Sensory Systems Practice Questions

Q: The Lewis triple response is caused due to:

Histamine. The **Lewis Triple Response** is a classic physiological reaction of the skin to mechanical injury or chemical irritation. It is primarily mediated by the release of **Histamine** from mast cells. ### Why Histamine is the Correct Answer When the skin is firmly stroked or injured, mast cells release histamine. This chemical mediator acts on H1 receptors to produce three distinct components: 1. **Red Reaction (Red Spot):** Localized capillary dilatation occurring within seconds. 2. **Flare:** A spreading redness caused by the **Axon Reflex**, where sensory nerve stimulation leads to the release of Substance P and CGRP, causing arteriolar dilatation. 3. **Wheal:** Localized edema (swelling) due to increased capillary permeability and exudation of fluid. ### Why Other Options are Incorrect * **B. Axon Reflex:** While the axon reflex is the *mechanism* responsible for the "Flare" component, it is not the primary *cause* of the entire triple response. Histamine is the chemical trigger that initiates the reflex. * **C. Injury to Endothelium:** While physical injury triggers the response, the physiological manifestation is a result of chemical mediators (histamine) rather than direct structural damage to the endothelium itself. * **D. Increased Permeability:** This is a *consequence* of histamine release (leading to the Wheal), not the underlying cause of the triple response. ### High-Yield NEET-PG Pearls * **Mediator:** Histamine is the chief mediator. * **Nerve Involvement:** The "Flare" is the only component that requires an intact nerve supply (Axon Reflex). If the sensory nerves are severed and allowed to degenerate, the flare will be absent. * **Antihistamines:** Pre-treatment with H1-receptor antagonists can significantly diminish all three components of the Lewis triple response.

Q: Which sensory system has the most direct connection to the brain?

Smell. **Explanation:** The correct answer is **Smell (Olfaction)**. **Why Smell is the correct answer:** The olfactory system is unique among the special senses because it is the only sensory pathway that reaches the cerebral cortex (specifically the piriform cortex) **without first synapsing in the Thalamus**. Olfactory receptor neurons in the nasal epithelium send axons through the cribriform plate directly to the olfactory bulb. From there, the olfactory tract projects directly to the primary olfactory cortex (limbic system). This anatomical arrangement makes it the most direct connection to the brain, bypassing the "sensory relay station" of the thalamus. **Why other options are incorrect:** * **A. Taste (Gustation):** Taste signals from the tongue (via Cranial Nerves VII, IX, and X) must synapse in the Nucleus Tractus Solitarius (NTS) and then the **Ventral Posteromedial (VPM) nucleus** of the thalamus before reaching the gustatory cortex. * **C. Vision:** Visual information from the retina travels via the optic nerve and tract to synapse in the **Lateral Geniculate Nucleus (LGN)** of the thalamus before reaching the primary visual cortex (Area 17). * **D. Hearing:** Auditory signals undergo a complex pathway (Cochlear nuclei → Superior Olive → Lateral Lemniscus → Inferior Colliculus) and must synapse in the **Medial Geniculate Nucleus (MGN)** of the thalamus before reaching the auditory cortex. **High-Yield NEET-PG Pearls:** * **Thalamic Relays:** Remember **L**GN for **L**ight (Vision) and **M**GN for **M**usic (Hearing). * **Limbic Connection:** Because olfaction projects directly to the amygdala and hippocampus, it is the sense most strongly linked to emotional memory. * **Clinical Correlation:** Anosmia (loss of smell) can be an early diagnostic marker for neurodegenerative diseases like Parkinson’s or Alzheimer’s, or a result of head trauma involving the cribriform plate.

Q: A 10-year-old girl with type I diabetes develops a neuropathy limited to sensory neurons with free nerve endings. Quantitative sensory testing will reveal higher-than-normal thresholds for the detection of which of the following?

Temperature. **Explanation:** The core of this question lies in matching specific sensory receptors with the modalities they transmit. **Free nerve endings** are the most basic type of sensory receptor and are primarily responsible for detecting **pain (nociception)** and **temperature (thermal sensation)**. They are also involved in crude touch and itch. 1. **Why Temperature is Correct:** Thermoreceptors (for both cold and heat) are histologically classified as free nerve endings. If a neuropathy selectively affects these endings, the patient will require a stronger stimulus (a higher threshold) to perceive changes in temperature. 2. **Why Other Options are Incorrect:** * **Fine Touch (A):** This is primarily mediated by **Meissner’s corpuscles** (in non-hairy skin) and **Merkel discs**. These are encapsulated or specialized receptors, not free nerve endings. * **Vibration (B):** High-frequency vibration is detected by **Pacinian corpuscles**, while low-frequency vibration is detected by Meissner’s corpuscles. * **Pressure (C):** Deep pressure is sensed by **Pacinian corpuscles** and sustained pressure by **Ruffini endings**. **High-Yield NEET-PG Pearls:** * **A-delta fibers:** Thinly myelinated; carry "fast" pain and cold temperature. * **C fibers:** Unmyelinated; carry "slow" pain and warm temperature. Both terminate as free nerve endings. * **Large Myelinated Fibers (A-beta):** Carry vibration, proprioception, and fine touch. These are usually affected first in common diabetic "glove and stocking" neuropathy, but this specific question describes a selective "small fiber" involvement. * **Mnemonic:** **P**acinian = **P**ressure/vibration; **M**eissner = **M**oving touch/vibration.

Q: A colorblind male has decreased vision for red light, which appears very light compared to other colors. Which of the following is the likely anomaly in him?

Protanomaly. ### Explanation **1. Why Protanomaly is the Correct Answer:** Color vision is mediated by three types of cone pigments: erythrolabe (red), chlorolabe (green), and cyanolabe (blue). In **Protanomaly**, there is a functional but **defective red-cone pigment** (L-cone). Because the sensitivity of the red cones is shifted toward shorter wavelengths, red light appears dimmer, less saturated, and "lighter" or greyish compared to normal vision. This specific clinical presentation—dimness or decreased vision for red light—is the hallmark of protan defects. **2. Analysis of Incorrect Options:** * **B. Deuteranomaly:** This is the most common type of color blindness. It involves a **defective green-cone pigment** (M-cone). While it causes red-green confusion, it does not typically result in the "dimming" of red light seen in protanopes. * **C. Tritanomaly:** This is a rare condition involving a **defective blue-cone pigment** (S-cone). Patients have difficulty distinguishing between blue and yellow. * **D. Butanomaly:** This is a **fictitious term** and does not exist in medical literature. **3. Clinical Pearls for NEET-PG:** * **Terminology:** Suffix **"-anomaly"** means the pigment is present but defective (Trichromat); suffix **"-anopia"** means the pigment is completely absent (Dichromat). * **Prefixes:** *Prot-* (1st/Red), *Deuter-* (2nd/Green), *Trit-* (3rd/Blue). * **Inheritance:** Red-green color blindness (Protan/Deuteran) is **X-linked recessive**, making it significantly more common in males. Tritan defects are usually autosomal dominant. * **Diagnosis:** The **Ishihara Pseudoisochromatic Plate** test is the most common screening tool, though it primarily detects red-green defects. The **Nagel Anomaloscope** is the gold standard for distinguishing between dichromacy and anomalous trichromacy.

Q: Facial nerve stimulation during testing is indicated by the contraction of which muscle?

Orbicularis oris. **Explanation:** The **Facial Nerve (Cranial Nerve VII)** is the primary motor nerve for the muscles of facial expression. These muscles are derived from the **second branchial arch**. **Why Orbicularis Oris is Correct:** The **Orbicularis oris** is a sphincter muscle surrounding the mouth responsible for closing and puckering the lips. It is directly innervated by the marginal mandibular and buccal branches of the facial nerve. During clinical testing (such as the Chvostek sign or direct nerve stimulation), contraction of this muscle confirms the functional integrity of the facial nerve's motor fibers. **Why Other Options are Incorrect:** * **A & B (Temporalis and Masseter):** These are **muscles of mastication**, which are derived from the first branchial arch. They are innervated by the mandibular branch of the **Trigeminal Nerve (CN V3)**, not the facial nerve. * **C (Sternocleidomastoid):** This muscle is responsible for head rotation and flexion. It is innervated by the **Spinal Accessory Nerve (CN XI)**. **High-Yield Clinical Pearls for NEET-PG:** * **Chvostek Sign:** Tapping the facial nerve in front of the tragus causes twitching of the facial muscles (including orbicularis oris); this is a classic sign of **hypocalcemia** (latent tetany). * **Bell’s Palsy:** Lower motor neuron (LMN) lesion of CN VII resulting in ipsilateral paralysis of all facial muscles, including the inability to close the eye (Orbicularis oculi) or whistle (Orbicularis oris). * **Mnemonic for Branches:** "To Zanzibar By Motor Car" (Temporal, Zygomatic, Buccal, Marginal Mandibular, Cervical).

Question 1

The Lewis triple response is caused due to:

Accepted Answer

Histamine

Answer

Axon reflex

Answer

Injury to endothelium

Answer

Increased permeability

Question 2

The ability to detect two points simultaneously applied to the skin is based on which physiologic mechanism?

Accepted Answer

Lateral inhibition

Answer

Presynaptic inhibition

Answer

Feedback inhibition

Answer

Feed-forward inhibition

Question 3

Taste receptors are a type of:

Accepted Answer

Chemoreceptor.

Answer

Primary taste sensations are spatially separated on the surface of the tongue.

Answer

Afferent fibres of V, VII, and IX.

Answer

All of the above.

Question 4

Which sensory system has the most direct connection to the brain?

Accepted Answer

Smell

Answer

Taste

Answer

Vision

Answer

Hearing

Question 5

Phantom limb phenomenon is best explained by which of the following laws?

Accepted Answer

Law of projection

Answer

Weber's law

Answer

Fechner's law of degeneration

Answer

Pascal's law

Question 6

A 10-year-old girl with type I diabetes develops a neuropathy limited to sensory neurons with free nerve endings. Quantitative sensory testing will reveal higher-than-normal thresholds for the detection of which of the following?

Accepted Answer

Temperature

Answer

Fine touch

Answer

Vibration

Answer

Pressure

Question 7

Joint position sense in the left lower limb is impaired in all EXCEPT which of the following?

Accepted Answer

Left thalamic lesion

Answer

Left dorsal column lesion in the thoracic region

Answer

Left cerebellar lesion

Answer

Right lenticulostriate artery thrombosis

Question 8

A colorblind male has decreased vision for red light, which appears very light compared to other colors. Which of the following is the likely anomaly in him?

Accepted Answer

Protanomaly

Answer

Dutanomaly

Answer

Tritanomaly

Answer

Butanomaly

Question 9

Facial nerve stimulation during testing is indicated by the contraction of which muscle?

Accepted Answer

Orbicularis oris

Answer

Temporalis

Answer

Masseter

Answer

Sternocleidomastoid

Question 10

During accommodation, what changes occur in the lens?

Accepted Answer

Reduction in the radius of curvature of the anterior surface of the lens only

Answer

Reduction in the radius of curvature of the posterior surface of the lens only

Answer

Increase in the radius of curvature of the posterior surface of the lens only

Answer

Both the radius of curvature of the anterior and posterior surfaces reduce

Sensory Systems — MCQs

Sensory Systems — MCQs

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