Neurophysiology — MCQs

Neurophysiology Indian Medical PG Practice Questions and MCQs

Question 21: In a monkey, following temporal lobectomy, which of the following functions is NOT observed?

A. Sexual desire
B. Visual agnosia
C. Oral tendency
D. Fearfulness (Correct Answer)

Explanation: ### Explanation The question describes the classic presentation of **Klüver-Bucy Syndrome**, which results from bilateral destruction of the **amygdala** and surrounding **temporal lobes**. #### Why "Fearfulness" is the Correct Answer: In Klüver-Bucy Syndrome, there is a profound **loss of fear and aggression** (placidity). The amygdala is the primary center for processing emotional responses, particularly fear. Following a temporal lobectomy, the animal no longer perceives threats or reacts to previously frightening stimuli (e.g., snakes or aggressive humans). Therefore, "fearfulness" is **not** observed; instead, the subject becomes abnormally calm. #### Analysis of Incorrect Options: * **A. Sexual Desire (Hypersexuality):** This is a hallmark feature. Subjects show increased libido and inappropriate sexual behavior, often directed toward inanimate objects or other species. * **B. Visual Agnosia (Psychic Blindness):** The subject can see but cannot recognize objects or their significance. This is due to damage to the visual association areas in the temporal cortex. * **C. Oral Tendency (Hyperorality):** There is a compulsive urge to examine all objects by mouth (licking, biting, or chewing) to identify them, compensating for visual agnosia. #### High-Yield Clinical Pearls for NEET-PG: * **Anatomical Site:** Bilateral temporal lobes, specifically the **amygdala**. * **Triad of Symptoms:** Hyperorality, Hypersexuality, and Placidity (loss of fear). * **Human Causes:** Most commonly seen as a sequela of **Herpes Simplex Encephalitis (HSE)**, which has a predilection for the temporal lobes. * **Memory Link:** Temporal lobectomy often involves the hippocampus, leading to **anterograde amnesia**.

Question 22: Wallerian degeneration is seen in which part of the axon?

A. Distal to the site of injury (Correct Answer)
B. Proximal to the site of injury
C. Both proximal and distal to the site of injury
D. In the cell body (soma)

Explanation: **Explanation:** **Wallerian Degeneration** (also known as orthograde degeneration) refers to the process where the nerve fiber undergoes disintegration following a traumatic injury or transection. **Why Option A is correct:** The axon depends on the cell body (soma) for the synthesis of proteins and organelles, which are transported down the axon via axoplasmic flow. When an axon is severed, the segment **distal to the site of injury** is cut off from its metabolic source. Within 24–48 hours, the cytoskeleton and membrane of this distal segment begin to fragment, and the myelin sheath unravels, eventually being cleared by macrophages and Schwann cells. **Why the other options are incorrect:** * **Option B & C:** Changes occurring **proximal** to the injury are termed **Retrograde Degeneration**. This involves changes moving toward the cell body, usually limited to the first few nodes of Ranvier (traumatic degeneration). Wallerian degeneration is strictly an anterograde (forward-moving) process. * **Option D:** Changes in the **cell body** are specifically called **Chromatolysis**. This involves swelling of the soma, displacement of the nucleus to the periphery, and disappearance of Nissl granules as the cell shifts its metabolism toward protein synthesis for regeneration. **High-Yield NEET-PG Pearls:** * **Timeline:** Wallerian degeneration begins within 24 hours; myelin sheath breakdown is prominent by day 3–5. * **Regeneration:** In the PNS, Schwann cells form **Bands of Büngner**, which act as a scaffold to guide the regenerating proximal axon sprout toward the target organ. * **Rate of Growth:** Regenerating axons typically grow at a rate of **1–3 mm/day**. * **CNS vs. PNS:** Wallerian degeneration is much slower in the CNS because oligodendrocytes do not provide the same growth-promoting environment as Schwann cells and may actually inhibit regrowth via Nogo proteins.

Question 23: In cases of recent memory loss, which brain region is the most probable site of a lesion?

A. Thalamus
B. Temporal lobe (Correct Answer)
C. Frontal lobe
D. All of the above

Explanation: **Explanation:** The **Temporal Lobe**, specifically the medial aspect containing the **Hippocampus** and the **Entorhinal cortex**, is the primary site for the consolidation of short-term memory into long-term memory (recent memory). Lesions in this area, particularly bilateral damage, result in profound **anterograde amnesia** (the inability to form new memories), which is the hallmark of recent memory loss. **Analysis of Options:** * **Temporal Lobe (Correct):** The hippocampus (located in the medial temporal lobe) is essential for declarative memory. Damage here prevents the encoding of new information, though remote (long-term) memories often remain intact. * **Thalamus:** While the Mediodorsal nucleus of the thalamus is involved in memory (part of the Papez circuit), lesions here typically present as part of Wernicke-Korsakoff syndrome, characterized by confabulation rather than isolated recent memory loss. * **Frontal Lobe:** This region is primarily responsible for **working memory** (short-term buffering of information) and executive functions. Frontal lesions affect attention and retrieval strategies but do not prevent the formation of permanent memory traces. **High-Yield NEET-PG Pearls:** * **Papez Circuit:** Hippocampus → Fornix → Mammillary bodies → Anterior Thalamic Nucleus → Cingulate Gyrus → Entorhinal Cortex → Hippocampus. * **Klüver-Bucy Syndrome:** Results from bilateral temporal lobe (amygdala) lesions; characterized by hyperorality, hypersexuality, and visual agnosia. * **Alzheimer’s Disease:** The hippocampus is one of the first structures to undergo atrophy, explaining why recent memory loss is the earliest clinical symptom.

Question 24: What is the peak time of a fast excitatory postsynaptic potential (EPSP) or fast inhibitory postsynaptic potential (IPSP) after stimulus application?

A. 0.5 ms
B. 0.5 to 1.5 ms (Correct Answer)
C. 11.5 ms
D. 100-500 ms

Explanation: **Explanation:** The correct answer is **B. 0.5 to 1.5 ms**. **1. Why Option B is Correct:** Fast EPSPs and IPSPs are mediated by **ionotropic receptors** (ligand-gated ion channels). When an action potential reaches the presynaptic terminal, neurotransmitters are released and bind to these receptors, causing an immediate change in membrane permeability. * In **Fast EPSPs**, there is an influx of $Na^+$ or $Ca^{2+}$. * In **Fast IPSPs**, there is an influx of $Cl^-$ or efflux of $K^+$. Because these channels open directly upon ligand binding without second messengers, the electrical response is rapid. The potential typically begins after a latent period of 0.5 ms (synaptic delay) and reaches its **peak magnitude within 0.5 to 1.5 ms**. **2. Why Other Options are Incorrect:** * **Option A (0.5 ms):** This represents the typical **synaptic delay** (the time taken for transmitter release and diffusion across the cleft) rather than the time to reach the peak potential. * **Option C (11.5 ms):** This is closer to the total duration of a fast EPSP (which typically decays over 10–15 ms) rather than the time to reach its peak. * **Option D (100-500 ms):** This timeframe is characteristic of **Slow EPSPs/IPSPs**, which are mediated by **metabotropic receptors** (G-protein coupled receptors). These involve secondary messengers and enzymatic cascades, leading to a significantly delayed peak and longer duration. **High-Yield NEET-PG Pearls:** * **Synaptic Delay:** Minimum time is **0.5 ms**; it is primarily due to the time required for $Ca^{2+}$ influx to trigger exocytosis of vesicles. * **Spatial vs. Temporal Summation:** EPSPs are graded potentials. **Spatial** involves multiple presynaptic neurons firing simultaneously, while **Temporal** involves a single neuron firing at high frequency. * **Inhibitory Neurotransmitter:** GABA and Glycine are the primary mediators of fast IPSPs. GABA-A is ionotropic (fast), while GABA-B is metabotropic (slow).

Question 25: The efferent limb of a myotatic reflex includes which of the following?

A. Anterior horn motor neuron (Correct Answer)
B. Lateral horn visceromotor nucleus
C. Muscle spindle
D. Preganglionic sympathetic neuron

Explanation: The **myotatic reflex** (stretch reflex) is the simplest functional unit of the nervous system, characterized as a **monosynaptic reflex** arc. It plays a crucial role in maintaining muscle tone and posture. ### **Explanation of the Correct Option** * **A. Anterior horn motor neuron:** The reflex arc begins when a muscle is stretched, stimulating the **Muscle Spindle** (receptor). The signal travels via **Type Ia afferent fibers** to the spinal cord, where they synapse directly onto the **Alpha Motor Neurons** located in the **Anterior Horn** of the gray matter. These neurons serve as the **efferent limb**, sending impulses back to the extrafusal muscle fibers to cause contraction. ### **Why Other Options are Incorrect** * **B & D. Lateral horn / Preganglionic sympathetic neuron:** The lateral horn contains the intermediolateral nucleus, which houses preganglionic sympathetic neurons. These are part of the **Autonomic Nervous System** and are involved in visceral functions, not somatic skeletal muscle reflexes. * **C. Muscle spindle:** This is the **sensory receptor** and the starting point of the **afferent limb**, not the efferent limb. ### **High-Yield Clinical Pearls for NEET-PG** * **Monosynaptic Nature:** The myotatic reflex is the only reflex in the body that is truly monosynaptic (no interneuron between afferent and efferent). * **Reciprocal Inhibition:** While the reflex itself is monosynaptic, it simultaneously involves a polysynaptic pathway where inhibitory interneurons relax the antagonist muscle. * **Clinical Correlation:** Testing Deep Tendon Reflexes (DTRs) like the knee-jerk assesses the integrity of this arc. **Lower Motor Neuron (LMN)** lesions result in hyporeflexia, while **Upper Motor Neuron (UMN)** lesions lead to hyperreflexia due to loss of descending inhibition. * **Gamma Motor Neurons:** These regulate the sensitivity of the muscle spindle but are not the primary efferent limb for the contraction itself.

Question 26: Prothrombin Time (PT) detects deficiency of all the following clotting factors EXCEPT?

A. II
B. V
C. VII
D. VIII (Correct Answer)

Explanation: **Explanation:** The **Prothrombin Time (PT)** is a laboratory test used to evaluate the integrity of the **Extrinsic** and **Common pathways** of the coagulation cascade. **1. Why Option D (Factor VIII) is the correct answer:** Factor VIII is a critical component of the **Intrinsic pathway**. Deficiencies in Factor VIII (as seen in Hemophilia A) or other intrinsic factors (XII, XI, IX) will prolong the **Activated Partial Thromboplastin Time (aPTT)**, but will leave the PT unaffected. Therefore, PT cannot detect a deficiency in Factor VIII. **2. Why the other options are incorrect:** The PT test specifically measures the activity of factors involved in the extrinsic and common pathways: * **Factor VII (Option C):** This is the key factor of the **Extrinsic pathway**. PT is the most sensitive test for Factor VII deficiency. * **Factors II, V, and X (Options A & B):** These factors belong to the **Common pathway**. Since the PT test monitors the progression from the extrinsic pathway through the common pathway to fibrin clot formation, deficiencies in Factor II (Prothrombin), Factor V, or Factor X will result in a prolonged PT. **Clinical Pearls for NEET-PG:** * **Reagent used for PT:** Tissue Thromboplastin (Tissue Factor) and Calcium. * **INR (International Normalized Ratio):** A standardized version of PT used primarily to monitor **Warfarin (Oral Anticoagulant)** therapy. * **Vitamin K Dependent Factors:** II, VII, IX, and X. Since Factor VII has the shortest half-life, PT is the first test to become prolonged in Vitamin K deficiency or early liver disease. * **Mnemonic for PT (Extrinsic):** "PET" — **P**T measures **E**xtrinsic pathway and involves Factor **T**en (X) and Seven (VII).

Question 27: Increased gamma efferent discharge is seen in all except:

A. Jendrassik's maneuver
B. Anxiety
C. Rapid shallow breathing (Correct Answer)
D. Stimulation of skin

Explanation: The **gamma motor system** regulates the sensitivity of muscle spindles. Increased gamma efferent discharge increases the tension on the intrafusal fibers, making the spindle more sensitive to stretch and thereby increasing muscle tone and reflex activity. ### **Why "Rapid Shallow Breathing" is the Correct Answer** Rapid shallow breathing (tachypnea) is typically a result of restrictive lung disease, pulmonary edema, or respiratory distress. It is primarily mediated by **J-receptors** (juxtacapillary receptors) in the alveolar walls. Stimulation of J-receptors via the vagus nerve leads to the **reflex inhibition of spinal motor neurons**, including a decrease in gamma efferent discharge. This results in muscle weakness and decreased reflex activity, rather than an increase. ### **Analysis of Incorrect Options** * **Jendrassik's Maneuver:** This involves a patient hooking their fingers together and pulling apart. This voluntary effort increases the general level of excitation in the spinal cord, specifically **increasing gamma efferent discharge**, which makes the muscle spindles more sensitive and reinforces the deep tendon reflexes. * **Anxiety:** Emotional stress and anxiety lead to increased activity in the **reticular activating system (RAS)**. The descending pathways from the RAS stimulate gamma motor neurons, leading to increased muscle tension and hyperactive reflexes. * **Stimulation of Skin:** Noxious or tactile stimulation of the skin (especially over a muscle) causes a reflex increase in gamma efferent discharge to that muscle to prepare for a protective withdrawal or postural adjustment. ### **High-Yield Clinical Pearls for NEET-PG** * **Alpha-Gamma Coactivation:** During voluntary movement, both alpha and gamma motor neurons are activated simultaneously to maintain spindle sensitivity during muscle contraction. * **Gamma Bias:** The level of gamma motor neuron activity is often referred to as "gamma bias" or "set." * **Supraspinal Control:** The **Pontine Reticular Formation** and **Vestibular Nuclei** increase gamma discharge, while the **Medullary Reticular Formation** inhibits it.

Question 28: Efferents from the cerebellum exit through which of the following cell types?

A. Golgi cells
B. Basket cells
C. Purkinje cells (Correct Answer)
D. Bipolar cells

Explanation: ### Explanation The cerebellar cortex is organized into a highly structured three-layered system consisting of five main cell types. Understanding the flow of information through these layers is crucial for NEET-PG. **Why Purkinje Cells are Correct:** The **Purkinje cell** is the functional unit of the cerebellar cortex and represents the **sole output** (efferent) from the cerebellar cortex. While most Purkinje cells send inhibitory (GABAergic) projections to the **Deep Cerebellar Nuclei** (Dentate, Emboliform, Globose, and Fastigial), some fibers from the vestibulocerebellum bypass these nuclei to project directly to the vestibular nuclei in the brainstem. **Analysis of Incorrect Options:** * **A. Golgi cells:** These are inhibitory interneurons located in the granular layer. They provide feedback inhibition to granule cells. * **B. Basket cells:** These are inhibitory interneurons located in the molecular layer. They provide lateral inhibition to Purkinje cells (projections run perpendicular to parallel fibers). * **D. Bipolar cells:** These are not a standard component of the cerebellar cortex; they are primarily found in the retina, olfactory epithelium, and vestibulocochlear nerve. **High-Yield Clinical Pearls for NEET-PG:** * **All cells** in the cerebellar cortex are **inhibitory (GABAergic)** except for **Granule cells**, which are excitatory (Glutamatergic). * **Afferent inputs:** The cerebellum receives two main types of excitatory inputs: **Climbing fibers** (from the Inferior Olivary Nucleus) and **Mossy fibers** (from all other sources like vestibulocerebellar and spinocerebellar tracts). * **Climbing fibers** have a 1:1 relationship with Purkinje cells and generate "complex spikes," whereas **Mossy fibers** synapse on granule cells to generate "simple spikes." * **Clinical Sign:** Lesions to Purkinje cells or the cerebellum result in **ipsilateral** motor deficits (e.g., hypotonia, ataxia, and intention tremors).

Question 29: Which one of the following statements about EEG is true?

A. Alpha rhythm is most marked at the parieto-occipital area. (Correct Answer)
B. Beta rhythm is most marked at the region below the mastoid.
C. In REM sleep, the patient is awake.
D. EPSP and IPSP of cortical cells act as dipoles.

Explanation: ### Explanation **1. Why Option A is Correct:** The **Alpha rhythm (8–13 Hz)** is the characteristic rhythm of an adult who is awake but relaxed with eyes closed. It is most prominent in the **parieto-occipital regions** of the brain. This rhythm represents "idling" of the visual cortex; it disappears immediately upon opening the eyes or focusing on a mental task (a phenomenon known as **Alpha Block** or Desynchronization). **2. Why the Other Options are Incorrect:** * **Option B:** Beta rhythm (13–30 Hz) is associated with mental activity and alertness. It is most prominent in the **frontal and parietal regions**, not the mastoid area. * **Option C:** In REM (Rapid Eye Movement) sleep, the patient is **asleep**, but the EEG shows low-voltage, high-frequency activity that resembles an awake state. This is why REM sleep is termed **"Paradoxical Sleep."** * **Option D:** While EPSPs and IPSPs are the basis of the EEG, they act as dipoles specifically in the **pyramidal cells** of the cerebral cortex. The statement in the option is technically incomplete/vague compared to the definitive localization of the Alpha rhythm. (Note: In some contexts, D is considered partially true, but A is the "most true" and classic textbook fact for NEET-PG). **3. Clinical Pearls & High-Yield Facts:** * **EEG Waves Frequency:** Delta (<4 Hz) < Theta (4–7 Hz) < Alpha (8–13 Hz) < Beta (13–30 Hz). * **Delta Waves:** Normal in deep sleep (Stage 3 NREM) and infancy; abnormal in awake adults (indicates brain injury/coma). * **Theta Waves:** Normal in children and during light sleep; seen in adults during emotional stress. * **EEG Basis:** The EEG does *not* record action potentials. It records the **summation of excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs)** in the dendrites of vertically oriented pyramidal cells.

Question 30: The pudendal nerve supplies motor innervation to the external sphincter. Which nerve roots contribute to this innervation?

A. L5-S1 roots
B. S1-S2 roots
C. L2-L3 roots
D. S2-S3 roots (Correct Answer)

Explanation: **Explanation:** The **pudendal nerve** is the primary nerve of the perineum and is a major branch of the sacral plexus. It originates from the anterior rami of the **S2, S3, and S4** spinal nerves. **Why S2-S3 is the correct answer:** While the pudendal nerve is classically described as S2-S4, its motor innervation to the **external urethral sphincter** and the **external anal sphincter** is primarily derived from the **S2 and S3** nerve roots. These fibers originate from a specific group of motor neurons in the ventral horn of the sacral spinal cord known as **Onuf’s nucleus**. This nucleus is unique as it possesses characteristics of both somatic and autonomic neurons, allowing for voluntary control over continence. **Analysis of Incorrect Options:** * **L5-S1 roots (Option A):** These roots primarily contribute to the sciatic nerve and the superior/inferior gluteal nerves, supplying muscles of the hip and lower limb. * **S1-S2 roots (Option B):** While S2 is involved, S1 is not a primary contributor to the pudendal nerve; it mainly supplies the gastrocnemius and small muscles of the foot. * **L2-L3 roots (Option C):** These roots contribute to the lumbar plexus (e.g., femoral and obturator nerves), supplying the anterior and medial thigh. **High-Yield Clinical Pearls for NEET-PG:** * **Onuf’s Nucleus:** Located at the S2 level; it is remarkably spared in Amyotrophic Lateral Sclerosis (ALS), which is why these patients often maintain sphincter control despite widespread paralysis. * **Pudendal Nerve Block:** Performed by injecting local anesthetic near the **ischial spine** (using the sacrospinous ligament as a landmark) to provide anesthesia during childbirth or perineal surgery. * **Alcock’s Canal:** The pudendal nerve travels through this fascial tunnel (pudendal canal) along the lateral wall of the ischioanal fossa.

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

Practice Questions

Hypothalamus and Limbic System

Practice Questions

Cerebral Cortex Functions

Practice Questions

Electroencephalography

Practice Questions

Neuroplasticity

Practice Questions

Sleep and Wakefulness

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free