Nerve and Muscle Physiology Practice Questions

Q: Which fibres are most sensitive to local anesthesia?

C fibres. ### Explanation The sensitivity of nerve fibers to local anesthetics (LA) is determined by the **size of the fiber** and the **presence of myelin**. Local anesthetics work by blocking voltage-gated sodium channels. **Why C Fibers are the most sensitive:** According to the traditional physiological classification, **C fibers** are the most sensitive to local anesthesia because they are the **smallest in diameter** and **unmyelinated**. Because they lack a myelin sheath, the drug can easily access the axonal membrane along its entire length. In contrast, myelinated fibers require the anesthetic to reach a high enough concentration to block at least three successive Nodes of Ranvier to stop conduction. **Analysis of Options:** * **Option A (A fibers):** These are large, myelinated fibers. Due to their large diameter and thick myelin sheath, they are the **least sensitive** (most resistant) to local anesthesia. Within this group, A-alpha (motor) are the hardest to block, while A-delta (pain/temperature) are blocked earlier. * **Option B (B fibers):** These are small, preganglionic autonomic myelinated fibers. While they are very sensitive (often blocked even before C fibers in clinical practice due to their anatomical location), in standard physiological testing based on fiber diameter alone, C fibers are considered the most sensitive. **High-Yield Clinical Pearls for NEET-PG:** 1. **Order of Blockade (Clinical):** Autonomic (B) > Pain/Temperature (C & A-delta) > Touch/Pressure (A-beta) > Motor (A-alpha). 2. **Order of Recovery:** Exactly the reverse of the blockade. 3. **Sensitivity to Hypoxia:** B fibers > A fibers > C fibers (C fibers are most resistant to pressure and hypoxia). 4. **Sensitivity to Pressure:** A fibers > B fibers > C fibers. 5. **Rule of Thumb:** Smaller diameter and lack of myelin generally increase sensitivity to local anesthetics.

Q: What is the resting membrane potential of a muscle fiber?

-90mV. The Resting Membrane Potential (RMP) is the electrical potential difference across the cell membrane when the cell is at rest. In skeletal muscle fibers, the RMP is typically **-90 mV**. ### Why -90 mV is Correct The RMP is primarily determined by the equilibrium potential of Potassium ($K^+$). According to the Nernst equation, the equilibrium potential for $K^+$ is approximately -94 mV. Because the resting muscle membrane is highly permeable to $K^+$ (via leak channels) and relatively impermeable to $Na^+$, the RMP sits very close to the $K^+$ equilibrium potential. The slight difference (-90 mV vs -94 mV) is due to a minor influx of $Na^+$ and the electrogenic contribution of the $Na^+-K^+$ ATPase pump. ### Analysis of Incorrect Options * **-70 mV (Option B):** This is the typical RMP for **large neurons**. Neurons have a slightly higher RMP than muscle fibers, making them more excitable. * **-60 mV (Option A):** This is closer to the RMP of **smooth muscle cells** (ranging from -50 to -60 mV) or the threshold potential for firing an action potential in many excitable tissues. * **-80 mV (Option C):** While some cardiac cells or specific fibers may show this value, -90 mV is the standard physiological value cited for skeletal muscle and ventricular myocytes. ### High-Yield NEET-PG Pearls * **Skeletal Muscle vs. Nerve:** Muscle fibers have a more negative RMP (-90 mV) compared to nerves (-70 mV). * **Ionic Basis:** $K^+$ efflux is the most important factor in establishing RMP. * **Maintenance:** The $Na^+-K^+$ ATPase pump maintains the concentration gradients but only contributes about -4 to -5 mV directly to the RMP. * **Clinical Correlation:** Hypokalemia hyperpolarizes the membrane (makes it more negative), making it harder to trigger an action potential, leading to muscle weakness.

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.

Question 1

Which fibres are most sensitive to local anesthesia?

Accepted Answer

C fibres

Answer

A fibres

Answer

B fibres

Answer

None of the above

Question 2

What is the resting membrane potential of a muscle fiber?

Accepted Answer

-90mV

Answer

-60mV

Answer

-70mV

Answer

-80mV

Question 3

Repolarization of a neuron is due to which of the following ionic movements?

Accepted Answer

Potassium influx

Answer

Sodium influx

Answer

Potassium efflux

Answer

All of the above

Question 4

Mutation of the gene coding for ryanodine receptors is implicated in malignant hyperthermia. Which of the following statements best explains the increased heat production in malignant hyperthermia?

Accepted Answer

Increased muscle metabolism due to excess calcium ions

Answer

Thermic effect of food

Answer

Increased sympathetic discharge

Answer

Mitochondrial thermogenesis

Question 5

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 6

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 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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