Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 291: The Lewis triple response is caused due to:

A. Histamine (Correct Answer)
B. Axon reflex
C. Injury to endothelium
D. Increased permeability

Explanation: 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.

Question 292: The ability to detect two points simultaneously applied to the skin is based on which physiologic mechanism?

A. Presynaptic inhibition
B. Feedback inhibition
C. Feed-forward inhibition
D. Lateral inhibition (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Lateral inhibition** **Mechanism:** Two-point discrimination is the ability to discern that two nearby objects touching the skin are truly two distinct points, rather than one. This is primarily achieved through **lateral inhibition**. When a stimulus touches the skin, it excites a central set of neurons. Simultaneously, via inhibitory interneurons, it inhibits the firing of adjacent (lateral) neurons. This process "sharpens" the sensory boundaries and increases the contrast between the stimulated area and the surrounding tissue, allowing the somatosensory cortex to distinguish two separate points of contact. **Analysis of Incorrect Options:** * **A. Presynaptic inhibition:** This occurs when an inhibitory neuron releases neurotransmitters (like GABA) onto the axon terminal of another neuron to reduce its neurotransmitter release. While it modulates signal intensity, it is not the primary mechanism for spatial resolution. * **B. Feedback inhibition:** This is a regulatory mechanism where the output of a pathway inhibits its own further activation (e.g., Renshaw cells in the spinal cord). It limits the duration and intensity of a signal rather than enhancing spatial contrast. * **C. Feed-forward inhibition:** This involves one neuron activating an inhibitory interneuron that then inhibits a downstream target. It is common in the cerebellum for coordinating timing but is not the basis for two-point discrimination. **High-Yield Facts for NEET-PG:** * **Weber’s Law:** Relates to the "just noticeable difference" in stimulus intensity. * **Density of Receptors:** Two-point discrimination is finest where receptor density is highest (e.g., fingertips, lips) and the receptive fields are smallest. * **Clinical Test:** The **Compass test** is used to measure the two-point threshold. The smallest distance is found on the fingertips (approx. 2mm) and the largest on the back (approx. 40-70mm). * **Pathway:** This information is carried via the **Dorsal Column-Medial Lemniscal (DCML) system**.

Question 293: Taste receptors are a type of:

A. Primary taste sensations are spatially separated on the surface of the tongue.
B. Chemoreceptor. (Correct Answer)
C. Afferent fibres of V, VII, and IX.
D. All of the above.

Explanation: ### Explanation **Correct Answer: B. Chemoreceptor** Taste receptors (gustatory receptors) are specialized epithelial cells located within taste buds. They are classified as **chemoreceptors** because they function by detecting specific chemical substances (tastants) dissolved in saliva. When a tastant binds to its specific receptor (G-protein coupled receptors for sweet, bitter, and umami; or ion channels for salty and sour), it triggers a receptor potential that leads to neurotransmitter release and subsequent activation of sensory neurons. **Analysis of Incorrect Options:** * **Option A:** This refers to the outdated "Tongue Map" theory. Modern physiology confirms that all areas of the tongue containing taste buds can respond to all five primary taste sensations. There is no strict spatial separation, though some areas may have slightly lower thresholds for certain tastes. * **Option C:** While taste involves cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus), the **Trigeminal nerve (V)** does not carry taste (gustatory) fibers. It carries general somatic sensations (touch, temperature, pain) from the tongue. * **Option D:** Since A and C are incorrect, "All of the above" is invalid. **High-Yield NEET-PG Pearls:** 1. **Innervation:** Anterior 2/3 of the tongue is supplied by the **Chorda Tympani (CN VII)**; Posterior 1/3 by the **Glossopharyngeal (CN IX)**; and the base of the tongue/epiglottis by the **Vagus (CN X)**. 2. **Receptor Type:** Taste receptors are **modified epithelial cells**, not neurons (unlike olfactory receptors, which are true bipolar neurons). 3. **Pathway:** All taste fibers synapse in the **Nucleus Tractus Solitarius (NTS)** in the medulla before ascending to the Thalamus (VPM nucleus) and the Gustatory Cortex (Insula). 4. **Papillae:** The **Vallate** and **Foliate** papillae contain numerous taste buds, while **Fungiform** papillae contain few. **Filiform** papillae contain no taste buds and are purely mechanical.

Question 294: Which sensory system has the most direct connection to the brain?

A. Taste
B. Smell (Correct Answer)
C. Vision
D. Hearing

Explanation: **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.

Question 295: Phantom limb phenomenon is best explained by which of the following laws?

A. Law of projection (Correct Answer)
B. Weber's law
C. Fechner's law of degeneration
D. Pascal's law

Explanation: **Explanation:** **1. Why Law of Projection is Correct:** The **Law of Projection** states that regardless of where a sensory pathway is stimulated along its course to the cortex, the conscious sensation produced is always referred (projected) to the location of the **specific receptor** where the pathway begins. In **Phantom Limb Syndrome**, even though the limb is absent, the remaining nerve fibers in the stump or the sensory neurons in the thalamus/cortex are stimulated (due to irritation or cortical remodeling). The brain interprets these signals as coming from the original site of the receptors (the missing hand or foot), leading the patient to feel sensations or pain in a non-existent limb. **2. Why Other Options are Incorrect:** * **Weber’s Law:** This relates to sensory perception and states that the "just noticeable difference" (JND) between two stimuli is proportional to the magnitude of the original stimulus. It deals with intensity discrimination, not localization. * **Fechner’s Law:** Derived from Weber’s Law, it states that the intensity of a sensation is proportional to the logarithm of the stimulus intensity ($S = k \log I$). * **Pascal’s Law:** This is a principle of fluid mechanics (physics) stating that pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid. It has no relevance to sensory physiology. **3. Clinical Pearls for NEET-PG:** * **Bell-Magendie Law:** States that dorsal roots are sensory and ventral roots are motor. * **Muller’s Law (Law of Specific Nerve Energies):** No matter how a specific sensory nerve is stimulated (electrically, mechanically, or thermally), the sensation perceived is always the same (e.g., stimulating the optic nerve always produces a sensation of light). * **Cortical Plasticity:** While the Law of Projection explains the *localization*, the underlying cause of phantom limb is often **reorganization of the somatosensory cortex (S1)**, where adjacent areas (like the face) "take over" the cortical area of the missing limb.

Question 296: 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?

A. Fine touch
B. Vibration
C. Pressure
D. Temperature (Correct Answer)

Explanation: **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.

Question 297: Joint position sense in the left lower limb is impaired in all EXCEPT which of the following?

A. Left dorsal column lesion in the thoracic region
B. Left cerebellar lesion
C. Left thalamic lesion (Correct Answer)
D. Right lenticulostriate artery thrombosis

Explanation: To understand this question, we must trace the pathway of **conscious proprioception** (joint position sense) via the **Dorsal Column-Medial Lemniscus (DCML)** system and **unconscious proprioception** via the **Spinocerebellar** tracts. ### 1. Why "Left Thalamic Lesion" is the Correct Answer Sensory information from the **left** side of the body crosses the midline (decussates) at the level of the **medulla** (internal arcuate fibers). Therefore, all structures above the medulla, including the **thalamus**, process sensory information from the **contralateral** (opposite) side. A **left** thalamic lesion would result in sensory loss on the **right** side of the body. Thus, it would not impair joint position sense in the left lower limb. ### 2. Analysis of Incorrect Options * **A. Left dorsal column lesion (Thoracic):** The DCML pathway remains **ipsilateral** (same side) in the spinal cord. A lesion in the left thoracic dorsal column interrupts fibers from the left lower limb before they decussate. * **B. Left cerebellar lesion:** The cerebellum receives **unconscious proprioception** via the spinocerebellar tracts, which are primarily **ipsilateral**. A left-sided lesion leads to ipsilateral ataxia and impaired joint positioning (dysmetria). * **D. Right lenticulostriate artery thrombosis:** These arteries supply the **Internal Capsule**. Since the DCML pathway has already crossed in the medulla, the sensory fibers for the left limb are now on the right side. A right-sided capsular stroke causes **contralateral** (left-sided) sensory loss. ### 3. High-Yield Clinical Pearls for NEET-PG * **Decussation Rule:** DCML decussates in the **Medulla**; Spinothalamic (pain/temp) decussates in the **Spinal Cord** (anterior commissure). * **Lesion Localization:** * Below Medulla = Ipsilateral loss. * Above Medulla (Pons, Midbrain, Thalamus, Cortex) = Contralateral loss. * **Tabes Dorsalis:** A classic NEET-PG topic involving bilateral destruction of dorsal columns (syphilis), leading to loss of position sense and a "stamping gait."

Question 298: 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?

A. Protanomaly (Correct Answer)
B. Dutanomaly
C. Tritanomaly
D. Butanomaly

Explanation: ### 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.

Question 299: Facial nerve stimulation during testing is indicated by the contraction of which muscle?

A. Temporalis
B. Masseter
C. Sternocleidomastoid
D. Orbicularis oris (Correct Answer)

Explanation: **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 300: During accommodation, what changes occur in the lens?

A. Reduction in the radius of curvature of the anterior surface of the lens only (Correct Answer)
B. Reduction in the radius of curvature of the posterior surface of the lens only
C. Increase in the radius of curvature of the posterior surface of the lens only
D. Both the radius of curvature of the anterior and posterior surfaces reduce

Explanation: ### Explanation **1. Why Option A is Correct:** Accommodation is the process by which the eye increases its refractive power to focus on near objects. According to the **Helmholtz Theory**, when we look at a near object, the ciliary muscles contract, leading to the relaxation of the suspensory ligaments (zonules). This releases the tension on the lens capsule, allowing the lens to assume its natural, more spherical shape due to its inherent elasticity. During this process, the **anterior surface of the lens** undergoes a significant change, becoming more convex. In geometric terms, an increase in convexity corresponds to a **reduction in the radius of curvature**. While the lens thickness increases (anteroposterior diameter), the change is almost exclusively localized to the anterior surface. **2. Why Other Options are Incorrect:** * **Options B & C:** The **posterior surface** of the lens is already highly curved and is relatively fixed against the vitreous humor. During accommodation, its curvature remains virtually unchanged. * **Option D:** While it is a common misconception that both surfaces change, physiological studies (and Purkinje image shifts) confirm that the posterior surface radius remains constant at approximately 6 mm, while the anterior radius reduces significantly from 10 mm to about 6 mm. **3. NEET-PG High-Yield Pearls:** * **Purkinje-Sanson Images:** During accommodation, the **3rd Purkinje image** (from the anterior lens surface) becomes smaller and moves forward, while the 4th image (from the posterior surface) remains unchanged. * **The Accommodation Triad:** 1. Pupillary constriction (miosis), 2. Convergence of eyeballs, 3. Contraction of ciliary muscle (leading to lens thickening). * **Presbyopia:** A condition where the lens loses its elasticity with age, leading to a failure of accommodation and a receding "near point." * **Innervation:** The ciliary muscle is supplied by **parasympathetic fibers** via the Short Ciliary Nerves (Cranial Nerve III).

Practice by Chapter

General Sensory Physiology

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Somatosensation

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Pain Physiology

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Vision and Optics

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Retinal Physiology

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Visual Pathways and Processing

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Auditory System

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Vestibular System

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Taste and Smell

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Sensory Integration

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