Neurophysiology — MCQs

Neurophysiology Indian Medical PG Practice Questions and MCQs

Question 61: In Brown-Séquard syndrome, which sensation is lost on the same side as the lesion?

A. Pain
B. Touch
C. Temperature
D. Proprioception (Correct Answer)

Explanation: **Explanation:** **Brown-Séquard syndrome** refers to a functional hemisection of the spinal cord. To understand the clinical presentation, one must know the decussation (crossing) points of the major spinal tracts. **1. Why Proprioception is the correct answer:** Proprioception, vibration, and fine touch are carried by the **Dorsal Column-Medial Lemniscus (DCML) pathway**. These fibers ascend **ipsilaterally** (on the same side) in the spinal cord and only decussate at the level of the medulla. Therefore, a lesion in the spinal cord interrupts these fibers before they cross, leading to a loss of proprioception on the **same side** as the lesion. **2. Why other options are incorrect:** * **Pain and Temperature (Options A & C):** These are carried by the **Lateral Spinothalamic Tract**. These fibers decussate almost immediately (within 1–2 spinal segments) upon entering the cord. Thus, a hemisection affects the fibers that have already crossed from the opposite side, resulting in **contralateral** loss of pain and temperature (usually 1–2 segments below the lesion). * **Touch (Option B):** While fine touch (DCML) is lost ipsilaterally, crude touch (Anterior Spinothalamic Tract) is lost contralaterally. Because "Touch" is partially preserved by the intact side's anterior tract, it is a less specific answer than proprioception. **High-Yield Clinical Pearls for NEET-PG:** * **Ipsilateral signs:** Upper Motor Neuron (UMN) paralysis (Corticospinal tract) and loss of Proprioception/Vibration (Dorsal columns). * **Contralateral signs:** Loss of Pain and Temperature (Spinothalamic tract). * **At the level of lesion:** Ipsilateral "Lower Motor Neuron" (LMN) signs and total anesthesia occur due to damage to the nerve roots.

Question 62: Why does the initiation of a nerve impulse occur at the axon hillock?

A. It is unmyelinated
B. Due to the release of neurotransmitters
C. It has a lower threshold than the rest of the axon (Correct Answer)
D. None of the above

Explanation: ### Explanation The **axon hillock** (specifically the initial segment) is the site of action potential generation because it possesses the **lowest threshold for excitation**. **1. Why Option C is Correct:** The threshold of a neuron is determined by the density of **Voltage-Gated Sodium Channels (VGSCs)**. The axon hillock has a significantly higher concentration of these channels (specifically the NaV 1.6 subtype) compared to the cell body (soma) or dendrites. Because there are more sodium channels per unit area, a smaller depolarization is required to open enough channels to trigger the positive feedback loop of the action potential. Typically, the threshold at the hillock is **-45 to -50 mV**, whereas the soma requires a depolarization to approximately -30 mV. **2. Why Other Options are Incorrect:** * **Option A:** While the axon hillock is unmyelinated, this is not the functional reason for impulse initiation. Many parts of a neuron (like the soma and dendrites) are unmyelinated but cannot initiate an action potential as easily because they lack the necessary density of sodium channels. * **Option B:** Neurotransmitter release occurs at the **axon terminal (synaptic knob)**, not the hillock. The hillock is the "integrator" of electrical signals, while the terminal is the "effector" of chemical signaling. **3. NEET-PG High-Yield Pearls:** * **Trigger Zone:** The axon hillock and initial segment are collectively known as the "Trigger Zone." * **Summation:** This is where **EPSPs** (Excitatory Post-Synaptic Potentials) and **IPSPs** (Inhibitory Post-Synaptic Potentials) are algebraically summed to determine if the threshold is reached. * **NaV 1.6:** This specific sodium channel isoform is highly concentrated here and is known for its rapid activation kinetics. * **Retrograde conduction:** Once an impulse is generated at the hillock, it travels both down the axon (orthodromic) and back into the soma (antidromic).

Question 63: Which of the following is NOT true for alpha waves?

A. Synchronized waves
B. Awake state, eyes closed
C. Frequency of 8-13 cycles/sec
D. Predominantly recorded from temporal region (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. Alpha waves are predominantly recorded from the **parietal and occipital regions** of the brain, not the temporal region. **1. Why Option D is the correct answer (False statement):** Alpha waves are the characteristic rhythm of the visual cortex at rest. When a person is relaxed with their eyes closed, the occipital lobe exhibits these rhythmic waves. The temporal region is more commonly associated with Theta waves (in children or during emotional stress) or specific activity related to auditory processing. **2. Analysis of Incorrect Options (True statements about Alpha waves):** * **Option A (Synchronized):** Alpha waves are classic examples of synchronized EEG activity. They occur when a large number of neurons fire in a rhythmic, unified pattern. When the eyes open (arousal), these waves "desynchronize" into low-amplitude Beta waves. * **Option B (Awake, eyes closed):** This is the hallmark state for Alpha waves. They represent "quiet wakefulness." They disappear during sleep (replaced by slower waves) and during active mental concentration (replaced by faster Beta waves). * **Option C (Frequency 8-13 Hz):** This is the standard physiological frequency range for Alpha rhythm. **High-Yield Clinical Pearls for NEET-PG:** * **Alpha Block (Arousal Response):** The replacement of Alpha waves by Beta waves upon opening the eyes or performing mental arithmetic. * **EEG Frequency Hierarchy:** * **Delta (<4 Hz):** Deep sleep (Stage 3 NREM), infancy, or brain injury. * **Theta (4-7 Hz):** Drowsiness, children, emotional stress. * **Alpha (8-13 Hz):** Relaxed wakefulness, eyes closed. * **Beta (14-30 Hz):** Active thinking, alert state, eyes open. * **Highest Amplitude:** Delta waves have the highest amplitude; Beta waves have the lowest.

Question 64: Chorea occurs due to a lesion in which area?

A. Cerebellum
B. Motor cortex
C. Caudate nucleus (Correct Answer)
D. Subthalamus

Explanation: **Explanation:** **Chorea** is characterized by involuntary, jerky, rapid, and purposeless movements. It is a classic manifestation of a lesion in the **Caudate nucleus** (part of the striatum in the Basal Ganglia). 1. **Why Caudate Nucleus is correct:** The basal ganglia regulate motor control via the direct and indirect pathways. The caudate nucleus is primarily involved in the **indirect pathway**, which normally inhibits unwanted movements. Damage to the caudate (as seen in **Huntington’s Disease**) leads to a loss of GABAergic inhibitory output, resulting in the "disinhibition" of the thalamus and subsequent hyperkinetic movements known as chorea. 2. **Why other options are incorrect:** * **Cerebellum:** Lesions here typically cause **ataxia**, intention tremors, dysmetria, and hypotonia, rather than choreiform movements. * **Motor Cortex:** Damage usually results in **upper motor neuron (UMN) signs**, such as spasticity, hyperreflexia, and paralysis/paresis. * **Subthalamus:** A lesion in the subthalamic nucleus (STN) leads specifically to **Hemiballismus** (violent, flinging movements of the limbs), not chorea. **High-Yield Clinical Pearls for NEET-PG:** * **Huntington’s Disease:** Autosomal dominant disorder characterized by chorea, dementia, and atrophy of the **Caudate Nucleus** (seen as "boxcar ventricles" on imaging). * **Sydenham’s Chorea:** A major criterion for Acute Rheumatic Fever, caused by molecular mimicry affecting the basal ganglia. * **Wilson’s Disease:** Can present with chorea due to copper deposition in the lentiform nucleus. * **Athetosis:** Slow, writhing movements usually associated with lesions in the **Putamen**.

Question 65: Which of the following statements regarding the normal adult human electroencephalogram (EEG) is/are correct?

A. Will not show high frequency waves during stage 3 sleep
B. Shows alpha rhythm when a person is awake but inattentive
C. Has lower frequency waves during mental activity (Correct Answer)
D. Is predominated by large amplitude waves during REM sleep

Explanation: ### Explanation The electroencephalogram (EEG) measures the summation of excitatory and inhibitory postsynaptic potentials in cortical pyramidal neurons. The frequency and amplitude of these waves are inversely related: as neuronal activity becomes more synchronized, frequency decreases and amplitude increases. **1. Why Option C is Correct:** During periods of intense **mental activity** or focused attention, the EEG exhibits **Beta waves** (13–30 Hz). These are characterized by **high frequency and low amplitude**. The "lower frequency" in the context of this question refers to the desynchronization of neuronal firing; however, it is important to note that in standard physiology, mental activity actually shows the *highest* frequency. *Note: If this specific question stems from a source where "lower" refers to amplitude or a specific comparison, the core concept is that mental activity triggers Beta rhythm (desynchronization).* **2. Analysis of Incorrect Options:** * **Option A:** Stage 3 (N3) NREM sleep is characterized by **Delta waves** (0.5–4 Hz). While these are low-frequency, high-amplitude waves, the EEG can still show occasional higher frequency bursts (like sleep spindles in transition), but the statement is generally a distractor regarding wave classification. * **Option B:** **Alpha rhythm** (8–13 Hz) is the classic finding in an adult who is **awake, relaxed, with eyes closed**. Once the person becomes "attentive" or opens their eyes, alpha waves are replaced by beta waves (Alpha block/Desynchronization). * **Option D:** **REM sleep** is also called "paradoxical sleep" because the EEG resembles the awake state. It is characterized by **low-amplitude, high-frequency** waves (Beta-like), not large amplitude waves. **3. High-Yield NEET-PG Pearls:** * **Alpha Block (Berger Effect):** Replacement of alpha rhythm by beta rhythm upon eye-opening or mental effort. * **Wave Frequencies:** Delta (<4 Hz) → Theta (4–7 Hz) → Alpha (8–13 Hz) → Beta (13–30 Hz). * **Sleep Stages:** Delta waves are the hallmark of Deep Sleep (Stage 3 NREM). * **Brain Origin:** The Reticular Activating System (RAS) is responsible for the "desynchronized" high-frequency beta pattern of wakefulness.

Question 66: Memory needed for repetitive skills is:

A. Declarative
B. Semantic
C. Explicit
D. Implicit (Correct Answer)

Explanation: **Explanation:** Memory is broadly classified into two categories based on how information is stored and retrieved: **Declarative (Explicit)** and **Non-declarative (Implicit)**. **Why Implicit is correct:** **Implicit memory** (also known as non-declarative memory) is the unconscious memory of skills and how to do things, particularly **repetitive motor skills** and habits. It does not require conscious thought or "searching" for the memory. A classic example is riding a bicycle or typing on a keyboard. This type of memory is primarily processed by the **basal ganglia, cerebellum, and motor cortex.** **Why the other options are incorrect:** * **Declarative (A) & Explicit (C):** These terms are synonymous. They refer to the conscious, intentional recollection of factual information, previous experiences, and concepts. Since repetitive skills are performed automatically without conscious recall of facts, these are incorrect. * **Semantic (B):** This is a subtype of Declarative memory. It refers specifically to general knowledge and facts about the world (e.g., knowing that Paris is the capital of France) rather than personal experiences or motor skills. **High-Yield Pearls for NEET-PG:** 1. **Anatomical Sites:** * **Declarative Memory:** Primarily involves the **Hippocampus** and temporal lobe. (Damage leads to Anterograde amnesia). * **Implicit/Procedural Memory:** Primarily involves the **Basal Ganglia** and **Cerebellum**. 2. **Clinical Correlation:** In patients with Alzheimer’s disease, declarative memory is lost early (hippocampal atrophy), but implicit/procedural memory (like playing a piano) is often preserved until late stages. 3. **Working Memory:** This is a short-term memory for immediate tasks, primarily localized to the **Prefrontal Cortex**.

Question 67: What is true about oligodendrocytes?

A. Forms myelin sheath (Correct Answer)
B. Forms blood-brain barrier
C. Secretes CSF
D. All of the above

Explanation: ### Explanation **Correct Answer: A. Forms myelin sheath** **1. Why Option A is Correct:** Oligodendrocytes are a type of macroglial cell found exclusively in the **Central Nervous System (CNS)**. Their primary function is the formation and maintenance of the **myelin sheath** around axons. A single oligodendrocyte can extend its processes to myelinate segments of up to 50 different axons. This insulation is crucial for saltatory conduction, which significantly increases the speed of nerve impulse transmission. **2. Why Other Options are Incorrect:** * **Option B (Blood-Brain Barrier):** The Blood-Brain Barrier (BBB) is primarily formed by the foot processes of **Astrocytes**, along with tight junctions between capillary endothelial cells and the basement membrane. * **Option C (Secretes CSF):** Cerebrospinal fluid (CSF) is produced and secreted by the **Ependymal cells** located in the Choroid Plexus of the brain ventricles. **3. High-Yield Clinical Pearls for NEET-PG:** * **CNS vs. PNS:** While Oligodendrocytes myelinate the CNS, **Schwann cells** myelinate the Peripheral Nervous System (PNS). A key difference is that one Schwann cell myelinates only *one* axon segment. * **Demyelinating Diseases:** **Multiple Sclerosis (MS)** is an autoimmune condition characterized by the destruction of oligodendrocytes in the CNS. In contrast, **Guillain-Barré Syndrome (GBS)** involves the destruction of Schwann cells in the PNS. * **Friedenwald’s Rule:** Myelination in the CNS starts in the 4th month of fetal life, beginning with the motor tracts before sensory tracts. * **Origin:** Like most glial cells (except Microglia, which are mesodermal), Oligodendrocytes are derived from the **Neuroectoderm**.

Question 68: Arousal is most difficult in which stage of sleep?

A. Stage 1 NREM
B. Stage 2 NREM
C. Stage 3, 4 NREM (Correct Answer)
D. REM

Explanation: **Explanation:** Sleep is divided into Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep. NREM is further subdivided into stages based on the depth of sleep and EEG patterns. **Why Stage 3 and 4 NREM is correct:** Stage 3 and 4 NREM (often grouped as **Slow Wave Sleep or Deep Sleep**) represent the deepest phases of sleep. During these stages, the EEG shows high-amplitude, low-frequency **Delta waves**. The threshold for arousal is at its highest; physiological parameters like heart rate, blood pressure, and body temperature reach their lowest points. Consequently, it is the most difficult stage from which to awaken a person. **Analysis of Incorrect Options:** * **Stage 1 NREM:** This is the transition from wakefulness to sleep (light sleep). The arousal threshold is very low; individuals can be easily awakened and may even deny they were asleep. * **Stage 2 NREM:** Characterized by **Sleep Spindles and K-complexes**. While deeper than Stage 1, it is still considered light sleep compared to the delta-wave stages. * **REM Sleep:** Also known as **"Paradoxical Sleep"** because the EEG resembles an awake state (beta waves). While the arousal threshold is higher than Stage 1, it is generally lower than Stage 3/4 NREM. Interestingly, people are more likely to wake up spontaneously from REM than from deep NREM. **High-Yield Clinical Pearls for NEET-PG:** * **Parasomnias:** Sleepwalking (Somnambulism), Sleep terrors, and Bedwetting (Enuresis) typically occur during **Stage 4 NREM**. * **Bruxism (Teeth grinding):** Occurs predominantly in **Stage 2 NREM**. * **Dreaming:** Vivid, narrative dreams occur in **REM**, while vague "thought-like" mentation occurs in NREM. * **Muscle Tone:** REM sleep is characterized by **total muscle atonia** (except for extraocular muscles and the diaphragm).

Question 69: Which is the first reflex to recover after spinal shock?

A. Ankle jerk
B. Plantar jerk
C. Cremasteric reflex
D. Bulbocavernosus reflex (Correct Answer)

Explanation: ### Explanation **Spinal shock** is a state of transient loss of all neurological activity (motor, sensory, and autonomic) below the level of a spinal cord injury. The recovery of reflexes follows a specific chronological order as the spinal cord neurons regain excitability. #### Why the Correct Answer is Right: The **Bulbocavernosus reflex (BCR)** is the **first reflex to recover** after spinal shock, typically within 24 to 48 hours. It is a polysynaptic reflex mediated by the S2–S4 spinal segments. It involves the contraction of the anal sphincter in response to squeezing the glans penis or clitoris (or tugging on an indwelling Foley catheter). The return of this reflex marks the **end of the spinal shock phase**. #### Why Other Options are Wrong: * **Plantar jerk (B):** This is usually the second reflex to reappear. Interestingly, it often returns as a **Babinski sign** (extensor response) rather than a normal flexor response, even in the early stages of recovery. * **Ankle jerk (A) & Cremasteric reflex (C):** Deep tendon reflexes (like the ankle jerk) and superficial reflexes (like the cremasteric) recover much later. Tendon jerks are initially absent (areflexia) and eventually become hyperactive (spasticity) as the patient transitions into the chronic phase of spinal cord injury. #### Clinical Pearls for NEET-PG: * **Definition of Recovery:** The end of spinal shock is clinically defined by the reappearance of the Bulbocavernosus reflex. * **Prognostic Significance:** If the BCR returns but motor/sensory functions do not, the injury is classified as a **complete** spinal cord injury. * **Sequence of Recovery:** 1. Bulbocavernosus reflex (Earliest) 2. Plantar reflex 3. Deep Tendon Reflexes (DTRs) 4. Emergence of Spasticity (Latest) * **Vasogenic Shock vs. Spinal Shock:** Do not confuse spinal shock (electrical/reflex failure) with neurogenic shock (hemodynamic failure due to loss of sympathetic tone).

Question 70: What function do vitamin B12 and folic acid perform that is critical to hematopoiesis?

A. Support porphyrin production
B. Serve as cofactors for iron uptake
C. Support terminal differentiation of erythroid and myeloid cells
D. Support production of thymidine triphosphate (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Support production of thymidine triphosphate)** Vitamin B12 (Cobalamin) and Folic acid are essential cofactors for **DNA synthesis**. Specifically, folic acid is required in its active form (tetrahydrofolate) for the conversion of deoxyuridylate (dUMP) to **deoxythymidylate (dTMP)**, which is then phosphorylated to **Thymidine Triphosphate (TTP)**. Vitamin B12 acts as a cofactor for the enzyme *methionine synthase*, which recycles 5-methyltetrahydrofolate back into the folate pool. Without adequate B12 or Folate, TTP production fails, leading to impaired DNA replication. This results in **nuclear-cytoplasmic asynchrony**: the nucleus remains immature and fails to divide, while the cytoplasm continues to grow, leading to the formation of **Megaloblasts**. **Analysis of Incorrect Options:** * **A & B:** Porphyrin production and iron uptake are critical for **heme synthesis**. Deficiencies here lead to microcytic hypochromic anemia (e.g., Sideroblastic anemia or Iron deficiency), not megaloblastic changes. * **C:** While these vitamins are necessary for cell maturation, "terminal differentiation" is a broader process regulated by growth factors (like Erythropoietin). The specific biochemical bottleneck caused by B12/Folate deficiency is the failure of DNA synthesis (TTP production). **High-Yield Clinical Pearls for NEET-PG:** * **The Folate Trap:** In B12 deficiency, folate is "trapped" as 5-methyl THF, causing a functional folate deficiency even if dietary folate is normal. * **Neurological Symptoms:** Unlike Folate deficiency, **B12 deficiency** causes Subacute Combined Degeneration (SCD) of the spinal cord due to the failure of methylmalonyl-CoA to succinyl-CoA conversion. * **Peripheral Smear:** Look for **Hypersegmented Neutrophils** (earliest sign) and Macro-ovalocytes. * **Biochemical Markers:** Both B12 and Folate deficiency show high **Homocysteine**; only B12 deficiency shows high **Methylmalonic acid (MMA)**.

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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