Nerve and Muscle Physiology Practice Questions

Q: What is the contribution of noncontractile muscle elements to total tension?

E. In skeletal muscle physiology, the **Total Tension** produced by a muscle is the sum of its **Active Tension** (generated by cross-bridge cycling) and **Passive Tension** (generated by noncontractile elastic elements). ### Explanation of the Correct Answer The correct answer is **E**. In a standard length-tension relationship graph: * **Passive Tension (Curve E):** This represents the tension developed by stretching the muscle's noncontractile components (such as the sarcolemma, connective tissue sheaths like endomysium/perimysium, and the protein **titin**) without any electrical stimulation. As the muscle is stretched beyond its resting length, this tension increases exponentially. * **Total Tension:** This is the sum of active and passive tension. At longer muscle lengths, the contribution of noncontractile elements (Curve E) becomes the dominant factor in total tension. ### Explanation of Incorrect Options * **Option A (Active Tension):** This curve typically shows a bell shape, peaking at the optimal resting length ($L_0$) where actin-myosin overlap is maximal, and decreasing as the muscle is overstretched. * **Option B & D:** These usually represent intermediate points or specific components of the active tension curve (like the descending limb) rather than the standalone contribution of elastic elements. * **Option C:** Often represents the "Total Tension" curve itself, which is the resultant of adding the active and passive components. ### High-Yield NEET-PG Pearls * **Titin:** The primary protein responsible for passive tension and for keeping thick filaments centered in the sarcomere. * **Frank-Starling Law:** In cardiac muscle, the passive tension curve is much steeper than in skeletal muscle, preventing overstretch and ensuring efficient pumping. * **Optimal Length ($L_0$):** The length at which active tension is maximal (approx. 2.0–2.2 μm per sarcomere).

Q: Small axons are concerned with all of the following functions, except:

Proprioception. **Explanation:** The classification of nerve fibers (Erlanger-Gasser classification) is based on the principle that **fiber diameter is directly proportional to conduction velocity.** **1. Why Proprioception is the Correct Answer:** Proprioception (the sense of self-movement and body position) requires extremely rapid transmission of information to the CNS to maintain balance and coordinate movement. Therefore, it is mediated by the **largest and most heavily myelinated fibers** (Type A-alpha). These fibers have the highest conduction velocities (70–120 m/s). Because proprioception is carried by large axons, it is the "except" in this question. **2. Analysis of Incorrect Options:** * **Pain and Temperature (Options A & B):** These sensations are carried by small-diameter fibers. Fast pain and cold temperature are carried by **Type A-delta** fibers (small, thinly myelinated), while slow pain and warmth are carried by **Type C** fibers (the smallest, unmyelinated axons). * **Autonomic Function (Option C):** Preganglionic autonomic fibers are **Type B** (small, myelinated), and postganglionic fibers are **Type C** (small, unmyelinated). Both are significantly smaller than the fibers used for proprioception. **3. High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Hypoxia:** Affects Type A fibers first (Large > Small). * **Pressure:** Affects Type A fibers first (Large > Small). This is why your foot "falls asleep" (loss of touch/proprioception) before you lose pain sensation. * **Local Anesthetics:** Affect Type C fibers first (Small > Large). This explains why pain is blocked before motor function during regional anesthesia. * **Type C fibers** are the only unmyelinated fibers and have the slowest conduction velocity (0.5–2 m/s). * **A-alpha fibers** are the thickest and fastest; they serve both motor (extrafusal muscle fibers) and sensory (proprioception) roles.

Q: Troponin C mediated function is of which of the following?

Actin. **Explanation:** The correct answer is **Actin**. In skeletal and cardiac muscle, contraction is regulated by the **Troponin-Tropomyosin complex** located on the thin (actin) filaments. **Troponin C (TnC)** is the calcium-binding subunit. When calcium binds to TnC, it induces a conformational change that pulls **Tropomyosin** away from the myosin-binding sites on the **Actin** filament. This exposure allows the myosin heads to bind to actin, forming cross-bridges and initiating contraction. Therefore, the function of Troponin C is fundamentally mediated through its interaction with the actin-tropomyosin complex. **Analysis of Incorrect Options:** * **Dystrophin:** This is a structural protein that links the internal cytoskeleton (actin) of a muscle fiber to the surrounding extracellular matrix through the cell membrane. It provides structural stability but does not mediate Troponin C function. * **Calmodulin:** This is a calcium-binding protein found in **smooth muscle** (which lacks troponin). While it is structurally homologous to Troponin C, it functions independently to activate Myosin Light Chain Kinase (MLCK). * **Calcineurin:** This is a calcium-dependent phosphatase involved in T-cell activation and cardiac hypertrophy signaling pathways; it is not a structural component of the contractile apparatus. **High-Yield NEET-PG Pearls:** * **Troponin Subunits:** **TnI** (Inhibitory - binds actin), **TnT** (Tropomyosin-binding), and **TnC** (Calcium-binding). * **Cardiac Biomarkers:** Troponin I and T are highly specific markers for myocardial infarction. * **Smooth Muscle:** Remember, smooth muscle uses **Calmodulin** and **Caldesmon** instead of the Troponin complex. * **Calcium Source:** In skeletal muscle, calcium comes solely from the Sarcoplasmic Reticulum (SR), whereas in cardiac muscle, it involves Calcium-Induced Calcium Release (CICR) from both ECF and SR.

Q: Fast pain has a conduction velocity of _____ m/sec?

15. **Explanation:** Pain is transmitted to the central nervous system via two distinct pathways involving different nerve fiber types. The correct answer is **15 m/sec**, which falls within the characteristic range for **Aδ (A-delta) fibers**. 1. **Why 15 m/sec is correct:** Fast pain (also known as "pricking" or "acute" pain) is carried by **Aδ fibers**. These are thinly myelinated fibers with a diameter of 2–5 μm. Their conduction velocity typically ranges from **6 to 30 m/sec**. Therefore, 15 m/sec is the most accurate representative value among the choices. 2. **Analysis of incorrect options:** * **1 m/sec (Option A):** This is the velocity for **C fibers**, which transmit **slow pain** (dull, aching, or burning). These fibers are small and unmyelinated (0.5–2 m/sec). * **5 m/sec (Option B):** This is too slow for Aδ fibers and too fast for typical C fibers; it does not represent the standard "fast pain" velocity. * **50 m/sec (Option D):** This velocity is characteristic of **Aβ (A-beta) fibers**, which carry non-noxious stimuli like touch and pressure. **High-Yield NEET-PG Pearls:** * **Double Pain Sensation:** When you stub your toe, the initial sharp sting is Fast Pain (Aδ), followed seconds later by a dull ache, which is Slow Pain (C fibers). * **Neurotransmitter:** Fast pain uses **Glutamate** (fast-acting), while slow pain uses **Substance P** (slow-releasing). * **Termination:** Fast pain fibers terminate mainly in **Laminae I and V** of the dorsal horn; slow pain fibers terminate in **Laminae II and III (Substantia Gelatinosa)**. * **Lateral vs. Medial:** Fast pain travels via the **Neospinothalamic tract**, whereas slow pain travels via the **Paleospinothalamic tract**.

Q: Which of the following best describes an attribute of visceral smooth muscle that is not shared by skeletal muscle?

Contracts in response to stretch. **Explanation:** The correct answer is **B. Contracts in response to stretch.** **Why it is correct:** Visceral (unitary) smooth muscle, found in the walls of hollow organs like the gut and ureters, exhibits a unique property called **stress-relaxation** and the **myogenic response**. When these muscles are stretched, mechanically gated calcium channels open, leading to depolarization and subsequent contraction. This allows organs to move contents forward (peristalsis) or maintain tone despite distension. In contrast, skeletal muscle is neurogenic; it requires a motor nerve impulse (ACh release) to initiate contraction and does not contract simply by being stretched. **Why the other options are incorrect:** * **A. Contraction is ATP dependent:** This is a shared feature. Both skeletal and smooth muscles require ATP for cross-bridge cycling and for the sequestration of calcium by the SERCA pump. * **C. Does not contain actin filaments:** This is incorrect. Both muscle types contain actin (thin) and myosin (thick) filaments. While smooth muscle lacks organized sarcomeres and troponin, it relies heavily on actin-myosin interaction for force generation. * **D. High rate of cross-bridge cycling:** This is a characteristic of **skeletal muscle**. Smooth muscle is known for its **low** rate of cross-bridge cycling, which allows for the "Latch State"—a mechanism where it maintains prolonged tension with minimal energy (ATP) consumption. **High-Yield NEET-PG Pearls:** * **Latch State:** Unique to smooth muscle; allows for sustained contraction (e.g., vascular tone) without fatigue. * **Calmodulin:** Smooth muscle lacks troponin; calcium binds to Calmodulin, which then activates **Myosin Light Chain Kinase (MLCK)** to initiate contraction. * **Caveolae:** These are the functional equivalents of T-tubules in smooth muscle.

Q: Calcium initiates skeletal muscle contraction by which mechanism?

Binding to troponin. **Explanation:** In skeletal muscle, contraction is regulated by the **Troponin-Tropomyosin complex** located on the actin (thin) filament. **1. Why the correct answer is right:** When an action potential reaches the muscle fiber, it triggers the release of Calcium ($Ca^{2+}$) from the **Sarcoplasmic Reticulum (SR)** into the sarcoplasm. This $Ca^{2+}$ binds specifically to **Troponin C**. This binding induces a conformational change in the entire troponin complex, which physically pulls the **tropomyosin** molecule away from the active sites on the actin filament. This exposes the binding sites, allowing the myosin heads to form cross-bridges with actin, initiating the power stroke. **2. Why the incorrect options are wrong:** * **Option A:** Calcium binds to Troponin, not Tropomyosin. Tropomyosin’s role is to physically mask the binding sites; it moves only after Troponin C is activated. * **Option B:** In skeletal muscle, the primary source of $Ca^{2+}$ is the **Sarcoplasmic Reticulum**, not the sarcolemma (extracellular fluid). This is a key distinction from cardiac muscle, which requires extracellular $Ca^{2+}$ influx. * **Option D:** Calcium *uncovers* the binding sites; it does not cover them. Tropomyosin is the protein that covers the sites in a relaxed state. **High-Yield Clinical Pearls for NEET-PG:** * **Troponin Subunits:** Remember **T** (binds to **T**ropomyosin), **I** (**I**nhibitory; binds to actin), and **C** (binds to **C**alcium). * **Rigor Mortis:** Occurs due to the lack of ATP, which is required for the *detachment* of myosin from actin. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR1)**, leading to excessive $Ca^{2+}$ release from the SR. * **Dihydropyridine Receptor (DHPR):** Acts as a voltage sensor in the T-tubule that triggers the RyR1.

Q: Which muscles are primarily used during the stance and swing phases of normal walking?

Gastrocnemius. **Explanation:** The gait cycle consists of two main phases: the **Stance phase** (60%) and the **Swing phase** (40%). Efficient walking requires coordinated muscle activation to provide propulsion and clearance. **Why Gastrocnemius is the correct answer:** The **Gastrocnemius** (along with the Soleus) is the primary muscle responsible for the **"Push-off"** or "Toe-off" stage at the end of the stance phase. It performs powerful plantarflexion, providing the necessary forward propulsive force to transition the limb into the swing phase. While many muscles contribute to walking, the plantarflexors are the chief contributors to the mechanical work required for forward progression. **Analysis of Incorrect Options:** * **Popliteus:** Known as the "Key to the knee," its primary role is to **unlock the knee** by medially rotating the tibia on the femur (or laterally rotating the femur on the tibia) to initiate flexion from a fully extended position. It is not a primary locomotor muscle. * **Tibialis Anterior:** This muscle is most active during the **initial swing phase** (for foot clearance/dorsiflexion) and at **heel strike** (to eccentrically control foot drop). It does not provide the primary power for the stance-to-swing transition. * **Iliopsoas:** This is a powerful hip flexor. While it helps initiate the swing phase by accelerating the thigh forward, it is not considered the primary driver of the gait cycle compared to the propulsive power of the posterior compartment of the leg. **High-Yield Clinical Pearls for NEET-PG:** * **Trendelenburg Gait:** Caused by weakness of Gluteus Medius/Minimus (hip abductors). * **Foot Drop:** Caused by paralysis of the Tibialis Anterior (Common Peroneal Nerve injury). * **Stance Phase Stages:** Heel strike → Foot flat → Mid-stance → Heel-off → Toe-off. * **Energy Efficiency:** The center of gravity follows a sinusoidal curve during walking to minimize energy expenditure.

Q: The action of acetylcholine at the neuromuscular junction is terminated primarily by?

Enzymatic breakdown by acetylcholinesterase. ### Explanation **1. Why Option D is Correct:** The primary mechanism for terminating the action of Acetylcholine (ACh) at the neuromuscular junction (NMJ) is **rapid enzymatic hydrolysis**. The enzyme **Acetylcholinesterase (AChE)** is concentrated in the synaptic cleft, specifically within the basal lamina. It breaks down ACh into **Choline and Acetate**. This process is incredibly efficient (one of the fastest enzymatic reactions in the body), ensuring that the muscle fiber can be repolarized quickly to respond to subsequent nerve impulses. **2. Why Other Options are Incorrect:** * **Option A (Diffusion):** While a small amount of ACh may diffuse away from the cleft, it is a minor contributor compared to the rapid enzymatic destruction. * **Option B (Uptake into nerve ending):** Unlike other neurotransmitters (like Norepinephrine or Serotonin), **ACh itself is not reuptaken**. Only the byproduct **Choline** is transported back into the presynaptic terminal via a Na⁺-choline symporter to synthesize new ACh. * **Option C (Uptake into muscle):** Muscle cells do not possess transport mechanisms to "uptake" intact ACh molecules. **3. NEET-PG High-Yield Clinical Pearls:** * **Myasthenia Gravis:** Caused by antibodies against post-synaptic ACh receptors. It is treated with **AChE inhibitors** (e.g., Neostigmine, Pyridostigmine) to increase the concentration and duration of ACh in the cleft. * **Organophosphate Poisoning:** These compounds irreversibly inhibit AChE, leading to a "cholinergic crisis" (SLUDGE syndrome) due to persistent ACh action. * **Pseudocholinesterase (Butyrylcholinesterase):** Found in the plasma (not the NMJ). It is responsible for the metabolism of drugs like **Succinylcholine**. Deficiency leads to prolonged apnea after anesthesia. * **Hemicholinium:** A drug that blocks the reuptake of Choline into the nerve terminal, depleting ACh stores.

Question 1

What is the contribution of noncontractile muscle elements to total tension?

Accepted Answer

E

Answer

A

Answer

C

Answer

B

Question 2

Small axons are concerned with all of the following functions, except:

Accepted Answer

Proprioception

Answer

Pain sensation

Answer

Temperature sensation

Answer

Autonomic function

Question 3

Troponin C mediated function is of which of the following?

Accepted Answer

Actin

Answer

Dystrophin

Answer

Calmodulin

Answer

Calcineurin

Question 4

Fast pain has a conduction velocity of _____ m/sec?

Accepted Answer

15

Answer

1

Answer

5

Answer

50

Question 5

Increased velocity of conduction in a nerve is caused by:

Accepted Answer

Increased capacitance

Answer

Decreased capacitance

Answer

Increased resistance

Answer

All

Question 6

Which of the following best describes an attribute of visceral smooth muscle that is not shared by skeletal muscle?

Accepted Answer

Contracts in response to stretch

Answer

Contraction is ATP dependent

Answer

Does not contain actin filaments

Answer

High rate of cross-bridge cycling

Question 7

Calcium initiates skeletal muscle contraction by which mechanism?

Accepted Answer

Binding to troponin

Answer

Binding to tropomyosin

Answer

Being released primarily from the sarcolemma during an action potential

Answer

Covering actin binding sites

Question 8

Which muscles are primarily used during the stance and swing phases of normal walking?

Accepted Answer

Gastrocnemius

Answer

Popliteus

Answer

Tibialis anterior

Answer

Iliopsoas

Question 9

Two electrodes are placed 4.5 cm apart. It takes 1.5 ms for current to propagate along the nerve from one electrode to the other. What is the velocity of nerve conduction?

Accepted Answer

30 m/s

Answer

60 m/s

Answer

45 m/s

Answer

80 m/s

Question 10

The action of acetylcholine at the neuromuscular junction is terminated primarily by?

Accepted Answer

Enzymatic breakdown by acetylcholinesterase

Answer

Diffusion into the surrounding extracellular fluid

Answer

Uptake into the nerve ending

Answer

Uptake into the muscle

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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