General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 211: The QRS complex on an electrocardiogram represents which physiological event?

A. Ventricular repolarization
B. Ventricular depolarization (Correct Answer)
C. Atrial repolarization
D. Atrial depolarization

Explanation: **Explanation:** The **QRS complex** represents the rapid **ventricular depolarization** (Option B). This electrical event triggers the contraction of the ventricular myocardium. In a normal ECG, the QRS complex follows the PR interval and typically lasts less than 0.12 seconds. It consists of the Q wave (initial downward deflection), R wave (first upward deflection), and S wave (downward deflection following the R wave). **Analysis of Incorrect Options:** * **Option A (Ventricular repolarization):** This is represented by the **T wave**. It reflects the recovery of the ventricles to their resting electrical state. * **Option C (Atrial repolarization):** This occurs simultaneously with ventricular depolarization. Because the ventricles have a much larger muscle mass, the electrical signal of atrial repolarization is buried within the high-amplitude QRS complex and is not visible on a standard ECG. * **Option D (Atrial depolarization):** This is represented by the **P wave**, which precedes the QRS complex. **High-Yield NEET-PG Pearls:** * **Duration:** A widened QRS (>0.12s) suggests a bundle branch block (RBBB/LBBB) or a ventricular origin of the impulse. * **Pathological Q waves:** Defined as >0.04s wide or >25% of the R-wave height; these are hallmarks of a **previous myocardial infarction**. * **Delta Wave:** A slurred upstroke of the QRS complex, pathognomonic for **Wolff-Parkinson-White (WPW) syndrome** due to an accessory pathway (Bundle of Kent). * **J-Wave (Osborn wave):** A deflection at the junction of the QRS and ST segment, classically seen in **hypothermia**.

Question 212: The Weber-Fechner law states that the magnitude of stimulus strength perceived is approximately proportional to the logarithm of the intensity of the stimulus.

A. Magnitude of stimulus strength perceived is approximately proportional to the log of the intensity of stimulus strength. (Correct Answer)
B. Magnitude of stimulus strength perceived is directly proportional to the intensity of stimulus strength.
C. Threshold of receptor is directly proportional to stimulus strength.
D. Threshold of receptor is inversely proportional to stimulus strength.

Explanation: The **Weber-Fechner Law** is a fundamental principle in sensory physiology that describes the relationship between the physical intensity of a stimulus and its perceived intensity. ### **Explanation of the Correct Answer (Option A)** The law states that the intensity of a sensation ($S$) is proportional to the logarithm of the stimulus intensity ($I$). Mathematically, this is expressed as: $$S = K \cdot \log(I)$$ This means that as the physical strength of a stimulus increases geometrically (e.g., 10, 100, 1000), the perceived sensation increases only arithmetically (e.g., 1, 2, 3). This logarithmic relationship allows our sensory systems (vision, hearing, touch) to process a massive range of stimulus intensities without saturating the receptors. ### **Analysis of Incorrect Options** * **Option B:** This describes a linear relationship. If perception were directly proportional to intensity, we would be unable to distinguish small changes at low intensities or would be overwhelmed by high intensities. * **Options C & D:** These options confuse "threshold" with "perception." The threshold is the minimum intensity required to elicit a response; while it relates to sensitivity, it does not define the mathematical progression of perceived magnitude described by Weber-Fechner. ### **High-Yield Clinical Pearls for NEET-PG** * **Weber’s Law:** The "Just Noticeable Difference" (JND) is a constant fraction of the original stimulus ($\Delta I / I = k$). * **Stevens' Power Law:** A more modern refinement suggesting that for some sensations (like pain/electric shock), the relationship is an exponential power function rather than logarithmic. * **Clinical Application:** This law explains why we can hear a whisper in a quiet room but cannot hear a shout in a noisy construction site—the background intensity shifts the threshold for perception.

Question 213: Which neurotransmitter is primarily associated with pain transmission?

A. Dopamine
B. Substance P (Correct Answer)
C. Platelet-activating factor (PAF)
D. None of the above

Explanation: **Explanation:** **Substance P** is the correct answer because it is a neuropeptide that acts as a primary neurotransmitter in the transmission of pain signals. It is released from the peripheral terminals of **nociceptors** (C-fibers) and their central terminals in the **substantia gelatinosa** (Lamina II) of the spinal cord's dorsal horn. It facilitates the transmission of pain impulses to the brain and is often co-released with Glutamate to enhance the pain response. **Analysis of Incorrect Options:** * **A. Dopamine:** This is a catecholamine primarily involved in the brain's reward system, motor control (basal ganglia), and executive functions. While it plays a role in pain modulation within the CNS, it is not the primary transmitter for pain transmission. * **C. Platelet-activating factor (PAF):** This is a phospholipid mediator involved in platelet aggregation, inflammation, and anaphylaxis. It is not a neurotransmitter for nociception. **High-Yield Clinical Pearls for NEET-PG:** * **Glutamate vs. Substance P:** Glutamate is the primary neurotransmitter for **fast, sharp pain** (A-delta fibers), while Substance P is associated with **slow, chronic, aching pain** (C-fibers). * **Capsaicin:** Found in chili peppers, it causes the release and subsequent depletion of Substance P from sensory neurons, which is why it is used topically as an analgesic for conditions like post-herpetic neuralgia. * **Enkephalins:** These are endogenous opioids that inhibit the release of Substance P in the dorsal horn, providing "presynaptic inhibition" of pain.

Question 214: Which of the following cellular junctions is NOT present in cardiac muscle?

A. Zonula occludens (Correct Answer)
B. Fascia adherens
C. Gap junctions
D. Macula adherens

Explanation: **Explanation:** The cardiac muscle cells (cardiomyocytes) are joined end-to-end by specialized structures called **Intercalated Discs**. These discs contain three specific types of junctions: Fascia adherens, Macula adherens (Desmosomes), and Gap junctions. **1. Why Zonula Occludens is the correct answer:** **Zonula occludens (Tight junctions)** are typically found in epithelial tissues (e.g., intestinal lining, blood-brain barrier) where they seal the intercellular space to prevent the leakage of molecules. They are **not present** in cardiac muscle because the heart requires rapid electrical conduction and mechanical cohesion rather than a waterproof seal between cells. **2. Analysis of Incorrect Options:** * **Fascia Adherens:** This is the most prominent junction in the vertical portion of the intercalated disc. It anchors actin filaments of the sarcomere to the cell membrane, transmitting contractile forces between cells. * **Gap Junctions (Nexus):** Located in the horizontal portions of the disc, these provide low-resistance electrical pathways. They allow ions to flow between cells, ensuring the heart functions as a **functional syncytium**. * **Macula Adherens (Desmosomes):** These provide strong mechanical attachment by anchoring intermediate filaments (desmin). They prevent the cardiomyocytes from pulling apart during the intense mechanical stress of contraction. **High-Yield NEET-PG Pearls:** * **Intercalated Discs** always coincide with the **Z-lines** of the cardiac muscle. * **Connexin 43** is the primary protein forming gap junctions in the ventricles. * **Arrhythmogenic Right Ventricular Dysplasia (ARVD)** is a clinical condition caused by mutations in desmosomal proteins (Macula adherens) in cardiac muscle.

Question 215: As atmospheric pressure is raised and lowered during the course of a day, blood flow through the brain would be expected to?

A. Change in the same direction as the atmospheric blood pressure because of the limited autoregulatory ability of the cerebral vessels.
B. Change in a direction opposite the change in mean atmospheric pressure.
C. Remain constant because cerebral vascular resistance changes in the same direction as atmospheric pressure. (Correct Answer)
D. Fluctuate widely, as both atmospheric pressure and brain neural activity status change.

Explanation: ### Explanation **1. Why Option C is Correct: The Concept of Cerebral Autoregulation** The primary physiological principle at play is **Cerebral Autoregulation**. The brain maintains a constant blood flow (CBF) despite changes in systemic arterial pressure (or atmospheric pressure fluctuations) within a mean arterial pressure (MAP) range of approximately **60 to 140 mmHg**. According to Poiseuille’s Law, Flow = Pressure / Resistance ($Q = \Delta P/R$). To keep flow ($Q$) constant when pressure ($P$) increases, the resistance ($R$) must increase proportionally. In the brain, this is achieved through the **myogenic mechanism**: an increase in pressure stretches vascular smooth muscle, triggering vasoconstriction (increased resistance), while a decrease in pressure triggers vasodilation. Thus, cerebral vascular resistance changes in the same direction as the pressure to ensure stable perfusion. **2. Why Other Options are Incorrect:** * **Option A:** This is incorrect because the brain has one of the most robust and sensitive autoregulatory systems in the body, not a "limited" one. * **Option B:** If flow changed in the opposite direction of pressure, it would imply an inverse relationship that contradicts basic hemodynamics and would lead to cerebral ischemia during pressure drops. * **Option C:** Atmospheric pressure changes are generally minor and well within the autoregulatory window. The brain does not "fluctuate widely" because such instability would lead to syncope or edema. **3. High-Yield Clinical Pearls for NEET-PG:** * **Normal CBF:** 50 ml/100g of brain tissue per minute (approx. 750 ml/min or 15% of Cardiac Output). * **Most Potent Regulator:** Arterial **$PCO_2$** is the most important chemical regulator of CBF. Hypercapnia causes marked vasodilation. * **Cerebral Perfusion Pressure (CPP):** $CPP = MAP - ICP$ (Intracranial Pressure). * **Cushing’s Reflex:** A triad of hypertension, bradycardia, and irregular respiration seen in response to increased ICP.

Question 216: Nerve depolarization is due to?

A. Opening of sodium channels (Correct Answer)
B. Opening of chloride channels
C. Opening of potassium channels
D. Opening of calcium channels

Explanation: **Explanation:** The Action Potential (AP) is a rapid change in the membrane potential of an excitable cell. In a resting state, the cell is polarized (approx. -70 to -90 mV). **1. Why Option A is Correct:** Depolarization is the phase where the membrane potential becomes more positive. This is primarily triggered by the **opening of voltage-gated sodium (Na⁺) channels**. Due to the steep electrochemical gradient, Na⁺ ions rush into the cell (influx). This rapid entry of positive charge reverses the resting membrane potential toward the equilibrium potential for sodium (+60 mV). **2. Why Incorrect Options are Wrong:** * **Option B (Chloride channels):** Opening of Cl⁻ channels leads to Cl⁻ influx. Since Cl⁻ is negatively charged, this makes the interior more negative, causing **Hyperpolarization** or Inhibitory Post-Synaptic Potentials (IPSP). * **Option C (Potassium channels):** Opening of K⁺ channels leads to K⁺ efflux (exit from the cell). This loss of positive charge results in **Repolarization** (returning to rest) or Hyperpolarization. * **Option D (Calcium channels):** While Ca²⁺ influx contributes to depolarization in cardiac pacemaker cells and smooth muscle, in a standard **nerve action potential**, sodium is the primary ion responsible for the initial rapid depolarization phase. **Clinical Pearls for NEET-PG:** * **Tetrodotoxin (Pufferfish) & Saxitoxin:** Block voltage-gated Na⁺ channels, preventing depolarization and causing paralysis. * **Local Anesthetics (Lignocaine):** Work by blocking these same Na⁺ channels from the inner side of the membrane. * **Overshoot:** The portion of the AP where the membrane potential is positive (>0 mV). * **Activation Gate:** The 'm' gate of the Na⁺ channel opens rapidly during depolarization; the 'h' gate (inactivation gate) closes slowly to end the phase.

Question 217: How many muscle fibers are typically found in one motor unit in the eye?

A. 15
B. 30
C. 5 (Correct Answer)
D. 50

Explanation: **Explanation:** The correct answer is **C (5)**. **1. Underlying Medical Concept:** A **motor unit** is defined as a single motor neuron and all the muscle fibers it innervates. The number of muscle fibers per motor unit determines the degree of **precision and control** over a movement. This is known as the **innervation ratio**. * **Low Innervation Ratio (Small Motor Units):** Found in muscles requiring fine, precise movements (e.g., extraocular muscles, hand muscles). In the eye, the ratio is typically very low, ranging from **3 to 6 muscle fibers per neuron**, with 5 being the standard average cited in physiological texts. * **High Innervation Ratio (Large Motor Units):** Found in large muscles requiring power rather than precision (e.g., Gastrocnemius), where one neuron may innervate up to 2,000 fibers. **2. Analysis of Incorrect Options:** * **Option A (15) & B (30):** While these represent relatively small motor units compared to postural muscles, they are too high for the extreme precision required for saccadic and smooth pursuit eye movements. * **Option D (50):** This ratio is more characteristic of the small muscles of the hand (e.g., interossei), which require fine motor control but not the microscopic accuracy of the extraocular muscles. **3. High-Yield Clinical Pearls for NEET-PG:** * **Smallest Motor Units:** Located in the **Extraocular muscles** (approx. 3–6 fibers/unit). * **Largest Motor Units:** Located in the **Gastrocnemius** or **Soleus** (approx. 1000–2000 fibers/unit). * **All-or-None Law:** This law applies to the individual motor unit; when the neuron fires, all fibers in that unit contract simultaneously. * **Recruitment (Henneman’s Size Principle):** Smaller motor units are recruited first during a contraction to allow for smooth increases in muscle tension.

Question 218: An index of the binding affinity of a hormone for its receptor can be obtained by examining which parameter?

A. The Y-intercept of a Scatchard plot
B. The slope of a Scatchard plot (Correct Answer)
C. The maximum point on a biological dose-response curve
D. The X-intercept of a Scatchard plot

Explanation: ### Explanation The binding affinity of a hormone for its receptor is a measure of how tightly the hormone binds to its receptor. In physiology and pharmacology, this is quantitatively analyzed using a **Scatchard Plot**. #### Why Option B is Correct The Scatchard plot graphs the ratio of **Bound/Free hormone (B/F)** on the Y-axis against the **Bound hormone (B)** on the X-axis. The relationship is expressed by the equation: **$B/F = (R_0 - B) / K_d$** Where $R_0$ is the total number of receptors and $K_d$ is the dissociation constant. * The **slope** of the resulting straight line is equal to **$-1/K_d$**. * Since affinity ($K_a$) is the reciprocal of the dissociation constant ($1/K_d$), the **slope directly represents the binding affinity**. A steeper negative slope indicates a higher affinity. #### Why Other Options are Incorrect * **Option A (Y-intercept):** The Y-intercept represents the ratio of Bound/Free hormone when the concentration of bound hormone is zero ($R_0/K_d$). It does not independently represent affinity. * **Option D (X-intercept):** The X-intercept represents the **$B_{max}$** (or $R_0$), which is the **total number of binding sites** (receptor capacity) available in the tissue. * **Option C (Dose-response curve):** The maximum point on a biological dose-response curve represents **Efficacy** ($V_{max}$), not affinity. Affinity is better reflected by the $EC_{50}$ (potency) on such a curve. #### High-Yield NEET-PG Pearls * **Scatchard Plot:** Used to distinguish between changes in receptor **number** (X-intercept) vs. receptor **affinity** (slope). * **Upregulation/Downregulation:** Primarily affects the **X-intercept** (number of receptors). * **Competitive Inhibition:** Changes the **slope** (decreases affinity) but not the X-intercept (number of receptors remains the same). * **Non-linear Scatchard Plot:** Suggests **cooperativity** in binding (e.g., heme-heme interaction in hemoglobin).

Question 219: Calcium initiates skeletal muscle contraction by which mechanism?

A. Binding to tropomyosin
B. Being released primarily from the sarcolemma during an action potential
C. Binding to troponin (Correct Answer)
D. Covering actin binding sites

Explanation: **Explanation:** In skeletal muscle, contraction is regulated by the **Troponin-Tropomyosin complex** located on the actin (thin) filament. **1. Why the correct answer is right:** When an action potential reaches the muscle fiber, it triggers the release of Calcium ($Ca^{2+}$) from the **Sarcoplasmic Reticulum (SR)** into the sarcoplasm. This $Ca^{2+}$ binds specifically to **Troponin C**. This binding induces a conformational change in the entire troponin complex, which physically pulls the **tropomyosin** molecule away from the active sites on the actin filament. This exposes the binding sites, allowing the myosin heads to form cross-bridges with actin, initiating the power stroke. **2. Why the incorrect options are wrong:** * **Option A:** Calcium binds to Troponin, not Tropomyosin. Tropomyosin’s role is to physically mask the binding sites; it moves only after Troponin C is activated. * **Option B:** In skeletal muscle, the primary source of $Ca^{2+}$ is the **Sarcoplasmic Reticulum**, not the sarcolemma (extracellular fluid). This is a key distinction from cardiac muscle, which requires extracellular $Ca^{2+}$ influx. * **Option D:** Calcium *uncovers* the binding sites; it does not cover them. Tropomyosin is the protein that covers the sites in a relaxed state. **High-Yield Clinical Pearls for NEET-PG:** * **Troponin Subunits:** Remember **T** (binds to **T**ropomyosin), **I** (**I**nhibitory; binds to actin), and **C** (binds to **C**alcium). * **Rigor Mortis:** Occurs due to the lack of ATP, which is required for the *detachment* of myosin from actin. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR1)**, leading to excessive $Ca^{2+}$ release from the SR. * **Dihydropyridine Receptor (DHPR):** Acts as a voltage sensor in the T-tubule that triggers the RyR1.

Question 220: The second messengers cyclic AMP and cyclic GMP:

A. Activate the same signal transduction pathways.
B. Are generated by the activation of cyclases. (Correct Answer)
C. Activate the same protein kinase.
D. Are important only in sensory transduction.

Explanation: ### Explanation **1. Why Option B is Correct:** Cyclic AMP (cAMP) and cyclic GMP (cGMP) are critical second messengers in cellular signaling. They are synthesized from ATP and GTP, respectively, through the action of **cyclase enzymes**. * **cAMP** is generated by **Adenylyl Cyclase**, which is typically activated by G-protein coupled receptors (specifically the Gs subunit). * **cGMP** is generated by **Guanylyl Cyclase**, which exists in two forms: membrane-bound (activated by Atrial Natriuretic Peptide) and soluble (activated by Nitric Oxide). **2. Why Other Options are Incorrect:** * **Option A:** They activate **distinct** signal transduction pathways. cAMP primarily regulates metabolic processes and ion channels, while cGMP is heavily involved in vasodilation and phototransduction. * **Option C:** They activate different kinases. cAMP activates **Protein Kinase A (PKA)**, whereas cGMP activates **Protein Kinase G (PKG)**. These kinases phosphorylate different target proteins. * **Option D:** While both are vital in sensory transduction (e.g., cGMP in vision, cAMP in olfaction), they are ubiquitous and regulate diverse functions including heart rate, smooth muscle relaxation, and hormone secretion throughout the body. **3. High-Yield NEET-PG Pearls:** * **Phosphodiesterases (PDE):** These enzymes degrade cAMP and cGMP. **Sildenafil** (Viagra) works by inhibiting PDE-5, thereby increasing cGMP levels to cause vasodilation. * **Bacterial Toxins:** *Vibrio cholerae* toxin causes permanent activation of Adenylyl Cyclase (via Gs), leading to massive cAMP levels and secretory diarrhea. * **ANP & NO:** Always associate Atrial Natriuretic Peptide and Nitric Oxide with the **cGMP** pathway. Most other hormones (ACTH, Glucagon, PTH) utilize the **cAMP** pathway.

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