase into the blood.

• This explains the high serum CK and progressive muscle weakness in the patient.

❌ Why other options are incorrect:

• Defective cross bridge formation → Caused by actin–myosin interaction problems, but dystrophin is not part of the cross-bridge cycle.

• Defective SERCA pump → Leads to impaired Ca²⁺ reuptake into the SR (seen in some metabolic myopathies), but not in dystrophin mutations.

•  Impaired acetylcholine release → This would be a neuromuscular junction issue (e.g., botulism), unrelated to dystrophin.

•  Reduced binding of calcium to troponin C → Would prevent contraction initiation, but dystrophin has no role in calcium–troponin binding.

Creatine phosphate (phosphocreatine) is the next immediate source of energy after ATP is depleted in muscle fibers during contraction. Muscle cells store a limited amount of ATP, enough for only about 3–5 seconds of maximal effort. Once this is used up, the phosphagen system kicks in, where creatine phosphate donates its phosphate group to ADP, rapidly regenerating ATP via the enzyme creatine kinase.

This process is anaerobic, extremely fast, and supports short bursts of high-intensity muscle activity — like sprinting or lifting a heavy weight — for an additional 8–10 seconds.

Why the other options are incorrect:

• Krebs citric acid cycle ❌
  This is part of aerobic metabolism and provides energy more slowly over longer periods. It’s not immediate and doesn’t kick in right after ATP runs out.

• Aerobic respiration ❌
  Highly efficient for ATP production but slower to activate. It’s used in endurance activities, not immediate muscle contraction.

• Myoglobin ❌
  Myoglobin is an oxygen-binding protein, not a direct energy source. It helps maintain aerobic respiration but doesn’t generate ATP itself.

• Anaerobic glycolysis ❌
  Provides ATP faster than aerobic pathways but slower than the creatine phosphate system. It’s the third line of energy support, especially during activities lasting longer than 10–15 seconds.

During excitation-contraction coupling in skeletal muscle, the Ryanodine Receptor (RyR1) channels located on the terminal cisternae of the sarcoplasmic reticulum (SR) are responsible for releasing stored calcium ions (Ca²⁺) into the cytoplasm. This rise in cytosolic calcium allows actin-myosin cross-bridge formation, initiating muscle contraction.

The process is triggered when an action potential travels down the T-tubule and activates dihydropyridine receptors (DHPRs), which are voltage-sensitive L-type calcium channels. In skeletal muscle, DHPRs are mechanically coupled to ryanodine receptors, not dependent on calcium influx from the outside. The mechanical activation of RyR opens the channels, releasing Ca²⁺ from the SR into the cytoplasm.

Why the other options are incorrect:

• Calmodulin ❌
  Calmodulin is a calcium-binding regulatory protein, especially important in smooth muscle for activating myosin light-chain kinase (MLCK), not a calcium release channel.

• Calsequestrin ❌
  Calsequestrin is a calcium-binding protein inside the SR that helps store calcium by buffering it, not releasing it.

• Dihydropyridine receptors (DHPRs) ❌
  These are voltage sensors on the T-tubules that trigger ryanodine receptors, but do not themselves release calcium in skeletal muscle.

• SERCA (SR Ca²⁺-ATPase) ❌
  SERCA is responsible for pumping calcium back into the SR after contraction, leading to muscle relaxation, not its release.

Parathyroid hormone (PTH) is primarily secreted by the chief cells of the parathyroid gland in response to low serum calcium levels. A secondary stimulus is high serum phosphate, which lowers free calcium by binding to it, thereby indirectly promoting PTH release.

PTH works to increase serum calcium by:

• Stimulating osteoclastic bone resorption (releasing calcium and phosphate),

• Enhancing renal calcium reabsorption,

• Promoting phosphate excretion in urine,

• Activating vitamin D to its active form (calcitriol), which increases intestinal calcium absorption.

❌ Low calcium and high vitamin D – While low calcium stimulates PTH, high vitamin D suppresses it.

❌ High calcium and low vitamin D – High calcium suppresses PTH secretion regardless of vitamin D level.

❌ High phosphate and high calcium – High calcium is a negative regulator of PTH release, even if phosphate is high.

❌ High phosphate and low vitamin D – Low vitamin D may support PTH release, but without low calcium, it’s not a strong enough stimulus alone.

✅ Correct Answer: Miniature end-plate potential

Explanation:

Miniature End-Plate Potential (MEPP) is the small, spontaneous depolarization observed at the neuromuscular junction (NMJ) when acetylcholine (ACh) is released in the absence of a nerve impulse.

These potentials are due to the random release of single vesicles of ACh from the presynaptic terminal. Each vesicle causes a small, sub-threshold depolarization of the muscle fiber membrane — not enough to trigger an action potential.

❌ Why other options are incorrect:

• Action potential: This requires a large depolarization that reaches the threshold. MEPPs are sub-threshold and do not trigger an action potential on their own.

• Resting membrane potential: This is the baseline electrical potential across the muscle membrane, maintained by ion pumps — not caused by ACh release.

• Post-tetanic potential: This refers to the increased excitability of a synapse after a high-frequency tetanic stimulation, not a spontaneous event.

• Threshold potential: This is the specific membrane voltage that must be reached to initiate an action potential — MEPPs are well below threshold.

✅ Correct Answer: Myosin

Correct Answer:

The H-zone is the central part of the A-band in a sarcomere of skeletal muscle that contains only thick filaments — i.e., myosin.

• During muscle contraction, the H-zone shortens as thin filaments slide over thick ones.

• No actin overlaps the thick filaments in the H-zone, which is why it appears lighter than the surrounding A-band under a microscope.

❌ Why the other options are incorrect:

• Nebulin: ❌ This is associated with thin filaments (actin) and does not reside in the H-zone. It’s more prominent near the Z-line.

• Actin: ❌ Actin (thin filaments) does not extend into the H-zone; it ends just before it.

• A-band: ❌ The A-band includes the entire length of the thick filaments (myosin), including the H-zone, but the H-zone is a part of the A-band, not a component within it.

• Z-band (Z-line): ❌ The Z-line marks the boundary between sarcomeres and anchors thin filaments; it’s nowhere near the H-zone.

Osteoblasts are specialized, bone-forming cells responsible for the synthesis of bone matrix and regulation of its mineralization. During bone remodeling, these cells do more than just secrete collagen—they also produce non-collagenous proteins that play regulatory roles.

🔍 Why Osteocalcin is Correct:

Osteocalcin is a non-collagenous protein uniquely secreted by osteoblasts. It binds to hydroxyapatite in bone and plays a critical role in regulating bone mineralization and calcium ion homeostasis. It’s also used as a biomarker for bone formation in clinical practice.

Additionally, osteocalcin is vitamin K–dependent and is implicated in endocrine functions, including modulation of insulin secretion and sensitivity, making it even more significant in systemic physiology.

❌ Why the Other Options Are Incorrect:

1. Procollagen Type 2:

   • Type II collagen is cartilage-specific, produced by chondrocytes, not osteoblasts.

   • It is not involved in bone remodeling.

2. Procollagen Type 1:

   • While osteoblasts do synthesize this, it is a precursor of Type I collagen, which is a structural protein.

   • However, it’s not the most specific secreted product reflecting bone remodeling activity.

3. Collagen Type 1 Propeptide (P1NP):

   • This is released when Type I collagen is processed.

   • It serves as a marker for collagen synthesis, but it’s more of a by-product of matrix formation than a key functional secretion of osteoblasts.

4. BALP (Bone-specific Alkaline Phosphatase):

   • This enzyme is involved in phosphate regulation and mineralization.

   • It’s also a useful marker, but it’s not as uniquely tied to bone formation as osteocalcin.

At rest, the neuronal cell membrane is most permeable to potassium ions (K⁺) due to the high number of leak channels for K⁺ that are always open.

• The resting membrane potential of most neurons is about −70 mV, which is very close to the equilibrium potential of K⁺ (~−90 mV).

• This is because K⁺ ions tend to diffuse out of the cell, following their concentration gradient, making the inside of the cell more negative.

• Since more K⁺ leak channels are open at rest than for any other ion, K⁺ permeability dominates the resting state.

❌ Why the Other Options Are Incorrect:

• Na⁺ (Sodium):
  Has limited permeability at rest due to fewer open channels. Its equilibrium potential (~+60 mV) is far from resting potential.

• Cl⁻ (Chloride):
  Permeability varies by cell type but does not majorly determine resting potential in most neurons. Its equilibrium potential is usually near −70 mV, but it follows rather than sets the resting potential.

• Ca²⁺ (Calcium):
  Extremely low intracellular levels and very low resting membrane permeability. Its equilibrium potential is highly positive (~+120 mV), very far from resting potential.

• HCO₃⁻ (Bicarbonate):
  Involved in buffering, not a major player in setting membrane potential or contributing to resting ionic gradients.

When a muscle cell is stimulated, the action potential travels down the T-tubules, triggering the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the sarcoplasm.

Once released, Ca²⁺ binds to a specific protein subunit of the troponin complex known as Troponin C. This binding initiates a conformational change in the troponin complex, particularly moving tropomyosin away from the actin binding sites, allowing myosin to attach to actin and initiate contraction.

• Troponin C is the calcium-binding component.

• Troponin I inhibits the actin-myosin interaction under resting conditions.

• Troponin T binds the troponin complex to tropomyosin, anchoring it to the thin filament.

❌ Why the Other Options Are Incorrect:

• Fibronectin:
  This is an extracellular matrix protein, unrelated to intracellular calcium regulation or muscle contraction.

• Titin:
  A structural protein that provides elasticity and passive tension in the sarcomere, not involved in calcium binding or release.

• Troponin I:
  This subunit inhibits actin-myosin binding in the absence of calcium but does not bind calcium itself.

• Troponin T:
  Anchors the troponin complex to tropomyosin, but it too does not bind calcium.

Tropomyosin is a regulatory protein that lies along the groove of actin filaments in skeletal and cardiac muscle fibers. In a resting muscle, tropomyosin blocks the myosin-binding sites on actin, thereby inhibiting cross-bridge formation between actin and myosin.

When calcium ions are released during an action potential, they bind to Troponin C, which causes a conformational change in the troponin complex. This shifts tropomyosin away from the myosin-binding sites on actin, allowing cross-bridge cycling and thus muscle contraction.

❌ Why the Other Options Are Incorrect:

• Calmodulin:
  A calcium-binding protein important in smooth muscle contraction, where it activates myosin light-chain kinase (MLCK). It is not involved in blocking myosin binding in skeletal muscle.

• Troponin:
  It is the calcium-sensing complex, not the blocking agent. It facilitates the movement of tropomyosin once calcium binds to it.

• Phospholamban:
  Regulates calcium uptake into the sarcoplasmic reticulum via the SERCA pump. It plays no direct role in actin-myosin interaction.

• Titin:
  A giant elastic protein that helps maintain sarcomere structure and provides passive elasticity. It does not inhibit cross-bridge formation.

After calcium is released from the sarcoplasmic reticulum (a specialized form of endoplasmic reticulum in muscle cells), it plays a key role in initiating muscle contraction by interacting with specific regulatory proteins on the actin filament.

Troponin C is the calcium-binding subunit of the troponin complex (which also includes Troponin I and Troponin T). When calcium binds to Troponin C, it causes a conformational change in the troponin complex, pulling tropomyosin away from the myosin-binding sites on actin.

This unblocking of the binding sites allows myosin heads to attach to actin filaments and begin the cross-bridge cycle, leading to muscle contraction.

❌ Why the Other Options Are Incorrect:

• Troponin I:
  This subunit inhibits actin-myosin interaction. It does not bind calcium but stabilizes the complex when calcium is absent.

• Myosin:
  A motor protein that interacts with actin after calcium binding has occurred. It does not bind calcium directly in this process.

• Troponin T:
  This subunit binds tropomyosin, helping to position the troponin complex along the thin filament. It does not interact with calcium.

• Tropomyosin:
  A regulatory protein that blocks the myosin-binding sites on actin when calcium is absent. It moves out of the way due to the effect of calcium-bound Troponin C but does not bind calcium itself.

The equilibrium potential (also known as the Nernst potential) is the electrical potential that exactly balances the concentration gradient of a particular ion across a membrane.

In simpler terms, it’s the membrane voltage at which there is no net movement of that ion across the membrane, even though there’s a concentration difference. The electrical driving force perfectly cancels the chemical driving force.

This concept is mathematically described by the Nernst equation, which gives the equilibrium potential for any ion based on its intra- and extracellular concentrations.

❌ Why the Other Options Are Wrong:

• Diffusion potential:
  This is the potential difference created by the passive movement of ions due to concentration gradients. It doesn’t necessarily stop ion movement—it just results from it.

• Resting membrane potential:
  This is the actual voltage across the membrane of a resting (non-excited) cell. It’s influenced by multiple ions, not just one. It’s not necessarily equal to any one ion’s equilibrium potential.

• Liquid junction potential:
  This is a potential difference that occurs when two different ionic solutions come into contact—not across a membrane, and not specific to physiological membranes.

• Electrical potential:
  This is a general term for any voltage difference. It doesn’t specifically refer to the point at which diffusion is halted.

Parathyroid hormone (PTH) plays a central role in calcium and phosphate homeostasis. It is secreted by the parathyroid glands in response to low blood calcium levels and acts through the following mechanisms:

1. Bone: Stimulates osteoclast activity indirectly, leading to bone resorption and release of calcium and phosphate into the blood.

2. Kidney:

   • Increases calcium reabsorption in the distal tubules.

   • Decreases phosphate reabsorption, promoting phosphate excretion in urine.

3. Vitamin D Activation: Stimulates 1-alpha hydroxylase in the kidney, increasing production of active vitamin D (calcitriol), which then enhances intestinal absorption of calcium and phosphate.

Together, these effects help restore calcium levels while limiting phosphate accumulation, maintaining a delicate balance between the two minerals.

❌ Why the Other Options Are Incorrect:

• Calcitonin:
  It lowers blood calcium levels by inhibiting osteoclasts, but its role is minor and short-term compared to PTH.

• Cortisol:
  A glucocorticoid hormone that has indirect effects on bone, and excess can lead to bone loss, but it is not a primary regulator of calcium-phosphorus metabolism.

• Insulin:
  Regulates glucose metabolism, not directly involved in calcium or phosphate balance.

• Thyroid hormone:
  Influences growth and development, but not directly responsible for regulating calcium or phosphate metabolism.

• Botulinum toxin (produced by Clostridium botulinum) acts at the neuromuscular junction.

• It cleaves SNARE proteins (e.g., SNAP-25, synaptobrevin, syntaxin) required for vesicular docking and fusion.

• As a result, acetylcholine cannot be released into the synaptic cleft, leading to flaccid paralysis.

Why the others are wrong:

• Breaks down acetylcholine → False, this is the action of acetylcholinesterase.

• Increases the release of acetylcholine → Opposite; botulinum prevents release.

• Decreases the formation of acetylcholine → False; synthesis in the presynaptic terminal remains normal.

• Blocks the uptake of acetylcholine → Not true; acetylcholine isn’t “taken up,” it’s synthesized inside the neuron.

• Local anesthetics (e.g., lidocaine) block voltage-gated sodium (Na⁺) channels in neuronal membranes.

• By preventing Na⁺ influx, the depolarization phase of the action potential cannot occur.

• Without depolarization, pain signals from nociceptive fibers cannot be conducted, leading to loss of sensation in the area.

🔴 Why the others are wrong

• Opening of ligand gated sodium channels → This actually produces excitatory postsynaptic potentials (EPSPs), promoting action potentials rather than blocking them.

• Blockage of ligand gated potassium channels → These are not the primary channels targeted by local anesthetics, and their blockage does not stop action potential initiation.

• Early closure of voltage gated potassium channels → This would delay repolarization, leading to prolonged depolarization, not inhibition of nerve conduction.

• Blockage of voltage gated potassium channels → This interferes with repolarization but does not prevent the initiation of an action potential (nerve could still fire, just abnormally).

• Schwann cells form the myelin sheath around axons in the peripheral nervous system (PNS).

• Myelin acts as an electrical insulator, preventing ion leakage and allowing the action potential to “jump” between the nodes of Ranvier via saltatory conduction.

• This process greatly enhances the speed of impulse transmission compared to unmyelinated fibers.

🔴 Why the others are wrong

• Fast transmission at synapse → Neurotransmitter release at synapses depends on calcium channels and vesicle fusion, not Schwann cells.

• Thermal insulation of neuronal axons → Myelin provides electrical, not thermal, insulation.

• Protect the neuronal soma from trauma → Schwann cells wrap axons, not neuronal cell bodies.

• Limit the speed of action potential → The opposite is true: Schwann cells increase, not limit, conduction speed.

• The condition described is Myasthenia Gravis (MG).

• It is an autoimmune neuromuscular disorder in which antibodies block or destroy nicotinic acetylcholine receptors (AChRs) at the neuromuscular junction.

• This reduces the effectiveness of neuromuscular transmission, leading to progressive weakness that worsens with activity and improves with rest (fatigability).

🔴 Why others are wrong

• Motor end plate → The receptors are on the motor end plate, but the direct target is the ACh receptor, not the entire end plate.

• Acetylcholine → The neurotransmitter itself is normal.

• Presynaptic membrane → A different condition (Lambert-Eaton Myasthenic Syndrome) involves antibodies against presynaptic calcium channels.

• Acetylcholinesterase → Inhibition of this enzyme (e.g., by organophosphates) causes excessive acetylcholine, not weakness due to receptor loss.

• During contraction, myosin (thick filaments) pull actin (thin filaments) inward.

• Thin filaments slide toward the M line (the midline of the sarcomere).

• This shortens the I band and H zone, but the A band remains constant in length.

• Thus, the ultimate movement is toward the M line at the center of the sarcomere.

🔴 Why the others are wrong

• A band → Length of thick filaments; it stays constant during contraction, not the direction of movement.

• I band → Contains thin filaments only; it shortens but is not the target of sliding.

• Z disc → Anchors thin filaments; during contraction, Z discs are pulled closer together but thin filaments do not move toward the Z disc.

• H zone → Region with only thick filaments; this disappears as thin filaments slide in, not the final destination.

• In skeletal muscle, the sarcoplasmic reticulum (SR) acts as a storage site for calcium ions.

• Calsequestrin is the main Ca²⁺-binding protein inside the SR.

• Its role is to bind large amounts of calcium with low affinity, allowing the SR to store high concentrations of Ca²⁺ without precipitating.

• This stored calcium is later released during excitation-contraction coupling to initiate muscle contraction.

🔴 Why the others are wrong

• Calexcitin → Found in invertebrate neurons, associated with learning and memory, not SR calcium storage.

• Calbindin → Calcium-binding protein found in cytosol of many tissues (e.g., intestine, kidney, brain), not the SR.

• Calmodulin → Calcium sensor protein in cytoplasm; activates enzymes like myosin light-chain kinase, not SR storage.

• Calcineurin → A Ca²⁺/calmodulin-dependent phosphatase involved in T-cell activation, not in skeletal muscle SR.

Neuronal action potentials are generated when the membrane potential reaches threshold. Certain ions can inhibit or prevent depolarization.

Chloride ions (Cl⁻) are negatively charged. When they enter the neuron through GABA_A or glycine receptor channels, they hyperpolarize the membrane, moving it further from the threshold and inhibiting action potential generation.

Other ions have different effects.

❌ Why the other options are incorrect:

• Influx of potassium: Normally potassium exits the cell to repolarize; inward movement is rare and does not inhibit AP effectively.

• Influx of bicarbonate: Minor pH effects, not a direct inhibitor of AP.

• Influx of calcium: Contributes to depolarization and synaptic activity.

• Influx of sodium: Directly triggers depolarization, not inhibition.

The Nernst equation relates the equilibrium potential (diffusion potential) of an ion to its concentration difference across a semipermeable membrane. It predicts the voltage at which there is no net movement of that particular ion because the electrochemical driving force is balanced.

It considers one ion at a time, not multiple ions simultaneously.

It applies to ions that are permeable through the membrane.

❌ Why the other options are incorrect:

• Counter transport: Refers to coupled movement of two ions in opposite directions; Nernst equation does not account for this.

• Concentration of multiple ions: That is described by the Goldman-Hodgkin-Katz equation, not Nernst.

• Facilitated diffusion: Mechanism of transport; Nernst describes equilibrium potential, not transport kinetics.

• Co-transport: Movement of two ions in the same direction; not what the Nernst equation addresses.

Lambert-Eaton Myasthenic Syndrome (LEMS) is an autoimmune disorder affecting the neuromuscular junction. In LEMS:

• Autoantibodies are produced against voltage-gated calcium channels (VGCCs) on the presynaptic nerve terminal.

• Blocking these channels reduces calcium influx, which is necessary for acetylcholine release.

• This leads to muscle weakness, especially in proximal muscles, and autonomic symptoms like dry mouth.

❌ Why the other options are incorrect:

• Sodium channels: Autoantibodies to these are involved in certain channelopathies, not LEMS.

• Potassium channels: Not targeted in LEMS; involved in repolarization of nerve/muscle membranes.

• Acetylcholine channels (nicotinic receptors): Targeted in myasthenia gravis, not LEMS.

• Muscle proteins: Autoantibodies against structural muscle proteins occur in inflammatory myopathies, not LEMS.

The Fenn effect states that the amount of work done by skeletal muscle is directly proportional to the amount of ATP hydrolyzed. In other words, when muscle does more work (shortening against a load), it consumes more ATP. This phenomenon shows the tight coupling between chemical energy (ATP hydrolysis) and mechanical energy (contraction) in muscle physiology.

❌ Why the other options are incorrect:

• Work done is inversely proportional to ATP cleaved: Opposite of the Fenn effect; more ATP hydrolysis equals more work, not less.

• Work done is directly proportional to ATP and Mg²⁺ cleaved: Mg²⁺ is a cofactor for ATPase activity but is not cleaved, so this is inaccurate.

• Work done is inversely proportional to ATP and Mg²⁺ cleaved: Same error as above, and also the relationship is direct, not inverse.

• Work done is directly proportional to Ca²⁺ cleaved:  Calcium is not “cleaved”; it regulates contraction by binding troponin, not by being broken down for energy.

Complete tetanus refers to a state of continuous, sustained contraction with no relaxation between successive stimuli. This happens when the frequency of stimulation is so rapid that the muscle does not have time to enter relaxation, resulting in a smooth, sustained contraction.

❌ Why the other options are incorrect:

• Incomplete tetanus: Here, partial relaxation is still present between contractions, so it is not continuous.

• Treppe (staircase phenomenon): Muscle contractions gradually increase in strength with repeated stimulation, but relaxation still occurs.

• Wave summation: This is the additive effect of successive stimuli before complete relaxation, but still not continuous contraction.

• Twitch: A single, quick contraction followed by relaxation — the opposite of sustained contraction.

The conduction velocity of an action potential varies by tissue:

Skeletal muscle fibers are large in diameter, so impulses travel relatively fast — but they are not myelinated, so they are slower than nerves.

Thus:

3–4 m/s is correct.

❌ Why the others are wrong

• 2–3 mm/s → far too slow (smooth muscle range)

• 20–30 m/s → nerve fiber speed, not muscle

• 2–3 m/s → slightly low

• 1–2 m/s → closer to cardiac muscle

The Nernst potential (equilibrium potential) is the diffusion potential that exactly balances and prevents net diffusion of a particular ion across a semipermeable membrane.
It represents the electrical force required to counteract the concentration gradient of that ion.

❌ Why the other options are incorrect:

• Inhibitory postsynaptic potential (IPSP): A graded potential caused by opening of Cl⁻ or K⁺ channels, making the neuron less excitable, not related to ion equilibrium.

• Action potential: A rapid, self-propagating change in membrane potential involving Na⁺ influx and K⁺ efflux, not an equilibrium potential.

• Postsynaptic potential: General term for excitatory or inhibitory changes in membrane potential after neurotransmitter binding, not specific to ion balance.

• Graded potential: Local, variable changes in membrane potential (depolarization or hyperpolarization), not the specific equilibrium point for an ion.

Skeletal muscle contraction begins at the neuromuscular junction (NMJ). The sequence is as follows:

1. Action potential arrives at the axon terminal of the motor neuron.

2. This depolarization opens voltage-gated calcium channels in the presynaptic membrane.

3. Calcium enters the terminal, binding to synaptic vesicles.

4. Vesicles fuse with the presynaptic membrane and release acetylcholine (ACh) into the synaptic cleft.

5. ACh binds to nicotinic receptors on the muscle end plate, causing depolarization and generation of an end plate potential.

6. This leads to propagation of an action potential in the muscle fiber, ultimately triggering contraction.

❌ Why the other options are incorrect:

• Binding of calcium with the neurotransmitter vesicles:
  This happens after the action potential arrives and calcium channels open. Not the first step.

• Excitation of the postsynaptic receptors:
  Occurs only after acetylcholine is released and binds to the motor end plate.

• Opening of the presynaptic calcium channel:
  Triggered by depolarization from the arriving action potential. It’s a subsequent, not initial, event.

• Release of neurotransmitter acetylcholine:
  Also comes later, after calcium influx causes vesicle fusion.

In smooth muscle, contraction is regulated differently than in skeletal muscle:

1. An increase in intracellular calcium occurs (from extracellular entry or sarcoplasmic reticulum release).

2. Calcium binds to calmodulin, a regulatory protein.

3. The calcium–calmodulin complex activates myosin light chain kinase (MLCK).

4. MLCK phosphorylates the regulatory light chains on myosin heads, allowing them to interact with actin.

5. Cross-bridge cycling occurs → contraction.

Thus, the initiating step is calcium binding to calmodulin.

❌ Why the other options are incorrect:

• Tropomyosin:
  Regulates contraction in skeletal and cardiac muscle by covering actin’s binding sites. In smooth muscle, contraction is not controlled this way.

• Myosin:
  Calcium does not bind directly to myosin. Instead, phosphorylation (via MLCK) regulates its interaction with actin.

• Actin:
  Calcium does not bind to actin in smooth muscle. Actin is the filament myosin interacts with after activation.

• Troponin:
  Absent in smooth muscle. In skeletal and cardiac muscle, calcium binds troponin C to initiate contraction.

During an action potential, the sequence of events is:

1. Resting state: Membrane at –70 mV, Na⁺ and K⁺ voltage-gated channels closed.

2. Depolarization (rising phase): Voltage-gated Na⁺ channels open, Na⁺ rushes in, membrane potential rises rapidly toward +30 mV.

3. Peak of action potential: As the membrane reaches maximum depolarization, Na⁺ channels begin to inactivate/close. This marks the end of the peak.

4. Repolarization (falling phase): Voltage-gated K⁺ channels open, K⁺ efflux repolarizes the cell.

5. Hyperpolarization: K⁺ channels remain open slightly longer, causing the membrane to dip below resting potential.

6. Restoration: Ion gradients restored via Na⁺/K⁺ pump.

❌ Why the other options are incorrect:

• Opening of Na⁺ channels:
  This causes the start of depolarization, not the end of the peak.

• Closing of Ca²⁺ channels:
  Relevant in cardiac action potentials (plateau phase), not in the typical neuronal action potential.

• Closing of K⁺ channels:
  Happens later, after repolarization, not at the peak.

• Opening of Ca²⁺ channels:
  Again specific to cardiac muscle (plateau phase), not the mechanism for neuronal peak termination.

When an action potential reaches the muscle fiber membrane (sarcolemma):

• The voltage-gated sodium channels open, allowing Na⁺ ions to rush inside the cell.

• This influx of Na⁺ makes the inside of the muscle fiber less negative (depolarization).

• Once depolarization reaches threshold, the action potential propagates across the sarcolemma and down the T-tubules, leading to excitation-contraction coupling.

So, depolarization in skeletal muscle is primarily caused by Na⁺ influx.

❌ Why the other options are incorrect:

• Ca²⁺:
  Essential for neurotransmitter release at the NMJ and for binding troponin to trigger contraction, but not the main ion for sarcolemma depolarization.

• Cl⁻:
  Chloride influx would make the inside more negative, leading to hyperpolarization, not depolarization.

• K⁺:
  Potassium leaves the cell during repolarization, restoring resting potential, not causing depolarization.

• Mg²⁺:
  Plays a role as an enzymatic cofactor (e.g., ATP-dependent processes) but not in depolarization of muscle membrane.

Skeletal muscle has four major properties:

1. Excitability – ability to respond to a stimulus (usually from a motor neuron).

2. Contractility – ability to shorten and produce force.

3. Extensibility – ability to stretch without being damaged.

4. Elasticity – ability to recoil to its original length after stretching.

❌ Why the other options are incorrect:

• Rhythmicity:
  Seen in cardiac and some smooth muscles (e.g., peristalsis in gut). Skeletal muscle requires nervous input to contract, so it is not rhythmic on its own.

• Syncytium:
  Refers to functional syncytium in cardiac muscle (intercalated discs, gap junctions). Skeletal muscle fibers are multinucleated but not a true syncytium.

• Latch system:
  A property of smooth muscle, where myosin heads remain attached for long periods, maintaining tone with minimal ATP. Not found in skeletal muscle.

• Self-excitation:
  A hallmark of cardiac pacemaker cells (SA node). Skeletal muscle cannot self-excite; it requires stimulation via somatic motor neurons.

The intracellular concentration of K+ is significantly higher than the extracellular concentration. Due to this steep concentration gradient, K+ ions have a strong tendency to move out of the cell.

The extracellular concentration of Na+ is much higher than the intracellular concentration. Na+ ions have a strong tendency to move into the cell.

The final resting membrane potential is a compromise between these two opposing forces. The constant outward flow of K+ dominates, setting the potential close to −90 mV, but the small inward leak of Na+ pulls the potential slightly back towards a more positive value. The −86 mV represents this precise balance—the membrane potential is not at the K+ equilibrium potential of −90 mV because of the continuous, though small, influx of Na+ ions.

❌ Why the other options are incorrect:

• Na+-K+ pump: While this pump is absolutely critical for long-term maintenance of the resting potential, it does not cause the −86 mV value. The pump’s role is to restore the ion gradients that are constantly being “run down” by the leaky channels.
• Voltage-gated Na+ channels: These channels are responsible for the rapid depolarization phase of an action potential. They are closed at rest and only open when the membrane potential reaches a specific threshold (e.g., −55 mV).
• Voltage-gated Ca2+ channels: These channels are also not involved in establishing the RMP. They are typically closed at rest and play a role in specific cellular functions, such as neurotransmitter release at the axon terminal or muscle contraction.
• All of these: This is incorrect because, as explained above, voltage-gated channels are closed at the resting potential and therefore do not contribute to it. The primary determinants are the passive leak channels.

• The sarcoplasmic reticulum (SR) is the muscle cell’s modified smooth ER, specialized for calcium storage and release.

• Structurally:

  • It forms longitudinal tubules and terminal cisternae (enlarged ends where calcium is stored).

  • It interacts with transverse (T)-tubules (invaginations of the sarcolemma).

  • Together, they form the triad in skeletal muscle (T-tubule sandwiched between two cisternae).

• Functionally, this setup allows the action potential in the T-tubule to trigger Ca²⁺ release from the SR cisternae → leading to excitation-contraction coupling.

❌ Why the Other Options Are Wrong

• Lysosomes → Digestive organelles, no cisternae or T-tubules.

• Sarcolemma → The plasma membrane of the muscle fiber; it gives rise to T-tubules but does not contain cisternae.

• Smooth endoplasmic reticulum (SER) → Has tubules and cisternae but is not associated with T-tubules; functions in lipid metabolism/detox.

• Rough endoplasmic reticulum (RER) → Has cisternae with ribosomes attached for protein synthesis, but again no T-tubules.

📝 Key Pearl

👉 Sarcoplasmic reticulum + T-tubules = “Triad” system in skeletal muscle (essential for excitation-contraction coupling

Parathyroid hormone (PTH) is secreted by the parathyroid glands in response to low blood calcium levels. Its primary function is to raise serum calcium concentration, and one of the key ways it does this is by increasing bone resorption.

🔹 Mechanism:

• PTH activates osteoblasts, which in turn stimulate osteoclast differentiation via RANKL signaling.

• The osteoclasts then resorb bone, releasing calcium and phosphate into the bloodstream.

❌ Why the Other Options Are Incorrect:

• Ascorbic acid (Vitamin C):
  → Important for collagen synthesis, not directly involved in calcium homeostasis or bone resorption.

• Calcitriol (Active Vitamin D):
  → Promotes calcium absorption from the gut; can also support bone resorption, but PTH is the primary hormone regulating this.

• Tocopherol (Vitamin E):
  → Antioxidant role; no direct effect on bone resorption or calcium regulation.

• Androgens:
  → Have bone-protective effects; promote bone formation, not resorption.

Skeletal muscle fibers are made up of both contractile and non-contractile (structural/regulatory) proteins.

🔹 Contractile Proteins:

These are directly involved in force generation and the sliding filament mechanism:

• Actin: Thin filament; interacts with myosin heads.

• Myosin: Thick filament; contains heads that bind to actin for cross-bridge formation.

🔹 Regulatory Proteins:

These control the interaction of actin and myosin:

• Troponin: Binds calcium and regulates tropomyosin.

• Tropomyosin: Covers myosin-binding sites on actin at rest.

🔹 Structural (Non-Contractile) Proteins:

These maintain sarcomere integrity and elasticity:

• Titin: A giant protein that anchors myosin to the Z-line and acts like a molecular spring, contributing to passive elasticity of muscle.

❌ Why the Other Options Are Incorrect:

• Actin – A contractile protein; forms the thin filament.

• Myosin – The main contractile protein; forms the thick filament.

• Troponin – A regulatory protein, but it indirectly controls contraction.

• Tropomyosin – Another regulatory protein that modulates access to actin’s binding sites.

Pain sensation is divided into two major types:

• Fast pain (sharp, well-localized, immediate)

• Slow pain (dull, aching, poorly localized)

These are carried by different fibers and pathways:

• Fast pain is transmitted by A-delta fibers, which are myelinated and conduct impulses rapidly.

• These signals synapse in lamina I (marginal zone) of the dorsal horn and ascend primarily via the neospinothalamic tract, a division of the larger anterolateral system (which includes the spinothalamic tract broadly).

• The neospinothalamic tract sends fibers directly to the thalamus, allowing for precise localization and discrimination of pain.

❌ Why the Other Options Are Incorrect:

• Cerebrospinal tract → Not a recognized sensory or pain pathway.

• Vestibulocochlear tract → Not involved in somatic sensory pathways; it’s part of the special sensory system for balance and hearing.

• Spinothalamic tract → This term generally includes both neo- and paleospinothalamic components. While technically not wrong, it’s less specific than “neospinothalamic tract” when the focus is on fast pain.

• Paleospinothalamic tract → Carries slow, chronic, dull pain via unmyelinated C fibers and involves multiple brainstem regions including the reticular formation, not the sharply localized fast pain pathway.

At the neuromuscular junction, acetylcholine is released from the presynaptic motor neuron terminal and binds to nicotinic acetylcholine receptors  located on the postsynaptic membrane (motor end plate) of the skeletal muscle fiber.

When ACh binds:

• The nicotinic receptor channel opens, allowing Na⁺ to flow in and a small amount of K⁺ to flow out, though Na⁺ influx dominates.

• This creates a local depolarization called the end-plate potential (EPP).

• If the EPP reaches threshold, it triggers the opening of nearby voltage-gated Na⁺ channels, propagating an action potential along the muscle membrane.

Thus, Na⁺ channels open first in response to ACh binding — specifically, ligand-gated Na⁺ channels that are part of the nicotinic receptor complex.

❌ Why the Other Options Are Incorrect:

• Ryanodine channels → Found on the sarcoplasmic reticulum; involved in Ca²⁺ release during excitation-contraction coupling, not at the synapse.

• Mg²⁺ channels → Not involved in NMJ transmission; magnesium mainly serves as a modulator of other processes like NMDA receptor activity in the CNS.

• Dihydropyridine channels → These are L-type voltage-gated Ca²⁺ channels in the T-tubules, activated later during excitation–contraction coupling, not during neurotransmission.

• K⁺ channels → These do open later to help repolarize the muscle membrane after the action potential, but they are not opened directly by ACh binding at the NMJ.

✅ A-delta fibers

• Small-diameter, thinly myelinated fibers.

• Conduct impulses at ~6–40 m/s.

• Responsible for fast, sharp, localized pain immediately felt after a painful stimulus (e.g., pinprick, needle, acute burn).

• They transmit to the spinal cord’s neospinothalamic tract, reaching the somatosensory cortex for precise localization.

Why the Other Options are Wrong

❌ A-beta fibers

• Large, myelinated fibers conducting at ~30–70 m/s.

• Mainly carry touch, pressure, and vibration sensation, not pain.

• Sometimes involved in abnormal pain perception in neuropathic states, but not normal fast pain fibers.

❌ 1B fibers

• Large, myelinated fibers.

• Innervate Golgi tendon organs, transmitting information about muscle tension.

• Not involved in pain transmission.

❌ A-alpha fibers

• The largest, fastest myelinated fibers.

• Carry motor signals to skeletal muscles and proprioception from muscle spindles.

• Not related to nociception.

❌ C fibers

• Small, unmyelinated fibers.

• Conduct impulses slowly (~0.5–2 m/s).

• Responsible for slow, dull, burning, and aching pain (chronic, poorly localized pain).

• Correct for slow pain, not fast pain.

At the neuromuscular junction (NMJ) — the synapse between a motor neuron and a skeletal muscle fiber — the key neurotransmitter is acetylcholine (ACh).

How it works:

1. A nerve impulse reaches the axon terminal of the motor neuron.

2. Calcium channels open, and calcium enters the neuron.

3. Acetylcholine is released into the synaptic cleft.

4. ACh binds to nicotinic receptors on the motor end plate of the muscle fiber.

5. This opens ligand-gated sodium channels, allowing Na⁺ influx, causing depolarization.

6. If threshold is reached, it triggers an action potential, leading to muscle contraction.

❌ Why the Other Options Are Incorrect

• Epinephrine:
  ❌ A hormone/neurotransmitter involved in the sympathetic nervous system, not the NMJ.

• Dopamine:
  ❌ Involved in central nervous system functions like motivation, reward, and movement — not skeletal muscle contraction.

• Serotonin:
  ❌ Regulates mood, appetite, and sleep in the central nervous system, not neuromuscular transmission.

• Glutamine:
  ❌ An amino acid, not a primary neurotransmitter. Glutamate (not glutamine) is a major excitatory neurotransmitter in the CNS — but not at the NMJ.

The muscle spindle is a sensory receptor embedded within skeletal muscle, specifically among intrafusal fibers.

Its primary function:

• Detects changes in muscle length, especially the rate of stretch (dynamic sensitivity)

• Helps in stretch reflexes and proprioception

It plays a vital role in maintaining muscle tone and posture via the monosynaptic stretch reflex (e.g., knee jerk).

Key Innervations:

• Sensory:

  • Type Ia fibers: detect rate of change in length (dynamic response)

  • Type II fibers: detect static length

• Motor:

  • Gamma motor neurons (not alpha) innervate intrafusal fibers to adjust spindle sensitivity

❌ Why the Other Options Are Incorrect

• Responds to tension in the muscle:
  ❌ That’s the function of the Golgi tendon organ, not the muscle spindle.

• Are innervated only by type II fibers:
  ❌ Muscle spindles are innervated by both Type Ia and Type II sensory fibers.

• Receives motor innervation from alpha fibers:
  ❌ Alpha motor neurons innervate extrafusal (contractile) muscle fibers, not the spindle.

  • Spindles receive motor innervation from gamma motor neurons.

• Are extrafusal:
  ❌ Muscle spindles are composed of intrafusal fibers, located within the muscle.

When an action potential is generated (usually at the axon hillock or any stimulated excitable membrane), it propagates along the cell membrane due to the local current flow that depolarizes adjacent areas, triggering the opening of voltage-gated Na⁺ channels.

Key Concept:

• Action potentials are not unidirectional by nature.

• If an action potential is initiated in the middle of an axon or muscle fiber, it can propagate in both directions from the stimulus point.

❌ Why Other Options Are Incorrect:

• “Can spread in only one direction from initial stimulus”
  ❌ This is true in vivo in neurons due to the refractory period, but in general physiology, the AP can spread bidirectionally from a central point if stimulated artificially.

• “Propagates due to opening of ligand gated ion channels along the membrane”
  ❌ Action potential propagation is mediated by voltage-gated Na⁺ and K⁺ channels, not ligand-gated ones.

• “Spreads along the membrane due to opening of K⁺ channels”
  ❌ K⁺ channels help in repolarization, not in propagation.

• “Action potential does not propagate along the membrane”
  ❌ False. Propagation is a fundamental property of action potentials in neurons and muscle fibers.

Myasthenia gravis is an autoimmune disorder in which the immune system produces antibodies that block or destroy nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction. This prevents acetylcholine from stimulating muscle contraction, leading to muscle weakness, especially after repeated use.

Why the other options are incorrect:

❌ Sarcoplasmic reticulum – Involved in calcium storage and release during muscle contraction, but it is not targeted in myasthenia gravis.

❌ Presynaptic terminal – This is affected in another condition called Lambert-Eaton myasthenic syndrome, not in myasthenia gravis.

❌ Muscle golgi apparatus – Involved in protein packaging; unrelated to the pathophysiology of myasthenia gravis.

❌ Acetylcholinesterase – This enzyme breaks down acetylcholine in the synaptic cleft. Inhibiting it is actually a treatment strategy, not a disease mechanism.

Nociceptors are the specialized receptors for pain sensation. They:

• Are free nerve endings found in skin, joints, muscles, and viscera

• Detect noxious mechanical, thermal, or chemical stimuli that signal potential tissue damage

• Transmit signals through A-delta fibers (sharp, fast pain) and C fibers (dull, slow pain)

• Serve a protective function by alerting the body to danger

❌ Why the Other Options Are Incorrect:

• Osmoreceptors → Detect changes in osmotic pressure, especially in the hypothalamus, and help regulate water balance and thirst. They are unrelated to pain.

• Mechanoreceptors → Respond to mechanical stimuli like touch, vibration, pressure, and stretch. They are for tactile and proprioceptive sensations, not pain.

• Photoreceptors → Specialized receptors in the retina (rods and cones) that detect light. They are for vision, not pain.

• Chemoreceptors → Respond to chemical stimuli, such as blood O₂/CO₂ levels (carotid and aortic bodies) or taste and smell. They do not directly sense pain, though chemical irritants may indirectly activate nociceptors.

• A sarcomere is the functional contractile unit of skeletal muscle, extending from one Z-line to the next Z-line.

• In a resting muscle fiber, its average length is about 2 µm (range: 1.6–2.2 µm).

• This length allows optimal overlap between actin (thin) filaments and myosin (thick) filaments, ensuring maximum cross-bridge formation and optimal force generation.

❌ Why the Other Options Are Incorrect:

• 1 µm → Too short; filaments would excessively overlap, reducing force.

• 3 µm → Too long; decreased overlap between actin and myosin.

• 4 µm / 6 µm → Far beyond physiological resting length; minimal or no overlap, so contraction efficiency would be very poor.

The Na⁺/K⁺-ATPase is electrogenic: for every cycle it moves 3 Na⁺ out and 2 K⁺ in, creating a net outward movement of one positive charge. This makes the inside of the cell slightly more negative, adding a small hyperpolarizing component to the resting membrane potential. Quantitatively, this electrogenic effect is about –4 mV, whereas the bulk of the resting potential (~–70 mV) comes from K⁺ leak channels and the K⁺ gradient (Goldman–Hodgkin–Katz considerations).

❌Why the other options are incorrect:

• –5 mV: Too negative. The pump’s effect is relatively minor and does not usually reach this value under physiological conditions.

• –3 mV: Close, but still an underestimate of the pump’s established electrogenic contribution (which is slightly greater, around –4 mV).

• –2 mV: Too small; this underplays the actual electrogenic role of the pump.

• –1 mV: Far too small; this would not adequately represent the real contribution of the pump to the resting potential.

Smooth muscles are involuntary muscles found in the walls of hollow organs like the gut, blood vessels, and the uterus. They can be stimulated by:

• Prostaglandins → local chemical mediators that induce contraction.

• Oxytocin → a hormone that stimulates uterine smooth muscle.

• Autonomic nerves → sympathetic and parasympathetic nerves directly control smooth muscle contraction.

• Internal stretching → myogenic response; stretching the muscle wall can trigger contraction.

Somatic nerves, however, innervate skeletal muscle only and cannot stimulate smooth muscles.

The Fenn effect describes the phenomenon where the energy liberated from ATP hydrolysis increases when a muscle shortens and performs work. In other words, the more ATP is cleaved by the myosin heads, the greater the mechanical work the muscle does. This effect highlights the relationship between chemical energy (ATP) and mechanical output in muscle contraction.

❌ Why the other options are incorrect:

• None of these: The relationship is well-established as directly proportional.

• Work done is inversely proportional to ATP cleaved: More ATP hydrolysis leads to more work, not less.

• Work done is directly proportional to Mg++ and ATP cleaved: Mg²⁺ is a cofactor for ATP hydrolysis, but the Fenn effect specifically describes the relationship between ATP cleavage and work, not magnesium concentration.

• Work done is inversely proportional to Mg++ and ATP cleaved: Both parts of this statement are contrary to the established relationship.

📘 Aα fibers are the fastest-conducting nerve fibers in the human body.

They are:

• Heavily myelinated

• Largest in diameter

• Responsible for motor innervation of skeletal muscle (extrafusal fibers)

• Crucial for proprioception from muscle spindles and Golgi tendon organs

Their speed is essential for immediate motor responses and reflexes like the knee-jerk reflex or postural adjustments.

❌ Why the Other Options Are Incorrect:

• Aβ fibers: Conduct pressure and touch—fast but slower than Aα.

• Aγ fibers: Supply muscle spindle intrafusal fibers—important for tone, but slower.

• AΔ fibers: Carry fast pain and cold sensations—smaller diameter and slower.

• C fibers: Unmyelinated and slowest—carry dull pain, warmth, and postganglionic autonomic signals.

• Calcitonin is a hormone secreted by the parafollicular cells (C cells) of the thyroid gland.

• Its main function is to lower blood calcium levels when they are too high.

• It does this by:

  1. Inhibiting osteoclast activity → reduces bone resorption.

  2. Promoting calcium deposition in bone.

  3. Increasing urinary calcium excretion.

Thus, calcitonin acts as a calcium-lowering hormone, opposing the effects of parathyroid hormone (PTH).

❌ Why the other options are wrong:

• Increases plasma calcium concentration → This is the role of PTH and calcitriol (active vitamin D), not calcitonin.

• Increases plasma calcitriol concentration → PTH stimulates calcitriol synthesis in kidneys; calcitonin does not.

• Decreases plasma magnesium concentration → Calcitonin doesn’t affect magnesium directly; magnesium homeostasis is largely regulated by kidneys.

• Increases plasma potassium concentration → Not related to calcitonin at all; potassium is regulated by renal handling and aldosterone.

Parathyroid hormone (PTH) is secreted by the chief cells of the parathyroid gland in response to low plasma calcium levels.
Its main effect is to increase plasma calcium concentration by:

1. Bone → stimulates osteoclast activity indirectly (via osteoblast signaling), increasing bone resorption and calcium release.

2. Kidneys → increases Ca²⁺ reabsorption in the distal tubule, while decreasing phosphate reabsorption (phosphaturia).

3. Intestine → indirectly increases Ca²⁺ absorption by stimulating calcitriol (active vitamin D) synthesis in kidneys.

Thus, overall: ↑ Calcium, ↓ Phosphate.

❌ Why the other options are wrong:

• Sodium → regulated mainly by aldosterone and kidney tubular mechanisms, not PTH.

• Phosphate → PTH actually lowers plasma phosphate by increasing its urinary excretion.

• Potassium → regulated by aldosterone and renal handling, no major role for PTH.

• Magnesium → PTH has a small effect to increase Mg²⁺ reabsorption, but the major and most tested mineral increased is calcium.