Nerve and Muscle Physiology Practice Questions

Q: Regulation of smooth muscle tone is persistently influenced by:

Latch bridge mechanism. **Explanation:** The **Latch bridge mechanism** is the physiological basis for the sustained, energy-efficient contraction (tone) characteristic of smooth muscle. Unlike skeletal muscle, smooth muscle can maintain high tension for long periods with very low ATP consumption. This occurs when **myosin light chain phosphatase (MLCP)** dephosphorylates the myosin head while it is still attached to actin. This "latches" the cross-bridge in place, significantly slowing the detachment rate. This allows the muscle to maintain **persistent tone** without requiring continuous high-frequency stimulation or massive ATP hydrolysis. **Analysis of Incorrect Options:** * **A. Calcium release from SR:** While calcium is essential for initiating contraction (via binding to Calmodulin), it is not the mechanism for *persistent* tone. In fact, smooth muscle relies heavily on extracellular calcium entry through voltage-gated channels rather than just SR release. * **B. Beta 1 receptor:** These receptors are primarily located in the **heart** (increasing heart rate and contractility). Smooth muscle tone in the bronchioles and blood vessels is more typically regulated by **Beta 2** (relaxation) or **Alpha 1** (contraction) receptors. * **C. Troponin:** This is a **high-yield negative fact**. Smooth muscle **lacks troponin**. Instead, it uses **Calmodulin** and **Caldesmon/Calponin** to regulate the interaction between actin and myosin. **NEET-PG High-Yield Pearls:** * **Calmodulin** is the functional analog of Troponin C in smooth muscle. * **Myosin Light Chain Kinase (MLCK)**: Phosphorylates myosin to *initiate* contraction. * **Myosin Light Chain Phosphatase (MLCP)**: Dephosphorylates myosin to *initiate* the latch state or relaxation. * Smooth muscle has the **slowest** cycling of cross-bridges but the **greatest** force of contraction per unit area compared to skeletal muscle.

Q: What does a muscle spindle primarily detect?

Length. **Explanation:** The **muscle spindle** is a specialized sensory receptor located within the belly of skeletal muscles. Its primary physiological role is to detect changes in **muscle length** and the **rate of change in length**. 1. **Why "Length" is correct:** Muscle spindles are arranged in **parallel** with extrafusal muscle fibers. When a muscle is stretched, the spindle is also stretched, triggering sensory impulses via **Type Ia (primary)** and **Type II (secondary)** afferent fibers. This mechanism is the basis for the **stretch reflex (myotatic reflex)**, which helps maintain muscle tone and posture. 2. **Why other options are incorrect:** * **Tension (Option A):** This is the primary function of the **Golgi Tendon Organ (GTO)**. GTOs are arranged in **series** with muscle fibers and protect the muscle from damage during excessive contraction. * **Proprioception (Option C):** While muscle spindles contribute significantly to proprioception (the sense of body position), "Proprioception" is a broad category, not a specific physical parameter. "Length" is the specific stimulus detected. * **Pressure (Option D):** Pressure is primarily detected by cutaneous and deep tissue receptors like **Pacinian corpuscles**. **High-Yield Clinical Pearls for NEET-PG:** * **Innervation:** Muscle spindles receive motor innervation from **Gamma ($\gamma$) motor neurons**, which maintain spindle sensitivity during muscle contraction (Alpha-Gamma co-activation). * **Reflex Arc:** The muscle spindle is the receptor for the **monosynaptic** deep tendon reflex (e.g., Knee jerk). * **Nuclear Bag vs. Chain:** Nuclear bag fibers detect dynamic changes (velocity), while nuclear chain fibers detect static changes (length).

Q: Which equation is used to measure the resting membrane potential in case of multiple ions?

Goldman equation. The resting membrane potential (RMP) is a fundamental concept in neurophysiology, representing the electrical potential difference across a cell membrane at rest. [1] ### **Explanation of the Correct Answer** **Option A (Goldman Equation):** The Goldman-Hodgkin-Katz (GHK) equation is the correct choice because it accounts for **multiple ions** (primarily $Na^+$, $K^+$, and $Cl^-$) simultaneously. Unlike simpler models, it considers two critical factors for each ion: 1. The **concentration gradient** across the membrane. [1] 2. The **relative permeability** ($P$) of the membrane to that specific ion. [1] Since the RMP of a neuron (typically -70 mV) is a collective result of all permeant ions, the Goldman equation provides the most accurate physiological measurement. ### **Explanation of Incorrect Options** * **Option B (Nernst Equation):** This equation calculates the **equilibrium potential for a single ion** only. [1] It determines the theoretical voltage at which the electrical and chemical gradients for one specific ion are perfectly balanced (e.g., -94 mV for $K^+$ or +61 mV for $Na^+$). It does not account for the membrane's simultaneous permeability to other ions. * **Options C & D:** These are incorrect as the two equations serve distinct purposes—one for individual ion equilibrium and the other for the total membrane potential. ### **NEET-PG High-Yield Pearls** * **The "K+ Rule":** The RMP is closest to the equilibrium potential of Potassium (-94 mV) because the resting membrane is 20–100 times more permeable to $K^+$ than to $Na^+$. * **The Na+-K+ Pump:** While the Goldman equation calculates the potential based on diffusion, the $Na^+$-$K^+$ ATPase pump is "electrogenic" and directly contributes about -4 mV to the RMP. * **Clinical Correlation:** Hypokalemia makes the RMP more negative (hyperpolarization), making neurons less excitable, which clinically manifests as muscle weakness or paralysis.

Q: What is true about summation?

Subthreshold stimuli are used.. **Explanation:** Summation is the process by which individual graded potentials (post-synaptic potentials) are added together to reach the threshold required to trigger an action potential. **1. Why Option C is Correct:** Summation specifically involves **subthreshold stimuli**. A single subthreshold stimulus is insufficient to reach the firing level (threshold) of a neuron. However, if multiple subthreshold stimuli are applied in rapid succession or at different locations simultaneously, their cumulative effect can depolarize the membrane to the threshold, resulting in an action potential. If a stimulus were already suprathreshold, it would trigger an action potential on its own, making summation unnecessary. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Spatial Summation**, not Temporal. Temporal summation occurs when a single presynaptic terminal fires **repeatedly in rapid succession** (one after another). * **Option B:** This describes **Temporal Summation**. Spatial summation occurs when **multiple different presynaptic terminals** fire simultaneously (together) at different locations on the same postsynaptic neuron. **NEET-PG High-Yield Pearls:** * **EPSP vs. IPSP:** Summation can be excitatory (EPSP) or inhibitory (IPSP). The net change in membrane potential depends on the algebraic sum of all inputs. * **Location:** Summation typically occurs at the **axon hillock**, which has the highest density of voltage-gated Na+ channels and the lowest threshold for firing. * **Refractory Period:** Temporal summation is possible because the postsynaptic potential lasts longer than the refractory period of the presynaptic action potential.

Q: Which troponin subunit binds to tropomyosin to form the troponin-tropomyosin complex?

Troponin T. **Explanation:** The troponin complex is a heterotrimeric protein located on the thin (actin) filaments of muscle fibers. It plays a crucial role in regulating muscle contraction by mediating the interaction between actin and myosin. **1. Why Troponin T is Correct:** * **Troponin T (T for Tropomyosin):** This subunit is responsible for binding the troponin complex to **tropomyosin**. It anchors the entire complex and helps position tropomyosin over the myosin-binding sites on the actin filament during the resting state. **2. Analysis of Incorrect Options:** * **Troponin I (I for Inhibitory):** This subunit binds to actin and inhibits the ATPase activity of the actomyosin complex, effectively preventing contraction by blocking the interaction between actin and myosin. * **Troponin C (C for Calcium):** This subunit contains binding sites for calcium ions ($Ca^{2+}$). When calcium binds to Troponin C, it undergoes a conformational change that pulls the troponin-tropomyosin complex away from the binding sites, initiating contraction. * **Troponin M:** This is a distractor; there is no such subunit in the troponin complex. **3. NEET-PG High-Yield Pearls:** * **Cardiac Biomarkers:** Cardiac-specific isoforms of **Troponin I and T** are the "Gold Standard" biomarkers for diagnosing Myocardial Infarction (MI) because they are released into the bloodstream when myocardial cells are damaged. * **Skeletal vs. Cardiac:** While Troponin C is identical in both slow-twitch skeletal and cardiac muscle, Troponin I and T have unique cardiac isoforms, making them highly specific for heart injury. * **Mechanism:** Remember the mnemonic: **T** (Tropomyosin), **I** (Inhibitory), **C** (Calcium).

Q: The hyperpolarization phase of the action potential is due to which of the following?

An outward potassium current. ### Explanation The hyperpolarization phase (also known as the "undershoot") occurs because voltage-gated potassium ($K^+$) channels are slow to close. **1. Why the Correct Answer is Right:** During the repolarization phase, voltage-gated $K^+$ channels open, allowing $K^+$ to flow out of the cell down its electrochemical gradient (**outward potassium current**). Unlike sodium channels, which inactivate rapidly, $K^+$ channels remain open for a prolonged period. This causes the membrane potential to move past the resting membrane potential (RMP) toward the **equilibrium potential of Potassium (approx. -94 mV)**. This transient state where the interior of the cell becomes more negative than the RMP is called hyperpolarization. **2. Why Incorrect Options are Wrong:** * **Option A & D:** Outward currents of positive ions (like $Ca^{2+}$ or $Na^+$) would technically cause repolarization, but these ions primarily move **inward** during an action potential due to their concentration gradients. There is no significant "outward" $Na^+$ or $Ca^{2+}$ current contributing to the hyperpolarization phase. * **Option B:** While an inward chloride ($Cl^-$) current would make the cell more negative (hyperpolarize it), $Cl^-$ channels are not the primary drivers of the action potential phases in standard nerve and muscle cells. **3. NEET-PG High-Yield Pearls:** * **Resting Membrane Potential (RMP):** Primarily determined by $K^+$ leak channels (not voltage-gated channels). * **Depolarization:** Due to $Na^+$ influx (inward current). * **Repolarization/Hyperpolarization:** Due to $K^+$ efflux (outward current). * **Na+-K+ ATPase:** Does NOT cause the phases of the action potential; it restores the ionic gradients *after* the event. * **Tetrodotoxin (TTX):** Blocks voltage-gated $Na^+$ channels, preventing depolarization.

Q: What is true about synaptic vesicles?

Small clear vesicles contain ACh and GABA.. ### Explanation Synaptic vesicles are specialized organelles that store and release neurotransmitters. They are broadly classified into two types based on their size and electron-microscopic appearance: **Small Clear Vesicles** and **Large Dense-Core Vesicles (LDCVs).** **1. Why Option A is Correct:** Small clear vesicles (approx. 40–60 nm) typically contain **small-molecule neurotransmitters** that are synthesized in the nerve terminal and mediate rapid synaptic transmission. Common examples include **Acetylcholine (ACh), GABA, Glutamate, and Glycine.** Therefore, the statement that small clear vesicles contain ACh and GABA is physiologically accurate. **2. Analysis of Incorrect Options:** * **Option B & D:** Neuropeptides (e.g., Substance P, Enkephalins) are synthesized in the cell body and transported to the terminal. They are exclusively stored in **Large Dense-Core Vesicles**, not small vesicles. * **Option C:** Glutamate is a classic small-molecule excitatory neurotransmitter and is stored in **small clear vesicles**, not large dense-core vesicles. Large vesicles typically contain catecholamines (like Norepinephrine) or neuropeptides. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **V-SNAREs (Synaptobrevin):** Proteins on the vesicle membrane that interact with T-SNAREs (Syntaxin, SNAP-25) on the presynaptic membrane to facilitate docking and fusion. * **Calcium Trigger:** The entry of $Ca^{2+}$ via voltage-gated channels is the essential trigger for exocytosis. * **Toxins:** *Clostridium botulinum* and *Clostridium tetani* toxins act by proteolytically cleaving SNARE proteins, thereby inhibiting neurotransmitter release. * **Recycling:** After exocytosis, vesicle membranes are recycled via **clathrin-mediated endocytosis**.

Q: Twitch of a single motor unit is called:

Fasciculation. ### Explanation **Correct Answer: B. Fasciculation** **Understanding the Concept:** A **fasciculation** is defined as the spontaneous, involuntary contraction of a **single motor unit** (one lower motor neuron and all the muscle fibers it innervates). Because these contractions involve an entire motor unit, they are often visible under the skin as small flickers or ripples but are usually insufficient to move a joint. In clinical practice, fasciculations are a hallmark sign of **Lower Motor Neuron (LMN) lesions**, such as Amyotrophic Lateral Sclerosis (ALS) or poliomyelitis, though they can occur benignly (e.g., due to caffeine or stress). **Analysis of Incorrect Options:** * **A. Myoclonic jerk:** This is a sudden, brief, shock-like contraction of a **muscle or group of muscles**. It originates in the central nervous system (cortex, brainstem, or spinal cord) rather than a single peripheral motor unit. * **C. Tremor:** This is a **rhythmic, oscillatory movement** produced by alternating or synchronous contractions of antagonistic muscles. It is not a single twitch but a continuous purposeless movement. * **D. Chorea:** This refers to **brief, semi-purposeful, irregular, and "dance-like" involuntary movements**. It involves multiple muscle groups and is typically associated with Basal Ganglia disorders (e.g., Huntington’s disease). **High-Yield Clinical Pearls for NEET-PG:** * **Fibrillation vs. Fasciculation:** While a fasciculation is the contraction of a *motor unit* (visible), a **fibrillation** is the spontaneous contraction of a *single muscle fiber* (not visible to the naked eye; detected only on EMG). * **LMN Lesion Signs:** Fasciculations, fibrillations, hypotonia, hyporeflexia, and significant muscle atrophy. * **Benign Fasciculations:** Most commonly seen in the orbicularis oculi (eyelid twitching).

Question 1

Regulation of smooth muscle tone is persistently influenced by:

Accepted Answer

Latch bridge mechanism

Answer

Calcium release from sarcoplasmic reticulum

Answer

Beta 1 receptor

Answer

Troponin

Question 2

What does a muscle spindle primarily detect?

Accepted Answer

Length

Answer

Tension

Answer

Proprioception

Answer

Pressure

Question 3

Which equation is used to measure the resting membrane potential in case of multiple ions?

Accepted Answer

Goldman equation

Answer

Nernst equation

Answer

Both Goldman and Nernst equations

Answer

Neither Goldman nor Nernst equation

Question 4

Which of the following statements is NOT true regarding action potential recording?

Accepted Answer

Compound action potential of a mixed nerve is recorded by cathode ray oscilloscope.

Answer

Biphasic action potential is recorded by a surface electrode.

Answer

Compound action potential is an algebraic summation of all the monophasic recordings from a mixed nerve.

Answer

A typical action potential is recorded by a microelectrode placed inside the nerve fiber.

Question 5

What is true about summation?

Accepted Answer

Subthreshold stimuli are used.

Answer

Temporal summation is the application of two stimuli together.

Answer

Spatial summation is the application of two stimuli one after another.

Answer

All are true.

Question 6

What property of the postsynaptic neuron would optimize the effectiveness of two closely spaced axodendritic synapses?

Accepted Answer

A high membrane resistance

Answer

A high dendritic cytoplasmic resistance

Answer

A small cross-sectional area

Answer

A small space constant

Question 7

Which troponin subunit binds to tropomyosin to form the troponin-tropomyosin complex?

Accepted Answer

Troponin T

Answer

Troponin I

Answer

Troponin C

Answer

Troponin M

Question 8

The hyperpolarization phase of the action potential is due to which of the following?

Accepted Answer

An outward potassium current

Answer

An outward calcium current

Answer

An inward chloride current

Answer

An outward sodium current

Question 9

What is true about synaptic vesicles?

Accepted Answer

Small clear vesicles contain ACh and GABA.

Answer

Small vesicles contain neuropeptides.

Answer

Large vesicles contain a dense core with glutamate.

Answer

Small clear vesicles contain neuropeptides.

Question 10

Twitch of a single motor unit is called:

Accepted Answer

Fasciculation

Answer

Myoclonic jerk

Answer

Tremor

Answer

Chorea

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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