Nerve and Muscle Physiology Practice Questions

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.

Q: Arrange the following nerve fibers sequentially in the descending order of nerve impulse transmission velocity: A. C fiber B. Aa fiber C. B fiber D. Ad fiber

B-D-C-A. **Explanation** The velocity of nerve impulse transmission is primarily determined by two factors: **fiber diameter** and the **presence of myelin**. According to the Erlanger-Gasser classification, thicker and more heavily myelinated fibers conduct impulses faster than thinner, unmyelinated ones. **The Correct Sequence (Descending Order):** 1. **Aα (Alpha) fiber:** The thickest (12–20 µm) and most heavily myelinated; conducts at 70–120 m/s. 2. **Aδ (Delta) fiber:** Thinner than Aα (2–5 µm) but still myelinated; conducts at 12–30 m/s. 3. **B fiber:** Small, preganglionic autonomic fibers ( Aδ > B > C**, which corresponds to sequence **B-D-C-A**. **Analysis of Options:** * **Option B (Correct):** Correctly follows the hierarchy of diameter and myelination. * **Options A, C, and D:** These are incorrect because they place C fibers (slowest) or B fibers (intermediate) ahead of A-group fibers in velocity. **High-Yield NEET-PG Pearls:** * **Sensitivity to Local Anesthetics:** The order of blockade is **B > C > Aδ > Aγ > Aβ > Aα**. Note that B fibers are blocked first despite being myelinated because of their small diameter and anatomy. * **Sensitivity to Pressure:** A fibers are most sensitive (e.g., "limb falling asleep"). * **Sensitivity to Hypoxia:** B fibers are most sensitive. * **Function of C fibers:** They carry "slow pain" (dull, aching), temperature, and postganglionic sympathetic signals. Aδ fibers carry "fast pain" (sharp, localized).

Q: Visceral noxious stimuli are mediated by which nerve fiber type?

C. **Explanation:** The correct answer is **D. C fibers**. **Underlying Medical Concept:** Sensory information from the viscera (internal organs) is primarily transmitted via two types of fibers: **A-delta (Aδ)** and **C fibers**. However, the vast majority of visceral afferents—especially those mediating dull, aching, and poorly localized **noxious stimuli** (pain)—are unmyelinated **C fibers**. These fibers have a small diameter and slow conduction velocity (0.5–2.0 m/s). In the viscera, these fibers respond to mechanical distortion, inflammation, and ischemia, often leading to the phenomenon of "referred pain" due to convergence in the spinal cord. **Analysis of Incorrect Options:** * **A. A-alpha (Aα):** These are the thickest, fastest myelinated fibers. They primarily carry proprioceptive information from muscles (muscle spindles) and tendons (Golgi tendon organs). * **B. A-beta (Aβ):** These are large, myelinated fibers that mediate "fine touch," pressure, and vibration. They are not involved in primary pain transmission. * **C. A-gamma (Aγ):** These are motor fibers (efferent) that innervate the intrafusal fibers of the muscle spindle to maintain spindle sensitivity; they do not carry sensory noxious stimuli. **NEET-PG High-Yield Pearls:** * **Fast Pain vs. Slow Pain:** Fast, sharp, localized pain (somatic) is carried by **A-delta** fibers. Slow, burning, chronic, or visceral pain is carried by **C fibers**. * **Fiber Sensitivity:** C fibers are the **most sensitive to local anesthetics**, while A-alpha fibers are the most sensitive to pressure/hypoxia. * **Visceral Pain Characteristics:** It is characterized by being poorly localized, often associated with autonomic symptoms (nausea, sweating), and typically referred to somatic structures (e.g., cardiac pain referred to the left inner arm).

Question 1

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 2

Titin is absent in which of the following?

Accepted Answer

Smooth muscle

Answer

Skeletal muscle

Answer

Cardiac muscle

Answer

All

Question 3

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 4

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 5

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 6

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

Question 7

Arrange the following nerve fibers sequentially in the descending order of nerve impulse transmission velocity: A. C fiber B. Aa fiber C. B fiber D. Ad fiber

Accepted Answer

B-D-C-A

Answer

A-D-C-B

Answer

A-C-D-B

Answer

C-A-B-D

Question 8

In Myasthenia gravis, what is the fundamental defect at the neuromuscular junction?

Accepted Answer

Failure of Acetylcholine (ACh) to attach to the postsynaptic receptor

Answer

Deficiency of Acetylcholine (ACh)

Answer

Defect in the release of Acetylcholine (ACh)

Answer

Impeded presynaptic release of Acetylcholine (ACh)

Question 9

Where does the action potential in a spinal motor neuron generate from?

Accepted Answer

Initial segment

Answer

Cell body

Answer

Dendrites

Answer

Axon

Question 10

Visceral noxious stimuli are mediated by which nerve fiber type?

Accepted Answer

C

Answer

A alpha

Answer

A beta

Answer

A gamma

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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