Nerve and Muscle Physiology Practice Questions

Q: Calmodulin acts by:

Phosphorylation of myosin by protein kinase. **Explanation:** **Mechanism of Action (Why Option B is correct):** Calmodulin is a calcium-binding messenger protein found in all eukaryotic cells. In smooth muscle contraction, it plays a pivotal role because smooth muscle lacks the troponin complex found in skeletal muscle. When intracellular $Ca^{2+}$ levels rise, four $Ca^{2+}$ ions bind to **Calmodulin**, forming a $Ca^{2+}$-Calmodulin complex. This complex activates an enzyme called **Myosin Light Chain Kinase (MLCK)**. MLCK then **phosphorylates** the regulatory light chain of the myosin head. This phosphorylation increases myosin ATPase activity, allowing the myosin head to bind to actin and initiate the cross-bridge cycle (contraction). **Why other options are incorrect:** * **Option A:** Sodium channel opening is typically mediated by voltage changes (action potentials) or ligand binding (e.g., Acetylcholine), not by Calmodulin. * **Option C:** The $Na^{+}$-$K^{+}$ pump is primarily regulated by ATP availability and specific hormones (like insulin or aldosterone), not directly by the Calmodulin-MLCK pathway. * **Option D:** Dephosphorylation of myosin leads to **relaxation**, not contraction. This process is mediated by **Myosin Light Chain Phosphatase (MLCP)**, which removes the phosphate group that MLCK added. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** 1 Calmodulin binds to **4 Calcium ions**. * **Smooth Muscle Relaxation:** Nitric Oxide (NO) causes relaxation by increasing cGMP, which inhibits MLCK and activates MLCP. * **Troponin vs. Calmodulin:** Remember the mnemonic: **S**keletal muscle uses **S**troke/Troponin; **S**mooth muscle uses **S**econd messenger/Calmodulin. * **Other functions:** Calmodulin also regulates the activation of phosphorylase kinase in glycogenolysis and modulates cyclic nucleotide phosphodiesterase.

Q: What is true about generator potential?

Graded. ### Explanation **Generator Potential** (also known as Receptor Potential) is the non-propagated local electrical response produced in a sensory receptor by a stimulus. **1. Why "Graded" is Correct:** Generator potentials are **graded**, meaning the amplitude of the potential is directly proportional to the intensity of the stimulus. Unlike action potentials, which have a fixed amplitude, a stronger stimulus results in a larger generator potential. Once this potential reaches a specific threshold, it triggers an action potential in the sensory nerve fiber. **2. Why the Other Options are Incorrect:** * **B. All or none:** This law applies to **Action Potentials**, not generator potentials. Generator potentials do not have a threshold for initiation and can vary in size. * **C. Propagated:** Generator potentials are **non-propagated** (local) responses. They spread passively via electrotonic conduction and decay over distance. Only action potentials are propagated along the axon. * **D. No summation:** Generator potentials **can be summated** (both temporally and spatially). If multiple stimuli are applied, the potentials add up to reach the threshold required to fire an action potential. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Usually involves the opening of non-specific cation channels (Na+ influx). * **Refractory Period:** Generator potentials have **no refractory period**, allowing for the summation of signals. * **Example:** The **Pacinian Corpuscle** is the classic model used to study generator potentials. If the first node of Ranvier is blocked (e.g., by local anesthetics), the generator potential still occurs, but the action potential is abolished. * **Key Distinction:** Generator potential = Graded, Local, Summable; Action potential = All-or-none, Propagated, Refractory period present.

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.

Question 1

Calmodulin acts by:

Accepted Answer

Phosphorylation of myosin by protein kinase

Answer

Opening of Na+ channels

Answer

Activation of the Na+-K+ pump

Answer

Dephosphorylation by protein kinase

Question 2

What is true about generator potential?

Accepted Answer

Graded

Answer

All or none

Answer

Propagated

Answer

No summation

Question 3

What type of cell junction is found in smooth muscle?

Accepted Answer

Gap junction

Answer

Zonula adherens

Answer

Macula adherens

Answer

Tight junction

Question 4

Which of the following muscle groups exhibits isometric contraction?

Accepted Answer

Antigravity muscles

Answer

Extraocular muscles

Answer

Small muscles of the hand

Answer

Abdominal muscles

Question 5

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 6

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 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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