Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 251: Taste sensation is carried by all nerves except:

A. Facial nerve
B. Vagus nerve
C. Trigeminal nerve (Correct Answer)
D. Glossopharyngeal nerve

Explanation: **Explanation:** The sensation of taste (gustation) is mediated by specialized chemoreceptors located in taste buds. These are innervated by three specific cranial nerves (CN VII, IX, and X). The **Trigeminal nerve (CN V)** is the correct answer because it does **not** carry special visceral afferent (taste) fibers; instead, it provides general somatic afference (touch, pain, and temperature) to the anterior two-thirds of the tongue via its lingual branch. **Analysis of Options:** * **Facial Nerve (CN VII):** Carries taste from the **anterior 2/3rd** of the tongue via the **chorda tympani** nerve. * **Glossopharyngeal Nerve (CN IX):** Carries taste from the **posterior 1/3rd** of the tongue (including the circumvallate papillae). * **Vagus Nerve (CN X):** Carries taste from the **extreme posterior part** of the tongue, the epiglottis, and the pharynx via the internal laryngeal nerve. * **Trigeminal Nerve (CN V):** While the lingual nerve (a branch of V3) physically carries the chorda tympani fibers to the tongue, the taste fibers themselves originate from the facial nerve, not the trigeminal. **High-Yield Clinical Pearls for NEET-PG:** * **Nucleus Tractus Solitarius (NTS):** All taste fibers from CN VII, IX, and X terminate in the gustatory portion (rostral part) of the NTS in the medulla. * **Ageusia:** Loss of taste sensation. * **Papillae:** Taste buds are found on fungiform, circumvallate, and foliate papillae. **Filiform papillae** are the most numerous but contain **no taste buds** (only tactile receptors). * **Pathway:** NTS → Thalamus (VPM nucleus) → Primary Gustatory Cortex (Insula and frontal operculum).

Question 252: What stimulates the macula?

A. Gravity
B. Head position change
C. Linear acceleration
D. All of the above (Correct Answer)

Explanation: The **maculae** are the sensory receptors located within the **utricle and saccule** of the inner ear. They are responsible for detecting **static equilibrium** and **linear acceleration**. ### **Explanation of the Correct Answer** The maculae consist of hair cells embedded in a gelatinous **otolithic membrane**, which contains calcium carbonate crystals called **otoconia**. * **Gravity & Head Position (Static Equilibrium):** When the head tilts, gravity pulls on the heavy otoconia. This shifts the otolithic membrane, bending the hair cell stereocilia and signaling the brain about the head's orientation relative to the vertical axis. * **Linear Acceleration (Dynamic Equilibrium):** When the body moves in a straight line (e.g., a car accelerating or an elevator rising), the inertia of the otoconia causes the membrane to lag behind the sensory epithelium. This displacement stimulates the hair cells. Since the macula responds to gravitational pull, changes in head tilt, and straight-line movement, **Option D (All of the above)** is correct. ### **Why other options are not "the only" answer** Options A, B, and C are all individual components of macular function. Selecting only one would be incomplete, as the vestibular system integrates all three stimuli to maintain balance and posture. ### **High-Yield Clinical Pearls for NEET-PG** * **Utricle vs. Saccule:** The Utricle detects **horizontal** acceleration (e.g., driving a car), while the Saccule detects **vertical** acceleration (e.g., riding an elevator). * **Semicircular Canals:** These detect **angular (rotational) acceleration**, not linear. Their sensory organ is the **Crista Ampullaris**. * **BPPV (Benign Paroxysmal Positional Vertigo):** This occurs when otoconia from the macula dislodge and enter the semicircular canals, causing brief episodes of vertigo. * **Striola:** The central landmark of the macula where hair cells change their orientation.

Question 253: Auditory receptors are found in which structure?

A. Semicircular canal
B. Organ of Corti (Correct Answer)
C. Ear ossicles
D. Tympanic membrane

Explanation: **Explanation:** The **Organ of Corti** is the correct answer because it is the specialized sensory organ of hearing located within the cochlea of the inner ear. It sits on the basilar membrane and contains **hair cells** (inner and outer), which are the actual auditory receptors. These receptors convert mechanical sound vibrations (fluid waves) into electrical nerve impulses via the vestibulocochlear nerve (CN VIII). **Analysis of Incorrect Options:** * **A. Semicircular canals:** These are part of the vestibular system, not the auditory system. They contain receptors (crista ampullaris) that detect **rotational acceleration** and maintain dynamic equilibrium. * **C. Ear ossicles:** These are three tiny bones (Malleus, Incus, Stapes) in the middle ear. Their function is mechanical—they **amplify and conduct** sound vibrations from the tympanic membrane to the oval window; they do not contain receptors. * **D. Tympanic membrane:** Also known as the eardrum, this structure marks the boundary between the outer and middle ear. It vibrates in response to sound waves but serves as a **transmitter**, not a sensory receptor site. **High-Yield NEET-PG Pearls:** * **Endolymph vs. Perilymph:** The Organ of Corti is bathed in **endolymph**, which is unique for being high in Potassium ($K^+$) and low in Sodium ($Na^+$). * **Tonotopic Organization:** The **base** of the cochlea (near the oval window) detects high-frequency sounds, while the **apex** (helicotrema) detects low-frequency sounds. * **Clinical Correlation:** Damage to the hair cells in the Organ of Corti (due to loud noise or ototoxic drugs like aminoglycosides) results in **sensorineural hearing loss**.

Question 254: A 50-year-old woman undergoes a neurologic exam that indicates loss of pain and temperature sensitivity, vibratory sense, and proprioception in the left side of the body. These symptoms could be explained by:

A. A tumor on the right medial lemniscal pathway in the spinal cord
B. A tumor on the left medial lemniscal pathway in the spinal cord
C. A tumor affecting the right postcentral gyrus (Correct Answer)
D. A large tumor in the right ventrolateral spinal cord

Explanation: ### Explanation **1. Why Option C is Correct:** The patient presents with a **global sensory loss** (pain, temperature, vibration, and proprioception) localized to the **left side of the body**. * **Anatomical Integration:** While different sensory modalities travel via different pathways in the spinal cord (Dorsal Column-Medial Lemniscal system for vibration/proprioception and Spinothalamic tract for pain/temperature), they all eventually converge in the **Thalamus** (VPL nucleus) and project to the **Primary Somatosensory Cortex (Postcentral Gyrus)**. * **Contralateral Representation:** Sensory information from the left side of the body is processed by the **right** cerebral hemisphere. A lesion in the right postcentral gyrus explains the loss of all sensory modalities on the contralateral (left) side. **2. Why the Other Options are Incorrect:** * **Options A & B:** The **Medial Lemniscus** only carries fine touch, vibration, and proprioception. It does **not** carry pain and temperature (which travel in the Spinothalamic tract). Furthermore, the medial lemniscus is a brainstem structure, not a spinal cord pathway. * **Option D:** A lesion in the **ventrolateral spinal cord** would affect the Lateral Spinothalamic Tract, causing loss of pain and temperature on the contralateral side. However, it would spare vibration and proprioception, as those fibers travel posteriorly in the Dorsal Columns. **3. Clinical Pearls for NEET-PG:** * **Dissociated Sensory Loss:** If a patient loses pain/temperature but retains vibration/proprioception (or vice versa), the lesion is usually in the **spinal cord** (e.g., Syringomyelia or Brown-Séquard Syndrome). * **Total Sensory Loss:** If all modalities are lost on one side of the body, the lesion is typically "high up" in the **Thalamus** or **Sensory Cortex**. * **Rule of Sides:** Below the medulla, dorsal column lesions are ipsilateral. Above the sensory decussation (medulla), all sensory lesions result in contralateral deficits.

Question 255: In dark adaptation, what occurs?

A. Breakdown of rhodopsin
B. Increased threshold for vision
C. No role for cones
D. No change in the dioptric power of the eye (Correct Answer)

Explanation: ### Explanation **Dark adaptation** is the process by which the eyes increase their sensitivity to light after moving from a bright environment to a dark one. **1. Why Option D is Correct:** The **dioptric power** of the eye refers to its refractive capacity (primarily the cornea and lens) to focus light on the retina. Dark adaptation is a **photochemical and neural process** occurring within the retina; it does not involve changes in the curvature of the cornea or the shape of the lens (accommodation). Therefore, the refractive power remains constant. **2. Analysis of Incorrect Options:** * **Option A (Breakdown of rhodopsin):** This occurs during **light adaptation** (bleaching). In dark adaptation, the primary event is the **regeneration (resynthesis) of rhodopsin** from opsin and retinene to increase sensitivity. * **Option B (Increased threshold for vision):** Dark adaptation involves a **decrease in the visual threshold**. As rhodopsin regenerates, the eye becomes more sensitive, meaning it requires *less* light (a lower threshold) to trigger a visual impulse. * **Option C (No role for cones):** Cones *do* play a role, especially in the first 5–10 minutes. Dark adaptation is biphasic: the first phase involves rapid but limited adaptation by **cones**, followed by a slower, more significant increase in sensitivity by **rods** (the rod-cone break). **3. High-Yield NEET-PG Pearls:** * **Timeframe:** Max sensitivity is reached in about 20–30 minutes. * **Vitamin A:** Essential for rhodopsin regeneration; deficiency leads to **Nyctalopia** (night blindness), which is an impairment of dark adaptation. * **Purkinje Shift:** During dark adaptation, the peak sensitivity of the eye shifts from longer wavelengths (yellow-green, 550nm) to shorter wavelengths (blue-green, 505nm). * **Red Light:** Used in darkrooms because rods are insensitive to long-wavelength red light, allowing a person to maintain dark adaptation while still seeing via cones.

Question 256: Hot water bottles relieve pain from abdominal spasms by which mechanism?

A. Stimulation of touch receptors (Correct Answer)
B. Stimulation of cholinergic fibers
C. Inhibition of cold receptors
D. Inhibition of heat receptors

Explanation: ### Explanation The relief of pain via hot water bottles is a classic clinical application of the **Gate Control Theory of Pain**, proposed by Melzack and Wall. **Why Option A is Correct:** When a hot water bottle is applied to the skin, it provides thermal and tactile stimuli. These stimuli activate large-diameter, myelinated **A-beta (Aβ) sensory fibers** (responsible for touch and pressure). These fibers enter the dorsal horn of the spinal cord and stimulate inhibitory interneurons (specifically in the *substantia gelatinosa*). These interneurons release enkephalins, which inhibit the transmission of pain signals from small, unmyelinated **C-fibers** (carrying dull, aching pain from spasms) to the second-order projection neurons. Essentially, the "gate" for pain is closed by the stimulation of non-nociceptive touch/tactile receptors. **Analysis of Incorrect Options:** * **B. Stimulation of cholinergic fibers:** Cholinergic fibers are primarily involved in the parasympathetic nervous system and neuromuscular junctions; they do not mediate the immediate sensory modulation of pain in the dorsal horn. * **C & D. Inhibition of receptors:** The mechanism is based on the **activation** of specific receptors (thermoreceptors and mechanoreceptors) to override pain, rather than the inhibition of cold or heat receptors themselves. **High-Yield Clinical Pearls for NEET-PG:** * **Gate Control Theory:** Large Aβ fibers "close" the gate; small C and Aδ fibers "open" the gate. * **Transcutaneous Electrical Nerve Stimulation (TENS):** Works on the same principle as the hot water bottle—stimulating Aβ fibers to suppress chronic pain. * **Substantia Gelatinosa (Lamina II):** The anatomical site in the dorsal horn where this "gating" occurs. * **Counter-irritation:** Rubbing a bumped knee or using liniments are other examples of Aβ fiber stimulation relieving pain.

Question 257: When light strikes the eye, there is an increase in which of the following?

A. The activity of transducin (Correct Answer)
B. The amount of transmitter released from the photoreceptors
C. The concentration of all-trans retinal within the photoreceptors
D. The concentration of calcium within the photoreceptors

Explanation: ### Explanation The visual transduction cascade is a unique process where light stimulation leads to the **hyperpolarization** of photoreceptors (rods and cones), rather than depolarization. **1. Why Option A is Correct:** When light (photons) strikes the retina, it is absorbed by rhodopsin, converting 11-cis retinal to all-trans retinal. This activates **metarhodopsin II**, which in turn activates a G-protein called **transducin**. Transducin then activates cGMP-phosphodiesterase (PDE), which breaks down cGMP. The decrease in cGMP levels causes the closure of Na⁺ channels, leading to hyperpolarization. **2. Why the Other Options are Incorrect:** * **Option B:** Because light causes hyperpolarization, it leads to a **decrease** in the release of the neurotransmitter (glutamate) from the synaptic terminals of photoreceptors. Photoreceptors release the most transmitter in the dark. * **Option C:** While 11-cis retinal is converted to all-trans retinal, the all-trans retinal is rapidly shuttled out of the photoreceptor to the **Retinal Pigment Epithelium (RPE)** for regeneration. Therefore, its concentration within the photoreceptor does not "increase" in a sustained manner during active signaling; rather, it is consumed. * **Option D:** The closure of cyclic nucleotide-gated channels (which are permeable to both Na⁺ and Ca²⁺) leads to a **decrease** in intracellular calcium concentration during light exposure. ### High-Yield NEET-PG Pearls * **Dark Current:** In the dark, photoreceptors are depolarized (~ -40 mV) due to constant Na⁺ influx. Light "shuts off" this current. * **Vitamin A:** It is the precursor for 11-cis retinal. Deficiency leads to Nyctalopia (night blindness). * **RPE Function:** The Retinal Pigment Epithelium is essential for the reisomerization of all-trans retinal back to 11-cis retinal (the visual cycle). * **Second Messenger:** In phototransduction, the second messenger is **cGMP**, not cAMP.

Question 258: Which of the following sensations is most affected by cortical lesions?

A. Proprioception (Correct Answer)
B. Temperature
C. Itch
D. Pain

Explanation: ### Explanation **1. Why Proprioception is the Correct Answer:** Sensory perception is divided into **crude (protopathic)** and **discriminative (epicritic)** sensations. * **Cortical Sensations:** These are complex, discriminative sensations that require processing by the **Primary Somatosensory Cortex (S1)** in the parietal lobe. They include proprioception (position sense), stereognosis, two-point discrimination, and graphesthesia. * **Subcortical Processing:** While the thalamus acts as a relay station, the conscious appreciation of fine spatial localization and limb position depends entirely on the integrity of the cortex. Therefore, a cortical lesion (like a stroke involving the postcentral gyrus) profoundly impairs proprioception while leaving crude sensations relatively intact. **2. Why the Other Options are Incorrect:** * **Pain and Temperature (B & D):** These are crude sensations carried by the **Lateral Spinothalamic Tract**. The **thalamus** is the primary center for the conscious appreciation of pain and temperature. Even if the sensory cortex is destroyed, a patient can still perceive pain and thermal changes (though they cannot localize them precisely), a phenomenon known as "thalamic awareness." * **Itch (C):** Itch (pruritus) is a primitive sensation closely related to pain pathways (Anterolateral system). Like pain, it is primarily processed at the subcortical and thalamic levels and is less dependent on cortical refinement. **3. High-Yield Clinical Pearls for NEET-PG:** * **Thalamic Syndrome (Dejerine-Roussy):** Characterized by a loss of sensation on the contralateral side, followed by the development of agonizing, burning pain (thalamic pain) because the thalamus is the "pain center." * **Cortical Sensory Loss:** If a patient has intact crude touch/pain but cannot identify an object in their hand (**Stereognosis**) or a number traced on their skin (**Graphesthesia**), the lesion is in the **Parietal Lobe**. * **Hierarchy:** Cortex = Discrimination; Thalamus = Crude Perception.

Question 259: The visual center is present in which lobe of the brain?

A. Parietal lobe
B. Occipital lobe (Correct Answer)
C. Frontal lobe
D. Temporal lobe

Explanation: **Explanation:** The primary visual center is located in the **Occipital lobe**, specifically in the area surrounding the calcarine sulcus. This region is designated as **Brodmann area 17** (Primary Visual Cortex or V1). It receives sensory input from the lateral geniculate nucleus (LGN) of the thalamus via optic radiations. Surrounding this are Brodmann areas 18 and 19 (Visual Association Areas), which are responsible for interpreting visual stimuli, such as recognizing objects and perceiving movement. **Analysis of Incorrect Options:** * **Parietal lobe:** Primarily houses the **Somatosensory cortex** (Brodmann areas 3, 1, 2). It is involved in processing tactile sensation, proprioception, and spatial awareness (the "where" pathway of vision). * **Frontal lobe:** Contains the **Motor cortex** and is responsible for executive functions, personality, and voluntary motor control. It also houses the **Frontal Eye Field (Area 8)**, which controls voluntary conjugate eye movements, but not visual perception. * **Temporal lobe:** Contains the **Primary Auditory cortex** (Areas 41, 42) and is involved in memory (hippocampus) and language comprehension (Wernicke’s area). It also processes the "what" pathway (object recognition) of vision. **High-Yield Clinical Pearls for NEET-PG:** * **Macular Sparing:** A lesion of the occipital cortex (e.g., PCA stroke) often results in contralateral homonymous hemianopia with macular sparing due to the dual blood supply (PCA and MCA) to the occipital pole. * **Meyer’s Loop:** Fibers of the optic radiation that pass through the **Temporal lobe**; a lesion here causes "pie in the sky" (superior quadrantanopia). * **Baum’s Loop:** Fibers passing through the **Parietal lobe**; a lesion here causes "pie on the floor" (inferior quadrantanopia).

Question 260: Perception of white light is due to:

A. Stimulation of red cones more than blue or green
B. Stimulation of blue cones more than red or green
C. Stimulation of green cones more than blue or red
D. Stimulation of red, blue and green cones equally (Correct Answer)

Explanation: ### Explanation **Core Concept: The Young-Helmholtz Trichromatic Theory** The perception of color is based on the **Trichromatic Theory**, which states that the human retina contains three types of cone photoreceptors, each sensitive to a specific range of wavelengths: **S-cones (Blue)**, **M-cones (Green)**, and **L-cones (Red)**. The sensation of **white light** is a result of the simultaneous and **equal stimulation** of all three types of cones. When the brain receives balanced signals from the red, green, and blue cones, it interprets the combined input as white. This is an additive process; conversely, the absence of any cone stimulation results in the perception of black. **Analysis of Options:** * **Option D (Correct):** Equal stimulation of all three cone types (Red, Blue, and Green) is the physiological requirement for the perception of white light. * **Options A, B, and C (Incorrect):** If one cone type is stimulated significantly more than the others, the brain perceives a specific hue rather than white. For example, predominant stimulation of L-cones results in the perception of red, M-cones in green, and S-cones in blue. **High-Yield Clinical Pearls for NEET-PG:** * **Photopigments:** Cones contain **Photopsins** (Iodopsin), while rods contain **Rhodopsin**. * **Color Blindness:** The most common type is **Red-Green color blindness**, which is an **X-linked recessive** condition. It is most frequently due to the absence of red (Protanopia) or green (Deuteranopia) cones. * **Ishihara Charts:** These are the gold standard clinical tool used to screen for color vision deficiencies. * **Fovea Centralis:** This area of the retina contains the highest concentration of cones and lacks rods, making it the point of highest visual acuity and color perception.

Practice by Chapter

General Sensory Physiology

Practice Questions

Somatosensation

Practice Questions

Pain Physiology

Practice Questions

Vision and Optics

Practice Questions

Retinal Physiology

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Visual Pathways and Processing

Practice Questions

Auditory System

Practice Questions

Vestibular System

Practice Questions

Taste and Smell

Practice Questions

Sensory Integration

Practice Questions

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