Neurophysiology — MCQs

Neurophysiology Indian Medical PG Practice Questions and MCQs

Question 111: Which ion causes repolarization?

A. Magnesium
B. Calcium
C. Potassium (Correct Answer)
D. Sodium

Explanation: ### Explanation **Correct Answer: C. Potassium** The action potential in a neuron or muscle cell consists of two primary phases: depolarization and repolarization. **Repolarization** is the process by which the cell membrane potential returns to its negative resting state after depolarization. This is primarily driven by the **efflux (outward movement) of Potassium ($K^+$) ions**. During the peak of an action potential, voltage-gated $Na^+$ channels close (inactivate) and voltage-gated $K^+$ channels open. Because the concentration of $K^+$ is much higher inside the cell (~140 mEq/L) than outside (~4 mEq/L), $K^+$ rushes out of the cell down its chemical gradient. This loss of positive charge restores the internal negativity of the cell. **Why other options are incorrect:** * **Sodium (D):** Sodium **influx** is responsible for **depolarization** (making the cell interior positive). * **Calcium (B):** Calcium influx is primarily involved in the **plateau phase** of the cardiac action potential and triggers neurotransmitter release at synaptic terminals. * **Magnesium (A):** Magnesium acts as a physiological calcium channel blocker and is a cofactor for the $Na^+$-$K^+$ ATPase pump, but it does not directly mediate the repolarization phase of a standard action potential. **NEET-PG High-Yield Pearls:** * **Hyperkalemia:** Increases membrane excitability initially (brings resting membrane potential closer to threshold) but eventually leads to inactivation of $Na^+$ channels, causing paralysis and cardiac arrhythmias. * **Hypokalemia:** Characterized by **U waves** on ECG and prolonged repolarization. * **TEA (Tetraethylammonium):** A classic pharmacological tool used to block voltage-gated $K^+$ channels, thereby inhibiting repolarization. * **Resting Membrane Potential (RMP):** Primarily determined by $K^+$ "leak" channels; the RMP of a typical neuron is approximately **-70 mV**.

Question 112: Which extrapyramidal tract acts along with the lateral spinothalamic tract?

A. Reticulospinal tract
B. Rubrospinal tract (Correct Answer)
C. Tectospinal tract
D. Vestibulospinal tract

Explanation: The **Rubrospinal tract** is the correct answer because of its unique anatomical and functional relationship with the lateral corticospinal tract (pyramidal system). ### **Why Rubrospinal Tract is Correct** In neuroanatomy, the spinal cord white matter is organized into columns. The **lateral column** contains both the **Lateral Corticospinal tract** (motor) and the **Lateral Spinothalamic tract** (sensory). Among the extrapyramidal tracts, the Rubrospinal tract is the only one that descends in the lateral column, positioned just anterior to the lateral corticospinal tract. Functionally, it facilitates flexor muscle tone and serves as an alternative pathway for voluntary motor control, "acting along" with the lateral system. ### **Analysis of Incorrect Options** * **A. Reticulospinal tract:** These tracts (medial and lateral) primarily descend in the **anterior and medial columns**. They are involved in posture and autonomic functions. * **C. Tectospinal tract:** This tract descends in the **anterior column** and mediates reflex head turning in response to visual and auditory stimuli. * **D. Vestibulospinal tract:** These descend in the **anterior column** and are primarily responsible for maintaining equilibrium and extensor tone (anti-gravity muscles). ### **High-Yield Clinical Pearls for NEET-PG** * **Lateral System:** Includes the Lateral Corticospinal and Rubrospinal tracts. They control distal limb muscles (fine movements). * **Medial System:** Includes Vestibulospinal, Tectospinal, and Reticulospinal tracts. They control axial and proximal muscles (posture). * **Decerebrate vs. Decorticate:** In decorticate posturing (lesion above red nucleus), the Rubrospinal tract is intact, leading to **flexion** of the upper limbs. In decerebrate posturing (lesion below red nucleus), the Rubrospinal influence is lost, leaving the Vestibulospinal tract unopposed, causing **extension**.

Question 113: A travelling nerve impulse does not depolarize the area immediately behind it, because:

A. It is hyperpolarized
B. It is refractory (Correct Answer)
C. It is not self-propagating
D. The condition is always orthodromic

Explanation: ### Explanation **1. Why the Correct Answer is Right: The Concept of Refractoriness** The unidirectional propagation of a nerve impulse is primarily due to the **Refractory Period**. When an action potential occurs, the voltage-gated $Na^+$ channels in that specific segment of the membrane undergo a conformational change to an **inactivated state**. * During the **Absolute Refractory Period**, these channels cannot be reopened regardless of the stimulus strength. * Consequently, even though the local current flows in both directions, the area "behind" the impulse cannot depolarize because its $Na^+$ channels are still recovering. This ensures the impulse moves forward toward the axon terminal rather than bouncing backward. **2. Why the Other Options are Wrong:** * **Option A (Hyperpolarized):** While many neurons experience an after-hyperpolarization phase (due to slow-closing $K^+$ channels), this is not the primary reason for unidirectional flow. A strong enough stimulus can overcome hyperpolarization, but it cannot overcome the absolute refractory period. * **Option C (Not self-propagating):** This is factually incorrect. Nerve impulses *are* self-propagating; once the threshold is reached, the regenerative nature of the action potential ensures it travels the length of the axon. * **Option D (Condition is always orthodromic):** This is a descriptive term, not a mechanism. "Orthodromic" means the impulse is traveling in the physiological direction. The *reason* it stays orthodromic is the refractory period. **3. High-Yield NEET-PG Pearls:** * **Absolute Refractory Period (ARP):** Corresponds to the period from the firing level until approximately one-third of repolarization is complete. It sets the **upper limit** for the frequency of discharge. * **Relative Refractory Period (RRP):** Corresponds to the period from the end of ARP to the start of after-hyperpolarization. A suprathreshold stimulus can trigger a second response here. * **Accommodation:** If a nerve is subjected to a slowly rising constant current, the threshold for activation increases; this is distinct from refractoriness.

Question 114: What is the neurotransmitter at the central synapse of the stretch reflex circuit?

A. Glycine
B. Glutamate (Correct Answer)
C. Substance P
D. None of the above

Explanation: The stretch reflex (myotatic reflex) is a monosynaptic reflex arc that maintains muscle tone and posture. Understanding its neurochemistry is high-yield for NEET-PG. ### **Explanation of the Correct Answer** The stretch reflex begins when a muscle spindle is stretched, activating **Type Ia afferent fibers**. these fibers enter the spinal cord via the dorsal root and synapse directly onto the **alpha motor neuron** in the anterior horn. * **Glutamate** is the primary excitatory neurotransmitter used by these primary sensory afferents. * When released into the synaptic cleft, glutamate binds to post-synaptic receptors (mainly AMPA), causing depolarization and subsequent muscle contraction. ### **Analysis of Incorrect Options** * **A. Glycine:** This is the primary **inhibitory** neurotransmitter in the spinal cord. In the context of the stretch reflex, glycine is released by **Renshaw cells** or IA inhibitory interneurons (during reciprocal inhibition) to inhibit antagonist muscles, but it is not the transmitter at the primary excitatory synapse. * **C. Substance P:** This neuropeptide is primarily associated with **pain transmission** (nociception) in the dorsal horn (Substantia Gelatinosa of Rolando). It is not involved in the rapid, excitatory transmission of the motor stretch reflex. ### **NEET-PG High-Yield Pearls** * **Monosynaptic Nature:** The stretch reflex is the *only* monosynaptic reflex in the human body (e.g., Knee jerk). * **Afferent vs. Efferent:** The afferent limb is the **Ia fiber** (fastest conducting); the efferent limb is the **Alpha motor neuron**. * **Reciprocal Inhibition:** While the stretch reflex itself is monosynaptic, it involves a polysynaptic component where inhibitory interneurons use **Glycine** to relax the antagonist muscle. * **Gamma Motor Neurons:** These regulate the sensitivity of the muscle spindle but are not part of the basic reflex arc itself.

Question 115: Which statement is not true regarding the withdrawal reflex?

A. It has a long latency.
B. Afferent fibres are Ia. (Correct Answer)
C. It is a polysynaptic reflex.
D. The response is non-linear and usually widespread.

Explanation: ### Explanation The **Withdrawal Reflex** (Flexor Reflex) is a protective mechanism designed to remove a body part from a noxious or painful stimulus. **1. Why "Afferent fibres are Ia" is the correct (false) statement:** The afferent fibers for the withdrawal reflex are **Group II, III, and IV (Aδ and C) fibers**, often referred to as **Flexor Reflex Afferents (FRA)**. These fibers carry pain and pressure sensations. In contrast, **Ia fibers** are large, myelinated sensory fibers originating from muscle spindles that mediate the **monosynaptic stretch reflex** (e.g., knee jerk), not the withdrawal reflex. **2. Analysis of other options:** * **Long Latency (Option A):** Unlike the monosynaptic stretch reflex, the withdrawal reflex involves multiple interneurons. This synaptic delay results in a longer latency between the stimulus and the motor response. * **Polysynaptic Reflex (Option C):** It involves a complex circuit of interneurons between the sensory afferent and the motor efferent. This allows for divergent signals, such as the **Crossed Extensor Reflex**, which maintains balance by extending the contralateral limb. * **Non-linear and Widespread Response (Option D):** The response is "graded" based on the intensity of the stimulus. Due to **spatial and temporal summation** and the phenomenon of **irradiation**, a strong stimulus causes the excitation of more motor units, leading to a widespread response involving multiple muscle groups. ### High-Yield Clinical Pearls for NEET-PG: * **After-discharge:** The withdrawal reflex often persists after the stimulus ceases due to prolonged firing in polysynaptic circuits (reverberating circuits). * **Reciprocal Inhibition:** While flexors of the stimulated limb contract, the extensors of the *same* limb are inhibited to facilitate withdrawal. * **Local Sign:** The pattern of withdrawal is specific to the site of the stimulus (e.g., if the medial side of the limb is stimulated, the limb moves laterally).

Question 116: Strychnine acts by

A. Exciting all the excitatory synapses in the cord
B. Blocking inhibitory synapses (Correct Answer)
C. Being incorporated as a substitute transmitter in monoaminergic synapses
D. Directly exciting skeletal muscle fibrosis

Explanation: **Explanation:** Strychnine is a potent neurotoxin that acts as a **selective competitive antagonist of glycine receptors**. Glycine is the primary inhibitory neurotransmitter in the spinal cord and brainstem. By binding to these receptors, strychnine prevents glycine from exerting its inhibitory effect on postsynaptic neurons. This leads to unchecked excitatory activity, resulting in severe muscle spasms and convulsions. **Analysis of Options:** * **Option B (Correct):** Strychnine blocks the inhibitory action of glycine at the level of the Renshaw cells and other inhibitory interneurons in the spinal cord. This "disinhibition" causes motor neurons to fire uncontrollably. * **Option A:** Strychnine does not directly stimulate excitatory synapses (like those using Glutamate); rather, it removes the "brakes" (inhibition) from the system. * **Option C:** This describes the mechanism of "false neurotransmitters" (e.g., alpha-methyldopa), not strychnine. Strychnine does not interfere with monoamine synthesis or substitution. * **Option D:** Strychnine acts centrally on the nervous system, not directly on the muscle fibers or connective tissue. **High-Yield Clinical Pearls for NEET-PG:** * **Renshaw Cells:** These are inhibitory interneurons in the spinal cord that use glycine to provide "recurrent inhibition" to alpha motor neurons. Strychnine specifically targets this pathway. * **Clinical Presentation:** Strychnine poisoning presents with **Opisthotonus** (hyperextension and arching of the back) and **Risus Sardonicus** (a fixed, sardonic grin), similar to Tetanus. * **Tetanus Toxin vs. Strychnine:** While both cause spasms, Tetanus toxin prevents the *release* of glycine/GABA, whereas Strychnine *blocks the receptor*. * **Antidote:** Management involves benzodiazepines (to enhance GABAergic inhibition) and neuromuscular blockers.

Question 117: Which of the following is false regarding REM sleep?

A. Dreaming sleep
B. Slow eye movements present (Correct Answer)
C. Paradoxical sleep
D. Alpha waves are seen

Explanation: **Explanation:** The correct answer is **B. Slow eye movements present**, as this statement is false. REM (Rapid Eye Movement) sleep is characterized by **rapid, jerky, saccadic eye movements**, not slow ones. Slow eye movements are typically seen during the transition from wakefulness to NREM Stage 1 sleep. **Why the other options are true:** * **A. Dreaming sleep:** Most vivid, narrative, and emotionally charged dreams occur during REM sleep. While some dreaming can occur in NREM, it is less frequent and less vivid. * **C. Paradoxical sleep:** REM is called "paradoxical" because the EEG shows high-frequency, low-voltage activity (similar to an awake state), yet the person is in a deep stage of sleep with profound muscle atonia (except for the diaphragm and extraocular muscles). * **D. Alpha waves are seen:** During REM, the EEG pattern is "desynchronized." It consists of low-voltage, high-frequency activity, including **alpha waves** and "sawtooth waves." This mimics the EEG of an alert, awake individual. **High-Yield Clinical Pearls for NEET-PG:** * **Muscle Atonia:** REM sleep is associated with a complete loss of muscle tone (glycine-mediated inhibition of spinal motor neurons). Failure of this mechanism leads to **REM Sleep Behavior Disorder**. * **PGO Spikes:** Ponto-Geniculo-Occipital spikes are the earliest signs of an upcoming REM cycle. * **Vital Signs:** Unlike NREM, REM sleep is characterized by **irregular** heart rate and respiration, and a loss of thermoregulation (poikilothermia). * **Duration:** REM periods lengthen as the night progresses; most REM sleep occurs in the last third of the night.

Question 118: Which of the following nerve fibers carries fast pain?

A. A-alfa
B. A-delta (Correct Answer)
C. B
D. C

Explanation: **Explanation:** Pain is transmitted via two distinct pathways based on the type of nerve fiber involved. **A-delta fibers** are responsible for **fast pain** (also known as sharp, pricking, or first pain). These are thin, myelinated fibers with a conduction velocity of 6–30 m/s. Their myelination allows for rapid signal transmission, enabling the body to react immediately to noxious stimuli (e.g., a needle prick). **Analysis of Options:** * **A-delta (Correct):** Small, myelinated fibers that carry fast pain and temperature. They secrete glutamate at the spinal cord level. * **A-alpha:** These are the thickest and fastest myelinated fibers. They primarily carry proprioception and somatic motor signals, not pain. * **B fibers:** These are medium-sized, myelinated preganglionic autonomic fibers. * **C fibers:** These are small, **unmyelinated** fibers that carry **slow pain** (dull, aching, or burning pain). They have the slowest conduction velocity (0.5–2 m/s) and secrete Substance P. **High-Yield NEET-PG Pearls:** 1. **Erlanger-Gasser Classification:** Remember that fiber diameter and myelination are directly proportional to conduction velocity (A > B > C). 2. **Fast vs. Slow:** Fast pain (A-delta) is well-localized and travels via the **Neospinothalamic tract**, while slow pain (C) is poorly localized and travels via the **Paleospinothalamic tract**. 3. **Sensitivity to Anesthesia:** Local anesthetics typically block **C fibers first** (smallest diameter), while pressure/hypoxia affects **A fibers first**.

Question 119: Which of the following is characteristic of the alpha rhythm?

A. Sleep with eyes closed and mind wandering
B. Mental activity
C. Awake with eyes open and relaxed (Correct Answer)
D. REM sleep

Explanation: ### Explanation The **Alpha rhythm** (8–13 Hz) is the characteristic EEG pattern of an adult who is **awake, relaxed, and at mental rest**, typically with the **eyes closed**. **Why Option C is the Correct Choice:** While alpha waves are most prominent when eyes are closed, they represent a state of "relaxed wakefulness." In the context of this question, "Awake and relaxed" is the defining physiological state for alpha activity. When a person opens their eyes or focuses on a mental task, the alpha rhythm is replaced by fast, low-voltage beta waves—a phenomenon known as **Alpha Block** or **Desynchronization**. **Analysis of Incorrect Options:** * **Option A (Sleep):** Sleep is characterized by slower waves. Stage N1 shows Theta waves (4–7 Hz), and Stage N3 (Deep Sleep) shows Delta waves (<4 Hz). Alpha waves disappear at the onset of sleep. * **Option B (Mental Activity):** Active thinking, problem-solving, or sensory stimulation triggers **Beta rhythms** (14–30 Hz). This represents a desynchronized EEG. * **Option D (REM Sleep):** The EEG during REM sleep is "paradoxical," meaning it resembles the awake state with low-amplitude, high-frequency activity (Beta-like), but it is not an alpha rhythm. **High-Yield NEET-PG Pearls:** 1. **Frequency Hierarchy:** Gamma (>30 Hz) > Beta (14-30) > Alpha (8-13) > Theta (4-7) > Delta (<4). 2. **Location:** Alpha waves are best recorded from the **parieto-occipital regions**. 3. **Berger Rhythm:** Another name for the Alpha rhythm, named after Hans Berger, the father of EEG. 4. **Delta Waves:** These are normal in deep sleep and infancy but pathological in awake adults (indicating brain injury or tumors).

Question 120: A lesion of the thalamus will not produce which of the following?

A. Sensory loss
B. Cogwheel rigidity (Correct Answer)
C. Sensory disturbance of one half of the body
D. Tingling sensation

Explanation: **Explanation:** The thalamus is the primary sensory relay station of the brain. A lesion here typically results in sensory deficits rather than motor rigidity. **1. Why Cogwheel Rigidity is the Correct Answer:** Cogwheel rigidity is a characteristic sign of **Extrapyramidal system** dysfunction, specifically involving the **Basal Ganglia** (e.g., Parkinson’s disease). It occurs due to a combination of lead-pipe rigidity and a resting tremor. While the thalamus has connections to the basal ganglia (via the VA/VL nuclei), a primary thalamic lesion typically presents with sensory loss or "Thalamic Pain Syndrome" (Dejerine-Roussy syndrome) rather than rigidity. **2. Analysis of Incorrect Options:** * **Sensory loss & Sensory disturbance (Options A & C):** Since all sensory pathways (except olfaction) relay through the **Ventral Posterolateral (VPL)** and **Ventral Posteromedial (VPM)** nuclei of the thalamus, a lesion will result in contralateral hemi-anesthesia (loss of touch, pain, and temperature) affecting one half of the body. * **Tingling sensation (Option D):** Thalamic lesions often cause paresthesia (tingling) or dysesthesia. In Dejerine-Roussy syndrome, patients experience agonizing burning pain or tingling in response to even light touch (allodynia). **Clinical Pearls for NEET-PG:** * **VPL Nucleus:** Relays sensory info from the **Body** (Medial Lemniscus, Spinothalamic tract). * **VPM Nucleus:** Relays sensory info from the **Face** (Trigeminal pathway) — *Mnemonic: **M**akeup goes on the **F**ace.* * **Thalamic Hand:** A clinical sign where the wrist is pronated/flexed and fingers are extended at the IP joints due to altered proprioception. * **Lateral Geniculate Body (LGB):** Relays **Visual** impulses (*L for Light*). * **Medial Geniculate Body (MGB):** Relays **Auditory** impulses (*M for Music*).

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