Neurophysiology — MCQs

Neurophysiology Indian Medical PG Practice Questions and MCQs

Question 431: During erythropoiesis, by which stage does hemoglobin begin to appear?

A. Proerythroblast
B. Early normoblast
C. Intermediate normoblast (Correct Answer)
D. Late normoblast

Explanation: **Explanation:** The correct answer is **C. Intermediate normoblast (Polychromatic erythroblast).** In the process of erythropoiesis, the **Intermediate normoblast** is the stage characterized by the first visible appearance of hemoglobin. At this stage, the cell’s cytoplasm exhibits "polychromasia" (a mix of pink and blue) because the increasing concentration of acidophilic hemoglobin begins to mask the basophilic RNA and ribosomes. **Analysis of Options:** * **A. Proerythroblast:** This is the first identifiable precursor. It contains no hemoglobin; the cytoplasm is deeply basophilic due to high RNA content. * **B. Early normoblast (Basophilic erythroblast):** While active synthesis of hemoglobin components begins here, the concentration is too low to be detected histologically. The cytoplasm remains intensely basophilic. * **D. Late normoblast (Orthochromatic erythroblast):** Hemoglobinization is nearly complete here. The cytoplasm is predominantly eosinophilic (pink), and the nucleus becomes pyknotic before being extruded. **High-Yield NEET-PG Pearls:** * **First appearance of Hemoglobin:** Intermediate normoblast. * **Last stage to divide (Mitosis):** Intermediate normoblast. * **Stage of Nucleus Extrusion:** Late normoblast (resulting in a Reticulocyte). * **Reticulocyte:** Contains "remnants of Golgi apparatus and RNA" (identified 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 432: A 24-year-old man presents with a broken leg. An initial blood test reveals a WBC count of 22 x 10^3/ml. Five hours later, a repeat blood test shows a WBC count of 7 x 10^3/ml. What is the most likely cause of the elevated WBC count in the initial test?

A. Increased production of WBCs by the bone marrow
B. Release of pre-formed, mature WBCs into the circulation (Correct Answer)
C. Decreased destruction of WBCs
D. Increased production of selectins

Explanation: ### Explanation The correct answer is **B. Release of pre-formed, mature WBCs into the circulation.** **1. Why Option B is Correct:** The rapid fluctuation in WBC count (from 22,000 to 7,000 in just five hours) is characteristic of **physiologic leukocytosis** triggered by acute stress or trauma (the broken leg). This occurs via two mechanisms: * **Demargination:** A significant portion of the body's neutrophils are "marginated" (adhered to the walls of blood vessels, especially in the lungs and spleen). Stress-induced catecholamine release (epinephrine) decreases the stickiness of these cells, causing them to move into the circulating pool. * **Release from Bone Marrow Storage Pool:** The bone marrow maintains a large reserve of mature, pre-formed leukocytes. Acute stress triggers their immediate release into the bloodstream. Because de novo production of WBCs takes several days, such a rapid spike and subsequent normalization can only be explained by the redistribution of existing cells. **2. Why Other Options are Incorrect:** * **Option A:** Increased production (leukopoiesis) takes days to occur. It cannot account for a massive spike within minutes or hours of an injury. * **Option C:** Decreased destruction is a slow process and does not result in a sudden, three-fold increase in cell count. * **Option D:** Selectins are adhesion molecules that facilitate the *rolling* of WBCs on endothelium. Increased selectin activity would actually lead to *more* margination (sequestration), which would **decrease** the circulating WBC count. **3. High-Yield Clinical Pearls for NEET-PG:** * **Pseudoleukocytosis:** Also known as "shift leukocytosis," this is a transient increase in WBCs without an actual increase in total body WBC mass. * **Glucocorticoid Effect:** Steroids also cause leukocytosis, but they do so by **inhibiting neutrophil egress** from the blood into tissues and stimulating release from the marrow. * **Timeframe:** Always look at the timeline. Changes occurring within minutes to hours are usually due to **redistribution**, whereas changes over days indicate **altered production**.

Question 433: Which of the following is NOT a peptide neurotransmitter?

A. Enkephalin
B. Substance P
C. Endorphin
D. Serotonin (Correct Answer)

Explanation: **Explanation:** The classification of neurotransmitters is a high-yield topic for NEET-PG. Neurotransmitters are broadly divided into **Small-molecule, rapidly acting transmitters** (e.g., Acetylcholine, Amines, Amino acids) and **Neuropeptides, slowly acting transmitters** (e.g., Opioids, Tachykinins). **Why Serotonin is the Correct Answer:** **Serotonin (5-Hydroxytryptamine)** is a **biogenic amine** (monoamine), not a peptide. It is synthesized from the amino acid **Tryptophan** in the cytosol of nerve terminals and stored in vesicles. Unlike neuropeptides, small-molecule transmitters like serotonin are typically recycled via reuptake mechanisms and produce rapid acute responses in the nervous system. **Analysis of Incorrect Options:** * **Enkephalin & Endorphin:** These are **Opioid Peptides**. They are synthesized as large precursor proteins (like Pro-enkephalin and POMC) in the cell body (ribosomes) and transported down the axon. They primarily modulate pain (nociception). * **Substance P:** This is a **Tachykinin neuropeptide**. It consists of 11 amino acids and is the primary neurotransmitter involved in transmitting pain signals from peripheral receptors to the dorsal horn of the spinal cord. **NEET-PG High-Yield Pearls:** * **Synthesis Site:** Small-molecule transmitters (Serotonin, GABA, Glycine) are synthesized in the **cytosol of axon terminals**, whereas Neuropeptides are synthesized in the **cell body (soma)**. * **Potency:** Neuropeptides are generally **1,000 times more potent** than small-molecule transmitters but have a slower, more prolonged action. * **Serotonin Metabolism:** The rate-limiting enzyme is *Tryptophan hydroxylase*, and its primary metabolite found in urine is **5-HIAA** (useful in diagnosing Carcinoid syndrome).

Question 434: Which of the following is NOT a low molecular weight neurotransmitter?

A. Glutamate
B. GABA
C. Dopamine
D. Orexins (Correct Answer)

Explanation: Neurotransmitters are broadly classified into two categories based on their molecular size and synthesis site: **Small-molecule, rapidly acting transmitters** and **Large-molecule, slowly acting neuropeptides**. ### 1. Why Orexins is the Correct Answer **Orexins (Hypocretins)** are **neuropeptides**. Unlike small-molecule transmitters synthesized in the cytosol of the axon terminal, neuropeptides are large molecules (chains of amino acids) synthesized as pre-prohormones in the neuronal cell body (ribosomes) and transported via axonal transport to the terminals. Orexins are specifically produced in the lateral hypothalamus and play a critical role in regulating wakefulness and appetite. ### 2. Why the Other Options are Incorrect * **Glutamate (Option A):** This is an amino acid and the primary excitatory neurotransmitter in the CNS. It is a classic small-molecule transmitter. * **GABA (Option B):** Gamma-aminobutyric acid is an amino acid derivative and the primary inhibitory neurotransmitter in the brain. It belongs to the small-molecule group. * **Dopamine (Option C):** This is a biogenic amine (specifically a catecholamine) derived from the amino acid tyrosine. All catecholamines are classified as low molecular weight, rapidly acting transmitters. ### 3. NEET-PG High-Yield Pearls * **Small-Molecule Transmitters:** Include Acetylcholine, Amino acids (Glutamate, GABA, Glycine), Biogenic amines (Dopamine, NE, Epinephrine, Serotonin, Histamine), and Purines (ATP). * **Neuropeptides:** Include Orexins, Substance P, Enkephalins, Endorphins, and hypothalamic-releasing hormones. * **Clinical Correlation:** A deficiency of **Orexin-A** in the cerebrospinal fluid is the hallmark of **Narcolepsy Type 1** (characterized by excessive daytime sleepiness and cataplexy). * **Synthesis Site:** Small molecules are recycled/synthesized in the **axon terminal**, whereas neuropeptides are synthesized in the **cell body (Soma)**.

Question 435: Learned movements are executed through which part of the brain?

A. Midbrain
B. Pons
C. Medulla
D. Cerebral Cortex (Correct Answer)

Explanation: **Explanation:** The execution of **learned, skilled, and voluntary movements** is a primary function of the **Cerebral Cortex**, specifically the Primary Motor Cortex (Brodmann area 4), Premotor Cortex (Area 6), and Supplementary Motor Area. These regions are responsible for planning, initiating, and refining complex motor sequences acquired through practice (e.g., writing, playing an instrument, or typing). The corticospinal (pyramidal) tract originates here to provide fine motor control to distal muscles. **Why other options are incorrect:** * **Midbrain:** Primarily involved in auditory and visual reflexes (superior and inferior colliculi) and contains the Substantia Nigra, which modulates the basal ganglia circuit, but it does not initiate learned movements. * **Pons:** Acts as a relay station between the cerebrum and cerebellum. It contains nuclei for cranial nerves (V-VIII) and respiratory centers (pneumotaxic/apneustic) but lacks the higher-order processing required for learned motor tasks. * **Medulla:** Responsible for vital autonomic functions such as cardiac, respiratory, and vasomotor control, as well as reflex activities like swallowing and vomiting. **High-Yield Facts for NEET-PG:** * **Motor Program Storage:** While the cerebral cortex executes the movement, the **Basal Ganglia** and **Cerebellum** are crucial for the "storage" and "coordination" of these motor programs. * **Ideomotor Apraxia:** Damage to the dominant parietal cortex or premotor area results in the inability to perform learned purposeful movements despite having normal muscle strength. * **Pyramidal vs. Extrapyramidal:** The Cerebral Cortex is the seat of the pyramidal system (voluntary), while the brainstem nuclei contribute to the extrapyramidal system (posture and muscle tone).

Question 436: What is the unit of a stretch reflex?

A. Muscle spindle
B. Alpha-motor neuron
C. Neuron
D. Reflex arc (Correct Answer)

Explanation: **Explanation:** The **reflex arc** is the fundamental functional unit of any reflex, including the stretch reflex (myotatic reflex). It represents the complete neural pathway required to produce an involuntary response to a stimulus. For a stretch reflex, this arc is **monosynaptic** and consists of five essential components: 1. **Receptor:** Muscle spindle (detects change in length). 2. **Afferent Nerve:** Type Ia sensory fibers. 3. **Integration Center:** The spinal cord synapse. 4. **Efferent Nerve:** Alpha (α) motor neuron. 5. **Effector:** Extrafusal muscle fibers (contraction). **Analysis of Options:** * **A. Muscle Spindle:** This is only the **sensory receptor** (the "input" device) of the reflex, not the entire functional unit. * **B. Alpha-motor neuron:** This is the **efferent limb** (the "output" device). While it is the "final common pathway" for motor control, it cannot function as a reflex unit in isolation. * **C. Neuron:** A single neuron is a structural unit of the nervous system, but a reflex requires a circuit involving at least two neurons (sensory and motor). **High-Yield Clinical Pearls for NEET-PG:** * **Monosynaptic Nature:** The stretch reflex is the *only* monosynaptic reflex in the human body (e.g., Knee jerk). * **Reciprocal Inhibition:** While the stretch reflex is monosynaptic, the simultaneous relaxation of the antagonist muscle involves an inhibitory interneuron (**polysynaptic**). * **Gamma (γ) Motor Neurons:** These regulate the sensitivity of the muscle spindle by contracting the intrafusal fibers, maintaining "spindle sensitivity" even during muscle contraction. * **Clinical Correlation:** Hyperreflexia (exaggerated stretch reflex) is a hallmark of **Upper Motor Neuron (UMN)** lesions.

Question 437: What is the best index of platelet function?

A. Bleeding time
B. Clotting time
C. Clot retraction time (Correct Answer)
D. Prothrombin time

Explanation: ### Explanation **Correct Answer: C. Clot retraction time** **Why it is the correct answer:** Clot retraction is the process where a blood clot shrinks and expresses serum. This process is directly dependent on the **quantity and quality of platelets**. Specifically, it requires the contractile protein **thrombosthenin** (actin and myosin) found within platelets. When platelets bind to fibrin strands via their **GPIIb/IIIa receptors**, they contract, pulling the fibrin mesh together. Therefore, Clot Retraction Time (CRT) serves as the most specific index of the functional integrity and contractile capacity of platelets. **Why the other options are incorrect:** * **Bleeding Time (BT):** While BT assesses the formation of the temporary platelet plug (primary hemostasis), it is influenced by both platelet function and **vascular wall integrity**. It is a screening test rather than a specific index of internal platelet contractile function. * **Clotting Time (CT):** This measures the time required for blood to coagulate via the **intrinsic and common pathways**. It primarily reflects the adequacy of plasma coagulation factors (like Factor VIII, IX, and XI) rather than platelet function. * **Prothrombin Time (PT):** This assesses the **extrinsic and common pathways** (Factors VII, X, V, II, and I). It is used to monitor oral anticoagulant therapy (Warfarin) and liver function, not platelet function. **High-Yield Clinical Pearls for NEET-PG:** * **Glanzmann’s Thrombasthenia:** A condition characterized by deficient GPIIb/IIIa receptors, leading to **absent or defective clot retraction** despite a normal platelet count. * **Thrombosthenin:** The specific contractile protein in platelets responsible for clot retraction. * **Normal CRT:** Typically begins within 30–60 minutes and is complete within 24 hours. * **Platelet Count vs. Function:** A normal platelet count does not guarantee normal function; CRT is superior for assessing the latter.

Question 438: Which substance does not cross the blood-brain barrier?

A. Insulin (Correct Answer)
B. Urea
C. Testosterone
D. Glucose

Explanation: **Explanation** The Blood-Brain Barrier (BBB) is a highly selective semipermeable border formed by capillary endothelial cells (with tight junctions), basement membrane, and astrocyte foot processes. Its primary function is to protect the brain from toxins and fluctuations in plasma composition. **Why Insulin is the Correct Answer:** Insulin is a **large, water-soluble polypeptide hormone**. Due to its high molecular weight and hydrophilic nature, it cannot cross the BBB via simple diffusion. While insulin can reach the brain through specific **receptor-mediated transcytosis** (saturable transport) in certain areas, it is generally considered "impermeable" compared to the small molecules or lipid-soluble substances listed in the other options. For the purpose of standard physiological classification in exams, large proteins like insulin and albumin do not freely cross the BBB. **Analysis of Incorrect Options:** * **Urea:** This is a small, uncharged molecule. While it crosses the BBB slowly (due to low lipid solubility), it does cross, and its slow equilibration is clinically significant in "Dialysis Disequilibrium Syndrome." * **Testosterone:** As a **steroid hormone**, it is highly lipid-soluble. Lipid-soluble substances (including O2, CO2, and steroid hormones) cross the BBB rapidly and easily by dissolving in the endothelial lipid bilayer. * **Glucose:** Although polar, glucose is the brain's primary fuel. It crosses the BBB rapidly via **facilitated diffusion** using the **GLUT-1** transporter. **High-Yield NEET-PG Pearls:** * **Circumventricular Organs (CVOs):** These are specific areas where the BBB is absent (e.g., Area Postrema, Median Eminence, Posterior Pituitary). * **Transport Mechanism:** Glucose = GLUT-1 (Deficiency leads to De Vivo Syndrome); Amino acids = Leucine-preferring (L-system) carriers. * **Lipid Solubility:** The most important factor determining the rate of diffusion of a non-electrolyte across the BBB is its lipid-to-water partition coefficient.

Question 439: Maximum number of sodium channels per square micrometer is present in which location?

A. Cell body
B. Axon terminal
C. Surface of myelin
D. Nodes of Ranvier (Correct Answer)

Explanation: **Explanation:** The density of voltage-gated sodium ($Na^+$) channels is the primary determinant of the threshold for excitation and the velocity of action potential propagation. **Why Nodes of Ranvier is correct:** In myelinated neurons, the axonal membrane is exposed to the extracellular fluid only at the **Nodes of Ranvier**. To facilitate **saltatory conduction** (the "jumping" of the action potential from node to node), $Na^+$ channels are concentrated at these gaps at an extremely high density—approximately **2,000 to 12,000 per $\mu m^2$**. This high concentration ensures a rapid influx of sodium ions, sufficient to depolarize the membrane and trigger an action potential that can bypass the insulated internodal segments. **Why the other options are incorrect:** * **Cell body (Soma):** Contains a relatively low density of $Na^+$ channels (approx. 50–75 per $\mu m^2$). While it integrates signals, it is not the primary site for action potential generation. * **Axon terminal:** While $Na^+$ channels are present to ensure the impulse reaches the synapse, the density is significantly lower than at the nodes. The terminal is more characterized by a high density of voltage-gated **Calcium ($Ca^{2+}$)** channels for neurotransmitter release. * **Surface of myelin:** Myelin is an insulator composed of lipid layers (Schwann cells or oligodendrocytes). It contains virtually **no $Na^+$ channels**, as its function is to prevent ion leakage and decrease membrane capacitance. **High-Yield Clinical Pearls for NEET-PG:** * **Axon Hillock:** This is the site with the **lowest threshold** for firing an action potential due to a high density of $Na^+$ channels (though still lower than the Nodes of Ranvier). * **Demyelinating Diseases:** In conditions like **Multiple Sclerosis** (CNS) or **Guillain-Barré Syndrome** (PNS), the loss of myelin exposes internodal membrane which lacks sufficient $Na^+$ channels, leading to conduction block or slowing. * **Internode Length:** Saltatory conduction is faster because depolarization "skips" the internodal segments, which can be up to 1 mm long.

Question 440: What is the commonest inhibitory neurotransmitter in the spinal cord?

A. GABA
B. Glycine (Correct Answer)
C. Substance P
D. Aspartate

Explanation: **Explanation:** In the central nervous system, inhibitory neurotransmission is primarily mediated by two amino acids: **Glycine** and **GABA**. Their distribution, however, follows a distinct anatomical pattern: 1. **Glycine (Correct Answer):** This is the **predominant inhibitory neurotransmitter in the spinal cord** and the brainstem. It acts by opening chloride channels (ionotropic receptors), leading to hyperpolarization of the postsynaptic membrane. It is essential for regulating motor neuron activity and mediating reciprocal inhibition in spinal reflexes (e.g., Renshaw cell activity). 2. **GABA (Option A):** While GABA is the most common inhibitory neurotransmitter in the **brain** (specifically the supraspinal levels), it is secondary to Glycine in the spinal cord. 3. **Substance P (Option B):** This is an **excitatory** neuropeptide. It is primarily involved in the transmission of pain signals (nociception) from the periphery to the dorsal horn of the spinal cord. 4. **Aspartate (Option D):** Along with Glutamate, Aspartate is a major **excitatory** neurotransmitter in the CNS. **High-Yield Clinical Pearls for NEET-PG:** * **Renshaw Cells:** These are inhibitory interneurons in the spinal cord that utilize **Glycine** to provide feedback inhibition to alpha motor neurons. * **Strychnine Poisoning:** Strychnine acts as a competitive antagonist of Glycine receptors. By blocking glycine-mediated inhibition in the spinal cord, it leads to unchecked excitatory activity, resulting in severe convulsions and "opisthotonus" (archback) posture. * **Tetanus Toxin:** This toxin prevents the release of Glycine (and GABA) from presynaptic terminals, leading to spastic paralysis and "lockjaw."

Practice by Chapter

Neurons and Glial Cells

Practice Questions

Synaptic Transmission

Practice Questions

Sensory Processing

Practice Questions

Motor Control Systems

Practice Questions

Autonomic Nervous System

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Hypothalamus and Limbic System

Practice Questions

Cerebral Cortex Functions

Practice Questions

Electroencephalography

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Neuroplasticity

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Sleep and Wakefulness

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