Neurophysiology — MCQs

Neurophysiology Indian Medical PG Practice Questions and MCQs

Question 511: The cell bodies of sympathetic preganglionic neurons are located in which part of the central nervous system?

A. Dorsal root ganglia
B. Sympathetic chain ganglia
C. Brain stem and spinal cord
D. Thoracic and Lumbar segments of the spinal cord (Correct Answer)

Explanation: **Explanation:** The sympathetic nervous system is characterized as the **thoracolumbar outflow** of the autonomic nervous system (ANS). The cell bodies of all sympathetic preganglionic neurons are located in the **Intermediolateral (IML) gray column** (lateral horn) of the spinal cord, specifically extending from segments **T1 to L2 (or L3)**. These neurons send their axons through the ventral roots to synapse with postganglionic neurons in the sympathetic ganglia. **Analysis of Options:** * **A. Dorsal root ganglia:** These contain the cell bodies of **pseudounipolar sensory neurons** (afferent), not motor or autonomic efferent neurons. * **B. Sympathetic chain ganglia:** These contain the cell bodies of **postganglionic** sympathetic neurons. Preganglionic fibers synapse here or pass through to reach collateral ganglia. * **C. Brain stem and spinal cord:** This describes the **Craniosacral outflow**, which is characteristic of the **Parasympathetic** nervous system (Cranial nerves III, VII, IX, X and sacral segments S2-S4). **High-Yield Facts for NEET-PG:** * **Neurotransmitter:** All preganglionic autonomic fibers (both sympathetic and parasympathetic) release **Acetylcholine (ACh)**, which acts on nicotinic receptors. * **Adrenal Medulla Exception:** The adrenal medulla is considered a modified sympathetic ganglion; it is innervated directly by sympathetic **preganglionic** fibers. * **White vs. Gray Rami:** Preganglionic fibers enter the sympathetic chain via **White rami communicantes** (myelinated), while postganglionic fibers exit via **Gray rami communicantes** (unmyelinated). * **Length:** In the sympathetic system, preganglionic fibers are generally **short**, and postganglionic fibers are **long**.

Question 512: What is the primary macrophage of the central nervous system?

A. Fibrous astrocyte
B. Microglia (Correct Answer)
C. Oligodendrocyte
D. Protoplasmic astrocyte

Explanation: **Explanation:** The correct answer is **Microglia**. **1. Why Microglia is correct:** Microglia are the resident immune cells and primary macrophages of the Central Nervous System (CNS). Unlike other glial cells which originate from the neuroectoderm, microglia are derived from **mesodermal yolk sac progenitors** that migrate into the brain during early embryonic development. They act as the first line of active immune defense, constantly scavenging the CNS for plaques, damaged neurons, and infectious agents. When they encounter a threat, they undergo "activation," changing shape and releasing inflammatory cytokines. **2. Why the other options are incorrect:** * **Astrocytes (Fibrous and Protoplasmic):** These are the most abundant glial cells. Their primary roles include maintaining the blood-brain barrier (BBB), regulating the extracellular ionic environment, and providing structural support. **Fibrous astrocytes** are found mainly in white matter, while **protoplasmic astrocytes** are found in gray matter. They are not phagocytic in a primary immunological sense. * **Oligodendrocytes:** These cells are responsible for the **myelination** of axons within the CNS (equivalent to Schwann cells in the PNS). One oligodendrocyte can myelinate multiple segments of several axons. **3. High-Yield Clinical Pearls for NEET-PG:** * **Origin:** Remember that Microglia are **Mesodermal** in origin, while all other macroglia (Astrocytes, Oligodendrocytes, Ependymal cells) are **Ectodermal**. * **HIV Pathology:** Microglia are the primary target of HIV in the brain; they fuse to form **multinucleated giant cells**, a hallmark of HIV-associated dementia. * **Glial Scars:** Astrocytes are responsible for **gliosis** (the CNS equivalent of scarring) following injury. * **Fried Egg Appearance:** On histology, Oligodendrocytes are often described as having a "fried egg" appearance (clear cytoplasm with a central nucleus).

Question 513: Hypotonia, tremor, and ataxia are seen in a lesion of which part of the brain?

A. Basal ganglia
B. Medullary pyramid
C. Cerebellum (Correct Answer)
D. Pons

Explanation: The cerebellum is the primary coordinator of motor activity, ensuring smooth, precise, and balanced movements. A lesion in the cerebellum disrupts this coordination, leading to the classic triad mentioned in the question. **1. Why Cerebellum is Correct:** * **Hypotonia:** The cerebellum maintains muscle tone via the gamma motor neuron system. Damage leads to "pendular" reflexes and decreased resistance to passive stretch. * **Tremor:** Specifically **Intention Tremor**. Unlike resting tremors, these occur during voluntary movement and worsen as the limb approaches its target. * **Ataxia:** This refers to the loss of full control of bodily movements. It manifests as a broad-based gait, dysmetria (past-pointing), and dysdiadochokinesia (inability to perform rapid alternating movements). **2. Why Other Options are Incorrect:** * **Basal Ganglia:** Lesions typically present with **Hypertonia** (rigidity) and **Resting Tremors** (e.g., Parkinson’s disease), rather than hypotonia and intention tremors. * **Medullary Pyramid:** This contains the corticospinal tracts. A lesion here results in **Upper Motor Neuron (UMN)** signs, characterized by spasticity (hypertonia) and paralysis, not ataxia. * **Pons:** While the pons contains pathways connecting to the cerebellum, a localized pontine lesion usually presents with cranial nerve palsies (VI, VII) and contralateral hemiplegia (Millard-Gubler Syndrome). **Clinical Pearls for NEET-PG:** * **VANIST Mnemonic** for Cerebellar signs: **V**ertigo, **A**taxia, **N**ystagmus (horizontal), **I**ntention tremor, **S**lurred speech (scanning speech), **T**remors/Hypotonia. * **Midline (Vermis) lesions** cause trunkal ataxia and gait instability. * **Lateral (Hemisphere) lesions** cause ipsilateral limb ataxia and dysmetria.

Question 514: Stimulation of which brain area can modulate the sensation of pain?

A. Superior olivary complex
B. Locus ceruleus
C. Periaqueductal gray (Correct Answer)
D. Amygdala

Explanation: **Explanation:** The **Periaqueductal Gray (PAG)**, located in the midbrain, is the primary control center for the **descending pain modulation system**. When stimulated, the PAG activates a descending pathway that inhibits pain transmission at the level of the spinal cord (the "Gate Control" mechanism). **Mechanism:** 1. Stimulation of the PAG activates the **Nucleus Raphe Magnus** (medulla). 2. These neurons release **Serotonin** onto the dorsal horn of the spinal cord. 3. This triggers the release of **Enkephalins** (endogenous opioids) from local interneurons. 4. Enkephalins cause pre- and post-synaptic inhibition of Type C and Type Aδ pain fibers, effectively "closing the gate" to pain signals. **Analysis of Incorrect Options:** * **A. Superior olivary complex:** Part of the **auditory pathway** in the pons; it is involved in sound localization. * **B. Locus ceruleus:** The primary site for **Norepinephrine** synthesis in the brain; while it has minor roles in stress and attention, it is not the primary modulator of pain compared to the PAG. * **D. Amygdala:** Part of the **limbic system**; it processes emotions (fear and aggression) rather than direct sensory modulation of pain. **High-Yield Clinical Pearls for NEET-PG:** * **Neurotransmitters:** The descending inhibitory pathway primarily uses **Serotonin** (from Raphe nuclei) and **Norepinephrine**. * **Opiate Receptors:** The PAG is densely packed with opioid receptors, which is why morphine and endogenous opioids are highly effective in this region. * **Surgical Application:** Deep brain stimulation (DBS) of the PAG is sometimes used for the management of chronic, intractable pain.

Question 515: What is the preganglionic neurotransmitter in the sympathetic division of the autonomic nervous system?

A. Noradrenergic
B. Adrenergic
C. Acetylcholine (Correct Answer)
D. Dopamine

Explanation: ### Explanation **Correct Answer: C. Acetylcholine** In the Autonomic Nervous System (ANS), the neurotransmitter released by **all preganglionic neurons**—both sympathetic and parasympathetic—is **Acetylcholine (ACh)**. These neurons are classified as cholinergic. ACh binds to **Nicotinic (Nn) receptors** located on the cell bodies of the postganglionic neurons within the autonomic ganglia. This triggers an excitatory postsynaptic potential, leading to the propagation of the impulse toward the effector organ. **Why the other options are incorrect:** * **Noradrenergic (A) & Adrenergic (B):** Norepinephrine (Noradrenaline) is the primary neurotransmitter released by **postganglionic** sympathetic neurons at the target organs (except for sweat glands). Adrenaline is primarily a hormone released by the adrenal medulla into the bloodstream. * **Dopamine (D):** While dopamine acts as a neurotransmitter in the CNS and specific peripheral sites (like renal vasodilation), it is not the standard neurotransmitter for preganglionic autonomic fibers. **High-Yield Clinical Pearls for NEET-PG:** 1. **The "Rule of All":** All preganglionic neurons (Sympathetic + Parasympathetic) and all parasympathetic postganglionic neurons use Acetylcholine. 2. **The Exception:** Sympathetic postganglionic neurons to **sweat glands** are **cholinergic** (release ACh), not noradrenergic. 3. **Adrenal Medulla:** It is considered a "modified sympathetic ganglion." The preganglionic fiber releases ACh directly onto the chromaffin cells, which then release Epinephrine (80%) and Norepinephrine (20%) into the circulation. 4. **Receptor Type:** Preganglionic transmission always involves **Nicotinic** receptors, whereas postganglionic parasympathetic transmission involves **Muscarinic** receptors.

Question 516: Which stage of sleep contributes the most to the total sleep time in an adult?

A. REM
B. NREM 1
C. NREM 2 (Correct Answer)
D. NREM 3

Explanation: **Explanation:** In a healthy adult, sleep is divided into two main phases: **Non-Rapid Eye Movement (NREM)** and **Rapid Eye Movement (REM)** sleep. These phases cycle throughout the night, typically lasting 90–120 minutes per cycle. **Why NREM 2 is the correct answer:** NREM Stage 2 (Light Sleep) is the longest stage of the sleep cycle. In a typical young adult, it accounts for approximately **45% to 55%** of the total sleep time. It is physiologically characterized by the presence of **Sleep Spindles** and **K-complexes** on EEG. **Analysis of Incorrect Options:** * **REM Sleep (Option A):** Occupies about **20–25%** of total sleep time. While it is crucial for memory consolidation and dreaming, it is significantly shorter than NREM 2. * **NREM 1 (Option B):** This is the transition phase from wakefulness to sleep. It is the shortest stage, accounting for only **5%** of total sleep time. * **NREM 3 (Option C):** Also known as Slow Wave Sleep (SWS) or Deep Sleep. It accounts for about **15–20%** of total sleep time. This stage is most prominent in the first third of the night and decreases with age. **NEET-PG High-Yield Pearls:** 1. **EEG Hallmarks:** * NREM 1: Theta waves. * NREM 2: Sleep spindles and K-complexes. * NREM 3: Delta waves (highest amplitude, lowest frequency). * REM: Beta-like activity (Sawtooth waves). 2. **Age-Related Changes:** As age increases, NREM 3 (Deep Sleep) and REM sleep decrease, while NREM 1 and 2 increase. 3. **Bruxism** (teeth grinding) typically occurs during NREM 2. 4. **Night Terrors and Somnambulism** (sleepwalking) occur during NREM 3.

Question 517: At what stage of consciousness is alpha rhythm typically observed in an EEG?

A. Awake, agitated, with eyes open and mind wandering
B. Awake, at rest, with eyes closed and mind wandering (Correct Answer)
C. Aroused with attention focused on a specific stimulus
D. Rapid eye movement (REM) sleep

Explanation: ### Explanation **1. Why Option B is Correct:** The **Alpha rhythm** (8–13 Hz) is the characteristic EEG pattern of an adult who is **awake but relaxed**, in a state of "quiet anticipation." The key physiological requirement for prominent alpha waves is the **closure of the eyes**. When a person closes their eyes and lets their mind wander, the visual cortex enters a synchronized resting state, producing these high-amplitude, rhythmic waves, primarily over the parietal and occipital lobes. **2. Analysis of Incorrect Options:** * **Options A & C:** When a person opens their eyes or focuses their attention on a specific mental task (like a math problem), the alpha rhythm is replaced by fast, low-voltage, irregular activity known as **Beta waves** (13–30 Hz). This phenomenon is called **Alpha Block** or **Desynchronization**. * **Option D:** **REM sleep** is characterized by "paradoxical" EEG activity. The waves are low-voltage and high-frequency, closely resembling the Beta waves of an alert, awake state, rather than the synchronized Alpha rhythm. **3. High-Yield NEET-PG Clinical Pearls:** * **Frequency Hierarchy:** Remember the mnemonic **"B-A-T-D"** (Beta > Alpha > Theta > Delta) for decreasing order of frequency. * **Delta Waves (<4 Hz):** Seen in Stage 3 NREM (Deep sleep) and are the highest amplitude waves. Their presence in an awake adult is always pathological (e.g., brain injury or coma). * **Theta Waves (4–7 Hz):** Normal in children and during light sleep (Stage 1 NREM) in adults. * **Vertex Sharp Waves & Sleep Spindles:** Characteristic of Stage 2 NREM sleep. * **Alpha Block:** This is a classic physiological example of **desynchronization**—moving from a synchronized (Alpha) to an asynchronous (Beta) state upon sensory stimulation.

Question 518: A lesion in the hippocampus primarily affects which of the following cognitive functions?

A. Memory consolidation from short-term to long-term memory (Correct Answer)
B. Retrieval of remote memories
C. Implicit memory formation
D. Working memory capacity

Explanation: ### Explanation **Correct Option: A. Memory consolidation from short-term to long-term memory** The hippocampus, located in the medial temporal lobe, is the critical structure for **declarative (explicit) memory consolidation**. It acts as a relay station that processes incoming sensory information from short-term memory and converts it into stable, long-term memory traces. This process is known as consolidation. A classic clinical example is the famous patient **H.M.**, who underwent bilateral medial temporal lobectomy and developed profound **anterograde amnesia** (inability to form new memories), while his older memories remained intact. **Analysis of Incorrect Options:** * **B. Retrieval of remote memories:** Once memories are consolidated, they are stored in various areas of the **cerebral cortex**. Therefore, a hippocampal lesion does not typically affect the retrieval of memories formed long before the injury. * **C. Implicit memory formation:** Implicit (non-declarative) memory, such as procedural skills (e.g., riding a bike) or classical conditioning, is mediated by the **basal ganglia, cerebellum, and amygdala**, not the hippocampus. * **D. Working memory capacity:** Working memory (holding information "in mind" for seconds) is primarily a function of the **prefrontal cortex**. Patients with hippocampal damage can often hold a conversation or remember a phone number for a few seconds, provided they are not distracted. **High-Yield NEET-PG Pearls:** * **Papez Circuit:** The hippocampus is a key component of this circuit, which is essential for emotional experience and memory. * **Kluver-Bucy Syndrome:** Results from bilateral destruction of the **amygdala** (often involving the temporal tips), characterized by hyperorality, hypersexuality, and visual agnosia. * **Wernicke-Korsakoff Syndrome:** Affects the **mammillary bodies** (part of the memory circuit) due to Thiamine (B1) deficiency, leading to confabulations and anterograde amnesia. * **Long-Term Potentiation (LTP):** The cellular mechanism of memory in the hippocampus, primarily involving **NMDA receptors**.

Question 519: Nerve depolarization is primarily due to which of the following ionic channel activities?

A. Opening of sodium channels (Correct Answer)
B. Opening of chloride channels
C. Opening of potassium channels
D. Opening of calcium channels

Explanation: **Explanation:** The generation of an action potential in a nerve fiber follows a specific sequence of ionic movements across the axonal membrane. **1. Why Option A is correct:** Depolarization is the phase where the negative resting membrane potential (RMP) becomes more positive. This is primarily triggered by the **opening of voltage-gated sodium (Na⁺) channels**. When these channels open, Na⁺ ions rush into the cell following their electrochemical gradient (influx). This rapid entry of positive charge reverses the membrane polarity, typically moving from -70mV toward +35mV. **2. Why the other options are incorrect:** * **Option B (Chloride channels):** Opening of Cl⁻ channels leads to an influx of negative ions, causing **hyperpolarization** (making the cell more negative), which inhibits action potential generation. * **Option C (Potassium channels):** The opening of voltage-gated K⁺ channels occurs during the **repolarization** phase. K⁺ ions exit the cell (efflux), restoring the negative resting potential. * **Option D (Calcium channels):** While Ca²⁺ influx is crucial for neurotransmitter release at the synapse and depolarization in cardiac pacemaker cells, it is not the primary driver for initial nerve fiber depolarization. **High-Yield Clinical Pearls for NEET-PG:** * **Tetrodotoxin (Pufferfish) & Saxitoxin:** Block voltage-gated Na⁺ channels, preventing depolarization and causing paralysis. * **Local Anesthetics (Lignocaine):** Act by blocking the inactivated state of voltage-gated Na⁺ channels, halting nerve conduction. * **The "All-or-None" Law:** Depolarization must reach a specific **threshold potential** (approx. -55mV) for the Na⁺ channels to open in a positive feedback loop. * **Hyperkalemia:** Initially increases excitability by bringing RMP closer to threshold, but persistent depolarization eventually inactivates Na⁺ channels, leading to muscle weakness.

Question 520: Which electrical event is characteristic of inhibitory synaptic interactions?

A. A neurotransmitter agent that selectively opens ligand-gated chloride channels is the basis for an inhibitory postsynaptic potential. (Correct Answer)
B. Because the Nernst potential for chloride is about -70 mV, chloride ions tend to move out of the cell along its electrochemical gradient.
C. A neurotransmitter that selectively opens potassium channels will allow potassium to move into the cell.
D. An increase in the extracellular sodium concentration usually leads directly to an inhibitory postsynaptic potential.

Explanation: ### Explanation **1. Why Option A is Correct:** Inhibitory Postsynaptic Potentials (IPSPs) function by moving the membrane potential away from the firing threshold (hyperpolarization). When inhibitory neurotransmitters (like GABA or Glycine) bind to their receptors, they open **ligand-gated chloride (Cl⁻) channels**. Since the equilibrium potential for chloride is typically more negative (approx. -70 to -90 mV) than the resting membrane potential (approx. -65 mV), Cl⁻ ions flow **into** the cell. This influx of negative charge makes the interior more negative, leading to hyperpolarization and inhibition of the neuron. **2. Why Other Options are Incorrect:** * **Option B:** The Nernst potential for chloride is indeed around -70 mV. However, because the resting membrane potential is slightly less negative (-65 mV), the electrochemical gradient favors the movement of chloride **into** the cell, not out. * **Option C:** Potassium (K⁺) has a much higher concentration inside the cell. Opening K⁺ channels allows K⁺ to move **out** of the cell (efflux) down its concentration gradient, which would cause hyperpolarization. * **Option D:** Sodium (Na⁺) influx is the primary mechanism for **Excitatory** Postsynaptic Potentials (EPSPs). An increase in extracellular sodium would increase the driving force for Na⁺ entry, leading to depolarization, not inhibition. **3. NEET-PG High-Yield Pearls:** * **Major Inhibitory Neuromitters:** **GABA** (Brain) and **Glycine** (Spinal Cord). * **GABA Receptors:** GABA-A is ionotropic (Cl⁻ channel), while GABA-B is metabotropic (K⁺ channel/G-protein linked). * **Strychnine:** A potent convulsant that acts by competitively inhibiting Glycine receptors in the spinal cord, leading to unchecked excitation. * **EPSP vs. IPSP:** EPSPs are caused by Na⁺ or Ca²⁺ influx; IPSPs are caused by Cl⁻ influx or K⁺ efflux.

Practice by Chapter

Neurons and Glial Cells

Practice Questions

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

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Cerebral Cortex Functions

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Electroencephalography

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Neuroplasticity

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Sleep and Wakefulness

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