Neurophysiology Practice Questions

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

Intermediate normoblast. **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).

Q: Which of the following is NOT a peptide neurotransmitter?

Serotonin. **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).

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

Orexins. 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)**.

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

Cerebral Cortex. **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).

Q: What is the unit of a stretch reflex?

Reflex arc. **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.

Q: What is the commonest inhibitory neurotransmitter in the spinal cord?

Glycine. **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."

Question 1

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

Accepted Answer

Intermediate normoblast

Answer

Proerythroblast

Answer

Early normoblast

Answer

Late normoblast

Question 2

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?

Accepted Answer

Release of pre-formed, mature WBCs into the circulation

Answer

Increased production of WBCs by the bone marrow

Answer

Decreased destruction of WBCs

Answer

Increased production of selectins

Question 3

Which of the following is NOT a peptide neurotransmitter?

Accepted Answer

Serotonin

Answer

Enkephalin

Answer

Substance P

Answer

Endorphin

Question 4

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

Accepted Answer

Orexins

Answer

Glutamate

Answer

GABA

Answer

Dopamine

Question 5

Learned movements are executed through which part of the brain?

Accepted Answer

Cerebral Cortex

Answer

Midbrain

Answer

Pons

Answer

Medulla

Question 6

What is the unit of a stretch reflex?

Accepted Answer

Reflex arc

Answer

Muscle spindle

Answer

Alpha-motor neuron

Answer

Neuron

Question 7

What is the best index of platelet function?

Accepted Answer

Clot retraction time

Answer

Bleeding time

Answer

Clotting time

Answer

Prothrombin time

Question 8

Which substance does not cross the blood-brain barrier?

Accepted Answer

Insulin

Answer

Urea

Answer

Testosterone

Answer

Glucose

Question 9

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

Accepted Answer

Nodes of Ranvier

Answer

Cell body

Answer

Axon terminal

Answer

Surface of myelin

Question 10

What is the commonest inhibitory neurotransmitter in the spinal cord?

Accepted Answer

Glycine

Answer

GABA

Answer

Substance P

Answer

Aspartate

Neurophysiology — MCQs

Neurophysiology — MCQs

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