Nerve and Muscle Physiology Practice Questions

Q: Which protein in muscle aids in relaxation?

Titin. **Explanation:** The correct answer is **Titin** (also known as connectin). Titin is the largest known protein in the human body and acts as a molecular spring within the sarcomere. It extends from the Z-disk to the M-line, anchoring the thick (myosin) filaments. During muscle contraction, titin stores elastic potential energy; once the contraction stimulus ceases, this stored energy provides the **passive recoil force** necessary to return the sarcomere to its resting length, thereby aiding in muscle relaxation. **Analysis of Incorrect Options:** * **Nebulin (A):** An actin-binding protein that acts as a "molecular ruler." It regulates the length of thin filaments during assembly but does not provide elastic recoil. * **Desmin (B):** A type III intermediate filament that links Z-disks of adjacent myofibrils to each other and to the plasma membrane. It maintains structural integrity and alignment rather than facilitating relaxation. * **Calcineurin (D):** A calcium-dependent phosphatase involved in signaling pathways (e.g., T-cell activation and muscle fiber type switching). It is an enzyme, not a structural protein involved in the mechanical recoil of the sarcomere. **High-Yield NEET-PG Pearls:** * **Titin:** Mutations in the *TTN* gene are a leading cause of **Dilated Cardiomyopathy (DCM)**. * **Dystrophin:** Often confused with these proteins, it links the cytoskeleton of a muscle fiber to the surrounding extracellular matrix (defective in Duchenne Muscular Dystrophy). * **Tropomyosin:** In the resting state, it covers the myosin-binding sites on actin, preventing contraction. Relaxation requires the re-uptake of $Ca^{2+}$ into the sarcoplasmic reticulum by **SERCA** pumps.

Q: All of the following are features of cardiac muscle EXCEPT:

Caldesmon is an important regulatory protein. **Explanation:** The correct answer is **D**. **Caldesmon** is a regulatory protein found specifically in **smooth muscle**, not cardiac muscle. It functions by binding to actin and tropomyosin, thereby inhibiting the ATPase activity of myosin and preventing contraction. In smooth muscle, contraction is initiated when calcium-calmodulin binds to caldesmon, releasing it from actin. **Analysis of Options:** * **A. Striated:** Cardiac muscle, like skeletal muscle, contains highly organized sarcomeres (the functional units of contraction), which give it a characteristic striated appearance under a microscope. * **B. Large T tubules:** Cardiac muscle features T-tubules that are significantly wider (up to 5 times the diameter) than those in skeletal muscle. They are located at the **Z-discs** (unlike skeletal muscle, where they are at the A-I junction). * **C. Troponin system present:** Cardiac contraction is regulated by the troponin complex (TnC, TnI, and TnT). Calcium binds to **Troponin C**, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Biomarkers:** Cardiac-specific isoforms of **Troponin I and T** are the gold-standard markers for diagnosing Myocardial Infarction (MI). * **Triads vs. Diads:** Skeletal muscle has "Triads" (1 T-tubule + 2 terminal cisternae), while cardiac muscle typically has **"Diads"** (1 T-tubule + 1 terminal cisterna). * **Smooth Muscle Regulation:** Remember that smooth muscle lacks troponin. Instead, it uses **Caldesmon, Calponin,** and the **Calmodulin-MLCK** (Myosin Light Chain Kinase) pathway.

Q: Initiation of nerve impulse occurs at the axon hillock because:

It has a lower threshold than the rest of the axon. The **axon hillock** (specifically the initial segment) is known as the **"Trigger Zone"** of the neuron. ### Why Option A is Correct The initiation of an action potential depends on the density of **voltage-gated sodium (Na+) channels**. The axon hillock has the highest concentration of these channels per unit area compared to the cell body or the rest of the axon. Because there are more channels available to open in response to depolarization, the **threshold for excitation is significantly lower** (approx. -45 mV) compared to the soma (approx. -30 mV). This makes it the site with the highest excitability, where graded potentials are summed to trigger an all-or-nothing impulse. ### Why Other Options are Incorrect * **Option B:** While the axon hillock is unmyelinated, this is not the functional reason for impulse *initiation*. Many parts of a neuron (like the dendrites and soma) are unmyelinated but cannot initiate an action potential as easily because they lack the necessary density of Na+ channels. * **Option C:** Neurotransmitter release occurs at the **axon terminals** (presynaptic boutons), not the hillock. The hillock is responsible for signal *generation*, while the terminal is responsible for signal *transmission*. ### High-Yield Facts for NEET-PG * **Initial Segment:** The actual site of impulse generation is the unmyelinated segment between the axon hillock and the first myelin sheath. * **Refractory Period:** This period is determined by the inactivation gate (h-gate) of the voltage-gated Na+ channels. * **Accommodation:** If a nerve is subjected to a slowly rising subthreshold stimulus, the threshold for firing increases; this is called accommodation. * **Safety Factor:** The ratio of action potential strength to the excitability threshold. In normal nerve fibers, this is usually greater than 1.

Q: As a neuron’s diameter increases, what effect manifests?

Conduction velocity increases. ***Conduction velocity increases*** - Increasing the diameter of an axon decreases the **internal (axial) resistance** ($R_i$) to passive current flow longitudinal to the axon. - Reduced internal resistance allows local current loops to spread further and faster, significantly increasing the **length constant**, thereby increasing conduction velocity. *Conduction velocity decreases* - This is incorrect, as larger diameter decreases internal resistance, leading directly to a **faster electrotonic spread** of depolarization and a higher conduction speed. - Decreased conduction velocity is typically observed in **small-diameter** or **demyelinated** axons where internal resistance is higher or membrane capacitance is altered. *Membrane resistance increases* - Membrane resistance ($R_m$) is determined by the density and activity of **leak ion channels** within the cell membrane, which is independent of the overall axon diameter. - While the total membrane area increases, the **specific membrane resistance** (resistance per unit area) does not change with diameter. *Action potential amplitude increases* - Action potential (AP) conduction is an **all-or-none** phenomenon, meaning the amplitude is fixed and determined by the electrochemical gradient of **voltage-gated sodium channels**. - Changes in axon diameter influence the **speed** of propagation (conduction velocity), but they do not alter the required peak voltage (amplitude) of the action potential.

Q: A 35-year-old man wakes up after sleeping with his arm draped over a chair and complains of pain. Which of the following accurately describes the order of susceptibility of nerve fibers in the given condition?

A > B > C. ***A > B > C***- The clinical scenario describes **neuropraxia** (transient functional block) due to **compression and ischemia**, such as in 'Saturday night palsy'.- A fibers have the largest diameter and the heaviest myelination, making them the most vulnerable to conduction block resulting from **focal demyelination** caused by mechanical stress. *A B > A*- This order represents the susceptibility of nerve fibers to **local anesthetic agents** (pharmacologic block), not mechanical compression. - Local anesthetics preferentially block smaller, unmyelinated C fibers (pain and temperature sensation), followed by B and then A fibers (motor/proprioception). *C B fibers > C fibers) to compression injury.

Q: Which of the following ion movements is primarily responsible for the repolarization phase (Phase 3) of an action potential, as depicted in the image?

Efflux of K ions. ***Efflux of K ions*** - Phase 3, the **repolarization** or falling phase, is initiated by the opening of voltage-gated **potassium (K+) channels** as the membrane potential peaks. - The outflow of positive K+ ions from the cell, known as **efflux**, causes the membrane potential to become negative again, returning it towards the resting state. *Efflux of Na ions* - The electrochemical gradient for **sodium (Na+)** strongly favors its movement into the cell (influx), not out of it (efflux). - While the **Na+/K+ pump** does move Na+ out of the cell, this is a slow, active process to maintain resting potential, not the cause of rapid repolarization. *Influx of Na ions* - The rapid influx (inflow) of **Na+** ions through voltage-gated channels is responsible for the **depolarization** phase (Phase 0), the sharp upstroke of the action potential. - During repolarization (Phase 3), these voltage-gated **Na+ channels** become inactivated, stopping the influx. *Resting membrane potential is maintained by the Na-K pump* - The **Na+/K+ pump** is crucial for establishing and maintaining the ion gradients for the **resting membrane potential** (Phase 4), not for the rapid repolarization phase itself. - Repolarization is a passive process resulting from ion flow through channels, which is much faster than the action of the Na+/K+ pump.

Question 1

Which protein in muscle aids in relaxation?

Accepted Answer

Titin

Answer

Nebulin

Answer

Desmin

Answer

Calcineurin

Question 2

Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction is/are due to?

Accepted Answer

IP3-DAG

Answer

CAMP

Answer

CGMP

Answer

CCMP

Question 3

All of the following are features of cardiac muscle EXCEPT:

Accepted Answer

Caldesmon is an important regulatory protein

Answer

Striated

Answer

Large T tubules

Answer

Troponin system present

Question 4

Initiation of nerve impulse occurs at the axon hillock because:

Accepted Answer

It has a lower threshold than the rest of the axon

Answer

It is unmyelinated

Answer

Neurotransmitter release occurs here

Answer

None of the above

Question 5

The thin filament of muscle consists of all the following except:

Accepted Answer

Myosin

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Question 6

What does the nerve impulse leading to smooth muscle contraction initiate?

Accepted Answer

Opening of calcium channels, leading to increased intracellular Ca+2 concentration.

Answer

Depolarization of both the plasma membrane and T-tubules.

Answer

Inhibition of Na+ entry into the sarcomere.

Answer

Binding of acetylcholine to receptors in the sarcoplasmic reticulum.

Question 7

As a neuron’s diameter increases, what effect manifests?

Accepted Answer

Conduction velocity increases

Answer

Conduction velocity decreases

Answer

Membrane resistance increases

Answer

Action potential amplitude increases

Question 8

In malignant hyperthermia, a genetic mutation in the ryanodine receptor (RYR1) leads to a life-threatening reaction to certain anesthetic agents. What is the primary ionic abnormality caused by this receptor defect?

Accepted Answer

Increased calcium release from the sarcoplasmic reticulum

Answer

Increased sodium influx into the muscle cell

Answer

Increased potassium uptake into the sarcoplasmic reticulum

Answer

Increased chloride efflux from the muscle cell

Question 9

A 35-year-old man wakes up after sleeping with his arm draped over a chair and complains of pain. Which of the following accurately describes the order of susceptibility of nerve fibers in the given condition?

Accepted Answer

A > B > C

Answer

C < B < A

Answer

A < B < C

Answer

C > B > A

Question 10

Which of the following ion movements is primarily responsible for the repolarization phase (Phase 3) of an action potential, as depicted in the image?

Accepted Answer

Efflux of K ions

Answer

Influx of Na ions

Answer

Efflux of Na ions

Answer

Resting membrane potential is maintained by the Na-K pump

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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