Nerve and Muscle Physiology Practice Questions

Q: Which of the following is associated with the 'All-or-none' law?

Action potential. ### Explanation **1. Why Action Potential is Correct:** The **All-or-None Law** states that if a stimulus is strong enough to reach the **threshold potential** (usually -55mV), an action potential of constant magnitude and shape will be generated. If the stimulus is sub-threshold, no action potential occurs at all. Once triggered, the amplitude and velocity of the action potential remain independent of the strength of the stimulus. Therefore, the cell either "fires" completely or not at all. **2. Why Other Options are Incorrect:** * **Resting Membrane Potential (RMP):** This is the static electrical potential across a cell membrane when the cell is not excited (typically -70mV to -90mV). It is maintained by ion pumps and leak channels and does not follow the all-or-none principle; it can fluctuate slightly based on metabolic conditions. * **Membrane Potential:** This is a general term for the voltage difference across a membrane at any given time. It includes graded potentials (like EPSPs or IPSPs), which are **proportional** to the stimulus intensity and do not follow the all-or-none law. **3. High-Yield Clinical Pearls for NEET-PG:** * **Graded Potentials:** Unlike action potentials, local potentials (e.g., end-plate potentials, receptor potentials) are **not** all-or-none; they are proportional to stimulus strength and can be summated. * **Refractory Period:** The absolute refractory period ensures that action potentials are discrete events, preventing them from merging. * **Exceptions:** While a single nerve fiber follows the all-or-none law, a **whole nerve trunk** (composed of many fibers) does not, as it shows "graded" responses due to the recruitment of different fibers with varying thresholds.

Q: A person wakes up with pain, paresthesia, and tingling in the arms after sleeping with their arm below their head. Which nerve fibers are involved?

Type A fibers. **Explanation:** The clinical scenario describes **"Saturday Night Palsy"** or **"Sleep Palsy,"** which results from prolonged mechanical **pressure** on a peripheral nerve. **Why Type A fibers are correct:** According to the **Gasser and Erlanger classification**, nerve fibers exhibit differential sensitivity to pressure, hypoxia, and local anesthetics. **Type A fibers** (large, myelinated) are the **most sensitive to pressure**. When a person sleeps on their arm, the mechanical compression leads to transient ischemia and conduction block in these fibers. Since Type A fibers include $A\alpha$ (motor), $A\beta$ (touch/pressure), and $A\delta$ (fast pain/temperature), their involvement results in the characteristic symptoms of motor weakness (heaviness), numbness, and tingling (paresthesia). **Why the other options are incorrect:** * **Type B fibers:** These are preganglionic autonomic fibers. They are the **most sensitive to hypoxia** but are less affected by direct mechanical pressure than Type A fibers. * **Type C fibers (Options C & D):** These are small, unmyelinated fibers. They are the **most sensitive to local anesthetics** but are the **least sensitive to pressure**. This is why, in cases of pressure-induced "pins and needles," slow/dull pain perception (carried by Type C) often remains intact even when touch and motor functions are lost. **High-Yield NEET-PG Pearls:** * **Sensitivity to Pressure:** A > B > C (Type A is most affected). * **Sensitivity to Hypoxia:** B > A > C (Type B is most affected). * **Sensitivity to Local Anesthesia:** C > B > A (Type C is most affected). * **Order of Loss in Spinal Anesthesia:** Pain > Temperature > Touch > Deep Pressure > Motor (C fibers are blocked first).

Q: All of the following are true regarding the Golgi tendon organ, EXCEPT:

Detects changes in muscle length. ### Explanation The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons. Its primary function is to monitor **muscle tension** rather than length. **Why Option B is the Correct Answer (The Exception):** The GTO is arranged **in series** with muscle fibers. When a muscle contracts, it pulls on the tendon, activating the GTO. It is highly sensitive to changes in **tension/force**. In contrast, the **Muscle Spindle** is arranged in parallel and is the receptor responsible for detecting changes in **muscle length** and the rate of change in length. **Analysis of Other Options:** * **Option A & C:** The GTO mediates the **Inverse Stretch Reflex** (Autogenic Inhibition). When excessive tension is detected, the GTO sends impulses via **Ib afferent fibers** to the spinal cord. These synapse with inhibitory interneurons that **inhibit the alpha motor neuron** of the same muscle, causing it to relax. This serves as a protective mechanism to prevent tendon avulsion or muscle tearing. * **Option D:** The GTO is indeed an **encapsulated sensory receptor** consisting of a network of collagen fibers enclosed in a capsule. **High-Yield Facts for NEET-PG:** * **Muscle Spindle:** Detects **Length**; Afferents: **Ia** (primary) and **II** (secondary); Reflex: **Stretch Reflex** (monosynaptic). * **Golgi Tendon Organ:** Detects **Tension**; Afferents: **Ib**; Reflex: **Inverse Stretch Reflex** (polysynaptic). * **Clasp-Knife Phenomenon:** This clinical sign (seen in UMN lesions) is attributed to the activation of the inverse stretch reflex mediated by GTOs when a spastic muscle is forcefully stretched.

Q: The major source of calcium for contraction of skeletal muscle is:

Sarcoplasmic reticulum (SR). **Explanation:** The correct answer is **Sarcoplasmic Reticulum (SR)**. In skeletal muscle, the process of excitation-contraction coupling relies almost exclusively on intracellular calcium stores. **1. Why Sarcoplasmic Reticulum is correct:** Skeletal muscle fibers contain an extensive network of Sarcoplasmic Reticulum (SR) that acts as a specialized reservoir for calcium ions ($Ca^{2+}$). When an action potential travels down the **T-tubules**, it activates **Dihydropyridine (DHP) receptors**. These receptors are mechanically linked to **Ryanodine receptors (RyR1)** on the SR membrane. This mechanical coupling triggers the massive release of $Ca^{2+}$ from the SR cisternae into the sarcoplasm, which then binds to **Troponin C** to initiate contraction. **2. Why other options are incorrect:** * **Extracellular Fluid (ECF):** Unlike cardiac or smooth muscle, skeletal muscle contraction is **not** dependent on ECF calcium. The DHP receptor acts as a voltage sensor, not a primary calcium channel for influx. * **Cytosol:** The cytosol is where calcium acts, but it is not the *source*. In a resting state, cytosolic calcium levels are kept extremely low to allow for muscle relaxation. * **Mitochondria:** While mitochondria can sequester some calcium, their primary role is ATP production; they do not provide the calcium required for the contractile cycle. **High-Yield NEET-PG Pearls:** * **Calsequestrin:** The protein inside the SR that binds and buffers calcium, allowing for high-capacity storage. * **SERCA Pump:** The $Ca^{2+}$-ATPase pump responsible for pumping calcium back into the SR to induce muscle relaxation. * **Malignant Hyperthermia:** A clinical condition caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive calcium release from the SR in response to certain anesthetics.

Q: Which type of muscle exercise involves the muscle changing length against a constant load?

Isotonic exercise. **Explanation:** The correct answer is **Isotonic exercise**. This type of muscle contraction occurs when the **tension remains constant** while the **muscle length changes** (either shortening or lengthening) to move a load. The term is derived from the Greek words *"iso"* (same) and *"tonos"* (tension). Isotonic exercises are further divided into **concentric** (muscle shortens, e.g., upward phase of a bicep curl) and **eccentric** (muscle lengthens under tension, e.g., controlled lowering of a weight). **Why other options are incorrect:** * **Isometric exercise:** In this type, the **length of the muscle remains constant** while the **tension increases**. No external work is performed (Work = Force × Displacement; here displacement is zero). An example is pushing against a stationary wall. * **Aerobic exercise:** This refers to the metabolic pathway used (oxidative phosphorylation) rather than the mechanics of muscle length or tension. While many isotonic exercises are aerobic, the terms are not synonymous. **High-Yield NEET-PG Pearls:** 1. **Isokinetic exercise:** A type of exercise where the muscle contracts at a **constant velocity** throughout the entire range of motion (requires specialized equipment). 2. **Energy Expenditure:** Isotonic contractions perform external work and consume more energy compared to isometric contractions. 3. **Clinical Correlation:** Isometric exercises cause a more significant rise in **mean arterial pressure (MAP)** due to the compression of blood vessels within the contracting muscle, leading to increased peripheral resistance. 4. **Golgi Tendon Organs:** These are sensitive to changes in muscle **tension** (prominent in isometric), whereas **Muscle Spindles** are sensitive to changes in muscle **length** (prominent in isotonic).

Q: Following traumatic peripheral nerve transection, re-growth usually occurs at which of the following rates?

1 mm per day. **Explanation:** The correct answer is **1 mm per day (Option B)**. Following a peripheral nerve injury (transection), the distal segment undergoes **Wallerian degeneration**, while the proximal segment undergoes **regeneration**. Once the axonal sprouts cross the lesion site and enter the distal endoneurial tubes, they grow toward the target organ. This regenerative process is driven by slow axonal transport and typically occurs at a rate of **1 mm/day** (or approximately 1 inch per month). **Analysis of Incorrect Options:** * **Option A (0.1 mm/day):** This rate is too slow. While initial "die-back" and the latent period before sprouting may delay visible progress, the actual growth rate is significantly higher. * **Option C (5 mm/day):** This is an overestimation. While some experimental conditions or very young patients might show slightly faster rates, the standard physiological average used in clinical practice is 1 mm/day. * **Option D (1 cm/day):** This is physiologically impossible for human nerve regeneration. Such a rapid rate would lead to recovery in days for injuries that typically take months. **High-Yield Facts for NEET-PG:** * **Hoffmann-Tinel Sign:** A clinical test where distal percussion over the regenerating nerve elicits a tingling sensation. The distal-most point of tingling indicates the extent of axonal regrowth, allowing clinicians to track the 1 mm/day progress. * **Factors affecting regeneration:** Regeneration is faster in **crush injuries** (Sunderland Grade 2) than in complete transections because the endoneurial sheath remains intact to guide the axons. * **Proximal vs. Distal:** Regeneration is generally faster in proximal segments compared to distal segments. * **CNS vs. PNS:** Regeneration occurs in the PNS (aided by Schwann cells) but is virtually absent in the CNS due to inhibitory factors like Nogo-A and the absence of a basement membrane.

Question 1

All of the following statements regarding electromechanical coupling are true, EXCEPT:

Accepted Answer

The major source of Ca2+ ions for contraction of skeletal muscle is influx of Ca2+ ions from the extracellular space following voltage-dependent opening of L-type Ca2+ channels.

Answer

In smooth muscle, Ca2+ ions bind to calmodulin.

Answer

Smooth muscle contraction is initiated by myosin light chain phosphorylation.

Answer

In cardiac muscle, Ca2+ ions bind to troponin.

Question 2

Which of the following is associated with the 'All-or-none' law?

Accepted Answer

Action potential

Answer

Resting potential

Answer

Membrane potential

Answer

None of the above

Question 3

What is saltatory conduction?

Accepted Answer

The jumping of depolarization from node to node in myelinated axons.

Answer

Conduction from a presynaptic to a postsynaptic neuron.

Answer

Conduction within the muscles of the T-tubule system.

Answer

Conduction of an impulse in both directions along an axon.

Question 4

A person wakes up with pain, paresthesia, and tingling in the arms after sleeping with their arm below their head. Which nerve fibers are involved?

Accepted Answer

Type A fibers

Answer

Type B fibers

Answer

Type C fibers (pain)

Answer

Type C fibers (postganglionic)

Question 5

Skeletal muscle contraction ends when:

Accepted Answer

Calcium levels decrease outside the sarcoplasmic reticulum

Answer

Ions move out of the cytoplasm

Answer

Acetylcholine is absorbed from the neuromuscular junction

Answer

Receptors close and are drawn inward

Question 6

All of the following are true regarding the Golgi tendon organ, EXCEPT:

Accepted Answer

Detects changes in muscle length

Answer

Mediates the inverse stretch reflex

Answer

Inhibits the alpha motor neuron

Answer

Is an encapsulated sensory receptor

Question 7

The major source of calcium for contraction of skeletal muscle is:

Accepted Answer

Sarcoplasmic reticulum (SR)

Answer

Extracellular fluid (ECF)

Answer

Cytosol

Answer

Mitochondria

Question 8

After-hyperpolarization during nerve conduction is due to:

Accepted Answer

Slow entry of K+

Answer

Slow entry of Na+

Answer

Pumping of Na+ outside

Answer

Pumping of K+ outside

Question 9

Which type of muscle exercise involves the muscle changing length against a constant load?

Accepted Answer

Isotonic exercise

Answer

Isometric exercise

Answer

Aerobic exercise

Answer

None of the above

Question 10

Following traumatic peripheral nerve transection, re-growth usually occurs at which of the following rates?

Accepted Answer

1 mm per day

Answer

0.1 mm per day

Answer

5 mm per day

Answer

1 cm per day

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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