Neurophysiology Practice Questions

Q: What is the major inhibitory neurotransmitter in the nervous system?

Gamma-amino butyric acid. **Explanation:** The correct answer is **Gamma-amino butyric acid (GABA)**. In the central nervous system (CNS), neurotransmitters are classified based on their effect on the post-synaptic membrane. GABA is the **primary inhibitory neurotransmitter in the brain**. It acts by binding to GABA-A (ionotropic) or GABA-B (metabotropic) receptors, leading to an influx of chloride ions or efflux of potassium ions, respectively. This causes **hyperpolarization** of the neuron, making it less likely to fire an action potential. **Analysis of Options:** * **Glutamate (Option A):** This is the major **excitatory** neurotransmitter in the CNS. It is a precursor to GABA (via the enzyme Glutamic Acid Decarboxylase). * **Aspartate (Option B):** Another excitatory neurotransmitter, primarily found in the visual cortex and spinal cord. * **Taurine (Option D):** While it has inhibitory properties, it is considered a neuromodulator rather than the "major" inhibitory neurotransmitter. **High-Yield Clinical Pearls for NEET-PG:** * **GABA vs. Glycine:** While GABA is the major inhibitor in the **brain**, **Glycine** is the major inhibitory neurotransmitter in the **spinal cord**. * **GABA-A Receptors:** These are the target for several important drug classes, including **Benzodiazepines** (increase frequency of Cl- channel opening) and **Barbiturates** (increase duration of Cl- channel opening). * **Strychnine Poisoning:** This toxin acts by antagonizing Glycine receptors, leading to unchecked muscle contractions and convulsions. * **Tetanus Toxin:** Inhibits the release of GABA and Glycine from Renshaw cells in the spinal cord, causing spastic paralysis.

Q: Lesion of the medial temporal lobe is associated with which of the following conditions?

Auditory amnesia. **Explanation:** The **medial temporal lobe** contains the primary auditory cortex (Heschl’s gyri) and the auditory association areas. Damage to these regions, particularly in the dominant hemisphere, impairs the brain's ability to process and interpret sounds despite intact hearing. This condition is known as **Auditory Amnesia** (or auditory agnosia), where the patient can hear sounds but cannot recognize or attach meaning to them (e.g., failing to recognize a ringing phone or spoken words). **Analysis of Options:** * **A. Auditory Amnesia (Correct):** As the temporal lobe houses the auditory processing centers, lesions here specifically disrupt the recognition of auditory stimuli. * **B. Agnosia:** This is a broad, non-specific term referring to the inability to interpret sensory information. While auditory amnesia is a *type* of agnosia, "Auditory Amnesia" is the more specific and clinically accurate answer for temporal lobe lesions. * **C. Visual Amnesia:** This is typically associated with lesions in the **occipital lobe** or the visual association areas (ventral stream), not the primary temporal auditory areas. * **D. Alexia:** Also known as "word blindness," this is the inability to read. It is usually caused by lesions in the **dominant angular gyrus** (parietal-temporal-occipital junction). **High-Yield Clinical Pearls for NEET-PG:** * **Kluver-Bucy Syndrome:** Bilateral medial temporal lobe (amygdala) lesions lead to hyperorality, hypersexuality, and visual agnosia. * **Wernicke’s Aphasia:** Caused by a lesion in the posterior part of the superior temporal gyrus; characterized by fluent but meaningless speech ("word salad"). * **Hippocampus:** Located in the medial temporal lobe; bilateral destruction leads to the inability to form new memories (**Anterograde Amnesia**).

Q: Cerebral ischemia occurs when cerebral blood flow is less than which of the following values?

20 ml/100g/minute. **Explanation:** The normal Cerebral Blood Flow (CBF) in an adult is approximately **50–55 ml/100g/min**. The brain is highly sensitive to fluctuations in blood flow, and symptoms of ischemia manifest progressively as flow decreases. **Why Option B is Correct:** When CBF falls below **20 ml/100g/min**, electrical activity in the neurons begins to fail. This threshold marks the onset of **cerebral ischemia**, where the brain can no longer maintain normal functional activity, leading to clinical symptoms like aphasia or hemiparesis. This is often referred to as the "threshold of electrical failure." **Analysis of Incorrect Options:** * **Option A (10 ml/100g/min):** This is the threshold for **irreversible neuronal death (infarction)**. At this level, ionic pumps fail, leading to massive calcium influx and cell necrosis. The zone between 10 and 20 ml/100g/min is known as the **Ischemic Penumbra**—tissue that is functionally silent but potentially salvageable. * **Option C (40 ml/100g/min):** At this level, the brain is still relatively well-perfused. While it is below the average normal, autoregulatory mechanisms usually prevent clinical ischemia at this stage. * **Option D (50 ml/100g/min):** This represents the **normal physiological CBF**. No ischemia occurs at this value. **High-Yield Clinical Pearls for NEET-PG:** * **Cerebral Perfusion Pressure (CPP):** Calculated as Mean Arterial Pressure (MAP) minus Intracranial Pressure (ICP). Normal CPP is 70–90 mmHg. * **Autoregulation:** The brain maintains constant CBF as long as the MAP stays between **60 and 160 mmHg**. * **Critical Thresholds:** * Normal: 50 ml/100g/min * Ischemia (Electrical failure): <20 ml/100g/min * Infarction (Membrane failure): <10 ml/100g/min

Q: What is the most important source of noradrenergic innervation to the cerebral cortex?

Locus coeruleus. **Explanation:** The **Locus coeruleus (LC)**, located in the posterior area of the rostral pons, is the primary source of **norepinephrine (noradrenaline)** in the central nervous system. It contains the largest collection of noradrenergic neurons in the brain. These neurons have highly branched axons that project widely to almost all parts of the neuraxis, including the entire cerebral cortex, cerebellum, and spinal cord. This system plays a crucial role in regulating arousal, attention, sleep-wake cycles, and the stress response. **Analysis of Incorrect Options:** * **Basal nucleus of Meynert:** This is the major source of **cholinergic (Acetylcholine)** innervation to the cerebral cortex. It is clinically significant in Alzheimer’s disease, where these neurons undergo degeneration. * **Caudate nucleus:** Part of the basal ganglia, it is primarily involved in motor control and executive functions. Its main neurotransmitters are **GABA** (output) and **Dopamine** (input from the substantia nigra). * **Raphe nucleus:** This is the principal source of **serotonergic (5-HT)** innervation to the brain and spinal cord, playing a key role in mood regulation and pain modulation. **High-Yield NEET-PG Pearls:** * **Locus Coeruleus:** Appears "blue" in gross specimens due to the presence of **neuromelanin** granules (formed by the polymerization of norepinephrine). * **Arousal:** The LC is a key component of the Reticular Activating System (RAS); it is most active during wakefulness and silent during REM sleep. * **Clinical Link:** Decreased activity in the LC is associated with depression, while overactivity is linked to anxiety and panic disorders.

Q: Intensity of sensory stimulation is directly related to:

Frequency of action potential. ### Explanation **1. Why "Frequency of Action Potential" is Correct:** In neurophysiology, the intensity of a stimulus is encoded through **Frequency Modulation**. According to the **Adrian-Zotterman Principle**, as the strength of a sensory stimulus increases, the magnitude of the receptor potential (graded potential) increases. This leads to a higher frequency of action potentials generated in the sensory nerve fiber. Since action potentials follow the "All-or-None Law," the nervous system cannot use the size of the spike to signal intensity; instead, it uses the number of spikes per unit of time. **2. Why Other Options are Incorrect:** * **Amplitude of Action Potential:** Action potentials are "All-or-None" phenomena. Once the threshold is reached, the amplitude remains constant regardless of the stimulus strength. Increasing intensity does not make the spike "taller." * **Duration of Action Potential:** The duration is determined by the kinetics of voltage-gated $Na^+$ and $K^+$ channels and the refractory period. It does not change in response to stimulus intensity. * **All of the Above:** This is incorrect because only frequency is variable in relation to intensity. **3. High-Yield Clinical Pearls for NEET-PG:** * **Weber-Fechner Law:** States that the intensity of a sensation is proportional to the logarithm of the stimulus intensity. * **Recruitment (Spatial Summation):** Besides frequency (temporal coding), the body also signals intensity by activating a greater number of sensory receptors in the stimulated area. * **Accommodation:** If a stimulus is applied constantly, the frequency of action potentials may decrease over time (e.g., rapidly adapting Pacinian corpuscles vs. slowly adapting Merkel discs). * **Refractory Period:** The upper limit of action potential frequency is determined by the **Absolute Refractory Period**.

Question 1

Cerebello-cerebral connections are important for all of the following functions except?

Accepted Answer

Detection and damping of oscillatory muscle movements

Answer

Error detection during locomotion

Answer

Comparison of cerebral cortical command and muscle action

Answer

Posture and equilibrium

Question 2

On electromyography, all of the following features suggest denervation, except?

Accepted Answer

Small short-duration polyphasic action potentials

Answer

Unregulated firing of individual muscle fibers

Answer

Presence of positive sharp waves

Answer

Spontaneous firing of motor units

Question 3

What is the major inhibitory neurotransmitter in the nervous system?

Accepted Answer

Gamma-amino butyric acid

Answer

Glutamate

Answer

Aspartate

Answer

Taurine

Question 4

Lesion of the medial temporal lobe is associated with which of the following conditions?

Accepted Answer

Auditory amnesia

Answer

Agnosia

Answer

Visual amnesia

Answer

Alexia

Question 5

The blood-brain barrier is thought to exist because capillaries in the central nervous system possess which of the following characteristics?

Accepted Answer

A few pinocytic vesicles

Answer

Discontinuous basal lamina

Answer

Fenestrae with diaphragms

Answer

Fenestrae without diaphragms

Question 6

Cerebral ischemia occurs when cerebral blood flow is less than which of the following values?

Accepted Answer

20 ml/100g/minute

Answer

10 ml/100g/minute

Answer

40 ml/100g/minute

Answer

50 ml/100g/minute

Question 7

Working memory can be affected due to lesion in which of the following structures?

Accepted Answer

Hypothalamus

Answer

Thalamus

Answer

Mammillary body

Answer

Dorsolateral frontal lobe

Question 8

What is the most important source of noradrenergic innervation to the cerebral cortex?

Accepted Answer

Locus coeruleus

Answer

Basal nucleus of Meynert

Answer

Caudate nucleus

Answer

Raphe nucleus

Question 9

The deep cerebellar nuclei exert their effect on other brain centers by which mechanism?

Accepted Answer

Comparing the inhibitory effects of Purkinje cells with the excitatory effects of afferent collateral axons.

Answer

Summating the inputs from mossy fibers and climbing fibers that synapse on the deep cerebellar nuclei.

Answer

Considering the temporal synaptic effects of granular and parallel fiber input.

Answer

Evaluating the Purkinje cell's excitatory influences and the granule cell's inhibitory input.

Question 10

Intensity of sensory stimulation is directly related to:

Accepted Answer

Frequency of action potential

Answer

Duration of action potential

Answer

Amplitude of action potential

Answer

All of the above.

Neurophysiology — MCQs

Neurophysiology — MCQs

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