Nerve and Muscle Physiology Practice Questions

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).

Q: NN Nicotinic acetylcholine receptors are present on which of the following structures?

Presynaptic glutamate-secreting axon terminals. **Explanation:** Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels categorized into two main subtypes: **$N_M$ (Muscle-type)** and **$N_N$ (Neuronal-type)**. **Why Option C is Correct:** $N_N$ receptors are primarily found in the central nervous system (CNS) and autonomic ganglia. In the CNS, they are frequently located on **presynaptic terminals** of non-cholinergic neurons (such as glutamate-secreting axons). When acetylcholine binds to these presynaptic $N_N$ receptors, it triggers an influx of $Ca^{2+}$, which facilitates the release of other neurotransmitters like glutamate, dopamine, or norepinephrine. This is a key mechanism of neuromodulation in the brain. **Analysis of Incorrect Options:** * **Options B & D (Motor endplates / Neuromuscular junction):** These contain **$N_M$ receptors**, not $N_N$. While both are nicotinic, $N_M$ receptors have a different subunit composition ($\alpha_1, \beta_1, \delta, \epsilon/\gamma$) and are specifically targeted by muscle relaxants like succinylcholine or vecuronium. * **Option A (Myocardium):** The heart contains **Muscarinic ($M_2$) receptors**, which are G-protein coupled receptors (GPCRs), not nicotinic ion channels. Activation of $M_2$ receptors leads to a decrease in heart rate (negative chronotropy). **High-Yield Facts for NEET-PG:** * **$N_N$ Location:** Autonomic ganglia (both sympathetic and parasympathetic), adrenal medulla, and CNS. * **$N_N$ Blockers:** Hexamethonium and Trimethaphan (Ganglionic blockers). * **$N_M$ Blockers:** d-Tubocurarine (Competitive) and Succinylcholine (Depolarizing). * **Key Difference:** $N_N$ receptors are primarily involved in ganglionic transmission and neurotransmitter modulation, whereas $N_M$ receptors mediate skeletal muscle contraction.

Q: Which of the following statements is NOT true regarding Type 1 muscle fibers?

High glycolytic capacity. **Explanation:** Type 1 muscle fibers, also known as **Slow-Twitch (Red) fibers**, are specialized for endurance and aerobic metabolism. **1. Why Option A is the Correct Answer (NOT true):** Type 1 fibers have a **low glycolytic capacity**. They rely primarily on oxidative phosphorylation (aerobic metabolism) rather than glycolysis (anaerobic metabolism) to generate ATP. High glycolytic capacity is a hallmark of **Type 2 (Fast-Twitch/White) fibers**, which are designed for short bursts of powerful activity and fatigue quickly. **2. Why the other options are incorrect (True for Type 1):** * **High capillary density (Option B):** To support continuous aerobic metabolism, Type 1 fibers require a rich blood supply for the steady delivery of oxygen and nutrients. * **High number of mitochondria (Option C):** Mitochondria are the sites of the Krebs cycle and the electron transport chain; their high density in Type 1 fibers facilitates efficient ATP production over long periods. * **High myoglobin content (Option D):** Myoglobin is an iron-binding protein that stores oxygen within the muscle. Its high concentration gives these fibers their characteristic red color and ensures oxygen availability during sustained contraction. **High-Yield NEET-PG Pearls:** * **Mnemonic:** **"One Slow Red Ox"** (Type **1**, **Slow**-twitch, **Red** color, **Ox**idative metabolism). * **Fatigability:** Type 1 fibers are highly resistant to fatigue (e.g., soleus, back muscles for posture), whereas Type 2B fibers fatigue rapidly. * **ATPase Activity:** Type 1 fibers have low myosin ATPase activity, leading to a slower velocity of contraction compared to Type 2. * **Lipid Content:** Type 1 fibers have higher triglyceride stores, while Type 2 fibers have higher glycogen stores.

Q: Each myosin molecule of a sarcomere is composed of how many monomers?

Six monomers. **Explanation:** The correct answer is **Six monomers (Option C)**. Myosin II, the type found in skeletal muscle, is a large hexameric protein (molecular weight ~480,000 Da). It is composed of **six polypeptide chains**: 1. **Two Heavy Chains:** These wrap around each other in a double helix to form the "tail" (rod) of the molecule. At one end, they fold outward to form two globular "heads." 2. **Four Light Chains:** Two light chains are associated with each myosin head. These are categorized as **Essential Light Chains** (provide structural stability) and **Regulatory Light Chains** (regulate ATPase activity). **Analysis of Incorrect Options:** * **Option A (Two):** This refers only to the number of heavy chains. * **Option B (Four):** This refers only to the number of light chains. * **Option D (Eight):** This is incorrect as the standard Myosin II molecule does not contain eight subunits. **High-Yield Facts for NEET-PG:** * **The Myosin Head:** Contains two critical sites: the **Actin-binding site** and the **Catalytic site (ATP-binding site)** which has ATPase activity. * **HMM vs. LMM:** When treated with the enzyme trypsin, myosin breaks into **Heavy Meromyosin (HMM)**, which contains the heads and short neck (cross-bridges), and **Light Meromyosin (LMM)**, which forms the tail. * **Power Stroke:** The actual "rowing" motion occurs when ADP is released from the myosin head, causing it to tilt toward the arm. * **Clinical Correlation:** Mutations in the genes encoding cardiac myosin heavy chains are a primary cause of **Familial Hypertrophic Cardiomyopathy (HCM)**.

Question 1

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 2

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 3

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 4

Twitch of a single motor unit is called:

Accepted Answer

Fasciculation

Answer

Myoclonic jerk

Answer

Tremor

Answer

Chorea

Question 5

NN Nicotinic acetylcholine receptors are present on which of the following structures?

Accepted Answer

Presynaptic glutamate-secreting axon terminals

Answer

Myocardium

Answer

Motor endplates

Answer

Neuromuscular junction

Question 6

Which of the following is a true statement about smooth muscle?

Accepted Answer

Sarcomeres are absent in smooth muscle.

Answer

Smooth muscle cells have multiple nuclei.

Answer

Smooth muscle cells have striations.

Answer

Cross bridges are formed in smooth muscle during contraction.

Question 7

If the refractory period of a nerve is 1/2500th of a second, what is the maximum frequency of excitation of the nerve?

Accepted Answer

2500 times/sec

Answer

100 times/sec

Answer

250 times/sec

Answer

2000 times/sec

Question 8

Which of the following statements is NOT true regarding Type 1 muscle fibers?

Accepted Answer

High glycolytic capacity

Answer

High capillary density

Answer

High number of mitochondria

Answer

High myoglobin content

Question 9

Which of the following events happens when a nerve impulse arrives at the Neuromuscular Junction?

Accepted Answer

Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

Answer

Release of Calcium from the Sarcoplasmic Reticulum

Answer

Efflux of calcium through synaptic vesicles into the synaptic cleft

Question 10

Each myosin molecule of a sarcomere is composed of how many monomers?

Accepted Answer

Six monomers

Answer

Two monomers

Answer

Four monomers

Answer

Eight monomers

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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