Nerve and Muscle Physiology Practice Questions

Q: A cross-sectional view of a skeletal muscle fiber through the H-zone would show the presence of what?

Myosin only. To understand the cross-sectional anatomy of a sarcomere, one must visualize the arrangement of myofilaments within its specific zones. ### **Explanation of the Correct Answer** The **H-zone** (Hensen’s zone) is the central region of the **A-band**. During muscle relaxation, the thin filaments (actin) do not extend all the way to the center of the sarcomere. Therefore, the H-zone contains **only thick filaments (myosin)**. In a cross-section through this specific area, you would see the hexagonal arrangement of myosin filaments without any overlapping actin. ### **Why Other Options are Incorrect** * **A. Actin and myosin:** This combination is found in the **outer regions of the A-band** (the overlap zone). This is the site where cross-bridges form. * **B. Titin and myosin:** While titin anchors myosin to the Z-discs, it is generally not the primary focus of cross-sectional identification in standard physiological models of the H-zone. * **C. Actin and titin:** This combination (along with other proteins like nebulin) is characteristic of the **I-band**, which contains only thin filaments and no myosin. ### **High-Yield NEET-PG Pearls** * **M-line:** Located in the dead center of the H-zone; it contains proteins (like myomesin) that hold the thick filaments together. * **Sarcomere Dynamics:** During muscle contraction (Sliding Filament Theory), the **H-zone and I-band shorten**, while the **A-band remains constant** in length. * **Pseudo-H Zone:** A narrow region in the center of the H-zone that lacks myosin heads (the "bare zone"). * **Ratio:** In the overlap zone of mammalian skeletal muscle, each thick filament is surrounded by **6 thin filaments**.

Q: In preload, which of the following can be seen?

Isotonic contraction with shortening of muscle fibers. ### Explanation In muscle physiology, **Preload** refers to the load applied to a muscle *before* it begins to contract. This load stretches the muscle to a certain resting length, determining the initial overlap between actin and myosin filaments. **Why Option B is Correct:** When a muscle is subjected to preload, it is stretched to a specific length. Upon stimulation, if the muscle develops enough tension to overcome this load, it undergoes **isotonic contraction**. In an isotonic contraction, the tension remains constant while the **muscle fibers shorten** to perform work (moving the load). Therefore, preload is fundamentally associated with the length-tension relationship and subsequent shortening during contraction. **Analysis of Incorrect Options:** * **Option A:** Isotonic contraction, by definition, involves a change in muscle length. A contraction "without shortening" cannot be isotonic if the muscle is performing work. * **Option C:** **Isometric contraction** occurs when the muscle develops tension without changing its length (e.g., pushing against a fixed wall). While preload determines the starting point for isometric tension, the hallmark of the *action* following preload in a standard experimental setup is the ability to shorten. * **Option D:** This is a physiological contradiction. Isometric means "same length"; therefore, shortening cannot occur during an isometric contraction. **High-Yield NEET-PG Pearls:** * **Preload vs. Afterload:** Preload sets the **resting length** (related to the Frank-Starling Law in the heart), while Afterload is the resistance the muscle must contract *against*. * **Isotonic vs. Isometric:** Remember: **Iso-metric** = Same Length (No work done, $W=0$); **Iso-tonic** = Same Tension (Work is done). * **Optimal Length ($L_0$):** The length at which the muscle develops maximum active tension. Preload helps the muscle reach this $L_0$.

Q: Action potential is generated in excitable cells. All the following are excitable cells, except?

Neuroglial cells. **Explanation:** The core concept of this question lies in the definition of **excitability**. An excitable cell is one that can generate and propagate an **action potential** (a rapid reversal of membrane potential) in response to a stimulus. **Why Neuroglial cells are the correct answer:** Neuroglial cells (astrocytes, oligodendrocytes, microglia, etc.) are considered **non-excitable cells**. While they possess a resting membrane potential and can communicate via chemical signaling or gap junctions, they lack a sufficient density of **voltage-gated sodium channels**. Consequently, they cannot generate a self-propagating action potential. Their role is primarily supportive, providing structural, metabolic, and immune defense for neurons. **Analysis of incorrect options:** * **Nerve cells (Neurons):** These are the classic examples of excitable cells. They generate action potentials at the axon hillock to transmit signals over long distances. * **Muscle cells:** All three types (skeletal, cardiac, and smooth) are excitable. In muscle, the action potential triggers the release of calcium, leading to contraction (excitation-contraction coupling). * **Glands:** Many endocrine and exocrine cells (e.g., pancreatic beta cells, anterior pituitary cells) are electrically excitable. They use action potentials to trigger the influx of calcium, which facilitates the exocytosis of hormones or secretory products. **High-Yield Facts for NEET-PG:** * **Resting Membrane Potential (RMP):** In neurons, it is typically -70 mV; in skeletal muscle, it is -90 mV. * **The "All-or-None" Law:** Action potentials follow this principle, whereas graded potentials (like those often seen in glia) do not. * **Glial Function:** While non-excitable, astrocytes play a crucial role in **K+ spatial buffering**, maintaining the excitability of surrounding neurons by absorbing excess extracellular potassium.

Q: Tetany is characterized by which of the following?

Hypertonicity of muscles. **Explanation:** **Tetany** is a clinical manifestation of increased neuromuscular excitability. The correct answer is **Hypertonicity of muscles** because the condition is characterized by involuntary muscle spasms, cramps, and sustained contractions (hypertonia) rather than relaxation. **1. Why Hypertonicity is Correct:** The underlying mechanism is usually **hypocalcemia**. Low extracellular calcium levels decrease the threshold for depolarization of nerve membranes. This leads to increased permeability to sodium ions, causing repetitive spontaneous firing of action potentials. This continuous stimulation results in sustained muscle contraction, clinically presenting as carpopedal spasm, laryngospasm, or generalized seizures. **2. Why Other Options are Incorrect:** * **Hypotonicity of muscles:** This refers to decreased muscle tone (flaccidity), which is the opposite of what occurs in tetany. Hypotonicity is more commonly seen in conditions like lower motor neuron lesions or hypercalcemia. * **Increased serum calcium concentration:** Hypercalcemia actually *decreases* neuromuscular excitability by raising the threshold for depolarization, leading to muscle weakness and lethargy. Tetany is classically associated with **decreased** serum calcium. **High-Yield Clinical Pearls for NEET-PG:** * **Trousseau’s Sign:** Induction of carpal spasm by inflating a BP cuff above systolic pressure for 3 minutes (more sensitive than Chvostek’s). * **Chvostek’s Sign:** Twitching of facial muscles elicited by tapping over the facial nerve. * **Acid-Base Link:** Alkalosis (e.g., hyperventilation) can trigger tetany even with normal total calcium levels because high pH increases calcium binding to albumin, reducing the physiologically active **ionized calcium (Ca²⁺)**. * **Hypomagnesemia:** Often co-exists with hypocalcemia and must be corrected to resolve tetany.

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**.

Question 1

Muscle weakness may be a feature of all of the following EXCEPT:

Accepted Answer

Physostigmine therapy

Answer

Progressive degeneration of muscle fibres

Answer

Magnesium deficiency

Answer

Myasthenia gravis

Question 2

All of the following are true about excitation-contraction coupling EXCEPT:

Accepted Answer

Calcium binds to troponin to initiate muscle contraction.

Answer

Acetylcholine is released at the nerve terminal.

Answer

Calcium is pumped back into the sarcoplasmic reticulum during relaxation.

Answer

Calcium is released from the sarcoplasmic reticulum during contraction.

Question 3

A cross-sectional view of a skeletal muscle fiber through the H-zone would show the presence of what?

Accepted Answer

Myosin only

Answer

Actin and myosin

Answer

Titin and myosin

Answer

Actin and titin

Question 4

In preload, which of the following can be seen?

Accepted Answer

Isotonic contraction with shortening of muscle fibers

Answer

Isotonic contraction without shortening of muscle fibers

Answer

Isometric contraction without shortening of muscle fibers

Answer

Isometric contraction with shortening of muscle fibers

Question 5

Action potential is generated in excitable cells. All the following are excitable cells, except?

Accepted Answer

Neuroglial cells

Answer

Nerve cells

Answer

Glands

Answer

Muscle

Question 6

Tetany is characterized by which of the following?

Accepted Answer

Hypertonicity of muscles

Answer

Hypotonicity of muscles

Answer

Increased serum calcium concentration

Answer

None of the above

Question 7

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

Accepted Answer

E

Answer

A

Answer

C

Answer

B

Question 8

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

Accepted Answer

Proprioception

Answer

Pain sensation

Answer

Temperature sensation

Answer

Autonomic function

Question 9

Troponin C mediated function is of which of the following?

Accepted Answer

Actin

Answer

Dystrophin

Answer

Calmodulin

Answer

Calcineurin

Question 10

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

Accepted Answer

15

Answer

1

Answer

5

Answer

50

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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