Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 101: Olfactory receptor cells belong to which group of cells?

A. Bipolar neurons (Correct Answer)
B. Fibroblasts
C. Modified epithelial cells
D. Multipolar neurons

Explanation: **Explanation:** **1. Why Bipolar Neurons is Correct:** Olfactory receptor cells are unique because they are **primary sensory neurons** located in the olfactory epithelium. They are classified as **bipolar neurons** because they possess two processes: * **Dendrite:** A short process that extends to the epithelial surface, ending in a knob-like "olfactory vesicle" with non-motile cilia containing odorant receptors. * **Axon:** An unmyelinated process that passes through the cribriform plate of the ethmoid bone to synapse directly in the olfactory bulb. Crucially, these are among the few neurons in the adult human body that undergo continuous **neurogenesis** (replacement every 30–60 days) from basal stem cells. **2. Why Other Options are Incorrect:** * **Fibroblasts:** These are connective tissue cells responsible for collagen synthesis, not excitable neural tissue. * **Modified Epithelial Cells:** While many sensory receptors (like taste buds or hair cells in the ear) are modified epithelial cells that synapse onto secondary neurons, olfactory receptors are **true neurons** themselves. * **Multipolar Neurons:** These have one axon and multiple dendrites (e.g., motor neurons, pyramidal cells). Olfactory receptors only have a single dendritic process. **3. High-Yield Clinical Pearls for NEET-PG:** * **Olfactory Pathway:** It is the **only** sensory modality that reaches the cerebral cortex (olfactory cortex) without a mandatory relay in the **thalamus**. * **Cranial Nerve I:** The collection of unmyelinated axons from these bipolar neurons forms the Olfactory Nerve. * **Anosmia:** Fracture of the cribriform plate can shear these bipolar axons, leading to a loss of smell. * **Bowman’s Glands:** Located in the olfactory mucosa, they secrete mucus to dissolve odorants.

Question 102: Which cells of the olfactory bulb function as local integrators?

A. Sustentacular cells
B. Hair cells
C. Granular cells
D. Mitral cells (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Mitral cells)** In the olfactory system, the olfactory bulb serves as the primary processing center. **Mitral cells** (along with tufted cells) are the primary output neurons of the olfactory bulb. They receive sensory input from the olfactory receptor neurons (ORNs) within complex synaptic structures called **glomeruli**. Mitral cells function as **local integrators** because they refine, modulate, and integrate diverse sensory signals before transmitting the processed information via the olfactory tract to the higher cortical centers (e.g., pyriform cortex, amygdala). **Analysis of Incorrect Options:** * **A. Sustentacular cells:** These are non-neuronal supporting cells located in the olfactory epithelium (not the bulb). They provide structural support, metabolic assistance, and maintain ion balance, similar to glial cells. * **B. Hair cells:** These are the sensory receptors for the auditory and vestibular systems (located in the cochlea and vestibular apparatus), not the olfactory system. The receptors for smell are termed olfactory receptor neurons (ORNs). * **C. Granular cells:** While present in the olfactory bulb, these are inhibitory interneurons that lack axons. They provide lateral inhibition to mitral cells to sharpen odor discrimination, but they are not the primary integrators/output units. **High-Yield Clinical Pearls for NEET-PG:** * **First-order neurons:** Olfactory receptor neurons (bipolar neurons) located in the olfactory epithelium. * **Second-order neurons:** Mitral and Tufted cells in the olfactory bulb. * **Unique Pathway:** Olfaction is the only sensory modality that reaches the cerebral cortex **without** a mandatory relay in the thalamus. * **Regeneration:** Olfactory receptor neurons are one of the few mammalian neurons that undergo continuous replacement throughout life from basal cells.

Question 103: The Weber-Fechner Law states that the magnitude of stimulus strength perceived is approximately proportional to the logarithm of the stimulus intensity?

A. Magnitude of stimulus strength perceived is approximately proportional to the log of the intensity of stimulus strength. (Correct Answer)
B. Magnitude of stimulus strength perceived is directly proportional to the intensity of stimulus strength.
C. Threshold of receptor is directly proportional to stimulus strength.
D. Threshold of receptor is inversely proportional to stimulus strength.

Explanation: ### Explanation The **Weber-Fechner Law** is a fundamental principle in psychophysics that describes the relationship between the physical intensity of a stimulus and its perceived intensity. **Why Option A is Correct:** The law states that the intensity of a sensation ($S$) is proportional to the logarithm of the intensity of the stimulus ($I$). Mathematically, this is expressed as: $$S = K \cdot \log(I)$$ This logarithmic relationship means that as a stimulus becomes stronger, much larger changes in physical intensity are required to produce a noticeable difference in perception. This allows our sensory systems (like hearing and vision) to operate over an incredibly wide range of intensities without saturating the receptors. **Analysis of Incorrect Options:** * **Option B:** This describes a linear relationship. If perception were directly proportional to intensity, our receptors would saturate very quickly, and we would be unable to distinguish between high-intensity stimuli (e.g., looking at a bright sky vs. a lightbulb). * **Options C & D:** These options confuse "threshold" with "perceived magnitude." The threshold is the minimum intensity required to elicit a response; while related to sensitivity, it does not define the mathematical scaling of perception described by Weber-Fechner. **High-Yield NEET-PG Pearls:** * **Weber’s Law:** The "Just Noticeable Difference" (JND) is a constant fraction of the original stimulus ($\Delta I / I = K$). * **Stevens’ Power Law:** A more modern refinement suggesting that for some sensations (like pain/electric shock), perception increases as a power function rather than a log function. * **Clinical Application:** This law explains why sound intensity is measured on a **logarithmic scale (Decibels)** and why we can perceive light ranging from a dim star to the bright midday sun.

Question 104: Which of the following are rapidly adapting receptors?

A. Merkel cells
B. Pacinian corpuscle (Correct Answer)
C. Free nerve endings
D. Ruffini endings

Explanation: **Explanation:** Mechanoreceptors in the skin are classified based on their adaptation rates: **Rapidly Adapting (RA)** receptors respond only at the onset and offset of a stimulus (detecting change/velocity), while **Slowly Adapting (SA)** receptors provide a continuous discharge during a sustained stimulus (detecting pressure/intensity). **Why Pacinian Corpuscle is Correct:** The **Pacinian corpuscle** is the most rapidly adapting receptor in the body. It is located in the deeper dermis and is specialized for detecting **high-frequency vibrations** (60–300 Hz) and rapid changes in mechanical displacement. Its rapid adaptation is due to its onion-like encapsulated structure, which filters out steady pressure, allowing only transient energy to reach the nerve ending. **Analysis of Incorrect Options:** * **Merkel cells (Option A):** These are **Slowly Adapting Type I (SAI)** receptors. They are located in the basal layer of the epidermis and are responsible for detecting fine touch, edges, and texture. * **Free nerve endings (Option C):** These primarily function as nociceptors (pain) and thermoreceptors. While their adaptation varies, they are generally not classified under the classic rapid mechanoreceptor category. * **Ruffini endings (Option D):** These are **Slowly Adapting Type II (SAII)** receptors. They respond to skin stretch and are crucial for perceiving finger position and grip. **NEET-PG High-Yield Pearls:** * **Meissner’s Corpuscles:** Also rapidly adapting, but detect **low-frequency vibrations** (flutter) and are located in the dermal papillae of hairless skin. * **Mnemonic for Adaptation:** **P**acinian and **M**eissner are **P**rompt (**Rapid**); **M**erkel and **R**uffini are **S**teady (**Slow**). * **Receptive Fields:** Meissner and Merkel have small, well-defined borders (Type I); Pacinian and Ruffini have large, ill-defined borders (Type II).

Question 105: Which sensory fibers are least susceptible to hypoxia?

A. Type I
B. Type II
C. Type III
D. Type IV (Correct Answer)

Explanation: **Explanation:** The susceptibility of nerve fibers to various insults depends on their physiological characteristics. The sensitivity of nerve fibers to hypoxia, pressure, and local anesthetics follows a specific order, often tested in NEET-PG. **Why Type IV is correct:** Type IV fibers (Erlanger-Gasser **Type C fibers**) are the least susceptible to hypoxia. These are small-diameter, unmyelinated fibers that conduct slow pain and temperature sensations. Because they have a lower metabolic rate and lack the complex machinery of thick myelin sheaths, they can withstand oxygen deprivation longer than larger, myelinated fibers. **Why the other options are incorrect:** * **Type I (A-alpha):** These are the largest, most heavily myelinated fibers (proprioception, somatic motor). They have the highest metabolic demand and are the **most susceptible to pressure**, but they are highly sensitive to hypoxia compared to Type IV. * **Type II (A-beta):** These are medium-sized myelinated fibers (touch, pressure). Like Type I, they are more sensitive to hypoxia than the unmyelinated Type IV fibers. * **Type III (A-delta):** These are small, thinly myelinated fibers (fast pain, cold). While more resistant than Type I, they are still more sensitive to hypoxia than Type IV. **High-Yield Clinical Pearls for NEET-PG:** To remember the sensitivity of nerve fibers, use the following hierarchy (from Most Sensitive to Least Sensitive): 1. **Hypoxia:** B > A > C (Type IV is C, thus least sensitive). 2. **Pressure:** A > B > C (Large fibers are compressed first; C is least sensitive). 3. **Local Anesthetics:** C > B > A (Small, unmyelinated fibers are blocked first). *Note: Type IV fibers are the only unmyelinated sensory fibers; Types I, II, and III are all subtypes of myelinated Type A fibers.*

Question 106: The bitter taste is mediated by the action of which of the following?

A. Guanyl cyclase
B. G protein (Correct Answer)
C. Tyrosine kinase
D. Epithelial Na+ channel

Explanation: **Explanation:** The sensation of taste (gustation) is mediated by specific receptors on the tongue that utilize different signal transduction pathways. **Why G protein is correct:** Bitter, sweet, and umami tastes are mediated by **G-protein coupled receptors (GPCRs)**. Specifically, bitter taste involves the **T2R family** of receptors. When a bitter tastand binds to these receptors, it activates a specialized G-protein called **Gustducin**. This triggers the phospholipase C (PLC-β2) pathway, increasing intracellular calcium, which leads to the release of neurotransmitters (primarily ATP) via hemichannels to activate gustatory afferent nerves. **Why the other options are incorrect:** * **Guanyl cyclase:** This is involved in cGMP signaling (e.g., Nitric Oxide or ANP pathways) but does not play a primary role in taste transduction. * **Tyrosine kinase:** These receptors are typically associated with growth factors and insulin signaling, not sensory perception of taste. * **Epithelial Na+ channel (ENaC):** These channels are responsible for **Salty** taste. Sodium ions enter the taste cell directly through amiloride-sensitive ENaC channels, causing depolarization. **High-Yield Facts for NEET-PG:** 1. **Salty & Sour:** Use **Ion Channels** (Salty: ENaC; Sour: H+ sensitive channels/OTOP1). 2. **Sweet, Bitter, & Umami:** Use **GPCRs** (Sweet: T1R2+T1R3; Umami: T1R1+T1R3; Bitter: T2R family). 3. **Gustducin:** The specific G-protein associated with bitter, sweet, and umami. 4. **Innervation:** Anterior 2/3 of the tongue is supplied by the Chorda Tympani (CN VII); Posterior 1/3 by the Glossopharyngeal nerve (CN IX). 5. **AGUESIA:** Loss of taste sensation; **DYSGEUSIA:** Distorted taste.

Question 107: A lesion in the posterior column of the spinal cord will affect which of the following modalities?

A. Vibration sense
B. Temperature sense (Correct Answer)
C. Fine touch
D. Crude touch sensation

Explanation: The posterior column-medial lemniscus (PCML) pathway and the anterolateral system (spinothalamic tract) are the two primary ascending pathways for sensory information. Understanding their distinct functions is crucial for localizing spinal cord lesions. **Explanation of the Correct Answer:** The question asks which modality is **affected** (impaired) by a lesion in the posterior column. However, based on classical physiology, **Temperature sense** is transmitted via the **Lateral Spinothalamic Tract** (part of the anterolateral system), not the posterior columns. In the context of standard NEET-PG patterns, if "Temperature sense" is marked as the correct answer for a lesion *affecting* the posterior column, it usually implies an "except" style question or a focus on what is **spared**. *Note: If the question asks what is LOST in a posterior column lesion, the answer should be Vibration or Fine Touch. If the answer key identifies Temperature, it indicates that Temperature is the modality **not** carried by this tract.* **Analysis of Options:** * **A. Vibration sense:** Carried by the posterior columns (Fasciculus Gracilis and Cuneatus). It would be lost in a posterior column lesion. * **C. Fine touch (Tactile discrimination):** Carried by the posterior columns. It would be lost. * **D. Crude touch sensation:** Carried primarily by the **Anterior Spinothalamic Tract**. While there is some overlap, it is generally spared in isolated posterior column lesions. * **B. Temperature sense:** Carried by the **Lateral Spinothalamic Tract**. It remains intact (spared) in a posterior column lesion. **NEET-PG High-Yield Pearls:** 1. **Posterior Column (PCML):** Carries Fine touch, Vibration, Proprioception, and Pressure. Fibers decussate in the **Medulla** (Internal arcuate fibers). 2. **Spinothalamic Tract (STT):** Carries Pain and Temperature (Lateral) and Crude touch (Anterior). Fibers decussate at the **level of entry** in the spinal cord. 3. **Tabes Dorsalis:** A classic clinical condition (neurosyphilis) involving selective destruction of the posterior columns, leading to sensory ataxia and loss of vibration/proprioception. 4. **Dissociated Sensory Loss:** Seen in Syringomyelia, where pain and temperature are lost but touch/vibration are preserved.

Question 108: Tapping the patellar tendon with a reflex hammer produces a brief contraction of the knee extensors. What is the cause of this muscle contraction?

A. Elastic rebound of muscle connective tissue
B. Golgi tendon organ response
C. Muscle spindle activation (Correct Answer)
D. Muscle spindle unloading

Explanation: **Explanation:** The patellar reflex (knee-jerk) is a classic example of a **monosynaptic stretch reflex**. When the patellar tendon is tapped, it causes a rapid, transient stretch of the quadriceps muscle. 1. **Why the correct answer is right:** The primary sensory receptors for detecting muscle stretch are the **muscle spindles** (intrafusal fibers). When the quadriceps is stretched, the muscle spindles are activated, sending afferent impulses via **Type Ia nerve fibers** to the spinal cord. These fibers synapse directly (monosynaptically) with **alpha-motor neurons** in the anterior horn, which then trigger the contraction of the extrafusal muscle fibers of the quadriceps. 2. **Why the incorrect options are wrong:** * **Elastic rebound:** This is a physical property of tissue, not a neural reflex. The force of contraction in a reflex is far greater than any simple elastic recoil. * **Golgi tendon organ (GTO) response:** GTOs (Type Ib fibers) respond to **muscle tension**, not stretch. Their activation typically causes *autogenic inhibition* (relaxation) to prevent tendon avulsion, which is the opposite of the observed contraction. * **Muscle spindle unloading:** This occurs during muscle **contraction** (shortening), which actually decreases the firing rate of the spindle. It is the *loading* (stretching) that triggers the reflex. **Clinical Pearls for NEET-PG:** * **Reflex Arc Components:** Receptor (Spindle) → Afferent (Ia) → Center (L3-L4 spinal cord) → Efferent (Alpha-motor neuron) → Effector (Quadriceps). * **Gamma-motor neurons:** These regulate the sensitivity of the muscle spindle. High gamma-discharge leads to hyperreflexia. * **Jendrassik Maneuver:** A reinforcement technique used to accentuate deep tendon reflexes by increasing the excitatory drive to the spinal motor pool.

Question 109: Where are odour receptors located?

A. Olfactory epithelium (Correct Answer)
B. Olfactory tract
C. Amygdala
D. Olfactory bulbs

Explanation: **Explanation:** **1. Why Olfactory Epithelium is Correct:** The **olfactory epithelium** is a specialized patch of pseudostratified columnar epithelium located in the roof of the nasal cavity. It contains the **olfactory receptor cells**, which are unique bipolar neurons. The apical ends of these neurons possess non-motile cilia (olfactory hairs) that contain the actual **G-protein coupled receptors (GPCRs)**. These receptors bind to odorant molecules dissolved in the overlying mucus, initiating the signal transduction pathway (via Adenyl cyclase and cAMP). **2. Why Other Options are Incorrect:** * **Olfactory Bulbs (D):** These are the primary processing centers where the axons of olfactory receptor cells synapse with **mitral and tufted cells** in structures called glomeruli. They do not contain the initial receptors. * **Olfactory Tract (B):** This is a bundle of axons (second-order neurons) that carries sensory information from the olfactory bulb to the brain. It is a conduction pathway, not a receptor site. * **Amygdala (C):** This is a part of the limbic system involved in the emotional processing of smells. It is a higher-order cortical destination, not the site of primary reception. **3. High-Yield Facts for NEET-PG:** * **Regeneration:** Olfactory receptor cells are one of the few neurons in the body that undergo continuous turnover (replacement) throughout life from **basal cells**. * **Pathway:** Olfaction is the **only** sensory system that reaches the cerebral cortex (olfactory cortex) without first relaying in the **thalamus**. * **Bowman’s Glands:** Located in the olfactory epithelium, they secrete the mucus that dissolves odorants. * **Clinical Correlation:** Fracture of the **cribriform plate** of the ethmoid bone can lead to **Anosmia** (loss of smell) and CSF rhinorrhea.

Question 110: Which of the following nerves serves as the afferent pathway for the light pupillary reflex?

A. Trigeminal nerve
B. Optic nerve (Correct Answer)
C. Abducent nerve
D. Ciliary nerve

Explanation: ### Explanation The **Light Pupillary Reflex** is an autonomic reflex that constricts the pupil (miosis) in response to light hitting the retina. Understanding the reflex arc is crucial for localizing neurological lesions. **Why the Optic Nerve is Correct:** The **Optic Nerve (CN II)** acts as the **afferent (sensory) limb**. When light enters the eye, it stimulates the retinal photoreceptors. The impulse travels via the optic nerve, through the optic chiasm and optic tract, bypassing the lateral geniculate body to reach the **Pretectal Nucleus** in the midbrain. From here, fibers project bilaterally to the Edinger-Westphal nuclei, initiating the efferent response. **Analysis of Incorrect Options:** * **Trigeminal Nerve (CN V):** This nerve provides sensory innervation to the face and is the afferent limb for the *corneal reflex*, not the light reflex. * **Abducent Nerve (CN VI):** This is a motor nerve that innervates the lateral rectus muscle for eye abduction. It has no role in pupillary constriction. * **Ciliary Nerve:** Short ciliary nerves carry the **efferent** parasympathetic fibers from the ciliary ganglion to the sphincter pupillae muscle. They are part of the motor output, not the sensory input. **High-Yield Clinical Pearls for NEET-PG:** * **Efferent Limb:** The **Oculomotor Nerve (CN III)** serves as the efferent pathway. * **Consensual Light Reflex:** Shining light in one eye causes constriction in both because pretectal fibers project to **both** Edinger-Westphal nuclei. * **Marcus Gunn Pupil:** Seen in optic nerve lesions (e.g., optic neuritis); characterized by a "Relative Afferent Pupillary Defect" (RAPD) where the pupil appears to dilate when light is moved from the normal eye to the affected eye during the swinging flashlight test. * **Argyll Robertson Pupil:** Pupil accommodates but does not react to light ("Prostitute’s Pupil"); classically associated with neurosyphilis (lesion in the pretectal nucleus).

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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