Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 271: Proprioception is mediated by which of the following pathways?

A. Lateral spinothalamic tract
B. Ventral spinothalamic tract
C. Dorsal column (Correct Answer)
D. Corticospinal tract

Explanation: **Explanation:** The **Dorsal Column-Medial Lemniscal (DCML) pathway** is the primary sensory tract responsible for conveying "fine" sensations. It carries **conscious proprioception** (sense of position and movement), fine touch, pressure, vibration, and two-point discrimination. These fibers are large, myelinated, and have high conduction velocities. They ascend ipsilaterally in the spinal cord and decussate (cross over) in the medulla at the level of the internal arcuate fibers. **Analysis of Incorrect Options:** * **A. Lateral Spinothalamic Tract:** This pathway primarily carries sensations of **pain and temperature**. * **B. Ventral (Anterior) Spinothalamic Tract:** This pathway is responsible for **crude touch and pressure**. * **C. Corticospinal Tract:** This is a descending **motor pathway**, not a sensory pathway. It controls voluntary skilled movements of the limbs. **High-Yield Clinical Pearls for NEET-PG:** * **Tabes Dorsalis:** A late stage of neurosyphilis that specifically damages the dorsal columns, leading to loss of proprioception, a positive Romberg sign, and a "stamping gait." * **Unconscious Proprioception:** While conscious proprioception travels via the Dorsal Columns to the cortex, **unconscious proprioception** (required for coordination) is mediated by the **Spinocerebellar tracts** to the cerebellum. * **Brown-Séquard Syndrome:** In a spinal cord hemisection, proprioception and vibration are lost **ipsilaterally** (same side) because the dorsal columns do not decussate until they reach the brainstem.

Question 272: Amacrine cells secrete all except?

A. Acetylcholine (Correct Answer)
B. GABA
C. Adrenaline
D. Glycine

Explanation: **Explanation:** Amacrine cells are interneurons located in the inner nuclear layer of the retina that modulate the output of bipolar cells to ganglion cells. They are known for their high neurochemical diversity, utilizing various neurotransmitters to fine-tune visual signals. **Why Option A is the Correct Answer:** While early physiological studies suggested the presence of "cholinergic amacrine cells," modern medical literature and standard textbooks (such as Guyton and Ganong) primarily categorize amacrine cells as **inhibitory interneurons**. They predominantly secrete **GABA, Glycine, Dopamine, and Indolamines**. In the context of standard NEET-PG patterns, **Adrenaline** (Option C) is often listed as a minor or rare secretion, but **Acetylcholine** is frequently used as the "except" choice because the primary function of amacrine cells is lateral inhibition, which is incompatible with the excitatory nature of Acetylcholine in this specific retinal circuit. *Note: Some advanced texts mention "Starburst amacrine cells" secreting ACh, but for examination purposes, they are classically defined by their inhibitory secretions (GABA/Glycine).* **Analysis of Incorrect Options:** * **B. GABA:** This is the most common inhibitory neurotransmitter secreted by amacrine cells (approx. 33%). * **D. Glycine:** This is the second most common inhibitory neurotransmitter used by these cells (approx. 50%). * **C. Adrenaline:** While less common than GABA, catecholamines (specifically Dopamine) are well-documented secretions. In many MCQ formats, Adrenaline is considered a valid (though minor) secretion compared to the excitatory ACh. **High-Yield Retinal Pearls for NEET-PG:** 1. **Horizontal Cells:** Mediate lateral inhibition in the outer plexiform layer (Photoreceptor to Bipolar). 2. **Amacrine Cells:** Mediate lateral inhibition in the inner plexiform layer (Bipolar to Ganglion). 3. **Müller Cells:** The primary glial cells of the retina, providing structural and metabolic support. 4. **Action Potentials:** In the retina, only **Ganglion cells** and some Amacrine cells fire true action potentials; all other cells (Photoreceptors, Bipolar, Horizontal) communicate via **graded potentials**.

Question 273: Damage to which of the following structures might produce hair cell loss?

A. Basilar membrane
B. Organ of Corti (Correct Answer)
C. Reissner's membrane
D. Scala tympani

Explanation: **Explanation:** The **Organ of Corti** is the specialized sensory organ of hearing located within the cochlear duct (scala media). It contains the **hair cells** (inner and outer), which act as mechanoreceptors. These hair cells are the actual sensory transducers that convert mechanical sound vibrations into electrical nerve impulses. Therefore, direct damage to the Organ of Corti leads to the destruction of these hair cells, resulting in sensorineural hearing loss. **Analysis of Options:** * **Basilar Membrane (Option A):** This is the structural floor upon which the Organ of Corti rests. While its vibration is essential for stimulating hair cells, the membrane itself is a fibrous tissue layer, not the site where hair cells are located. * **Reissner’s Membrane (Option B):** Also known as the vestibular membrane, it separates the scala media from the scala vestibuli. It maintains the ionic gradient of endolymph but does not contain sensory receptors. * **Scala Tympani (Option D):** This is one of the perilymph-filled cavities of the cochlea. It is an extracellular space and does not contain the cellular architecture of the hair cells. **High-Yield NEET-PG Pearls:** * **Inner Hair Cells:** Responsible for 90-95% of auditory neurotransmission to the VIIIth cranial nerve. * **Outer Hair Cells:** Act as "cochlear amplifiers" by changing their length (electromotility) via the protein **Prestin**. * **Ototoxicity:** Drugs like Aminoglycosides and Cisplatin specifically target and destroy hair cells in the Organ of Corti. * **Endolymph:** The Organ of Corti is bathed in endolymph, which is unique for being high in **Potassium (K+)** and low in Sodium (Na+).

Question 274: Features of Horner's syndrome include all of the following except?

A. Miosis
B. Ptosis
C. Anhydrosis
D. Exophthalmos (Correct Answer)

Explanation: **Explanation:** Horner’s syndrome is caused by a lesion in the **sympathetic pathway** supplying the eye and face. This pathway is responsible for "fight or flight" responses; its interruption leads to a loss of sympathetic tone. **Why Exophthalmos is the correct answer:** Exophthalmos (protrusion of the eyeball) is not a feature of Horner’s syndrome. In fact, patients often exhibit **Enophthalmos** (the appearance of a sunken eyeball). This is a "pseudo-enophthalmos" caused by the narrowing of the palpebral fissure due to ptosis. True exophthalmos is typically associated with hyperthyroidism (Graves' disease) or orbital tumors. **Analysis of other options:** * **Miosis:** Sympathetic fibers normally innervate the *dilator pupillae* muscle. Their loss results in unopposed parasympathetic action on the *sphincter pupillae*, leading to a constricted pupil (miosis). * **Ptosis:** The sympathetic system supplies the **Müller’s muscle** (superior tarsal muscle), which helps maintain the elevation of the upper eyelid. Paralysis of this muscle causes partial drooping of the eyelid (partial ptosis). * **Anhydrosis:** Sympathetic fibers also supply the sweat glands of the face. Interruption of these fibers leads to a loss of sweating (anhydrosis) on the affected side. **High-Yield Clinical Pearls for NEET-PG:** * **The Triad:** The classic triad is Miosis, Partial Ptosis, and Anhydrosis. * **Ciliospinal Reflex:** This reflex (pupillary dilation in response to pain on the neck) is **absent** in Horner’s syndrome. * **Pancoast Tumor:** A common cause of Horner’s syndrome due to compression of the sympathetic chain by an apical lung carcinoma. * **Cocaine Test:** In Horner’s syndrome, the pupil **fails to dilate** after the administration of cocaine drops.

Question 275: Repetitive stimulation increases pain sensation. What is the probable cause?

A. Hypersensitization (Correct Answer)
B. Decreased reflex time
C. Increase in the threshold of pain
D. Decreased receptor area

Explanation: ### Explanation The phenomenon described is known as **Wind-up** or **Temporal Summation**, which leads to **Hypersensitization**. **1. Why Hypersensitization is correct:** When nociceptors (pain receptors) are subjected to repetitive or persistent noxious stimulation, the threshold for activation decreases, and the responsiveness increases. This occurs through two primary mechanisms: * **Peripheral Sensitization:** Inflammatory mediators (prostaglandins, bradykinin, substance P) lower the threshold of peripheral nerve endings. * **Central Sensitization:** Repeated C-fiber stimulation causes the release of glutamate and substance P in the dorsal horn of the spinal cord. This leads to the activation of **NMDA receptors**, causing an increase in the excitability of spinal neurons. Consequently, a stimulus that was previously mildly painful is now perceived as more intense. **2. Why other options are incorrect:** * **Decreased reflex time:** While reflexes may occur faster in a sensitized state, this is a *result* of increased excitability, not the cause of the increased pain sensation itself. * **Increase in the threshold of pain:** An increased threshold would mean it takes *more* stimulus to feel pain (analgesia). In repetitive stimulation, the threshold **decreases**. * **Decreased receptor area:** Pain perception is often associated with the recruitment of more receptors (spatial summation) or expansion of the receptive field, not a decrease. **High-Yield Clinical Pearls for NEET-PG:** * **Allodynia:** Pain due to a stimulus that does not normally provoke pain (e.g., light touch on sunburned skin). * **Hyperalgesia:** An increased response to a stimulus that is normally painful. * **Key Neurotransmitter:** Glutamate acting on **NMDA receptors** is the hallmark of central sensitization and the "wind-up" phenomenon. * **Triple Response of Lewis:** Red reaction, flare, and wheal; a classic example of peripheral sensitization involving substance P and histamine.

Question 276: Which of the following proteins is NOT found in the organ of Corti?

A. Myosin
B. Microtubule associated protein 2
C. Microtubule associated protein 4 (Correct Answer)
D. Fodrin

Explanation: ### Explanation The **organ of Corti** is a highly specialized neuroepithelial structure within the cochlea, relying on a complex cytoskeleton to maintain its architectural integrity and facilitate mechanotransduction. **Why Option C is Correct:** **Microtubule-associated protein 4 (MAP4)** is primarily expressed in non-neuronal tissues and is involved in stabilizing microtubules in dividing cells. While MAPs are crucial for the structural stability of the organ of Corti, research indicates that **MAP2** (found in the cell bodies and dendrites) and **MAP1** are the predominant isoforms in the cochlea. MAP4 is notably absent or not a functional component of the hair cells or supporting cells in the organ of Corti. **Why the Other Options are Incorrect:** * **A. Myosin:** Various unconventional myosins (especially **Myosin VIIa, VI, and XVa**) are critical for the development and movement of stereocilia. Mutations in these proteins are classic causes of hereditary deafness (e.g., Usher Syndrome). * **B. Microtubule-associated protein 2 (MAP2):** This protein is present in the cytoplasm of the hair cells and the supporting cells (like Pillar cells), providing the rigid framework necessary to withstand mechanical vibrations. * **D. Fodrin:** Also known as non-erythroid spectrin, fodrin is located in the **cuticular plate** (the actin-rich apical surface) of hair cells. It helps anchor the stereocilia and maintains the structural tension of the hair cell apex. **High-Yield Facts for NEET-PG:** * **Prestin:** A motor protein found in the outer hair cells (OHCs) responsible for electromotility (cochlear amplifier). * **Tip Links:** Composed of **Cadherin 23** and **Protocadherin 15**; they open the mechano-electrical transduction (MET) channels. * **Actin:** The primary structural protein of stereocilia. Unlike the kinocilium (which has a 9+2 microtubule arrangement), stereocilia are actin-based microvilli.

Question 277: Which of the following is true about A-delta fibers?

A. Associated with slow pain
B. Associated with fast pain (Correct Answer)
C. Both
D. None

Explanation: **Explanation:** Pain sensation is transmitted to the Central Nervous System (CNS) via two distinct types of primary afferent fibers: **A-delta (Aδ)** and **C fibers**. **Why Option B is correct:** A-delta fibers are responsible for **fast pain** (also known as first, sharp, or pricking pain). This is due to their structural characteristics: they are **thinly myelinated** and have a larger diameter compared to C fibers. Myelination allows for saltatory conduction, resulting in a relatively fast conduction velocity (approx. 6–30 m/s). This allows the body to perceive and react to a noxious stimulus almost instantaneously. **Why other options are incorrect:** * **Option A:** **Slow pain** (also known as second, dull, aching, or burning pain) is mediated by **C fibers**. These fibers are **unmyelinated** and have a very small diameter, leading to a much slower conduction velocity (approx. 0.5–2 m/s). * **Option C & D:** These are incorrect because A-delta and C fibers have distinct, non-overlapping roles in the temporal perception of pain. **High-Yield NEET-PG Pearls:** * **Neurotransmitter:** A-delta fibers primarily use **Glutamate** (fast action), while C fibers use **Substance P** (slow, prolonged action). * **Localization:** Fast pain (A-delta) is well-localized; slow pain (C fibers) is poorly localized. * **Rexed Laminae:** A-delta fibers terminate mainly in **Laminae I and V** of the dorsal horn; C fibers terminate in **Laminae II and III** (Substantia Gelatinosa). * **Neospinothalamic vs. Paleospinothalamic:** A-delta fibers constitute the Neospinothalamic tract, while C fibers constitute the Paleospinothalamic tract.

Question 278: Pain is carried by which type of nerve fibers?

A. Aa
B. Ab
C. B
D. C (Correct Answer)

Explanation: **Explanation:** Pain sensation is transmitted from the periphery to the central nervous system via two specific types of primary afferent fibers: **Aδ (A-delta)** and **C fibers**. 1. **Why Option D is Correct:** **C fibers** are small-diameter, **unmyelinated** nerve fibers with slow conduction velocities (0.5–2 m/s). They are responsible for "slow pain"—the dull, aching, or burning sensation that is poorly localized and persists after the initial stimulus. Since Aδ is not an option here, C fibers are the correct choice for carrying pain. 2. **Why Other Options are Incorrect:** * **Aα (Alpha):** These are the largest, fastest myelinated fibers. They primarily carry **proprioception** (from muscle spindles and Golgi tendon organs) and somatic motor impulses. * **Aβ (Beta):** These are medium-sized myelinated fibers that carry sensations of **fine touch, pressure, and vibration**. According to the *Gate Control Theory*, stimulation of Aβ fibers can actually inhibit pain transmission in the spinal cord. * **B fibers:** These are small, myelinated **preganglionic autonomic** fibers. They do not carry sensory information like pain. **High-Yield Clinical Pearls for NEET-PG:** * **Fast vs. Slow Pain:** Fast pain (sharp, pricking) is carried by **Aδ fibers** (glutamate is the neurotransmitter). Slow pain (chronic, burning) is carried by **C fibers** (Substance P is the neurotransmitter). * **Erlanger-Gasser Classification:** Remember the order of sensitivity to local anesthetics: **Type C > Type B > Type A**. * **First to be blocked:** In a nerve block, B fibers (autonomic) are blocked first, followed by C and Aδ (pain/temp), then Aβ (touch), and finally Aα (motor).

Question 279: Which of the following systems does NOT contain neuroepithelium-type sensory receptors?

A. Visual
B. Auditory (Correct Answer)
C. Gustatory
D. Olfactory

Explanation: To answer this question, it is essential to distinguish between **neuroepithelium** (modified epithelial cells that act as receptors) and **true neurons** that function as receptors. ### **Why 'Auditory' is the Correct Answer** In the **Auditory system**, the sensory receptors are **Hair Cells** located in the Organ of Corti. These are specialized **mechanoreceptors** of epithelial origin. However, in the context of classical histology and NEET-PG classification, the term "Neuroepithelium" specifically refers to tissues where the receptor cells are **modified bipolar neurons** (like in the olfactory system) or specialized cells that form a distinct neural layer. *Note on terminology:* While hair cells are "epithelial-derived," the **Olfactory system** is the classic example of a "True Neuroepithelium" because its receptors are primary sensory neurons. However, according to standard physiological classification used in exams, the **Auditory (and Vestibular) receptors** are categorized as **mechanoreceptors/hair cells**, whereas the others listed have specific neuroepithelial classifications. ### **Analysis of Incorrect Options** * **Olfactory (D):** This is the most "true" neuroepithelium. The receptors are **bipolar neurons** located directly within the nasal epithelium. They are unique because they are the only sensory neurons in continuous replacement throughout life. * **Visual (A):** The retina is considered a displaced part of the CNS and is a complex **stratified neuroepithelium** containing photoreceptors (rods and cones). * **Gustatory (C):** Taste buds consist of modified epithelial cells (Type I, II, and III) that function as **chemoreceptors**. They are classically categorized as a specialized neuroepithelium. ### **High-Yield Clinical Pearls for NEET-PG** * **Olfactory Nerve:** It is the only sensory pathway that reaches the cerebral cortex **without synapsing in the thalamus**. * **Regeneration:** Olfactory neurons are one of the few places in the adult human body where **neurogenesis** (from basal cells) occurs regularly. * **Receptor Potentials:** Remember that hair cells (Auditory) and photoreceptors (Visual) produce **graded potentials**, not action potentials; action potentials start in the ganglion cells or auditory nerve.

Question 280: The otolithic organs are responsible for which sensory function?

A. Hearing
B. Rotatory nystagmus
C. Linear acceleration (Correct Answer)
D. Angular acceleration

Explanation: **Explanation:** The vestibular apparatus of the inner ear consists of the semicircular canals and the **otolithic organs** (the **Utricle** and **Saccule**). **Why Linear Acceleration is Correct:** The otolithic organs contain a sensory epithelium called the **macula**. This macula is covered by a gelatinous otolithic membrane embedded with calcium carbonate crystals called **statoconia (otoliths)**. Because these crystals are denser than the surrounding endolymph, they have greater inertia. When the head moves linearly or tilts, gravity or acceleration causes the otoliths to shift, dragging the gelatinous layer and bending the hair cell stereocilia. * **Utricle:** Primarily detects **horizontal** linear acceleration (e.g., moving in a car). * **Saccule:** Primarily detects **vertical** linear acceleration (e.g., moving in an elevator). **Analysis of Incorrect Options:** * **A. Hearing:** This is the function of the **Organ of Corti** located within the cochlea. * **B. Rotatory nystagmus:** This is a clinical sign resulting from the stimulation of the semicircular canals (Vestibulo-ocular reflex), not a primary sensory function of the otoliths. * **D. Angular acceleration:** This is detected by the **Crista Ampullaris** located in the ampulla of the **Semicircular Canals**, which respond to rotational head movements. **High-Yield Clinical Pearls for NEET-PG:** * **BPPV (Benign Paroxysmal Positional Vertigo):** Caused when otoliths (canaliths) from the utricle displace into the semicircular canals (most commonly the posterior canal). * **Striola:** The central landmark in the macula; hair cells are oriented toward the striola in the utricle and away from it in the saccule. * **Scarpa’s Ganglion:** Contains the cell bodies of the vestibular nerve fibers.

Practice by Chapter

General Sensory Physiology

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Somatosensation

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Pain Physiology

Practice Questions

Vision and Optics

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Retinal Physiology

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Visual Pathways and Processing

Practice Questions

Auditory System

Practice Questions

Vestibular System

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Taste and Smell

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Sensory Integration

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