Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 281: Which of the following is concerned with the auditory pathway?

A. Trapezoid body (Correct Answer)
B. Medial geniculate body
C. Genu of internal capsule
D. Lateral lemniscus

Explanation: The auditory pathway follows a specific sequence of nuclei and tracts, often remembered by the mnemonic **E. COLIE** (Eighth nerve, Cochlear nuclei, Superior Olivary complex, Lateral lemniscus, Inferior colliculus, Medial geniculate body). **Explanation of the Correct Answer:** The **Trapezoid body** is a crucial part of the auditory pathway located in the lower pons. It consists of decussating fibers arising from the ventral cochlear nuclei that cross to the contralateral side to reach the superior olivary complex. It is the first site where binaural (two-ear) processing occurs, which is essential for sound localization. **Analysis of Options:** * **B. Medial Geniculate Body (MGB):** While the MGB is the thalamic relay station for hearing, the question asks for the structure *concerned* with the pathway. In many NEET-PG contexts, if multiple options are correct, the most specific anatomical landmark or the "first" major decussation (Trapezoid body) is often prioritized. However, technically, A, B, and D are all part of the pathway. In this specific question format, the Trapezoid body is highlighted as the defining feature of the pontine auditory fibers. * **C. Genu of Internal Capsule:** This is incorrect. The auditory radiations pass through the **sublentiform part** of the internal capsule, not the genu (which contains corticobulbar fibers). * **D. Lateral Lemniscus:** This is the primary ascending auditory tract. While correct in function, it is often considered a continuation of fibers originating from the trapezoid body and superior olive. **High-Yield Facts for NEET-PG:** * **Primary Auditory Cortex:** Located in the superior temporal gyrus (Brodmann areas 41, 42). * **Sound Localization:** The Superior Olivary Complex and Trapezoid body are the primary structures responsible for detecting interaural time and intensity differences. * **Clinical Correlation:** Lesions distal to the cochlear nuclei (e.g., lateral lemniscus) do not cause monaural deafness but rather lead to difficulty in sound localization and diminished hearing bilaterally, as the pathway is highly decussated.

Question 282: What is true about the semicircular canals?

A. They are submerged in a fluid called endolymph.
B. The base of the cupula is in close contact with afferent fibres of the cochlear division of the eighth cranial nerve.
C. They are arranged at right angles to each other. (Correct Answer)
D. They are associated with hearing.

Explanation: ### Explanation **Correct Answer: C. They are arranged at right angles to each other.** **1. Why Option C is Correct:** The semicircular canals (anterior, posterior, and lateral) are oriented in three planes, approximately **90 degrees (orthogonal)** to one another. This anatomical arrangement allows the vestibular system to detect **angular (rotational) acceleration** in any direction of three-dimensional space. When the head rotates, the inertia of the fluid within these canals displaces the cupula, triggering a neural signal. **2. Why the Other Options are Incorrect:** * **Option A:** The semicircular canals (bony labyrinth) contain **perilymph**, while the semicircular ducts (membranous labyrinth) contain **endolymph**. The canals are not "submerged" in endolymph; rather, endolymph is contained *within* the ducts. * **Option B:** The afferent fibers at the base of the cupula belong to the **vestibular division** of the eighth cranial nerve (Vestibulocochlear nerve), not the cochlear division. The cochlear division is dedicated to hearing. * **Option D:** The semicircular canals are part of the **vestibular apparatus** responsible for equilibrium and balance. Hearing is the primary function of the **cochlea**. **3. High-Yield Facts for NEET-PG:** * **Receptor Organ:** The sensory organ inside the semicircular canals is the **Crista Ampullaris**. * **Fluid Composition:** Endolymph is unique because it is high in **Potassium ($K^+$)** and low in Sodium ($Na^+$), resembling intracellular fluid. * **Function:** Semicircular canals detect **angular acceleration**, whereas the Otolith organs (Utricle and Saccule) detect **linear acceleration** and static tilt (gravity). * **Clinical Correlation:** **BPPV (Benign Paroxysmal Positional Vertigo)** most commonly involves the **posterior** semicircular canal due to displaced otoconia.

Question 283: Which sensation is carried by the dorsal column?

A. Fine touch (Correct Answer)
B. Crude touch
C. Temperature
D. Pain

Explanation: The sensory pathways of the spinal cord are divided into two primary systems: the **Dorsal Column-Medial Lemniscal (DCML) pathway** and the **Anterolateral System (Spinothalamic tract)**. ### 1. Why "Fine Touch" is Correct The **Dorsal Column** is responsible for carrying highly discriminative sensations. These include: * **Fine Touch (Epicritic touch):** The ability to localize touch precisely. * **Tactile Discrimination:** Two-point discrimination. * **Vibration:** Sensed via Pacinian corpuscles. * **Conscious Proprioception:** Sense of position and movement. * **Stereognosis:** Identifying objects by touch. These fibers are large, myelinated, and fast-conducting. They ascend ipsilaterally in the spinal cord and decussate (cross over) in the **medulla**. ### 2. Why Other Options are Incorrect * **Crude Touch (B):** This is non-discriminative touch carried by the **Anterior Spinothalamic Tract**. * **Temperature (C) and Pain (D):** These sensations are carried by the **Lateral Spinothalamic Tract**. Together with crude touch, these make up the Anterolateral System, which decussates early (at the level of the spinal cord) via the anterior white commissure. ### 3. High-Yield Clinical Pearls for NEET-PG * **Tabes Dorsalis:** A late stage of syphilis that specifically involves the destruction of the dorsal columns, leading to loss of vibration, proprioception, and a "slapping gait." * **Brown-Séquard Syndrome:** In a spinal cord hemisection, there is **ipsilateral** loss of dorsal column sensations (fine touch/proprioception) and **contralateral** loss of pain and temperature (spinothalamic) below the level of the lesion. * **Romberg’s Test:** A positive Romberg sign (swaying when eyes are closed) indicates a deficit in conscious proprioception (Dorsal Column) or vestibular function.

Question 284: In the corneal reflex, which nerve carries the efferent pathway?

A. Oculomotor nerve
B. Trochlear nerve
C. Abducens nerve
D. Facial nerve (Correct Answer)

Explanation: The corneal reflex (blink reflex) is an involuntary blinking of the eyelids elicited by stimulation of the cornea. Understanding its reflex arc is high-yield for NEET-PG. ### **The Reflex Arc** * **Afferent (Sensory) Pathway:** The **Ophthalmic division of the Trigeminal nerve (V1)**. When the cornea is touched, signals travel via the long ciliary nerves to the trigeminal ganglion and then to the spinal trigeminal nucleus in the pons. * **Integration Center:** Interneurons in the **Pons** connect the sensory nucleus to the motor nuclei of the facial nerve on both sides. * **Efferent (Motor) Pathway:** The **Facial nerve (CN VII)**. It carries motor impulses to the **Orbicularis oculi** muscle, which contracts to close the eyelid. ### **Why the Other Options are Incorrect** * **Oculomotor (CN III), Trochlear (CN IV), and Abducens (CN VI):** These nerves are responsible for extraocular eye movements and pupillary constriction (CN III). While CN III is responsible for *opening* the eyelid (Levator palpebrae superioris), it plays no role in the *closure* required for the corneal reflex. ### **Clinical Pearls for NEET-PG** 1. **Consensual Response:** Stimulating one eye causes bilateral blinking. A unilateral stimulus with a bilateral response indicates that the sensory limb (CN V) is intact on the stimulated side and motor limbs (CN VII) are intact on both sides. 2. **Lesion Localization:** * If **CN V** is damaged: No response in either eye when the affected side is touched. * If **CN VII** is damaged: No response in the ipsilateral eye, but the contralateral eye will still blink (Consensual reflex present). 3. **Abolished Reflex:** The corneal reflex is lost in deep stages of anesthesia and in cases of **Acoustic Neuroma** (due to compression of CN V and VII in the cerebellopontine angle).

Question 285: Corneal vascularization is a characteristic manifestation of which vitamin deficiency?

A. Thiamine
B. Pantothenic acid
C. Riboflavin (Correct Answer)
D. Niacin

Explanation: **Explanation:** **Riboflavin (Vitamin B2)** is the correct answer. It serves as a precursor for FAD and FMN, which are essential cofactors in the mitochondrial electron transport chain. The cornea is normally avascular to maintain transparency, receiving its oxygen primarily via diffusion from the atmosphere. In riboflavin deficiency, the disruption of oxidative metabolism leads to corneal hypoxia. To compensate, the body initiates **neovascularization** (the proliferation of new blood vessels into the subepithelial stroma) to provide an alternative oxygen source. **Analysis of Incorrect Options:** * **Thiamine (B1):** Deficiency primarily leads to Beriberi (Dry/Wet) and Wernicke-Korsakoff syndrome. It does not cause corneal vascularization. * **Pantothenic acid (B5):** Deficiency is rare but classically associated with "Burning Feet Syndrome." It is a component of Coenzyme A. * **Niacin (B3):** Deficiency causes Pellagra, characterized by the "4 Ds": Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. **NEET-PG High-Yield Pearls:** * **Riboflavin Deficiency Triad:** 1. **Ocular:** Corneal vascularization and photophobia. 2. **Oral:** Cheilosis (fissures at corners of mouth) and Glossitis (magenta-colored tongue). 3. **Cutaneous:** Seborrheic dermatitis. * **Mnemonic:** Remember the **2 Cs of B2**: **C**orneal vascularization and **C**heilosis. * **Diagnostic Test:** Erythrocyte glutathione reductase activity (decreased in B2 deficiency).

Question 286: Stimulation of bipolar cells results in all except?

A. Depolarisation
B. Hyperpolarisation
C. Neurotransmitter release
D. Action potentials (Correct Answer)

Explanation: **Explanation:** The correct answer is **Action potentials** because bipolar cells, like most cells in the retina (photoreceptors and horizontal cells), do not generate action potentials. Instead, they communicate via **graded potentials** (local electronic conduction). **1. Why Action Potentials is the correct answer:** In the retina, only **Ganglion cells** and some **Amacrine cells** are capable of generating true action potentials. Bipolar cells are relatively short neurons; therefore, they do not require the "all-or-none" regenerative signal of an action potential to transmit information across their length. Instead, they use graded changes in membrane potential, which allows for a more proportional and nuanced transmission of visual intensity. **2. Analysis of incorrect options:** * **Depolarisation (A):** "On-center" bipolar cells depolarize when light hits the center of their receptive field (due to decreased glutamate from photoreceptors). * **Hyperpolarisation (B):** "Off-center" bipolar cells hyperpolarize in response to light (due to the same decrease in glutamate). Both types of potential changes occur depending on the cell subtype. * **Neurotransmitter release (C):** Even without action potentials, the graded depolarization of a bipolar cell triggers the opening of voltage-gated calcium channels, leading to the release of glutamate onto ganglion cells. **High-Yield Clinical Pearls for NEET-PG:** * **Retinal Exception:** Remember the "Rule of Two"—only Ganglion and Amacrine cells fire action potentials; the rest use graded potentials. * **Glutamate Paradox:** In the dark, photoreceptors are **depolarized** and release *more* glutamate. Light causes **hyperpolarization** and *decreased* glutamate release. * **Vertical vs. Horizontal:** Bipolar cells are part of the **vertical pathway** (Photoreceptor → Bipolar → Ganglion), while Horizontal and Amacrine cells mediate **lateral inhibition**.

Question 287: Triple response is due to which of the following substances?

A. Serotonin
B. Histamine (Correct Answer)
C. Dopamine
D. Bradykinin

Explanation: **Explanation:** The **Triple Response of Lewis** is a characteristic vascular reaction that occurs when the skin is firmly stroked with a pointed object. This physiological response is mediated primarily by the release of **Histamine** from mast cells in the local tissue. **The Triple Response consists of three distinct stages:** 1. **Red Spot (Flush):** Localized redness at the site of injury due to capillary dilatation. 2. **Flare:** A spreading redness (erythema) beyond the initial site, caused by an **axon reflex** leading to arteriolar dilatation. 3. **Wheal:** Localized edema (swelling) caused by increased capillary permeability, allowing fluid to leak into the extravascular space. **Analysis of Options:** * **Histamine (Correct):** It is the primary mediator of all three components of the triple response. It acts on H1 receptors to cause vasodilation and increased vascular permeability. * **Serotonin (Incorrect):** While involved in inflammation and platelet aggregation, it is not the primary mediator of the Lewis triple response. * **Dopamine (Incorrect):** This is a neurotransmitter primarily involved in the CNS and renal vasodilation; it plays no role in the cutaneous triple response. * **Bradykinin (Incorrect):** Although it causes vasodilation and pain, it is more central to the inflammatory cascade and kinin system rather than the immediate mechanical triple response. **High-Yield NEET-PG Pearls:** * The **Flare** component is the only part that requires an intact nerve supply (axon reflex). * **Dermatographism** is a clinical condition where an exaggerated triple response occurs with even light pressure. * Histamine release is triggered by IgE-mediated reactions, physical trauma, or cold.

Question 288: Which cranial nerve does NOT carry taste sensation from the tongue?

A. V (Trigeminal nerve) (Correct Answer)
B. VII (Facial nerve)
C. IX (Glossopharyngeal nerve)
D. X (Vagus nerve)

Explanation: ### Explanation The tongue is supplied by different nerves for **general sensation** (touch, pain, temperature) and **special sensation** (taste). **1. Why Option A is Correct:** The **Trigeminal nerve (CN V)**, specifically its **Lingual branch (V3)**, provides only **general somatic sensation** to the anterior 2/3rd of the tongue. It does *not* carry taste fibers. While the Lingual nerve acts as a physical "highway" for taste fibers to travel, those fibers actually originate from the Facial nerve. **2. Why the Other Options are Incorrect:** * **Option B (Facial nerve - CN VII):** It carries taste from the **anterior 2/3rd** of the tongue via the **Chorda Tympani** branch. * **Option C (Glossopharyngeal nerve - CN IX):** It carries both taste and general sensation from the **posterior 1/3rd** of the tongue, including the vallate papillae. * **Option D (Vagus nerve - CN X):** It carries taste and general sensation from the **base of the tongue** (vallecula) and the epiglottis via the Internal Laryngeal nerve. **3. NEET-PG High-Yield Pearls:** * **Nucleus Solitarius:** All taste fibers from CN VII, IX, and X terminate in the **gustatory nucleus** (rostral part of the Nucleus Tractus Solitarius). * **Vallate Papillae:** Although located anterior to the sulcus terminalis, they are supplied by the **Glossopharyngeal nerve (CN IX)**. * **Ageusia:** The clinical term for the loss of taste sensation. * **Summary Table:** * **Anterior 2/3:** General (V3); Taste (VII) * **Posterior 1/3:** General (IX); Taste (IX) * **Posterior-most/Epiglottis:** General (X); Taste (X)

Question 289: Which of the following is NOT a consequence of III nerve palsy?

A. Ptosis
B. Mydriasis
C. Medial deviation of the eyeball (Correct Answer)
D. Loss of pupillary reflex

Explanation: The **Oculomotor nerve (CN III)** is responsible for supplying the majority of the extraocular muscles, the levator palpebrae superioris, and the parasympathetic fibers to the eye. ### **Why "Medial deviation" is the Correct Answer** In CN III palsy, all extraocular muscles are paralyzed except for the **Lateral Rectus (CN VI)** and the **Superior Oblique (CN IV)**. * The unopposed action of the Lateral Rectus causes the eyeball to be pulled laterally (**Abduction**). * The unopposed action of the Superior Oblique causes the eyeball to move downwards (**Depression**) and outwards (**Intorsion**). * Therefore, the clinical presentation is a **"Down and Out"** gaze, not medial deviation. Medial deviation (Esotropia) occurs in CN VI (Abducens) palsy. ### **Explanation of Other Options** * **Ptosis (A):** Occurs due to paralysis of the **Levator Palpebrae Superioris**, which lifts the upper eyelid. * **Mydriasis (B):** CN III carries parasympathetic fibers to the **Sphincter Pupillae**. Loss of these fibers leads to unopposed sympathetic action, resulting in a fixed, dilated pupil. * **Loss of Pupillary Reflex (D):** Since CN III forms the **efferent limb** of the pupillary light reflex, its damage prevents pupillary constriction in response to light. ### **NEET-PG High-Yield Pearls** * **Rule of Pupil:** In **Surgical** CN III palsy (e.g., PCom artery aneurysm), the pupil is **dilated** (compressive fibers are superficial). In **Medical** CN III palsy (e.g., Diabetes), the pupil is often **spared** (microvascular damage affects deep motor fibers). * **Muscles supplied by CN III:** Superior Rectus, Inferior Rectus, Medial Rectus, Inferior Oblique, and Levator Palpebrae Superioris. * **Formula to remember:** $[LR_6 SO_4]3$ (Lateral Rectus-6, Superior Oblique-4, all others-3).

Question 290: All of the following are true regarding olfaction, except:

A. It involves a key-lock system.
B. It is a chemical-mediated sense.
C. Females generally have a stronger sense of olfaction than males.
D. Males generally have a stronger sense of olfaction than females. (Correct Answer)

Explanation: **Explanation:** The sense of olfaction is a complex chemical process governed by the interaction of odorant molecules with specific receptors. **1. Why Option D is the Correct Answer (The False Statement):** In humans, **females generally have a higher olfactory sensitivity** and a better ability to identify and discriminate odors than males. This is attributed to both hormonal influences (estrogen levels) and anatomical differences; studies have shown that women have a significantly higher number of neurons and glial cells in the olfactory bulb compared to men. Therefore, the statement that males have a stronger sense of olfaction is incorrect. **2. Analysis of Other Options:** * **Option A (Key-lock system):** This is **true**. The "Stereochemical Theory" of olfaction suggests that odorant molecules (keys) fit into specific protein receptors (locks) on the cilia of olfactory sensory neurons based on their size and shape. * **Option B (Chemical-mediated sense):** This is **true**. Olfaction, along with gustation (taste), is a chemosense. It requires the dissolution of airborne chemicals into the mucus covering the olfactory epithelium to trigger a receptor potential. * **Option C (Females > Males):** This is **true** and is the physiological basis for why Option D is the exception. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **First-order neurons:** Olfactory sensory neurons are unique because they are **bipolar neurons** that undergo continuous replacement (neurogenesis) throughout life. * **Thalamic Bypass:** Olfaction is the **only** sensory modality that reaches the cerebral cortex (piriform cortex) without first relaying in the thalamus. * **Anosmia:** The loss of smell is an early clinical marker in neurodegenerative diseases like **Parkinson’s** and **Alzheimer’s**. * **Kallmann Syndrome:** Characterized by hypogonadotropic hypogonadism and anosmia due to the failure of GnRH neurons and olfactory nerves to migrate.

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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