Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 131: Which of the following tracts is concerned with pain and temperature sensation?

A. Pyramidal tract
B. Anterior spinothalamic tract
C. Lateral spinothalamic tract (Correct Answer)
D. Dorsal spinocerebellar tract

Explanation: **Explanation:** The **Lateral Spinothalamic Tract (LSTT)** is the primary ascending pathway for **pain and temperature** sensations. The pathway begins with first-order neurons in the dorsal root ganglion, which synapse in the dorsal horn (Substantia Gelatinosa). Second-order neurons decussate (cross over) at the same spinal level through the anterior white commissure and ascend in the lateral funiculus to the thalamus (VPL nucleus). **Analysis of Options:** * **A. Pyramidal tract:** This is a **descending motor pathway** (Corticospinal tract) responsible for voluntary motor control, not sensory perception. * **B. Anterior spinothalamic tract:** This tract primarily carries **crude touch and pressure** sensations. While related to the LSTT, it is functionally distinct. * **C. Lateral spinothalamic tract (Correct):** Specifically dedicated to nociception (pain) and thermoreception (temperature). * **D. Dorsal spinocerebellar tract:** This pathway carries **unconscious proprioception** from the lower limbs to the cerebellum to coordinate posture and gait. **High-Yield Clinical Pearls for NEET-PG:** * **Brown-Séquard Syndrome:** A hemisection of the spinal cord results in **contralateral** loss of pain and temperature (due to the LSTT crossing at the spinal level) and **ipsilateral** loss of vibration/proprioception (Dorsal columns cross at the medulla). * **Syringomyelia:** This condition involves a cyst in the central canal that compresses the anterior white commissure, leading to a classic **"cape-like" bilateral loss** of pain and temperature, while sparing touch (dissociated sensory loss). * **Rexed Laminae:** The second-order neurons of the LSTT primarily originate in Laminae I, IV, and V.

Question 132: Sour taste is mediated by which of the following?

A. Na+ selective channel (ENac)
B. G - protein coupled receptors
C. H+ ions permeable to ENaCs (Correct Answer)
D. Glutamate receptors

Explanation: **Explanation:** The perception of taste (gustation) involves different transduction mechanisms for the five primary taste qualities. **Sour taste** is triggered by acids, specifically the concentration of **H+ ions**. **Why Option C is Correct:** Sour taste transduction occurs when H+ ions (protons) interact with the apical membrane of taste cells. These ions act in two ways: 1. They directly enter the cell through **Amiloride-sensitive Sodium Channels (ENaCs)** or specialized proton channels (like OTOP1). 2. They bind to and block **K+ selective channels**, preventing K+ efflux. Both mechanisms lead to depolarization of the taste cell, opening of voltage-gated Ca2+ channels, and neurotransmitter release. **Analysis of Incorrect Options:** * **Option A (Na+ selective channel):** While ENaCs are involved in sour taste, they are the primary mediators for **Salty taste** when they transport Na+ ions. * **Option B (G-protein coupled receptors):** GPCRs (specifically T1R and T2R families) mediate **Sweet, Bitter, and Umami** tastes via the second messenger IP3 and release of intracellular Ca2+. * **Option D (Glutamate receptors):** These are specifically associated with **Umami (savory) taste**, triggered by L-glutamate (found in MSG). **High-Yield NEET-PG Pearls:** * **Salty & Sour:** Use **Ion Channels** (Direct depolarization). * **Sweet, Bitter, Umami:** Use **GPCRs** (Metabotropic receptors). * **Taste Pathway:** Anterior 2/3 of tongue (CN VII via Chorda tympani) → Posterior 1/3 (CN IX) → Epiglottis/Pharynx (CN X) → Nucleus Tractus Solitarius (NTS) → Thalamus (VPM nucleus) → Gustatory Cortex (Insula). * **Agraphia/Ageusia:** Loss of taste sensation, often tested in the context of Zinc deficiency or cranial nerve lesions.

Question 133: What is true about the electroretinogram?

A. The a-wave is a positive wave.
B. The a-wave arises from the pigmented epithelium.
C. The b-wave arises from rods and cones.
D. The c-wave is a positive wave. (Correct Answer)

Explanation: ### Explanation: Electroretinogram (ERG) The Electroretinogram (ERG) is a recording of the electrical potentials generated by the retina in response to a light stimulus. It consists of three primary waves: **a, b, and c**. **Why Option D is Correct:** The **c-wave** is a prolonged, **positive** deflection. It is the slowest component of the ERG and is generated by the **Pigment Epithelium** of the retina. It reflects the metabolic activity and ionic changes (specifically potassium levels) in the subretinal space following light exposure. **Why the Other Options are Incorrect:** * **Option A:** The **a-wave** is the first component and is a **negative** wave. It represents the "receptor potential." * **Option B:** The **a-wave** arises from the **photoreceptors** (rods and cones), not the pigment epithelium. * **Option C:** The **b-wave** is a large positive wave that arises from the **inner retinal layers**, specifically the **Bipolar cells** (with contributions from Müller cells), not the rods and cones themselves. --- ### High-Yield Facts for NEET-PG: * **Sequence of Waves:** 1. **a-wave (Negative):** Photoreceptors (Rods/Cones). 2. **b-wave (Positive):** Bipolar cells/Müller cells. (This is the most clinically significant wave). 3. **c-wave (Positive):** Retinal Pigment Epithelium (RPE). 4. **d-wave (Positive):** Occurs at the cessation of light (off-response). * **Clinical Utility:** ERG is used to diagnose generalized retinal disorders. It is **normal in Macular Degeneration** (because the macula is a small area) but **extinguished (flat)** in **Retinitis Pigmentosa** and Vitamin A deficiency. * **EOG (Electro-oculogram):** Measures the standing potential between the front and back of the eye; it is the gold standard for diagnosing **Best’s Disease**.

Question 134: Which cells are the first-order neurons for visual sensations?

A. Rods and cones
B. Bipolar cells (Correct Answer)
C. Ganglion cells
D. None of the above

Explanation: In the visual pathway, the transmission of signals follows a specific anatomical sequence. To identify the "order" of neurons, we look for the cells that conduct the first nerve impulses toward the brain. ### **Why Bipolar Cells are the Correct Answer** In the retina, **Rods and Cones** (photoreceptors) are specialized neuroepithelial cells, not neurons. They act as **sensory receptors** that convert light into electrical potentials (hyperpolarization). The **Bipolar cells** receive this signal and are the first cells in the pathway to function as true neurons, transmitting the impulse to the next layer. Therefore, they are classified as the **First-order neurons**. ### **Analysis of Incorrect Options** * **A. Rods and Cones:** These are the **photoreceptors**. While they initiate the visual process through phototransduction, they are considered receptors rather than first-order neurons in the classical neuroanatomical hierarchy. * **C. Ganglion Cells:** These are the **Second-order neurons**. Their axons converge to form the optic nerve, which carries the signal to the Lateral Geniculate Body (LGB). * **LGB (Lateral Geniculate Body):** Though not an option here, it is important to remember that the neurons in the LGB are the **Third-order neurons**, which then project to the primary visual cortex. ### **High-Yield Facts for NEET-PG** * **The Exception:** In most sensory systems (like touch or pain), the first-order neuron is located in the Dorsal Root Ganglion. In vision, the first three neurons of the pathway are uniquely located **entirely within the retina**. * **Action Potentials:** Rods, cones, and bipolar cells do **not** fire action potentials; they communicate via **graded potentials**. The **Ganglion cells** are the first cells in the visual pathway to fire true action potentials. * **Neurotransmitter:** Glutamate is the primary neurotransmitter released by photoreceptors in the dark.

Question 135: Which of the following statements regarding the lateral geniculate body is FALSE?

A. It is part of the thalamus.
B. M cells are responsible for depth perception.
C. P cells are responsible for detecting movement. (Correct Answer)
D. None of the above.

Explanation: The **Lateral Geniculate Body (LGB)** is the primary relay center for visual information from the retina to the primary visual cortex. Understanding the functional segregation of its layers is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer (Option C)** Option C is **FALSE** because it swaps the functions of P and M cells. * **P cells (Parvocellular pathway):** These cells are located in layers 3, 4, 5, and 6 of the LGB. They have small receptive fields and are responsible for **color vision, fine detail, and shape (high spatial resolution).** * **M cells (Magnocellular pathway):** These cells are located in layers 1 and 2. They have large receptive fields and are specialized for detecting **movement, flicker, and depth perception (high temporal resolution).** ### **Analysis of Other Options** * **Option A (True):** The LGB is indeed a sensory nucleus of the **thalamus** (specifically the metathalamus), acting as the "gatekeeper" for visual signals. * **Option B (True):** As part of the Magnocellular pathway, M cells are crucial for processing motion and stereopsis (**depth perception**). ### **High-Yield NEET-PG Pearls** * **LGB Layer Organization:** * **Layers 1, 4, 6:** Receive input from the **contralateral** eye (nasal retina). * **Layers 2, 3, 5:** Receive input from the **ipsilateral** eye (temporal retina). * *Mnemonic: "See (C) the 1, 4, 6" (C for Contralateral).* * **Koniocellular (K) Cells:** These are found between the M and P layers and are involved in blue-yellow color vision. * **Lesion:** A lesion in the LGB results in **contralateral homonymous hemianopia** with macular sparing (though sparing is more characteristic of cortical lesions).

Question 136: Most of the afferents from the lateral geniculate body terminate in which layer of the visual cortex?

A. Layer 1
B. Layers 2 & 3
C. Layer 4 (Correct Answer)
D. Layers 5 & 6

Explanation: **Explanation:** The primary visual cortex (V1 or Brodmann area 17) is organized into six distinct horizontal layers. The **Lateral Geniculate Body (LGB)** of the thalamus serves as the primary relay station for visual information. In the sensory systems, it is a fundamental physiological principle that most **thalamocortical afferents** terminate in **Layer 4** (the internal granular layer). In the visual cortex, Layer 4 is highly developed and further subdivided. Specifically, the parvocellular and magnocellular fibers from the LGB terminate predominantly in **Layer 4C**. This layer is rich in stellate cells, which receive the sensory input before distributing it to other layers for processing. **Analysis of Incorrect Options:** * **Layer 1 (Molecular Layer):** Contains mostly dendrites and axons from other layers; it is a site for modulatory input rather than primary sensory termination. * **Layers 2 & 3 (External Granular/Pyramidal):** These layers primarily function in **cortico-cortical communication**, sending projections to other areas of the cerebral cortex. * **Layers 5 & 6 (Internal Pyramidal/Multiform):** These are the **output layers**. Layer 5 sends projections to subcortical structures (like the superior colliculus), and Layer 6 provides **feedback projections** back to the Thalamus (LGB). **High-Yield NEET-PG Pearls:** * **Histology:** The visual cortex is called the "Striate Cortex" because the dense termination of afferents in Layer 4 creates a visible white line known as the **Stria of Gennari**. * **Functional Rule:** Remember the "Input-Output" rule: **Layer 4 is Input** (from Thalamus), **Layer 5/6 is Output** (to subcortical/thalamus). * **LGB Structure:** Layers 1-2 are Magnocellular (motion), while 3-6 are Parvocellular (color/detail). All eventually target Layer 4 of the cortex.

Question 137: What is the typical membrane potential of a hair cell?

A. -20 mV
B. -40 mV
C. -60 mV (Correct Answer)
D. -80 mV

Explanation: **Explanation:** The resting membrane potential (RMP) of a hair cell (the sensory receptor of the auditory and vestibular systems) is approximately **-60 mV**. Unlike typical neurons (RMP -70 mV) or skeletal muscle (RMP -90 mV), hair cells exist in a unique ionic environment. The apical portion of the hair cell is bathed in **endolymph** (high $K^+$, +80 mV potential), while the basal portion is bathed in **perilymph** (low $K^+$, 0 mV potential). This creates a massive electrical gradient of 140 mV across the stereocilia, allowing for rapid depolarization when mechanically gated $K^+$ channels open. **Analysis of Options:** * **A (-20 mV):** This is too depolarized for a resting state. Hair cells may reach this potential during active stimulation, but it is not the baseline. * **B (-40 mV):** While some texts cite a range of -40 to -60 mV for specific vestibular cells, **-60 mV** is the standard physiological value taught for the cochlear hair cells in major textbooks like Guyton and Ganong. * **D (-80 mV):** This is closer to the RMP of large nerve fibers or cardiac myocytes. Hair cells are more depolarized than these cells at rest to allow for "tonic" neurotransmitter release. **High-Yield Clinical Pearls for NEET-PG:** * **Endocochlear Potential:** The endolymph has a positive potential of **+80 mV**, the highest transepithelial potential in the body, maintained by the **Stria Vascularis**. * **Tip Links:** These protein filaments connect stereocilia; their stretching opens **MET (Mechano-Electrical Transducer)** channels. * **Unique Depolarization:** In hair cells, **$K^+$ influx** causes depolarization (unlike neurons where $Na^+$ influx causes it). * **Outer Hair Cells (OHC):** Act as "cochlear amplifiers" via the protein **Prestin**, which provides electromotility.

Question 138: Odorant molecules bind to which type of receptors?

A. Cyclic AMP (cAMP)
B. G protein-coupled membrane receptors (Correct Answer)
C. JAK kinase pathway
D. Nuclear factor (NF) receptors

Explanation: ### Explanation **1. Why G protein-coupled membrane receptors (GPCRs) are correct:** The olfactory system utilizes a sophisticated signal transduction pathway. Odorant molecules bind to specific **G protein-coupled receptors (GPCRs)** located on the cilia of olfactory receptor neurons. These receptors belong to the largest family of GPCRs (Type II). Upon binding, the G-protein (specifically **$G_{olf}$**) is activated, which then stimulates the enzyme **adenylyl cyclase**. This leads to the production of cAMP, which opens cyclic nucleotide-gated channels, causing an influx of $Na^+$ and $Ca^{2+}$, resulting in depolarization and the generation of an action potential. **2. Why the other options are incorrect:** * **A. Cyclic AMP (cAMP):** This is a **second messenger**, not a receptor. While cAMP is crucial for the olfactory signaling cascade, it is produced *after* the odorant binds to the GPCR. * **C. JAK kinase pathway:** This pathway is typically associated with **cytokine receptors** and growth factors (e.g., Erythropoietin, Growth Hormone, Prolactin), not sensory perception. * **D. Nuclear factor (NF) receptors:** These are intracellular receptors (like steroid or thyroid hormone receptors) that act as transcription factors. Odorants are generally small, volatile molecules that act on the cell surface, not the nucleus. **3. High-Yield Clinical Pearls for NEET-PG:** * **Linda Buck and Richard Axel** won the Nobel Prize for discovering the large family of genes (approx. 1,000) encoding these olfactory GPCRs. * **Anosmia:** The loss of smell. Common causes include head trauma (shearing of olfactory filia at the cribriform plate) and viral infections (e.g., COVID-19). * **Adaptation:** Olfactory receptors adapt rapidly (about 50% in the first second), explaining why we stop noticing a persistent smell quickly. * **Olfactory Pathway:** It is the only sensory system that reaches the cerebral cortex (olfactory cortex) **without** a mandatory relay in the thalamus.

Question 139: Metabotropic glutamate receptor 4 (mGluR4) is associated with which taste sensation?

A. Sweet
B. Sour
C. Bitter
D. Umami (Correct Answer)

Explanation: **Explanation:** The correct answer is **Umami (D)**. Taste transduction occurs through specific receptors on the tongue. **Umami**, the savory taste associated with L-glutamate (found in MSG and protein-rich foods), utilizes two primary types of G-protein coupled receptors (GPCRs): 1. **T1R1 + T1R3 heterodimer:** The primary receptor for most umami-tasting amino acids. 2. **mGluR4 (metabotropic Glutamate Receptor 4):** A truncated version of this receptor is specifically expressed in taste buds and functions as a specialized umami sensor. **Analysis of Incorrect Options:** * **Sweet (A):** Mediated by a heterodimer of **T1R2 + T1R3** GPCRs. These respond to sugars, saccharin, and certain proteins. * **Sour (B):** Mediated by ion channels, specifically the **OTOP1 (Otopetrin-1)** proton channel, which detects the acidity (H+ ions) of substances. * **Bitter (C):** Mediated by the **T2R** family of GPCRs. There are approximately 30 different T2R receptors to detect a wide variety of potentially toxic bitter substances. **High-Yield Facts for NEET-PG:** * **Salt taste** is primarily mediated by **ENaC** (Epithelial Sodium Channels). * **Gustducin** is the specialized G-protein involved in the transduction of Sweet, Bitter, and Umami tastes. * **Ageusia** is the loss of taste, while **Dysgeusia** is a distorted sense of taste (often a side effect of drugs like Captopril or Metronidazole). * The **anterior 2/3** of the tongue is supplied by the Chorda Tympani (CN VII), and the **posterior 1/3** by the Glossopharyngeal nerve (CN IX).

Question 140: Semicircular canals are stimulated by?

A. Gravity
B. Linear acceleration
C. Rotation (Correct Answer)
D. Sound

Explanation: **Explanation:** The vestibular apparatus in the inner ear is responsible for maintaining equilibrium and posture. It consists of the **Semicircular Canals (SCC)** and the **Otolith organs** (Utricle and Saccule). **Why Rotation is Correct:** The Semicircular Canals are specifically designed to detect **angular (rotational) acceleration**. There are three canals (Anterior, Posterior, and Lateral) oriented at right angles to each other, allowing for the detection of rotation in any plane. When the head rotates, the **endolymph** (fluid) within the canals moves due to inertia, which displaces the **cupula** in the ampulla. This bends the hair cells, triggering a neural impulse. **Why Other Options are Incorrect:** * **Gravity & Linear Acceleration (Options A & B):** These are detected by the **Otolith organs**. The **Utricle** detects horizontal linear acceleration, while the **Saccule** detects vertical linear acceleration (including gravity). These organs contain calcium carbonate crystals called *otoconia* that shift in response to linear movement. * **Sound (Option D):** Sound waves are processed by the **Cochlea**, specifically the Organ of Corti, which is the sensory organ for hearing, not balance. **High-Yield Clinical Pearls for NEET-PG:** * **BPPV (Benign Paroxysmal Positional Vertigo):** Caused by displaced otoconia from the utricle entering the semicircular canals (most commonly the **Posterior canal**). It is diagnosed by the **Dix-Hallpike maneuver** and treated with the **Epley maneuver**. * **Caloric Reflex Test:** Used to test SCC function. Remember the mnemonic **COWS**: **C**old **O**pposite, **W**arm **S**ame (refers to the direction of the fast phase of nystagmus). * **Scarpa’s Ganglion:** The sensory ganglion for the vestibular nerve.

Practice by Chapter

General Sensory Physiology

Practice Questions

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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