Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 91: The receptor cells of the olfactory epithelium are?

A. Bipolar neurons (Correct Answer)
B. Unipolar neurons
C. Multipolar neurons
D. Stellate cells

Explanation: ### Explanation **1. Why Bipolar Neurons are Correct:** The olfactory receptor cells are unique because they are **primary sensory neurons** located directly within the olfactory epithelium. They are classic examples of **bipolar neurons**, characterized by a single dendrite extending to the epithelial surface (ending in an olfactory vesicle with cilia) and a single unmyelinated axon that penetrates the cribriform plate to reach the olfactory bulb. Notably, these are among the few neurons in the human body that undergo continuous turnover and replacement throughout life from basal stem cells. **2. Why Other Options are Incorrect:** * **Unipolar/Pseudounipolar neurons:** These are typically found in the dorsal root ganglia of spinal nerves and sensory ganglia of cranial nerves. They have a single process that divides into peripheral and central branches. * **Multipolar neurons:** These are the most common type of neurons in the CNS (e.g., motor neurons, pyramidal cells). They possess one axon and multiple dendrites, which is not the morphology of olfactory receptors. * **Stellate cells:** These are star-shaped interneurons found in the cerebral cortex and cerebellum (molecular layer). They serve inhibitory functions rather than primary sensory transduction. **3. High-Yield Clinical Pearls for NEET-PG:** * **First-order neurons:** The olfactory bipolar cells themselves are the first-order neurons (unlike the visual system where photoreceptors are not neurons). * **Regeneration:** Olfactory neurons have a lifespan of about 30–60 days and are replaced by **Basal cells**. * **Anosmia:** Fracture of the **cribriform plate** of the ethmoid bone can shear these bipolar axons, leading to a loss of smell (anosmia) and potentially CSF rhinorrhea. * **Bowman’s Glands:** Located in the olfactory mucosa, they secrete mucus to dissolve odorants; they are *not* the receptors themselves.

Question 92: Synovial fluid is characterized by all the following features EXCEPT:

A. It is secreted by Type A synoviocytes. (Correct Answer)
B. It exhibits non-Newtonian fluid behavior.
C. It contains hyaluronic acid.
D. Its viscosity is variable.

Explanation: **Explanation:** The correct answer is **A** because it is a factual inaccuracy regarding the cellular functions of the synovium. The synovial membrane consists of two distinct types of synoviocytes: * **Type B synoviocytes (Fibroblast-like):** These are the cells responsible for the **secretion** of synovial fluid components, specifically hyaluronic acid and lubricin. * **Type A synoviocytes (Macrophage-like):** These are derived from blood monocytes and are primarily responsible for **phagocytosis** and removing debris from the joint cavity. **Analysis of other options:** * **Option B & D:** Synovial fluid is a **non-Newtonian fluid**, meaning its viscosity is not constant. Its viscosity is **variable** and inversely proportional to the shear rate (rate of joint movement). At rest, it is thick to protect the cartilage; during rapid movement, it becomes less viscous to reduce friction. * **Option C:** **Hyaluronic acid** (hyaluronan) is a critical constituent secreted by Type B cells. It provides the fluid with its characteristic high viscosity and lubricating properties. **NEET-PG High-Yield Pearls:** * **Origin:** Synovial fluid is an ultrafiltrate of plasma plus secretions from Type B synoviocytes. * **Appearance:** Normal fluid is clear, straw-colored, and "stringy" (due to high hyaluronate content). * **Clinical Correlation:** In inflammatory conditions like Rheumatoid Arthritis, the viscosity decreases because inflammatory enzymes break down hyaluronic acid. * **Glucose levels:** Normally slightly lower than plasma; significantly decreased in septic arthritis.

Question 93: Which cortical area controls vestibular function?

A. Frontal lobe
B. Parietal lobe (Correct Answer)
C. Occipital lobe
D. All of the above

Explanation: **Explanation:** The primary cortical representation of the vestibular system is located in the **Parietal Lobe**. Specifically, vestibular signals from the semicircular canals and otolith organs travel via the vestibulocochlear nerve (CN VIII) to the vestibular nuclei in the brainstem, then to the ventral posterior nucleus of the thalamus, and finally to the **parieto-insular vestibular cortex (PIVC)**. This area, located near the junction of the parietal lobe and the insula (close to the postcentral gyrus), is responsible for the conscious perception of balance, spatial orientation, and self-motion. **Analysis of Options:** * **B. Parietal Lobe (Correct):** It houses the PIVC and Brodmann area 2v and 3a, which integrate vestibular, proprioceptive, and visual inputs to maintain equilibrium. * **A. Frontal Lobe:** While the frontal eye fields (FEF) are involved in voluntary eye movements (saccades), they do not serve as the primary sensory processing center for vestibular function. * **C. Occipital Lobe:** This is the primary center for visual processing. Although vision is crucial for balance, the vestibular sensory cortex itself is not located here. **High-Yield Facts for NEET-PG:** * **Primary Vestibular Cortex:** Often described as the **Parieto-Insular Vestibular Cortex (PIVC)**. * **Pathway:** Vestibular hair cells → Scarpa’s ganglion → Vestibular nuclei (Medulla/Pons) → Thalamus (Ventral Posterior Nucleus) → Parietal Lobe. * **Clinical Correlation:** Lesions in the parietal vestibular areas can lead to **"Pusher Syndrome"** or a tilted perception of the subjective visual vertical (SVV), where patients feel they are upright when they are actually tilted.

Question 94: Which of the following is an example of a direct chemoreceptor?

A. Olfactory receptor neuron
B. Taste buds (Correct Answer)
C. Vomeronasal organ
D. All of the above

Explanation: **Explanation:** In sensory physiology, chemoreceptors are classified based on how they interact with chemical stimuli. The distinction between "direct" and "indirect" (or distance) chemoreceptors depends on the proximity and state of the stimulant. **Why Taste Buds are the Correct Answer:** **Taste buds** are considered **direct chemoreceptors** (contact chemoreceptors). For a substance to be tasted, it must be dissolved in saliva and come into direct physical contact with the microvilli of the gustatory receptor cells. These receptors detect chemicals in the immediate environment of the oral cavity, facilitating the evaluation of ingested substances. **Analysis of Incorrect Options:** * **Olfactory Receptor Neurons (Option A):** These are classified as **distance chemoreceptors** (telereceptors). They detect volatile, airborne chemical molecules (odorants) that originate from a source away from the body. * **Vomeronasal Organ (Option C):** Similar to the olfactory system, this organ (though vestigial in humans) detects pheromones and chemical signals from the environment, typically functioning as a distance or specialized accessory olfactory receptor rather than a direct contact receptor. * **Option D:** Incorrect because only taste buds fulfill the criteria for direct contact chemoreception. **High-Yield Clinical Pearls for NEET-PG:** * **Type of Receptor:** Taste receptors are **modified epithelial cells** (G-protein coupled or ion channels), whereas olfactory receptors are **true bipolar neurons** (the only neurons that regularly undergo turnover). * **Signal Transduction:** Sweet, Bitter, and Umami use **G-protein coupled receptors (Gustducin)**; Salty and Sour act via **ion channels** (ENaC and H+ sensitive channels respectively). * **Innervation:** Remember the "Rule of 7, 9, 10" for taste: Anterior 2/3 of tongue (CN VII), Posterior 1/3 (CN IX), and Epiglottis/Pharynx (CN X).

Question 95: Which substance evokes the umami taste sensation in humans?

A. Glucose
B. Glutamic acid (Correct Answer)
C. Glucosamine
D. Gelatin

Explanation: ### Explanation **1. Why Glutamic Acid is Correct:** Umami, often described as a "savory" or "meaty" flavor, is one of the five basic taste sensations. It is primarily triggered by **L-glutamate (glutamic acid)** and certain nucleotides like inosine monophosphate (IMP) and guanosine monophosphate (GMP). In the tongue, specific G-protein coupled receptors (GPCRs), namely the **T1R1 + T1R3 heterodimer** and the **metabotropic glutamate receptor (mGluR4)**, detect glutamate. When these receptors are activated, they trigger a signal transduction pathway involving gustducin, leading to the perception of the umami taste. **2. Why the Other Options are Incorrect:** * **A. Glucose:** This is a simple sugar that binds to the **T1R2 + T1R3** receptor complex to evoke a **sweet** taste sensation. * **C. Glucosamine:** While chemically related to glucose and amino acids, it is an amino sugar used primarily in cartilage synthesis and does not serve as a primary ligand for the umami taste receptor. * **D. Gelatin:** This is a protein derived from collagen. While proteins contain amino acids, taste receptors respond to free amino acids (like glutamate) rather than large, intact protein molecules. **3. High-Yield Clinical Pearls for NEET-PG:** * **Receptor Summary:** * Sweet: T1R2 + T1R3 * Umami: T1R1 + T1R3 (Tip: **1** is for Umami, **2** is for Sweet) * Bitter: T2R family (~30 types) * **Monosodium Glutamate (MSG):** The sodium salt of glutamic acid, commonly used as a flavor enhancer to intensify umami. * **Signal Transduction:** All GPCR-mediated tastes (Sweet, Bitter, Umami) utilize the secondary messenger **IP3** and increase intracellular **Calcium**, which opens **TRPM5** channels to depolarize the cell.

Question 96: Ability to perceive shape and size is lost due to the lesion of which tract?

A. Tractus gracilis
B. Tractus cuneatus (Correct Answer)
C. Spinothalamic tract
D. Spinoreticular tract

Explanation: **Explanation:** The ability to perceive the shape and size of an object through touch is known as **Stereognosis**. This is a complex sensory function mediated by the **Dorsal Column-Medial Lemniscal (DCML) pathway**. **Why Tractus Cuneatus is correct:** The DCML pathway is responsible for fine touch, vibration, proprioception, and discriminative touch (including stereognosis). It is composed of two tracts: the *Tractus Gracilis* (medial) and the *Tractus Cuneatus* (lateral). While both carry similar modalities, the **Tractus Cuneatus** specifically carries sensory information from the **upper body (above T6)**, including the hands. Since stereognosis is primarily a function tested and performed using the hands, a lesion in the Tractus Cuneatus leads to the loss of this ability (Astereognosis). **Analysis of Incorrect Options:** * **Tractus Gracilis:** This tract carries sensations from the **lower limbs and lower trunk (below T6)**. While it belongs to the DCML system, it is not the primary tract involved in the manual perception of shape and size. * **Spinothalamic Tract:** This pathway carries **lateral** (pain and temperature) and **anterior** (crude touch and pressure) sensations. It does not mediate discriminative touch or stereognosis. * **Spinoreticular Tract:** This is an indirect pathway involved in the emotional and arousal aspects of **deep, chronic pain**; it has no role in fine sensory perception. **High-Yield Clinical Pearls for NEET-PG:** * **Stereognosis vs. Graphesthesia:** Stereognosis is identifying objects by touch; Graphesthesia is identifying letters/numbers traced on the skin. Both are DCML functions. * **Lesion Localization:** If stereognosis is lost but basic touch is intact, the lesion is likely in the **Sensory Cortex (Parietal Lobe)** rather than the spinal tracts. * **Rule of Thumb:** Gracilis is "G" for Ground (legs); Cuneatus is for the upper body.

Question 97: Prosopagnosia is associated with the lesion of which area?

A. Cingulate cortex gyrus
B. Posterior parietal cortex
C. Hippocampus, amygdala and entorhinal cortex
D. Fusiform gyrus and lingual gyrus (Correct Answer)

Explanation: **Explanation:** **Prosopagnosia**, also known as "face blindness," is a cognitive disorder characterized by the inability to recognize familiar faces, while other aspects of visual processing and intellectual functioning remain intact. **1. Why Option D is Correct:** The visual processing of objects follows two main pathways: the dorsal "where" stream and the **ventral "what" stream**. The ventral stream travels from the primary visual cortex to the inferior temporal lobe. A specialized area within this stream, located in the **fusiform gyrus** (specifically the Fusiform Face Area or FFA) and the adjacent **lingual gyrus**, is dedicated to facial recognition. Bilateral lesions (or sometimes right-sided lesions) in these areas result in the inability to link a face to a specific identity. **2. Why Other Options are Incorrect:** * **A. Cingulate cortex gyrus:** Part of the limbic system involved in emotion processing, learning, and memory, but not visual recognition. * **B. Posterior parietal cortex:** Part of the dorsal stream ("where" pathway). Lesions here lead to **spatial neglect**, optic ataxia, or Bálint’s syndrome, rather than recognition deficits. * **C. Hippocampus, amygdala, and entorhinal cortex:** These structures are central to memory consolidation and emotional responses (Klüver-Bucy syndrome). While they help remember *who* a person is, they do not perform the primary visual synthesis of a face. **Clinical Pearls for NEET-PG:** * **Ventral Stream:** Occipital to Temporal (Object/Face recognition). * **Dorsal Stream:** Occipital to Parietal (Spatial orientation/Motion). * **Achromatopsia:** Often co-occurs with prosopagnosia due to damage in the nearby V4 area of the lingual/fusiform gyri. * **Anton’s Syndrome:** Cortical blindness where the patient denies they are blind (occipital lobe lesion).

Question 98: Optokinetic nystagmus is due to a defect in which lobe of the brain?

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

Explanation: **Explanation:** **Optokinetic Nystagmus (OKN)** is a physiological rhythmic eye movement triggered by a moving visual field (e.g., looking out of a moving train). It consists of two phases: a **slow pursuit phase** (following the object) and a **fast corrective saccade** (resetting the eyes). **Why the Parietal Lobe is correct:** The **Parietal Lobe** (specifically the posterior parietal cortex) is the primary integration center for the "where" pathway of visual processing. It controls the **slow pursuit phase** of OKN. Lesions in the parietal lobe, particularly those involving the angular and supramarginal gyri, disrupt the smooth tracking of objects moving toward the side of the lesion, leading to an abolished or impaired OKN response. **Analysis of Incorrect Options:** * **Frontal Lobe:** This area contains the Frontal Eye Fields (FEF), which are primarily responsible for **voluntary saccades** (fast eye movements). While the frontal lobe is involved in the fast phase of OKN, the clinical localization for a defective OKN reflex is traditionally linked to parietal dysfunction. * **Occipital Lobe:** This is the primary visual cortex (V1). While it receives visual input, it does not coordinate the complex motor integration required for the pursuit reflex. * **Temporal Lobe:** This lobe is involved in the "what" pathway (object recognition) and auditory processing; it does not play a primary role in the optokinetic reflex. **High-Yield Clinical Pearls for NEET-PG:** * **OKN Test:** Used to detect malingering in patients claiming total blindness. If OKN is present, the visual pathways are intact. * **Deep Parietal Lesions:** A classic board finding is a patient with a homonymous hemianopia and an **asymmetric OKN**; this strongly localizes the lesion to the parietal lobe rather than the occipital lobe. * **Rule of Thumb:** Frontal Lobe = Saccades; Parietal Lobe = Pursuit.

Question 99: Endolymph is rich in which ion?

A. Na+
B. Cl-
C. HCO-3
D. K+ (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. K+**. **Underlying Concept:** Endolymph is a unique extracellular fluid found in the inner ear (membranous labyrinth). Unlike most extracellular fluids in the body, endolymph has an electrolyte composition similar to **intracellular fluid**. It is characterized by a very high concentration of **Potassium (K+)** and a very low concentration of Sodium (Na+). This high potassium concentration is actively maintained by the **Stria Vascularis** of the cochlear duct. The resulting electrochemical gradient (endocochlear potential of approximately +80 mV) is essential for the depolarization of hair cells during auditory and vestibular transduction. **Analysis of Incorrect Options:** * **A. Na+:** Sodium is the primary cation of **Perilymph** (which resembles ECF or CSF) and typical extracellular fluid. In endolymph, Na+ levels are extremely low. * **B. Cl-:** While chloride is present in endolymph to maintain electroneutrality, it is not the defining characteristic or the "rich" cation that distinguishes endolymph from other fluids in a physiological context. * **C. HCO3-:** Bicarbonate acts as a buffer in various body fluids but is not the predominant ion in endolymph. **High-Yield Clinical Pearls for NEET-PG:** * **Source of Endolymph:** Secreted by the **Stria Vascularis**. * **Perilymph vs. Endolymph:** Perilymph is rich in **Na+** (resembles ECF); Endolymph is rich in **K+** (resembles ICF). * **Meniere’s Disease:** Caused by the distension of the membranous labyrinth due to excess accumulation of endolymph (**Endolymphatic Hydrops**). * **Endocochlear Potential:** The +80 mV potential is the highest resting potential in the body, providing the driving force for sound transduction.

Question 100: Which of the following nerves serves as an afferent in the mediation of the corneal reflex?

A. Lacrimal nerve
B. Frontal nerve
C. Nasociliary nerve (Correct Answer)
D. Auriculotemporal nerve

Explanation: **Explanation:** The **corneal reflex** (or blink reflex) is an involuntary blinking of the eyelids elicited by stimulation of the cornea. It is a polysynaptic reflex arc consisting of: 1. **Afferent limb:** Trigeminal nerve (Ophthalmic division - V1). 2. **Center:** Pons. 3. **Efferent limb:** Facial nerve (CN VII), which supplies the orbicularis oculi muscle to close the eyelid. **Why Nasociliary Nerve is Correct:** The **Nasociliary nerve** is a major branch of the Ophthalmic nerve (V1). It gives off the **long ciliary nerves**, which provide sensory innervation directly to the cornea. When the cornea is touched, impulses travel through the long ciliary nerves to the nasociliary nerve, then to the trigeminal ganglion, making it the specific afferent mediator. **Analysis of Incorrect Options:** * **A & B (Lacrimal and Frontal nerves):** While these are branches of the Ophthalmic nerve (V1), they supply the lacrimal gland, forehead, and upper eyelid skin, respectively. They do not provide sensory fibers to the cornea. * **D (Auriculotemporal nerve):** This is a branch of the Mandibular nerve (V3). it supplies the TMJ, external auditory meatus, and temple region, playing no role in the corneal reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Consensual Response:** Stimulating one eye causes bilateral blinking. A unilateral absent response helps localize lesions (e.g., CN V lesion = no response in either eye; CN VII lesion = no response on the affected side). * **Contact Lens Users:** May have a diminished or absent corneal reflex due to decreased corneal sensitivity. * **Reflex Center:** The sensory nucleus of the trigeminal nerve in the **Pons** communicates with the facial nerve nuclei via interneurons.

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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