Neurophysiology — MCQs

Neurophysiology Indian Medical PG Practice Questions and MCQs

Question 581: Projections from the substantia nigra pars compacta to the striatum release which neurotransmitter?

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

Explanation: **Explanation:** The correct answer is **Dopamine**. The **Nigrostriatal pathway** is one of the major dopaminergic pathways in the brain. It consists of neurons that originate in the **Substantia Nigra pars compacta (SNpc)** and project their axons to the **Striatum** (comprising the Caudate nucleus and Putamen). These neurons release dopamine, which plays a critical role in the modulation of the Basal Ganglia circuitry. Dopamine exerts a dual effect: it excites the Direct Pathway (via D1 receptors) and inhibits the Indirect Pathway (via D2 receptors), both of which facilitate motor movement. **Analysis of Incorrect Options:** * **GABA:** This is the primary inhibitory neurotransmitter of the Basal Ganglia. It is released by the projections *from* the Striatum to the Globus Pallidus and Substantia Nigra pars reticulata (SNpr), but not by the SNpc. * **Glutamate:** This is an excitatory neurotransmitter. In the Basal Ganglia, it is released by the **Subthalamic Nucleus** and the cortical projections to the striatum. * **Aspartate:** While an excitatory neurotransmitter in the CNS, it is not the primary transmitter involved in the nigrostriatal projection. **Clinical Pearls for NEET-PG:** * **Parkinson’s Disease:** Caused by the progressive degeneration of dopaminergic neurons in the SNpc, leading to a depletion of dopamine in the striatum. * **Histology:** SNpc neurons contain **neuromelanin**, which gives the structure its characteristic dark (nigra) appearance. * **MPTP:** A neurotoxin that selectively destroys these dopaminergic neurons, inducing permanent Parkinsonian symptoms.

Question 582: Which of the following synaptic transmitters is NOT a peptide, polypeptide, or protein?

A. Substance P
B. Met-enkephalin
C. b-Endorphin
D. Serotonin (Correct Answer)

Explanation: ### Explanation Neurotransmitters are broadly classified into two categories based on their molecular size and synthesis: **Small-molecule, rapidly acting transmitters** and **Neuropeptides** (larger, slow-acting). #### Why Serotonin is the Correct Answer **Serotonin (5-Hydroxytryptamine)** belongs to the **biogenic amines** category of small-molecule transmitters. It is synthesized in the cytosol of the presynaptic terminal from the amino acid **tryptophan** via a two-step enzymatic process. Unlike peptides, it is not synthesized on ribosomes and does not consist of a chain of amino acids linked by peptide bonds. #### Analysis of Incorrect Options * **Substance P (Option A):** An 11-amino acid polypeptide. It is a key neuropeptide involved in pain transmission (nociception) in the spinal cord. * **Met-enkephalin (Option B):** A pentapeptide (5 amino acids). It is an endogenous opioid peptide involved in the "gate control" mechanism of pain inhibition. * **β-Endorphin (Option C):** A larger polypeptide (31 amino acids) derived from the precursor molecule **Pro-opiomelanocortin (POMC)**. It acts as a potent natural analgesic. #### NEET-PG High-Yield Pearls 1. **Synthesis Site:** Small-molecule transmitters (like Serotonin, Dopamine, GABA) are synthesized in the **axon terminal**, whereas neuropeptides are synthesized in the **cell body (ribosomes)** and transported via fast axonal transport. 2. **Vesicle Recycling:** Vesicles containing small-molecule transmitters are recycled; however, neuropeptide vesicles are used only once and are not recycled. 3. **Serotonin Clinical Link:** Low levels are associated with depression; hence, **SSRIs** (Selective Serotonin Reuptake Inhibitors) are the first-line treatment. 4. **Rate-limiting step:** For Serotonin, it is the enzyme **Tryptophan hydroxylase**.

Question 583: Which of the following sleep stages is characterized by the presence of delta waves?

A. Stage A
B. Stage B
C. Stage C (Correct Answer)
D. Stage D

Explanation: **Explanation:** The question refers to the classification of sleep stages based on EEG patterns. In the classic Loomis classification (and often referenced in physiological texts like Ganong), sleep is divided into stages A through E. **Correct Answer: Stage C** Stage C is known as **"Deep Sleep"** or the beginning of slow-wave sleep. It is characterized by the appearance of **Delta waves** (high amplitude, low frequency: 0.5–4 Hz) and the presence of **Sleep Spindles**. As sleep progresses into Stages D and E, delta waves become the predominant feature of the EEG. **Analysis of Incorrect Options:** * **Stage A:** This represents the **relaxed wakeful state** with eyes closed, dominated by **Alpha rhythms** (8–13 Hz), primarily in the parieto-occipital region. * **Stage B:** This corresponds to **Drowsiness** (Stage N1). The alpha rhythm disappears and is replaced by low-voltage, asynchronous **Theta waves** (4–7 Hz). * **Stage D:** While Stage D also contains delta waves, it is characterized by "Deep Sleep" where delta waves are more extensive than in Stage C. However, in many standardized physiological assessments, **Stage C** is the specific milestone where delta activity is first significantly noted alongside spindles. **NEET-PG High-Yield Pearls:** 1. **Bruxism** (teeth grinding) typically occurs in Stage N2. 2. **Somnambulism** (sleepwalking), **Sleep Terrors**, and **Enuresis** (bedwetting) occur during **Stage N3** (Slow Wave Sleep/Delta Sleep). 3. **Ponto-Geniculo-Occipital (PGO) spikes** are the earliest signs of REM sleep. 4. **Sawtooth waves** are a characteristic EEG finding of REM sleep. 5. The neurotransmitter **Adenosine** promotes sleep, while **Orexin (Hypocretin)** maintains wakefulness. Deficiency of Orexin leads to Narcolepsy.

Question 584: Which of the following statements is true regarding the autonomic nervous system?

A. The primary center for autonomic integration is the medulla oblongata.
B. Conduction in autonomic nerve fibers is identical to that in somatic motor fibers.
C. Preganglionic parasympathetic fibers are generally longer than preganglionic sympathetic fibers. (Correct Answer)
D. The ratio of preganglionic to postganglionic fibers in the autonomic system is approximately 20:1.

Explanation: **Explanation:** **1. Why Option C is correct:** The anatomical arrangement of the autonomic nervous system (ANS) differs significantly between the two divisions. In the **parasympathetic system**, the ganglia are located very close to or within the walls of the effector organs (terminal ganglia). Consequently, the **preganglionic fibers are long**, while the postganglionic fibers are short. In contrast, the **sympathetic system** has ganglia located near the spinal cord (paravertebral/prevertebral chains), resulting in **short preganglionic** and long postganglionic fibers. **2. Why the other options are incorrect:** * **Option A:** While the medulla contains vital reflex centers (cardiovascular/respiratory), the **Hypothalamus** is the "head office" or the primary higher center for autonomic integration, coordinating endocrine, emotional, and visceral responses. * **Option B:** Somatic motor fibers are heavily myelinated (Type Aα) and conduct impulses rapidly. Autonomic fibers are slower; preganglionic fibers are lightly myelinated (Type B), and postganglionic fibers are unmyelinated (Type C). * **Option D:** The ratio of preganglionic to postganglionic fibers is generally **1: many** (divergence), especially in the sympathetic system (approx. 1:20), to allow for mass discharge. The option incorrectly suggests more preganglionic than postganglionic fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Outflow:** Sympathetic is **Thoracolumbar** (T1–L2/L3); Parasympathetic is **Craniosacral** (CN III, VII, IX, X and S2–S4). * **Neurotransmitters:** All preganglionic neurons (both systems) release **Acetylcholine (ACh)** acting on Nicotinic receptors. * **Exception:** Sympathetic postganglionic fibers to **sweat glands** are cholinergic (ACh), not adrenergic. * **Adrenal Medulla:** Considered a modified sympathetic ganglion; it receives direct preganglionic sympathetic innervation.

Question 585: What is the normal concentration of glucose in cerebrospinal fluid (CSF) relative to plasma glucose?

A. One-half of plasma glucose
B. Two-thirds of plasma glucose (Correct Answer)
C. One-third of plasma glucose
D. Equal to plasma glucose

Explanation: ### Explanation **1. Understanding the Correct Answer (Option B):** The concentration of glucose in the Cerebrospinal Fluid (CSF) is physiologically maintained at approximately **60% to 70% (roughly two-thirds)** of the simultaneous plasma glucose level. In a healthy adult, if the plasma glucose is 100 mg/dL, the CSF glucose should be around 60–70 mg/dL. This gradient is maintained by **facilitated diffusion** via **GLUT-1 transporters** located in the blood-brain barrier (choroid plexus). Because it is a carrier-mediated process, it is saturable and always results in a lower concentration than plasma. **2. Analysis of Incorrect Options:** * **Option A (One-half):** This is too low. A CSF/Plasma glucose ratio of 0.5 or less is considered pathological and is a hallmark of bacterial meningitis. * **Option C (One-third):** This represents severe **hypoglycorrhachia** (low CSF glucose), typically seen in advanced bacterial or tubercular meningitis. * **Option D (Equal):** Glucose does not cross the blood-brain barrier by simple diffusion; the metabolic consumption of glucose by the brain and the nature of facilitated transport ensure that CSF levels never equilibrate with plasma. **3. Clinical Pearls for NEET-PG:** * **Hypoglycorrhachia:** A decrease in CSF glucose is a critical diagnostic marker. It occurs in **Bacterial, Tubercular, and Fungal meningitis** (due to inhibited transport and consumption by bacteria/WBCs). * **Viral Meningitis:** CSF glucose is typically **normal** in viral infections (Aseptic Meningitis). * **Equilibration Time:** It takes about 30–60 minutes for changes in plasma glucose to be reflected in the CSF. Therefore, blood glucose should ideally be measured 1 hour before a lumbar puncture for accurate comparison. * **Normal Range:** 45–80 mg/dL (absolute value).

Question 586: A 70-year-old male patient with intentional tremor, drunken gait, and past pointing has a lesion in which of the following structures?

A. Basal ganglia
B. Pyramidal tract
C. Cerebellum (Correct Answer)
D. Thalamus

Explanation: ### Explanation The clinical triad of **intentional tremor**, **drunken gait (ataxia)**, and **past pointing (dysmetria)** is pathognomonic for a **cerebellar lesion**. **1. Why Cerebellum is Correct:** The cerebellum is responsible for the coordination of voluntary movements, maintenance of posture, and equilibrium. * **Intentional Tremor:** Unlike resting tremors, these occur during active, goal-directed movement, indicating a failure of the cerebellum to "smooth out" motor signals. * **Drunken Gait:** Reflects cerebellar ataxia, where the patient lacks coordination and balance (vermis involvement). * **Past Pointing (Dysmetria):** The inability to judge distance, leading to overshooting or undershooting a target, caused by a breakdown in the timing of agonist/antagonist muscle activation. **2. Why Other Options are Incorrect:** * **Basal Ganglia:** Lesions here (e.g., Parkinson’s) typically present with **resting tremors**, rigidity, and bradykinesia, rather than intentional tremors. * **Pyramidal Tract:** Damage to the corticospinal tract results in **upper motor neuron (UMN) signs** such as spasticity, hyperreflexia, and a positive Babinski sign, not incoordination. * **Thalamus:** Primarily a sensory relay station. Thalamic lesions usually present with sensory loss or "Thalamic Pain Syndrome" (Dejerine-Roussy syndrome). **3. High-Yield Clinical Pearls for NEET-PG:** * **DANISH Mnemonic** for Cerebellar signs: **D**ysmetria, **A**taxia, **N**ystagmus, **I**ntention tremor, **S**lurred speech (Scanning dysarthria), **H**ypotonia. * **Ipsilateral Presentation:** Cerebellar lesions affect the **same side** of the body (unlike cerebral lesions) because of the double decussation of fibers. * **Archicerebellum:** Primarily involved in balance (Vestibulocerebellum). * **Neocerebellum:** Primarily involved in planning and coordination of fine movements.

Question 587: EEG cabins should be completely shielded by a continuous sheet of wire mesh of copper to avoid picking up noise from external electromagnetic disturbances. What is this type of shielding called?

A. Maxwell cage
B. Edison's cage
C. Faraday cage (Correct Answer)
D. Ohm's cage

Explanation: **Explanation:** **Why Faraday Cage is Correct:** The recording of an Electroencephalogram (EEG) involves measuring extremely low-amplitude electrical potentials (microvolts) from the scalp. These signals are highly susceptible to interference from external electromagnetic radiation, such as power lines (60 Hz/50 Hz interference), radio waves, and electronic equipment. A **Faraday cage** is an enclosure made of a continuous sheet or mesh of conductive material (like copper). According to the principles of electrostatics, the external electrical charges redistribute themselves on the exterior of the conductor such that they cancel the electric field's effect within the cage's interior. This "shielding" ensures that the EEG electrodes pick up only the physiological signals from the brain, preventing "noise" or artifacts. **Analysis of Incorrect Options:** * **Maxwell cage:** James Clerk Maxwell formulated the classical theory of electromagnetic radiation, but there is no specific "Maxwell cage" used for shielding. * **Edison’s cage:** Thomas Edison was a pioneer in electricity and light bulbs, but he did not develop this shielding method. * **Ohm’s cage:** Georg Simon Ohm is known for Ohm’s Law ($V=IR$); the term is fictitious in the context of electromagnetic shielding. **Clinical Pearls for NEET-PG:** * **Artifacts:** The most common external artifact in EEG is the **60 Hz (or 50 Hz) power line interference**, which a Faraday cage helps minimize. * **Material:** Copper is preferred due to its high conductivity. * **Other Applications:** Faraday cages are also essential in **MRI rooms** to prevent external RF (radiofrequency) pulses from distorting the image. * **EEG Basics:** Remember that EEG primarily records **post-synaptic potentials** (not action potentials) from the pyramidal cells of the cerebral cortex.

Question 588: Renshaw cells are inhibitory interneurons found in which location?

A. Gray matter of the brain
B. White matter of the brain
C. Gray matter of the spinal cord (Correct Answer)
D. White matter of the spinal cord

Explanation: **Explanation:** **Correct Option: C. Gray matter of the spinal cord** Renshaw cells are specialized **inhibitory interneurons** located in the **ventral (anterior) horn** of the spinal cord gray matter. They are part of a negative feedback mechanism known as **recurrent inhibition**. When an alpha motor neuron fires, it sends a collateral branch to a Renshaw cell. The Renshaw cell, in turn, releases the inhibitory neurotransmitter **glycine**, which acts back on the same motor neuron (and neighboring ones) to inhibit their firing. This mechanism prevents excessive muscular contraction and sharpens the focus of motor activity by limiting "lateral" excitation. **Why other options are incorrect:** * **Options A & B (Brain):** While the brain contains various inhibitory interneurons (like basket or stellate cells), Renshaw cells are anatomically and functionally specific to the spinal cord's motor control circuits. * **Option D (White matter of the spinal cord):** White matter consists of myelinated axonal tracts (ascending and descending). Neuronal cell bodies, including interneurons like Renshaw cells, are exclusively located within the **gray matter**. **High-Yield NEET-PG Pearls:** * **Neurotransmitter:** The primary neurotransmitter of Renshaw cells is **Glycine**. * **Clinical Correlation (Strychnine Poisoning):** Strychnine acts as a competitive antagonist of glycine at the postsynaptic receptors. By inhibiting Renshaw cell activity, it leads to unchecked motor neuron firing, resulting in severe muscle spasms and convulsions. * **Tetanus Toxin:** This toxin prevents the *release* of glycine from Renshaw cells, leading to the characteristic spastic paralysis (e.g., lockjaw). * **Function:** They provide "recurrent inhibition" to stabilize the firing rate of motor neurons.

Question 589: Which neurotransmitter is synthesized from choline and acetyl-CoA?

A. Acetylcholine (Correct Answer)
B. Glutamine
C. Aspartate
D. Nitric Oxide

Explanation: ### Explanation **Correct Answer: A. Acetylcholine** **Mechanism of Synthesis:** Acetylcholine (ACh) is the primary neurotransmitter of the parasympathetic nervous system, neuromuscular junctions, and preganglionic autonomic neurons. It is synthesized in the nerve terminal from two precursors: **Choline** (derived from dietary sources and recycled from the synaptic cleft) and **Acetyl-CoA** (produced in mitochondria during glucose metabolism). The reaction is catalyzed by the enzyme **Choline Acetyltransferase (ChAT)**. Once synthesized, ACh is stored in vesicles by the vesicular acetylcholine transporter (VAChT). **Analysis of Incorrect Options:** * **B. Glutamine:** This is a non-essential amino acid that serves as a precursor for **Glutamate** (the brain's primary excitatory neurotransmitter) via the enzyme glutaminase. * **C. Aspartate:** This is an excitatory amino acid neurotransmitter synthesized from **Oxaloacetate** (a Krebs cycle intermediate) via transamination. * **D. Nitric Oxide (NO):** This is a gaseous neurotransmitter synthesized from the amino acid **L-arginine** by the enzyme Nitric Oxide Synthase (NOS). It is not stored in vesicles but is produced on demand. **High-Yield Clinical Pearls for NEET-PG:** * **Termination of Action:** Unlike most neurotransmitters that undergo reuptake, ACh is primarily inactivated by enzymatic degradation via **Acetylcholinesterase (AChE)** in the synaptic cleft. * **Rate-limiting Step:** The uptake of **Choline** into the presynaptic nerve terminal is the rate-limiting step in ACh synthesis. This step can be inhibited by the drug **Hemicholinium**. * **Vesicular Release:** **Botulinum toxin** prevents the release of ACh by cleaving SNARE proteins, leading to flaccid paralysis. * **Clinical Correlation:** Myasthenia Gravis involves autoantibodies against post-synaptic nicotinic ACh receptors, treated with AChE inhibitors (e.g., Neostigmine).

Question 590: Which part of a nerve cell is considered the generator area?

A. Cell body
B. Initial segment (Correct Answer)
C. Axon
D. Dendrites

Explanation: ### Explanation The **initial segment** of the axon (the region between the axon hillock and the first myelin sheath) is known as the **generator area** or the **spike trigger zone**. **1. Why the Initial Segment is Correct:** The initial segment has the **lowest threshold** for firing an action potential. This is due to an exceptionally high density of **voltage-gated sodium (Na+) channels** (approximately 100 to 1000 times higher than in the cell body or dendrites). When graded potentials (EPSPs and IPSPs) from the dendrites and soma summate, they reach this zone; because of the high channel density, even a small depolarization is sufficient to open enough Na+ channels to trigger a self-propagating action potential. **2. Why the Other Options are Incorrect:** * **Cell Body (Soma):** While it integrates signals, it has a relatively low density of voltage-gated Na+ channels, making its threshold for firing much higher than the initial segment. * **Axon:** The rest of the axon is responsible for the **propagation** (conduction) of the action potential, not its primary generation. * **Dendrites:** These are the primary sites for **receiving** inputs. They generate local, graded potentials (electronic conduction) but generally lack the voltage-gated channel density to initiate a full action potential. **High-Yield NEET-PG Pearls:** * **Axon Hillock vs. Initial Segment:** While often used interchangeably, the *initial segment* is the actual site of action potential initiation, whereas the *axon hillock* is the anatomical funneling region of the soma. * **Threshold Value:** The threshold at the initial segment is roughly **-35 to -45 mV**, compared to the -10 to -15 mV required at the soma. * **Nodes of Ranvier:** In myelinated axons, these are the only sites where Na+ channels are concentrated after the initial segment, allowing for **saltatory conduction**.

Practice by Chapter

Neurons and Glial Cells

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Synaptic Transmission

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Sensory Processing

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Motor Control Systems

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Autonomic Nervous System

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Hypothalamus and Limbic System

Practice Questions

Cerebral Cortex Functions

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Electroencephalography

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Neuroplasticity

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Sleep and Wakefulness

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