General Physiology Practice Questions

Q: Calmodulin acts by:

Phosphorylation of myosin by protein kinase. **Explanation:** **Mechanism of Action (Why Option B is correct):** Calmodulin is a calcium-binding messenger protein found in all eukaryotic cells. In smooth muscle contraction, it plays a pivotal role because smooth muscle lacks the troponin complex found in skeletal muscle. When intracellular $Ca^{2+}$ levels rise, four $Ca^{2+}$ ions bind to **Calmodulin**, forming a $Ca^{2+}$-Calmodulin complex. This complex activates an enzyme called **Myosin Light Chain Kinase (MLCK)**. MLCK then **phosphorylates** the regulatory light chain of the myosin head. This phosphorylation increases myosin ATPase activity, allowing the myosin head to bind to actin and initiate the cross-bridge cycle (contraction). **Why other options are incorrect:** * **Option A:** Sodium channel opening is typically mediated by voltage changes (action potentials) or ligand binding (e.g., Acetylcholine), not by Calmodulin. * **Option C:** The $Na^{+}$-$K^{+}$ pump is primarily regulated by ATP availability and specific hormones (like insulin or aldosterone), not directly by the Calmodulin-MLCK pathway. * **Option D:** Dephosphorylation of myosin leads to **relaxation**, not contraction. This process is mediated by **Myosin Light Chain Phosphatase (MLCP)**, which removes the phosphate group that MLCK added. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** 1 Calmodulin binds to **4 Calcium ions**. * **Smooth Muscle Relaxation:** Nitric Oxide (NO) causes relaxation by increasing cGMP, which inhibits MLCK and activates MLCP. * **Troponin vs. Calmodulin:** Remember the mnemonic: **S**keletal muscle uses **S**troke/Troponin; **S**mooth muscle uses **S**econd messenger/Calmodulin. * **Other functions:** Calmodulin also regulates the activation of phosphorylase kinase in glycogenolysis and modulates cyclic nucleotide phosphodiesterase.

Q: Which of the following is a compensatory mechanism in a patient with hypovolemic shock?

Decreased cutaneous blood flow. **Explanation:** In hypovolemic shock, the primary physiological goal is to maintain perfusion to vital organs (brain and heart) despite a drop in cardiac output. This is achieved through the activation of the **sympathetic nervous system** and the **Renin-Angiotensin-Aldosterone System (RAAS)**. **Why Option D is Correct:** The sympathetic surge causes widespread peripheral vasoconstriction via **α1-adrenergic receptors**. Blood is shunted away from "non-essential" vascular beds, such as the skin and skeletal muscle, to prioritize the cerebral and coronary circulation. This **decreased cutaneous blood flow** manifests clinically as cold, clammy skin—a hallmark sign of shock. **Why the other options are incorrect:** * **A. Increased renal blood flow:** In shock, renal blood flow **decreases** due to vasoconstriction of the afferent arterioles (mediated by sympathetic nerves and Angiotensin II). This leads to oliguria, a key clinical indicator of the severity of shock. * **B. Decrease in cortisol:** Stress triggers the hypothalamic-pituitary-adrenal (HPA) axis, leading to an **increase** in cortisol levels to help maintain vascular tone and metabolic homeostasis. * **C. Decrease in vasopressin:** Low blood pressure and reduced atrial stretch trigger a significant **increase** in Vasopressin (ADH) release from the posterior pituitary to promote water retention and peripheral vasoconstriction (via V1 receptors). **NEET-PG High-Yield Pearls:** * **The "Golden Rule" of Shock:** Vital organs (Brain/Heart) have poor α1-receptor density and exhibit **autoregulation**, allowing them to maintain flow while the skin and kidneys suffer ischemia. * **Baroreceptor Reflex:** This is the earliest compensatory mechanism, leading to tachycardia and increased systemic vascular resistance (SVR). * **Reverse Stress Relaxation:** A delayed compensatory mechanism where blood vessels gradually constrict around the remaining volume.

Q: Which of the following are molecular motors?

All of the above. **Explanation:** Molecular motors are specialized proteins that convert chemical energy (ATP) into mechanical work to facilitate intracellular transport and movement. They move along cytoskeletal tracks—either microtubules or actin filaments. * **Kinesin:** These are microtubule-based motors that typically move toward the **plus-end** of the microtubule (anterograde transport). They are essential for moving vesicles and organelles from the cell body toward the periphery (e.g., axonal transport). * **Dynein:** These are also microtubule-based motors but move toward the **minus-end** (retrograde transport). Cytoplasmic dyneins carry cargo toward the cell center, while axonemal dyneins are responsible for the beating of cilia and flagella. * **Myosin:** These are **actin-based** motors. Myosin II is the primary motor involved in muscle contraction, while other isoforms (like Myosin V) are involved in short-range vesicle transport along actin filaments. Since all three proteins function as biological motors using ATP hydrolysis, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Axonemal Dynein**, leading to immobile cilia, bronchiectasis, and situs inversus. * **Directionality:** Remember **"K"**inesin moves **"K"**ick-away (Anterograde/Plus-end) and **"D"**ynein moves **"D"**rag-in (Retrograde/Minus-end). * **Fast Axonal Transport:** Occurs at a rate of 400 mm/day and utilizes both Kinesin and Dynein. * **Viral Transport:** Certain viruses like Herpes and Rabies exploit **Dynein** for retrograde transport to reach the CNS.

Q: Which of the following is true regarding a system which has oscillatory responses?

Positive feedback system. ### Explanation **Correct Answer: D. Positive Feedback System** In physiology, **positive feedback systems** are characterized by a response that reinforces or amplifies the initial stimulus. This creates a "vicious cycle" or an **oscillatory response** where the output continues to increase or fluctuate until a specific climax or external intervention occurs. Unlike negative feedback, which seeks stability (homeostasis), positive feedback drives the system away from the steady state, leading to instability or rapid physiological changes. **Why other options are incorrect:** * **A & B (Lesser/Greater Gain):** "Gain" refers to the efficiency of a control system (calculated as Correction/Error). While negative feedback systems have a high gain to maintain stability, the term "gain" is primarily used to describe the effectiveness of homeostatic regulation. Oscillatory behavior is a functional characteristic of the feedback loop's direction, not merely the magnitude of its gain. * **C (Proportional Component):** This is a term used in control systems (PID controllers) where the output is proportional to the current error. While it helps in stabilizing negative feedback, it does not inherently define the oscillatory nature of positive feedback loops. **High-Yield Clinical Pearls for NEET-PG:** * **Physiological Positive Feedback:** Most feedback in the body is negative. Key exceptions (Positive) include: * **LH Surge:** Leading to ovulation. * **Oxytocin:** During childbirth (Ferguson reflex). * **Blood Clotting Cascade:** Activation of one factor leads to the activation of many. * **Nerve Action Potential:** Opening of Na+ channels causes more Na+ channels to open (Hodgkin cycle). * **Negative Feedback:** The most common regulatory mechanism (e.g., Thyroxine regulation, BP control via baroreceptors). It is characterized by **stability** rather than oscillation.

Q: Transcellular fluid is seen in which location?

Body cavities lined by epithelium. **Explanation:** **Understanding Transcellular Fluid** Transcellular fluid is a specialized sub-compartment of the **Extracellular Fluid (ECF)**. It is defined as the fluid contained within body cavities that are lined by **epithelial cells**. Unlike interstitial fluid, which is formed by simple filtration, transcellular fluid is often produced by the active secretory activities of these epithelial cells, resulting in a composition that differs significantly from plasma. * **Why Option A is Correct:** Body cavities such as the pleural, peritoneal, and pericardial spaces, as well as the synovial joints and the cerebrospinal fluid (CSF) space, are all lined by specialized epithelium (e.g., mesothelium or ependyma). The fluid within these spaces is the classic definition of transcellular fluid. * **Why Option B is Incorrect:** Intracellular fluid (ICF) refers to the fluid contained *within* the cell membrane. Transcellular fluid is located *outside* the cells, making it a component of the ECF. * **Why Option C is Incorrect:** Intercellular (interstitial) fluid is the fluid that bathes the cells in the tissues. While it is part of the ECF, it is not separated by an epithelial barrier and is distinct from the transcellular compartment. **High-Yield Facts for NEET-PG:** 1. **Volume:** Transcellular fluid accounts for approximately **1–2 liters** (about 1–3% of total body water). 2. **Examples:** CSF, intraocular fluid (aqueous humor), synovial fluid, digestive secretions, and fluid in the serous cavities (pleural, peritoneal). 3. **Clinical Significance:** In pathological states, transcellular fluid can increase significantly, leading to conditions like **ascites, pleural effusion, or joint effusion** (often referred to as "third-spacing").

Q: Which of the following is best for measuring total body water?

Tritium oxide. **Explanation:** The measurement of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = \frac{Amount\ of\ substance}{Concentration}$). To measure a specific compartment, the indicator must distribute evenly within that compartment and not cross into others. **Why Tritium Oxide is Correct:** Total Body Water (TBW) includes both intracellular and extracellular fluids. To measure TBW, an indicator must be able to cross all cell membranes and distribute uniformly throughout all fluid compartments. **Tritium oxide ($H^3_2O$)** and **Deuterium oxide ($D_2O$)** are isotopes of water; they behave exactly like physiological water, making them the gold standard for TBW measurement. **Antipyrine** is another non-isotopic substance used for this purpose as it is highly lipid-soluble. **Analysis of Incorrect Options:** * **A. Evan’s Blue (T-1824):** This dye binds strongly to plasma albumin. Since it stays within the vascular system, it is used to measure **Plasma Volume**. * **B. $I^{131}$ (Radio-iodinated Serum Albumin):** Similar to Evan's blue, it remains in the intravascular space and is used to measure **Plasma Volume**. * **D. $P^{32}$ (Radioactive Phosphorus):** This is used to label Red Blood Cells (RBCs) to measure **Total Blood Volume**. **High-Yield Clinical Pearls for NEET-PG:** * **Extracellular Fluid (ECF) Volume:** Measured using Inulin (Gold Standard), Mannitol, or Sucrose. * **Intracellular Fluid (ICF) Volume:** Cannot be measured directly. It is calculated as: $ICF = TBW - ECF$. * **Interstitial Fluid Volume:** Cannot be measured directly. It is calculated as: $ECF - Plasma\ Volume$. * **Rule of Thumb:** TBW is approximately 60% of body weight in males and 50% in females.

Question 1

Calmodulin acts by:

Accepted Answer

Phosphorylation of myosin by protein kinase

Answer

Opening of Na+ channels

Answer

Activation of the Na+-K+ pump

Answer

Dephosphorylation by protein kinase

Question 2

Which of the following is a compensatory mechanism in a patient with hypovolemic shock?

Accepted Answer

Decreased cutaneous blood flow

Answer

Increased renal blood flow

Answer

Decrease in cortisol

Answer

Decrease in vasopressin

Question 3

Which of the following are molecular motors?

Accepted Answer

All of the above

Answer

Kinesin

Answer

Dynein

Answer

Myosin

Question 4

What do tyrosine kinase receptors mediate?

Accepted Answer

They mediate cellular processes involved in growth and differentiation.

Answer

They have constitutively active tyrosine kinase domains.

Answer

They phosphorylate and activate ras directly.

Answer

They are not phosphorylated upon activation.

Question 5

What type of cell junction is found in smooth muscle?

Accepted Answer

Gap junction

Answer

Zonula adherens

Answer

Macula adherens

Answer

Tight junction

Question 6

Which of the following is true regarding a system which has oscillatory responses?

Accepted Answer

Positive feedback system

Answer

Has a lesser gain

Answer

Has a greater gain

Answer

Proportional component

Question 7

What is the staircase effect in muscle contraction?

Accepted Answer

After a long period of rest, the initial strength of contraction is less than subsequent contractions.

Answer

After a long period of rest, the initial strength of contraction is greater than subsequent contractions.

Answer

After a long period of work, the strength of contraction is less than subsequent contractions.

Answer

After a long period of work, the strength of contraction is greater than subsequent contractions.

Question 8

Which hormone acts on nuclear receptors?

Accepted Answer

Thyroxine

Answer

Calcitonin

Answer

Glucocorticoids

Answer

Vasopressin

Question 9

Transcellular fluid is seen in which location?

Accepted Answer

Body cavities lined by epithelium

Answer

Intracellularly

Answer

Intercellularly

Answer

None of the above

Question 10

Which of the following is best for measuring total body water?

Accepted Answer

Tritium oxide

Answer

Evan's blue

Answer

I131

Answer

P 32

General Physiology — MCQs

General Physiology — MCQs

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