Nerve and Muscle Physiology Practice Questions

Q: Which of the following is referred to as a 'relaxation protein'?

Tropomyosin. ### Explanation **Correct Option: C. Tropomyosin** **Why Tropomyosin is the 'Relaxation Protein':** In a resting muscle fiber, the interaction between actin and myosin is physically blocked. **Tropomyosin** is a long, filamentous protein that wraps around the actin filament, covering the active myosin-binding sites. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing the power stroke and keeping the muscle in a state of relaxation. Contraction only occurs when Calcium binds to Troponin C, causing a conformational change that pulls tropomyosin away from these binding sites. Therefore, because its primary role is to **inhibit contraction**, it is classically referred to as the relaxation protein. **Analysis of Incorrect Options:** * **A. Actin:** Known as the **thin filament**. It is a contractile protein that provides the binding sites for myosin; it does not facilitate relaxation. * **B. Myosin:** Known as the **thick filament**. It is the primary contractile protein (motor protein) that utilizes ATP to generate force. * **D. Dystrophin:** This is a **structural protein** that links the actin cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. Its clinical significance lies in Duchenne Muscular Dystrophy (DMD). **High-Yield NEET-PG Pearls:** * **Troponin Complex:** Consists of three subunits: **Troponin I** (Inhibitory), **Troponin T** (Tropomyosin-binding), and **Troponin C** (Calcium-binding). * **The "Fenn Effect":** The increased energy consumption (ATP) that occurs when a muscle performs work. * **Rigor Mortis:** Occurs due to the depletion of ATP; without ATP, the myosin head cannot detach from actin, leading to permanent cross-bridge formation. * **Calsequestrin:** A protein in the Sarcoplasmic Reticulum that binds calcium, allowing for high-capacity storage.

Q: Which of the following structures contains Group B nerve fibers?

Autonomic preganglionic fibers. ### Explanation The classification of nerve fibers is a high-yield topic for NEET-PG, primarily based on the **Erlanger-Gasser classification**, which categorizes fibers according to diameter, myelination, and conduction velocity. **Why Option C is Correct:** **Group B fibers** are characterized as medium-diameter, **lightly myelinated** fibers with a moderate conduction velocity (3–15 m/s). The classic anatomical location for Group B fibers is the **autonomic preganglionic neurons** (both sympathetic and parasympathetic). Their light myelination allows for efficient signal transmission to autonomic ganglia before the signal transitions to unmyelinated Group C postganglionic fibers. **Analysis of Incorrect Options:** * **Option A (Intrafusal fibers):** These are innervated by **A-gamma (Aγ)** motor neurons. These are myelinated fibers responsible for maintaining muscle spindle sensitivity. * **Option B (Golgi tendon apparatus):** These are associated with **Group Ib** sensory fibers. These are large, heavily myelinated fibers with the fastest conduction velocities. * **Option D (Spinothalamic tracts):** These tracts primarily carry pain and temperature sensations via **A-delta (Aδ)** (fast pain) and **Group C** (slow pain) fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Hypoxia:** Type A > Type B > Type C (Large fibers suffer first). * **Pressure:** Type A > Type B > Type C (e.g., Saturday Night Palsy). * **Local Anesthetics:** Type C > Type B > Type A (Small, unmyelinated fibers are blocked first). * **Fastest vs. Slowest:** Type **A-alpha (Aα)** are the fastest and thickest; Type **C** are the slowest, thinnest, and the only **unmyelinated** fibers. * **Postganglionic Autonomic Fibers:** Always remember these are **Type C** fibers.

Q: What does the 'Lloyds classification' primarily categorize?

Sensory nerve fibers. **Explanation:** The **Lloyd-Hunt classification** (often referred to as Lloyds classification) is a system specifically designed to categorize **sensory (afferent) nerve fibers** based on their diameter and conduction velocity. This classification uses Roman numerals (I, II, III, and IV) and is primarily used to describe sensory input from muscles and joints (proprioception and mechanoreception). * **Why Option A is correct:** Unlike the Erlanger-Gasser classification (which uses letters like A, B, and C), the Lloyd classification applies **exclusively to sensory fibers**. For example, Type Ia fibers originate from muscle spindle primary endings, and Type Ib fibers originate from Golgi tendon organs. * **Why Options B and C are incorrect:** Motor (efferent) fibers are categorized only under the **Erlanger-Gasser classification** (e.g., Alpha-motor neurons are Aα; Gamma-motor neurons are Aγ). There is no "Type I" or "Type II" motor neuron in the Lloyd system. * **Why Option D is incorrect:** The classification is a fundamental physiological tool used to describe the speed and size of sensory neurons. **High-Yield Facts for NEET-PG:** 1. **Correlation Table:** * **Type Ia/Ib:** Corresponds to **Aα** (Fastest, largest, myelinated). * **Type II:** Corresponds to **Aβ** (Touch, pressure). * **Type III:** Corresponds to **Aδ** (Fast pain, temperature). * **Type IV:** Corresponds to **C fibers** (Slow pain, unmyelinated, smallest). 2. **Clinical Pearl:** Type IV (C fibers) are the only unmyelinated fibers in the body and are the first to be blocked by local anesthetics but the last to be affected by pressure/hypoxia. 3. **Memory Aid:** **L**loyd is for **L**earning what we feel (**Sensory**).

Q: Pricking pain, cold, and crude touch are conducted by which type of nerve fibers?

A delta fibers. ### Explanation The correct answer is **D. A delta fibers**. **1. Why A delta fibers are correct:** Nerve fibers are classified based on diameter and myelination (Erlanger-Gasser classification). **A delta (Aδ) fibers** are thin, myelinated fibers that conduct impulses at a moderate velocity (5–30 m/s). They are responsible for "fast pain" (sharp, pricking sensation), cold temperature, and crude touch. Because they are myelinated, they allow for the rapid localization of a stimulus, unlike the slow, unmyelinated C fibers which carry "slow pain" (dull, aching). **2. Why the other options are incorrect:** * **A alpha (Aα) fibers:** These are the thickest and fastest myelinated fibers. they primarily carry **proprioception** (muscle spindles and Golgi tendon organs) and somatic motor signals. * **A beta (Aβ) fibers:** These are large, myelinated fibers that carry **fine touch**, pressure, and vibration. They are associated with mechanoreceptors like Meissner’s and Pacinian corpuscles. * **A gamma (Aγ) fibers:** These are medium-sized myelinated fibers that specifically innervate the **intrafusal fibers** of the muscle spindle, regulating muscle tone. **3. NEET-PG High-Yield Pearls:** * **Pain Dualism:** Remember that pain has two components: **Aδ** (Fast, pricking, localized) and **C fibers** (Slow, burning/aching, poorly localized). * **Sensitivity to Anesthesia:** Local anesthetics block small, unmyelinated fibers first. The order of loss is: **Pain > Temperature > Touch > Pressure**. * **Susceptibility:** **Type A** fibers are most susceptible to **pressure/hypoxia**, while **Type C** fibers are most susceptible to **local anesthetics**. * **Cold vs. Warm:** Cold is carried by **Aδ**, while warmth is carried by **C fibers**.

Q: Resting membrane potential is close to the equilibrium potential of which ion?

Chloride (Cl-). ### Explanation The **Resting Membrane Potential (RMP)** of a cell is determined by the relative permeability of the membrane to specific ions and their respective equilibrium potentials (calculated by the Nernst equation). **Why Chloride (Cl-) is the correct answer:** In most neurons and muscle cells, the RMP typically ranges between **-70 mV to -90 mV**. The equilibrium potential for **Chloride ($E_{Cl}$)** is approximately **-70 mV to -80 mV**. Because the cell membrane at rest is highly permeable to Chloride and its equilibrium potential aligns almost exactly with the measured RMP, it is considered the ion whose equilibrium potential is closest to the RMP. **Analysis of Incorrect Options:** * **Potassium ($K^+$):** While $K^+$ is the *primary* determinant of RMP because the membrane is most permeable to it at rest, its equilibrium potential ($E_K$) is approximately **-94 mV**. While close, $E_{Cl}$ is numerically closer to the actual RMP of -70 mV. * **Sodium ($Na^+$):** The equilibrium potential for Sodium ($E_{Na}$) is approximately **+60 mV**. This is far from the RMP because the membrane has very low permeability to $Na^+$ at rest. * **Magnesium ($Mg^{2+}$):** Magnesium is primarily an intracellular cation and does not play a significant role in determining the RMP of excitable tissues. **High-Yield NEET-PG Pearls:** 1. **Goldman-Hodgkin-Katz Equation:** Unlike the Nernst equation (one ion), this equation calculates RMP by considering the permeability and concentration gradients of all major ions ($Na^+$, $K^+$, and $Cl^-$). 2. **The $Na^+$-$K^+$ Pump:** This pump is "electrogenic"; it contributes about **-4 to -10 mV** directly to the RMP by pumping 3 $Na^+$ out for every 2 $K^+$ in. 3. **Gibbs-Donnan Effect:** The presence of non-diffusible intracellular proteins (anions) influences the distribution of $Cl^-$ and $K^+$, helping establish the RMP.

Q: Titin is absent in which of the following?

Smooth muscle. Explanation: Titin (also known as connectin) is a giant protein that functions as a molecular spring. It is the third most abundant protein in striated muscle after myosin and actin. 1. Why Smooth Muscle is the Correct Answer: Titin is specifically associated with the sarcomere, the functional unit of striated muscle [1]. It extends from the Z-disc to the M-line, anchoring the thick (myosin) filaments and providing passive elasticity. Smooth muscle lacks sarcomeres (it has dense bodies instead) and does not possess a regular arrangement of thick and thin filaments that requires a titin scaffold. Therefore, titin is absent in smooth muscle. 2. Why Other Options are Incorrect: * Skeletal Muscle: Titin is a vital component of the skeletal muscle sarcomere [1]. It prevents overextension of the muscle and ensures the myosin filaments remain centered during contraction. * Cardiac Muscle: Titin is present in cardiac muscle and is even more critical here for determining myocardial stiffness and diastolic filling. Mutations in the titin gene (TTN) are a leading cause of Dilated Cardiomyopathy (DCM). High-Yield NEET-PG Pearls: * Largest Protein: Titin is the largest known single polypeptide chain in the human body (approx. 3800 kDa). * Function: It acts as a "template" for thick filament assembly and provides passive tension when the muscle is stretched. * Clinical Correlation: Mutations in the TTN gene are the most common genetic cause of Dilated Cardiomyopathy. * Striated vs. Non-striated: Remember, Titin = Striated (Skeletal + Cardiac). No Titin = Non-striated (Smooth).

Q: What is true about troponin?

All of the above.. Troponin is a complex of three regulatory proteins integral to muscle contraction in skeletal and cardiac muscle. It is located on the thin (actin) filament and works in conjunction with tropomyosin to regulate the interaction between actin and myosin. **Explanation of Options:** * **Option A:** Troponin is indeed a heterotrimeric complex consisting of **three subunits**: Troponin T, Troponin I, and Troponin C. * **Option B:** **Troponin C (TnC)** is the calcium-binding subunit. In cardiac muscle, it has one high-affinity binding site for $Ca^{2+}$. When calcium binds to TnC, it induces a conformational change that moves the troponin-tropomyosin complex, exposing the myosin-binding sites on actin. * **Option C:** **Troponin I (TnI)** is the inhibitory subunit. It binds to **actin** to inhibit the ATPase activity of the actomyosin complex. It also interacts with **tropomyosin** to keep it in a position that blocks the cross-bridge cycle during relaxation. * **Troponin T (TnT):** (Though not a separate option) It binds to **tropomyosin**, anchoring the entire troponin complex to the thin filament. **Clinical Pearls for NEET-PG:** 1. **Cardiac Biomarkers:** Cardiac-specific isoforms (cTnI and cTnT) are the "gold standard" biomarkers for diagnosing Myocardial Infarction (MI) due to their high sensitivity and specificity. 2. **Troponin C Exception:** Unlike TnI and TnT, Troponin C is identical in both cardiac and slow-twitch skeletal muscle; therefore, it is **not** used as a specific diagnostic marker for MI. 3. **Smooth Muscle:** Troponin is **absent** in smooth muscle. Instead, calcium binds to **calmodulin**, which then activates Myosin Light Chain Kinase (MLCK).

Q: How is an electrical synapse different from a chemical synapse?

Connexons are involved in electrical synapses.. ### Explanation **1. Why the correct answer is right:** Electrical synapses are characterized by the presence of **Gap Junctions**, which provide direct **cytoplasmic continuity** between the presynaptic and postsynaptic neurons. These gap junctions are formed by clusters of transmembrane protein channels called **Connexons**. Each connexon is composed of six subunits called **connexins**. This physical bridge allows ions to flow directly from one cell to another, ensuring rapid signal transmission. **2. Why the incorrect options are wrong:** * **Option A:** In electrical synapses, there **is** cytoplasmic continuity via gap junctions. Chemical synapses, however, have a distinct synaptic cleft (20–40 nm) with no physical continuity. * **Option C:** Electrical synapses are typically **bidirectional**, allowing signals to flow in either direction. Chemical synapses are strictly **unidirectional** (one-way) due to the release of neurotransmitters from the presynaptic terminal to the postsynaptic receptors. * **Option D:** Electrical synapses have **virtually no synaptic delay** (latent period), making them much faster than chemical synapses. Chemical synapses have a delay (approx. 0.5 ms) due to the time required for neurotransmitter release, diffusion, and receptor binding. ### NEET-PG High-Yield Pearls: * **Location:** Electrical synapses are common in the **cardiac muscle** (intercalated discs), **smooth muscle** (unitary type), and specific brain regions like the **inferior olive**. * **Function:** They are essential for **synchronizing** the activity of a group of neurons or muscle fibers (e.g., "all-or-none" contraction of the heart). * **Comparison:** Chemical synapses are the most common type in the human CNS and allow for **signal amplification** and **plasticity**, which electrical synapses lack.

Question 1

Which of the following is referred to as a 'relaxation protein'?

Accepted Answer

Tropomyosin

Answer

Actin

Answer

Myosin

Answer

Dystrophin

Question 2

Which of the following structures contains Group B nerve fibers?

Accepted Answer

Autonomic preganglionic fibers

Answer

Intrafusal fibers of the muscle spindle

Answer

Golgi tendon apparatus

Answer

Spinothalamic tracts

Question 3

What does the 'Lloyds classification' primarily categorize?

Accepted Answer

Sensory nerve fibers

Answer

Motor nerve fibers

Answer

Both sensory and motor nerve fibers

Answer

Neither sensory nor motor nerve fibers

Question 4

Pricking pain, cold, and crude touch are conducted by which type of nerve fibers?

Accepted Answer

A delta fibers

Answer

A alpha fibers

Answer

A beta fibers

Answer

A gamma fibers

Question 5

Resting membrane potential is close to the equilibrium potential of which ion?

Accepted Answer

Chloride (Cl-)

Answer

Sodium (Na+)

Answer

Potassium (K+)

Answer

Magnesium (Mg2+)

Question 6

Titin is absent in which of the following?

Accepted Answer

Smooth muscle

Answer

Skeletal muscle

Answer

Cardiac muscle

Answer

All

Question 7

What is true about troponin?

Accepted Answer

All of the above.

Answer

It has 3 subunits.

Answer

Troponin C binds to calcium.

Answer

Troponin I binds actin and tropomyosin.

Question 8

How is an electrical synapse different from a chemical synapse?

Accepted Answer

Connexons are involved in electrical synapses.

Answer

There is no cytoplasmic continuity between the presynaptic and postsynaptic neuron.

Answer

The direction of impulse transmission is unidirectional.

Answer

Electrical synapses have a longer latent period than chemical synapses.

Question 9

A 12-year-old girl fractures her right radius and ulna. Her arm is placed in a cast extending over the elbow, which nearly eliminates movement of her forearm muscles. Which of the following will occur in her forearm muscles after several weeks in the cast?

Accepted Answer

Decreased capillarity

Answer

Decreased intracellular pO2

Answer

Decreased myoglobin oxygen saturation

Answer

Increased metabolic rate

Question 10

Active tension in a muscle depends upon which of the following?

Accepted Answer

Number of motor units recruited

Answer

Number of muscle fibers

Answer

Aerobic capacity of muscle

Answer

Action potential amplitude

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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