General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 181: Which of the following can act as both an excitatory and inhibitory neurotransmitter?

A. Acetylcholine
B. Glutamate
C. Norepinephrine
D. Dopamine (Correct Answer)

Explanation: **Explanation:** The action of a neurotransmitter is not determined solely by its chemical structure, but by the **type of receptor** it binds to on the postsynaptic membrane. **Dopamine** is the classic example of a neurotransmitter that exhibits dual activity. Its effect depends on the G-protein coupled receptor subtype it activates: * **Excitatory:** Binding to **D1 and D5** receptors increases intracellular cAMP (via Gs proteins), leading to excitation. * **Inhibitory:** Binding to **D2, D3, and D4** receptors decreases cAMP (via Gi proteins), leading to inhibition. **Analysis of Incorrect Options:** * **Glutamate:** The primary **excitatory** neurotransmitter in the CNS. It acts via ionotropic (AMPA, NMDA) and metabotropic receptors to depolarize neurons. * **Acetylcholine:** While it can be inhibitory in specific peripheral tissues (e.g., M2 receptors in the heart), in the context of General Physiology and CNS neurotransmission, it is predominantly classified as **excitatory**, especially at the neuromuscular junction (Nicotinic) and in the autonomic ganglia. * **Norepinephrine:** Generally functions as an **excitatory** neurotransmitter in the brain and sympathetic nervous system, though it can have complex modulatory effects. **High-Yield Clinical Pearls for NEET-PG:** * **GABA & Glycine:** The major **inhibitory** neurotransmitters in the CNS (GABA in the brain, Glycine in the spinal cord). * **Parkinson’s Disease:** Results from a loss of dopaminergic neurons in the Substantia Nigra pars compacta, disrupting the balance between excitatory and inhibitory pathways in the basal ganglia. * **Schizophrenia:** Associated with overactivity of dopamine in the mesolimbic pathway (D2 receptors).

Question 182: Transfer of solutes between cells occurs via which of the following structures?

A. Desmosomes
B. Gap Junctions (Correct Answer)
C. Fascia adherens
D. Zonula occludens

Explanation: **Explanation:** The correct answer is **Gap Junctions**. **1. Why Gap Junctions are correct:** Gap junctions (communicating junctions) are specialized intercellular connections composed of clusters of channels called **connexons**. Each connexon is formed by six protein subunits called **connexins**. These channels create a direct physical bridge between the cytoplasm of adjacent cells, allowing the passage of ions, small water-soluble molecules, and secondary messengers (like cAMP or $Ca^{2+}$). This facilitates **electrical and metabolic coupling**, which is essential for coordinated activities like the rhythmic contraction of cardiac muscle and smooth muscle. **2. Why other options are incorrect:** * **Desmosomes (Macula Adherens):** These are "spot welds" that provide mechanical strength by anchoring intermediate filaments (keratin) of adjacent cells. They prevent cells from pulling apart but do not allow solute transfer. * **Fascia Adherens:** Found in the intercalated discs of cardiac muscle, these anchor actin filaments and help transmit contractile forces between cells. * **Zonula Occludens (Tight Junctions):** These are "sealing junctions" located at the apical end of epithelial cells. Their primary function is to prevent the paracellular movement of solutes and maintain cell polarity, acting as a barrier rather than a channel. **High-Yield Clinical Pearls for NEET-PG:** * **Connexin 26:** Mutations are a leading cause of congenital non-syndromic deafness. * **Connexin 32:** Mutations are associated with Charcot-Marie-Tooth disease (X-linked). * **Cardiac Synctium:** Gap junctions are the structural basis for the functional syncytium of the heart, allowing rapid spread of action potentials.

Question 183: In which stage of erythropoiesis does hemoglobin first appear?

A. Basophil erythroblast (Correct Answer)
B. Proerythroblast
C. Orthochromatic erythroblast
D. Polychromatophil erythroblast

Explanation: **Explanation:** The correct answer is **Basophil erythroblast** (also known as the Early Normoblast). **1. Why Basophil Erythroblast is correct:** During erythropoiesis, the synthesis of hemoglobin begins early in the lineage. While the genetic machinery for hemoglobin production is activated in the proerythroblast stage, the **first visible appearance** of hemoglobin (at the microscopic/biochemical level) occurs in the **Basophil erythroblast**. Although the cell appears intensely blue (basophilic) due to an abundance of RNA and ribosomes, small amounts of hemoglobin are being actively synthesized within the cytoplasm. **2. Analysis of Incorrect Options:** * **Proerythroblast:** This is the first committed precursor. It contains no hemoglobin yet; it is characterized by a large nucleus and nucleoli, focusing on cellular division and organelle synthesis. * **Polychromatophil erythroblast:** In this stage, hemoglobin levels increase significantly. The "polychromatic" appearance (pinkish-grey) is due to the mixture of pink hemoglobin and blue ribosomal RNA. It is the stage where hemoglobin becomes **prominent**, but not where it first appears. * **Orthochromatic erythroblast:** Also known as the Late Normoblast. Here, the cytoplasm is increasingly eosinophilic (pink) because it is saturated with hemoglobin, and the nucleus becomes pyknotic before extrusion. **3. NEET-PG High-Yield Pearls:** * **First appearance of Hb:** Basophil erythroblast. * **Hemoglobin becomes visible/prominent:** Polychromatophil erythroblast. * **Nucleus is extruded at:** Orthochromatic erythroblast stage (to form a Reticulocyte). * **Reticulocyte:** Contains remnants of the Golgi apparatus and mitochondria (detected by supra-vital stains like New Methylene Blue). * **Total duration of Erythropoiesis:** Approximately 7 days (5 days to reticulocyte + 2 days to mature RBC).

Question 184: Which of the following statements is true regarding total heat production during muscle contraction?

A. Activation heat is produced when a muscle is contracting. (Correct Answer)
B. Initial heat is due to basal metabolic processes.
C. Relaxation heat is present in both isotonic and isometric contractions.
D. Recovery heat of muscle is greater than the initial heat.

Explanation: ### Explanation The total heat produced during muscle contraction is categorized into **Initial Heat** (produced during the contraction process) and **Recovery Heat** (produced after contraction). **1. Why Option A is Correct:** **Activation heat** is a component of initial heat. It is the heat generated as the muscle transitions from a resting state to an active state. It specifically represents the energy expended during the release of calcium from the sarcoplasmic reticulum and its subsequent binding to Troponin C, initiating the cross-bridge cycle. Therefore, it occurs as the muscle is contracting. **2. Analysis of Incorrect Options:** * **Option B:** Initial heat is independent of oxygen (anaerobic) and is caused by the actual process of contraction (activation, shortening, and maintenance). **Basal heat** is the heat produced at rest due to basal metabolic processes, not initial heat. * **Option C:** **Relaxation heat** is unique to **isotonic contractions**. In isotonic contraction, work is done to lift a load; when the muscle relaxes, the potential energy stored in that load is converted into heat. In isometric contractions, no external work is done, so relaxation heat is absent. * **Option D:** This is a common misconception. **Recovery heat** (produced during the oxidative restoration of ATP and creatine phosphate) is approximately **equal** to the total initial heat. In some physiological contexts, it may be slightly less or equal, but it is not characteristically "greater" in a way that defines the thermodynamic profile of a single twitch. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Fenn Effect:** The total energy expenditure (heat + work) is greater when a muscle performs work (shortens) than when it does not. * **Efficiency:** Muscle efficiency is roughly 20-25%; the remaining 75-80% of energy is dissipated as heat. * **Sequence of Heat:** Activation Heat $\rightarrow$ Shortening Heat $\rightarrow$ Relaxation Heat (Isotonic only) $\rightarrow$ Recovery Heat.

Question 185: Transport across the nucleus is mediated by all of the following except?

A. Caveolins
B. Local signals
C. Importins
D. Rat proteins (Correct Answer)

Explanation: **Explanation:** Nucleocytoplasmic transport is a highly regulated process occurring through **Nuclear Pore Complexes (NPCs)**. The question asks for the exception among mediators of nuclear transport. **Why "Rat proteins" is the correct answer:** There is no such entity as "Rat proteins" involved in nuclear transport. This is a distractor designed to mimic **Ran proteins** (Ras-related nuclear proteins). Ran is a GTPase essential for providing the energy and directionality required for nuclear import and export. **Analysis of other options:** * **Importins (and Exportins):** These are transport receptors (karyopherins) that recognize specific cargo. Importins bind to proteins in the cytoplasm to carry them into the nucleus, while exportins facilitate movement out of the nucleus. * **Local signals:** Proteins destined for the nucleus contain specific amino acid sequences known as **Nuclear Localization Signals (NLS)**. Conversely, those exiting the nucleus contain **Nuclear Export Signals (NES)**. These "local signals" are essential for recognition by karyopherins. * **Caveolins:** While primarily known for forming "caveolae" in the plasma membrane (clathrin-independent endocytosis), recent research indicates that caveolins (specifically Caveolin-1) can translocate to the nuclear envelope and modulate signal transduction, making them recognized mediators in the broader context of nuclear-related transport. **High-Yield Clinical Pearls for NEET-PG:** * **Ran-GTP Gradient:** High concentration of Ran-GTP is found in the **nucleus**, while Ran-GDP is high in the **cytoplasm**. This gradient drives the direction of transport. * **Nuclear Pore Complex (NPC):** A massive structure composed of ~30 different proteins called **nucleoporins**. * **Size Limit:** Molecules smaller than **40–60 kDa** can typically diffuse passively through the NPC, while larger proteins require active transport via the Ran-GTPase cycle.

Question 186: cAMP acts by:

A. Increasing Na+ and K+ secretion
B. Increasing K+ secretion
C. Increasing Cl- secretion (Correct Answer)
D. Increasing Na+ secretion

Explanation: **Explanation:** The correct answer is **C. Increasing Cl- secretion.** Cyclic adenosine monophosphate (cAMP) is a vital second messenger that regulates ion transport across epithelial membranes. In the intestinal and respiratory mucosa, an increase in intracellular cAMP activates **Protein Kinase A (PKA)**. PKA subsequently phosphorylates the **Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)**, which is a cAMP-gated chloride channel located on the apical membrane. This phosphorylation opens the channel, leading to a massive efflux of Cl- ions into the lumen. Sodium (Na+) and water follow passively due to the electrochemical gradient and osmotic pull, resulting in secretory activity. **Analysis of Incorrect Options:** * **Options A, B, and D:** While Na+ and K+ movements often accompany Cl- secretion to maintain electroneutrality and osmotic balance, cAMP does not primarily or directly act by increasing their secretion. Na+ secretion is typically a passive process following the active transport of anions like Cl-. **Clinical Pearls for NEET-PG:** * **Cholera Toxin Mechanism:** *Vibrio cholerae* toxin causes permanent activation of Gs proteins, leading to constitutively high cAMP levels. This results in the overactivation of CFTR channels, causing the characteristic "rice-water" diarrhea due to massive Cl- and water loss. * **Cystic Fibrosis:** This condition is caused by a mutation in the CFTR gene. The lack of functional cAMP-responsive Cl- channels leads to thick, dehydrated secretions in the lungs and pancreas. * **Other cAMP-mediated effects:** Beyond ion transport, cAMP is involved in bronchodilation (via β2 receptors) and increased heart rate/contractility (via β1 receptors).

Question 187: Bilirubin is bound inside the hepatocyte to which of the following?

A. Albumin
B. Ubiquinone
C. Ligandin (Correct Answer)
D. Globulin

Explanation: **Explanation:** The transport of bilirubin from the blood into the bile involves three distinct phases: uptake, conjugation, and excretion. **Why Ligandin is Correct:** Once unconjugated bilirubin (UCB) is taken up by the hepatocyte via facilitated diffusion (OATP transporters), it must be prevented from diffusing back into the plasma. Inside the hepatocyte, bilirubin binds to **Ligandin** (also known as **Y-protein** or Glutathione S-transferase B). This binding serves two purposes: it "traps" the bilirubin within the cell and transports it to the smooth endoplasmic reticulum for conjugation with glucuronic acid. **Analysis of Incorrect Options:** * **A. Albumin:** While albumin is the primary carrier for unconjugated bilirubin in the **systemic circulation** (blood), it does not enter the hepatocyte. Bilirubin dissociates from albumin at the sinusoidal membrane before entering the cell. * **B. Ubiquinone:** Also known as Coenzyme Q10, this is a component of the electron transport chain in the mitochondria and has no role in bilirubin transport. * **C. Globulin:** These are plasma proteins (like Alpha, Beta, and Gamma globulins) involved in immunity and transport of other substances (e.g., steroid hormones), but they are not the specific intracellular binders for bilirubin. **High-Yield Clinical Pearls for NEET-PG:** * **Z-protein:** Another intracellular binding protein, though it has a lower affinity for bilirubin than Ligandin. * **Crigler-Najjar & Gilbert Syndrome:** These involve defects in the *conjugation* enzyme (UGT1A1), not the binding proteins. * **Rate-limiting step:** The excretion of conjugated bilirubin into the bile canaliculi (via MRP2) is the rate-limiting step of bilirubin metabolism, which is defective in **Dubin-Johnson Syndrome**.

Question 188: In skeletal muscle, actin is bound to the Z line by which of the following?

A. Titin
B. Desmin
C. Actinin (Correct Answer)
D. Tropomyosin

Explanation: ### Explanation The correct answer is **Actinin (specifically α-actinin)**. **1. Why Actinin is Correct:** The sarcomere is the functional unit of skeletal muscle, bounded by two **Z lines** (or Z disks). The thin filaments, primarily composed of **actin**, must be anchored firmly to the Z line to transmit the force of contraction. **α-actinin** is a rod-shaped protein that cross-links actin filaments and binds them directly to the Z line, ensuring structural stability during the power stroke. **2. Why Other Options are Incorrect:** * **Titin:** This is the largest known protein. It acts as a molecular spring that anchors **myosin** (thick filaments) to the Z line, providing passive elasticity to the muscle. It does not bind actin to the Z line. * **Desmin:** This is an intermediate filament that links adjacent myofibrils to each other and to the sarcolemma at the level of the Z line. It maintains the **lateral alignment** of sarcomeres but is not the primary anchor for actin. * **Tropomyosin:** This is a regulatory protein that wraps around the actin filament. In a resting state, it covers the myosin-binding sites on actin to prevent contraction. It is part of the thin filament complex but not an anchoring protein for the Z line. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dystrophin:** A crucial protein that links the actin cytoskeleton to the extracellular matrix via the glycoprotein complex. Mutations lead to **Duchenne Muscular Dystrophy**. * **Nebulin:** Acts as a "molecular ruler" that regulates the length of the actin (thin) filaments during assembly. * **M-line:** The central part of the sarcomere where thick filaments (myosin) are anchored by the protein **myomesin**. * **H-zone:** The region of the sarcomere containing *only* thick filaments (disappears during maximal contraction).

Question 189: In which organs can the whole range of cyclooxygenase (COX) products be synthesized?

A. Spleen and Liver
B. Lung and Liver
C. Spleen and Lung (Correct Answer)
D. Spleen and Blood vessel

Explanation: **Explanation:** The synthesis of eicosanoids (prostaglandins, thromboxanes, and prostacyclins) via the **Cyclooxygenase (COX) pathway** is tissue-specific. While most cells possess the COX enzyme, they usually lack the downstream isomerases required to produce all types of prostanoids. For instance, platelets primarily produce Thromboxane $A_2$ ($TXA_2$), while endothelial cells primarily produce Prostacyclin ($PGI_2$). **Why Spleen and Lung are correct:** The **Spleen and Lungs** are unique because they possess a comprehensive array of terminal enzymes (isomerases and reductases). Consequently, they are the only organs capable of synthesizing the **entire spectrum** of COX products, including $PGE_2$, $PGF_{2\alpha}$, $PGD_2$, $PGI_2$, and $TXA_2$. This is a high-yield physiological fact often tested in competitive exams. **Analysis of Incorrect Options:** * **Liver:** While metabolically active, the liver primarily focuses on the degradation and conjugation of eicosanoids rather than the synthesis of the full range. * **Blood Vessels:** Vascular endothelium is specialized. It predominantly produces **Prostacyclin ($PGI_2$)**, which acts as a potent vasodilator and inhibitor of platelet aggregation. It does not synthesize the full range of COX products. **NEET-PG High-Yield Pearls:** * **Rate-limiting step:** The release of Arachidonic acid from membrane phospholipids by **Phospholipase $A_2$**. * **COX-1 vs. COX-2:** COX-1 is constitutive (housekeeping), while COX-2 is inducible (inflammatory). * **Aspirin:** Irreversibly inhibits COX by acetylation, leading to a prolonged anti-platelet effect because platelets cannot synthesize new enzymes.

Question 190: A 37-year-old male patient diagnosed with sarcoidosis of the lung was treated with glucocorticoids. Glucocorticoids may result in bone loss primarily by which of the following mechanisms?

A. Increasing calcium absorption from the gastrointestinal tract
B. Increasing osteoblast growth
C. Inhibiting bone formation (Correct Answer)
D. Inhibiting bone resorption

Explanation: **Explanation:** Glucocorticoid-induced osteoporosis (GIO) is the most common cause of secondary osteoporosis. The primary mechanism by which glucocorticoids cause bone loss is the **inhibition of bone formation**. **Why the correct answer is right:** Glucocorticoids exert a direct inhibitory effect on **osteoblasts** (bone-forming cells). They decrease osteoblast proliferation, reduce their differentiation from mesenchymal stem cells, and increase the apoptosis of both osteoblasts and osteocytes. This leads to a significant reduction in the bone formation rate. While they also transiently increase bone resorption, the sustained suppression of bone formation is the hallmark of glucocorticoid action on bone. **Why the incorrect options are wrong:** * **Option A:** Glucocorticoids actually **decrease** calcium absorption from the GI tract by antagonizing Vitamin D action. This leads to secondary hyperparathyroidism, further contributing to bone loss. * **Option B:** Glucocorticoids **inhibit** (rather than increase) osteoblast growth and activity. * **Option D:** Glucocorticoids **stimulate** bone resorption initially by increasing the expression of RANK ligand (RANKL) and decreasing Osteoprotegerin (OPG), which activates osteoclasts. **NEET-PG High-Yield Pearls:** * **Biphasic Pattern:** GIO has a rapid early phase (increased resorption) followed by a slower, chronic phase (decreased formation). * **Site Predilection:** Loss is most rapid in **trabecular bone** (e.g., vertebrae and ribs) compared to cortical bone. * **Clinical Management:** Bisphosphonates are the first-line treatment for prevention and therapy of GIO. * **Key Lab Finding:** Unlike primary osteoporosis, GIO often presents with low levels of biochemical markers of bone formation (e.g., serum osteocalcin).

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