Neurophysiology Practice Questions

Q: Which ion causes repolarization?

Potassium. ### 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**.

Q: Which extrapyramidal tract acts along with the lateral spinothalamic tract?

Rubrospinal tract. 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**.

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

Glutamate. 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.

Q: Strychnine acts by

Blocking inhibitory synapses. **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.

Q: Which of the following is false regarding REM sleep?

Slow eye movements present. **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.

Q: Which of the following nerve fibers carries fast pain?

A-delta. **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**.

Q: Which of the following is characteristic of the alpha rhythm?

Awake with eyes open and relaxed. ### 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).

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

Cogwheel rigidity. **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*).

Question 1

Which ion causes repolarization?

Accepted Answer

Potassium

Answer

Magnesium

Answer

Calcium

Answer

Sodium

Question 2

Which extrapyramidal tract acts along with the lateral spinothalamic tract?

Accepted Answer

Rubrospinal tract

Answer

Reticulospinal tract

Answer

Tectospinal tract

Answer

Vestibulospinal tract

Question 3

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

Accepted Answer

It is refractory

Answer

It is hyperpolarized

Answer

It is not self-propagating

Answer

The condition is always orthodromic

Question 4

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

Accepted Answer

Glutamate

Answer

Glycine

Answer

Substance P

Answer

None of the above

Question 5

Which statement is not true regarding the withdrawal reflex?

Accepted Answer

Afferent fibres are Ia.

Answer

It has a long latency.

Answer

It is a polysynaptic reflex.

Answer

The response is non-linear and usually widespread.

Question 6

Strychnine acts by

Accepted Answer

Blocking inhibitory synapses

Answer

Exciting all the excitatory synapses in the cord

Answer

Being incorporated as a substitute transmitter in monoaminergic synapses

Answer

Directly exciting skeletal muscle fibrosis

Question 7

Which of the following is false regarding REM sleep?

Accepted Answer

Slow eye movements present

Answer

Dreaming sleep

Answer

Paradoxical sleep

Answer

Alpha waves are seen

Question 8

Which of the following nerve fibers carries fast pain?

Accepted Answer

A-delta

Answer

A-alfa

Answer

B

Answer

C

Question 9

Which of the following is characteristic of the alpha rhythm?

Accepted Answer

Awake with eyes open and relaxed

Answer

Sleep with eyes closed and mind wandering

Answer

Mental activity

Answer

REM sleep

Question 10

A lesion of the thalamus will not produce which of the following?

Accepted Answer

Cogwheel rigidity

Answer

Sensory loss

Answer

Sensory disturbance of one half of the body

Answer

Tingling sensation

Neurophysiology — MCQs

Neurophysiology — MCQs

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