Neurophysiology Practice Questions

Q: In Cerebrospinal Fluid (CSF), what substance is NOT typically lower in concentration when compared to blood plasma?

Chloride. **Explanation:** The composition of Cerebrospinal Fluid (CSF) is tightly regulated by the blood-CSF barrier. While CSF is an ultrafiltrate of plasma, it is not identical to it. **1. Why Chloride is the Correct Answer:** In CSF, the concentration of **Chloride (Cl⁻)** and **Magnesium (Mg²⁺)** is actually **higher** than in the plasma. This is primarily due to the Gibbs-Donnan effect and active transport mechanisms. Since CSF has significantly lower protein content (anions) compared to plasma, more chloride ions move into the CSF to maintain electrochemical neutrality. **2. Analysis of Incorrect Options:** * **Sodium (Na⁺):** While sodium levels are very similar to plasma to maintain osmolarity, they are technically slightly lower or nearly equal; however, they are never higher. * **Calcium (Ca²⁺):** The concentration of Calcium in the CSF is significantly **lower** (about 50%) than in the plasma because only the non-protein-bound (ionized) fraction can cross the blood-CSF barrier. * **Cells:** Normal CSF is virtually acellular (0-5 WBCs/mm³). Plasma contains a high concentration of RBCs and WBCs, making this option incorrect as cell count is much lower in CSF. **3. High-Yield Clinical Pearls for NEET-PG:** * **Higher in CSF:** Chloride, Magnesium, and PCO₂. * **Lower in CSF:** Glucose (approx. 60% of plasma levels), Protein (15–45 mg/dL vs 6-8 g/dL in plasma), Calcium, Potassium, and pH (CSF is slightly more acidic). * **Equal in CSF:** Osmolarity (approx. 290 mOsm/L). * **Diagnostic Rule:** If CSF glucose is <40% of plasma glucose, suspect bacterial meningitis.

Q: Which of the following is NOT a peptide neurotransmitter?

Serotonin. **Explanation:** The classification of neurotransmitters is a high-yield topic for NEET-PG. Neurotransmitters are broadly divided into **Small-molecule, rapidly acting transmitters** (e.g., Acetylcholine, Amines, Amino acids) and **Neuropeptides, slowly acting transmitters** (e.g., Opioids, Tachykinins). **Why Serotonin is the Correct Answer:** **Serotonin (5-Hydroxytryptamine)** is a **biogenic amine** (monoamine), not a peptide. It is synthesized from the amino acid **Tryptophan** in the cytosol of nerve terminals and stored in vesicles. Unlike neuropeptides, small-molecule transmitters like serotonin are typically recycled via reuptake mechanisms and produce rapid acute responses in the nervous system. **Analysis of Incorrect Options:** * **Enkephalin & Endorphin:** These are **Opioid Peptides**. They are synthesized as large precursor proteins (like Pro-enkephalin and POMC) in the cell body (ribosomes) and transported down the axon. They primarily modulate pain (nociception). * **Substance P:** This is a **Tachykinin neuropeptide**. It consists of 11 amino acids and is the primary neurotransmitter involved in transmitting pain signals from peripheral receptors to the dorsal horn of the spinal cord. **NEET-PG High-Yield Pearls:** * **Synthesis Site:** Small-molecule transmitters (Serotonin, GABA, Glycine) are synthesized in the **cytosol of axon terminals**, whereas Neuropeptides are synthesized in the **cell body (soma)**. * **Potency:** Neuropeptides are generally **1,000 times more potent** than small-molecule transmitters but have a slower, more prolonged action. * **Serotonin Metabolism:** The rate-limiting enzyme is *Tryptophan hydroxylase*, and its primary metabolite found in urine is **5-HIAA** (useful in diagnosing Carcinoid syndrome).

Q: Which of the following is NOT a low molecular weight neurotransmitter?

Orexins. Neurotransmitters are broadly classified into two categories based on their molecular size and synthesis site: **Small-molecule, rapidly acting transmitters** and **Large-molecule, slowly acting neuropeptides**. ### 1. Why Orexins is the Correct Answer **Orexins (Hypocretins)** are **neuropeptides**. Unlike small-molecule transmitters synthesized in the cytosol of the axon terminal, neuropeptides are large molecules (chains of amino acids) synthesized as pre-prohormones in the neuronal cell body (ribosomes) and transported via axonal transport to the terminals. Orexins are specifically produced in the lateral hypothalamus and play a critical role in regulating wakefulness and appetite. ### 2. Why the Other Options are Incorrect * **Glutamate (Option A):** This is an amino acid and the primary excitatory neurotransmitter in the CNS. It is a classic small-molecule transmitter. * **GABA (Option B):** Gamma-aminobutyric acid is an amino acid derivative and the primary inhibitory neurotransmitter in the brain. It belongs to the small-molecule group. * **Dopamine (Option C):** This is a biogenic amine (specifically a catecholamine) derived from the amino acid tyrosine. All catecholamines are classified as low molecular weight, rapidly acting transmitters. ### 3. NEET-PG High-Yield Pearls * **Small-Molecule Transmitters:** Include Acetylcholine, Amino acids (Glutamate, GABA, Glycine), Biogenic amines (Dopamine, NE, Epinephrine, Serotonin, Histamine), and Purines (ATP). * **Neuropeptides:** Include Orexins, Substance P, Enkephalins, Endorphins, and hypothalamic-releasing hormones. * **Clinical Correlation:** A deficiency of **Orexin-A** in the cerebrospinal fluid is the hallmark of **Narcolepsy Type 1** (characterized by excessive daytime sleepiness and cataplexy). * **Synthesis Site:** Small molecules are recycled/synthesized in the **axon terminal**, whereas neuropeptides are synthesized in the **cell body (Soma)**.

Q: Cerebrospinal fluid is principally secreted by which structure?

Choroid plexus. **Explanation:** **Correct Answer: A. Choroid plexus** Cerebrospinal fluid (CSF) is primarily produced by the **choroid plexus**, which is a network of specialized ependymal cells and capillaries located within the lateral, third, and fourth ventricles. Approximately **70-80%** of CSF is secreted here through a combination of active transport (primarily involving the Na+/K+ ATPase pump) and ultrafiltration of plasma. The remaining 20% is formed by the brain parenchyma and the ependymal lining of the ventricles. **Analysis of Incorrect Options:** * **B. Arachnoid granulation:** These are the primary sites for the **absorption** (drainage) of CSF into the dural venous sinuses (mainly the superior sagittal sinus). They do not secrete CSF. * **C. Floor of fourth ventricle:** While the choroid plexus is present in the roof of the fourth ventricle, the floor (rhomboid fossa) contains vital cranial nerve nuclei and the area postrema, but it is not a principal site of CSF secretion. * **D. Periaqueductal grey:** This is a nucleus in the midbrain involved in pain modulation and descending inhibitory pathways; it has no role in CSF production. **High-Yield Facts for NEET-PG:** * **Rate of secretion:** Approximately **0.35 ml/min** or **500 ml/day**. * **Total Volume:** The adult human body contains about **150 ml** of CSF at any given time. * **Flow Pathway:** Lateral ventricles → Foramen of Monro → 3rd Ventricle → Aqueduct of Sylvius → 4th Ventricle → Foramina of Luschka/Magendie → Subarachnoid space. * **Composition:** Compared to plasma, CSF has **higher** concentrations of Chloride and Magnesium, and **lower** concentrations of Glucose, Protein, and Potassium.

Q: Renshaw cell inhibition is an example of which type of mechanism?

Feedback inhibition. **Explanation:** **Renshaw cell inhibition** is a classic example of **Feedback inhibition** (specifically, recurrent inhibition). In this mechanism, an Alpha motor neuron in the spinal cord sends an axon collateral to an inhibitory interneuron called the **Renshaw cell**. When the motor neuron fires, it simultaneously excites the Renshaw cell (using acetylcholine). The Renshaw cell then sends an inhibitory signal (using glycine) back to the same motor neuron and its neighbors. This creates a negative feedback loop that limits the duration and frequency of the motor neuron's discharge, preventing muscle over-activity and ensuring "lateral inhibition" for motor precision. **Why other options are incorrect:** * **Feed-forward inhibition:** This occurs when an afferent signal triggers an inhibitory interneuron to inhibit an antagonist muscle *before* or during the activation of the agonist (e.g., reciprocal inhibition in the stretch reflex). It does not involve a return loop to the original neuron. * **Direct inhibition:** This generally refers to post-synaptic inhibition where an inhibitory neurotransmitter directly hyperpolarizes a neuron (like GABA acting on a receptor), but it doesn't describe the specific structural circuit architecture seen with Renshaw cells. **High-Yield Clinical Pearls for NEET-PG:** * **Neurotransmitter:** Renshaw cells use **Glycine** as their primary inhibitory neurotransmitter. * **Clinical Correlation (Strychnine Poisoning):** Strychnine acts as a competitive antagonist of glycine receptors. By blocking Renshaw cell inhibition, it leads to unchecked motor neuron firing, resulting in severe muscle spasms and convulsions (Opisthotonus). * **Tetanus Toxin:** Prevents the release of glycine from Renshaw cells, leading to "lockjaw" and spastic paralysis.

Q: What is the normal CSF pressure (lumbar)?

70 - 180 mm H2O. **Explanation:** The normal Cerebrospinal Fluid (CSF) pressure in a healthy adult, when measured via lumbar puncture in the **lateral recumbent position**, typically ranges from **70 to 180 mm H₂O** (or 5–15 mmHg). This pressure is a reflection of the balance between CSF production by the choroid plexus and its absorption through the arachnoid villi into the dural venous sinuses. **Analysis of Options:** * **Option A (Correct):** 70–180 mm H₂O is the standard physiological range. In some texts, up to 200 mm H₂O is considered the upper limit of normal in relaxed adults. * **Option B:** 50–100 mm H₂O is too narrow and represents the lower end of the spectrum. Pressures consistently below 60 mm H₂O may indicate CSF hypotension (e.g., CSF leak). * **Option C:** > 200 mm H₂O is clinically defined as **Intracranial Hypertension**. This can be seen in conditions like idiopathic intracranial hypertension (IIH), meningitis, or intracranial space-occupying lesions. * **Option D:** 150–200 mm H₂O represents only the high-normal range and excludes the typical baseline values. **High-Yield Clinical Pearls for NEET-PG:** 1. **Positioning:** CSF pressure is significantly higher in the sitting position (approx. 200–300 mm H₂O) due to hydrostatic pressure; hence, diagnostic measurements are standardized to the lateral decubitus position. 2. **Queckenstedt's Test:** Historically used to identify spinal canal obstruction; pressing on jugular veins normally causes a rapid rise in CSF pressure. 3. **Pediatric Values:** Normal CSF pressure in infants is lower, generally ranging from 10 to 100 mm H₂O. 4. **Conversion:** 1.36 mm H₂O ≈ 1 mmHg. (Remember: CSF pressure is usually recorded in mm H₂O, while blood pressure is in mmHg).

Question 1

In Cerebrospinal Fluid (CSF), what substance is NOT typically lower in concentration when compared to blood plasma?

Accepted Answer

Chloride

Answer

Calcium (Ca++)

Answer

Sodium (Na+)

Answer

Cells

Question 2

Which of the following is NOT a peptide neurotransmitter?

Accepted Answer

Serotonin

Answer

Enkephalin

Answer

Substance P

Answer

Endorphin

Question 3

Which of the following is NOT a low molecular weight neurotransmitter?

Accepted Answer

Orexins

Answer

Glutamate

Answer

GABA

Answer

Dopamine

Question 4

What is true about Galvanic Skin Response?

Accepted Answer

It is an indicator of thermal sweating.

Answer

It is affected by sweating because sweat contains electrolytes.

Answer

It falls when sympathetic activity is high.

Answer

It falls in association with yogic practice.

Question 5

Which substance does not cross the blood-brain barrier?

Accepted Answer

Insulin

Answer

Urea

Answer

Testosterone

Answer

Glucose

Question 6

Cerebrospinal fluid is principally secreted by which structure?

Accepted Answer

Choroid plexus

Answer

Arachnoid granulation

Answer

Floor of fourth ventricle

Answer

Periaqueductal grey

Question 7

Renshaw cell inhibition is an example of which type of mechanism?

Accepted Answer

Feedback inhibition

Answer

Feed forward inhibition

Answer

Direct inhibition

Answer

None of the above

Question 8

A man loses his right hand in a farm accident. Four years later, he experiences episodes of severe pain in the missing hand (phantom limb pain). A detailed PET scan study of his cerebral cortex might be expected to show what?

Accepted Answer

Projection of fibers from neighboring sensory areas into the right hand area of his left SI.

Answer

Expansion of the right hand area in his right somatic sensory area I (SI).

Answer

Expansion of the right hand area in his left SI.

Answer

Projection of fibers from neighboring sensory areas into the right hand area of his right SI.

Question 9

What is the normal CSF pressure (lumbar)?

Accepted Answer

70 - 180 mm H2O

Answer

50 - 100 mm H2O

Answer

> 200 mm H2O

Answer

150 - 200 mm H2O

Question 10

The mechanism of short-term to long-term memory formation occurs in which brain structure and is called what?

Accepted Answer

Hippocampus; Long-term potentiation

Answer

Neocortex; Post-tetanic potentiation

Answer

Amygdala; Long-term depression

Answer

Cerebellum; Post-term potentiation

Neurophysiology — MCQs

Neurophysiology — MCQs

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