Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 171: Endolymph in the inner ear is secreted by which of the following?

A. A filtrate of blood serum
B. Stria vascularis (Correct Answer)
C. Basilar membrane
D. Hair cells

Explanation: **Explanation:** The correct answer is **Stria vascularis**. **1. Why Stria Vascularis is correct:** The **stria vascularis**, located on the lateral wall of the cochlear duct (scala media), is a highly vascularized layer of specialized epithelium. It is responsible for the secretion and maintenance of **endolymph**. Unlike most extracellular fluids, endolymph is unique because it is rich in **Potassium (K+)** and low in Sodium (Na+), creating a high positive endocochlear potential (+80 mV). This potential is essential for the transduction of sound, as it provides the driving force for K+ ions to enter hair cells during stimulation. **2. Why other options are incorrect:** * **A. Filtrate of blood serum:** This describes **Perilymph**, which is found in the scala tympani and scala vestibuli. Perilymph is similar to ECF or CSF (high Na+, low K+). * **C. Basilar membrane:** This is a structural fibrous layer that supports the Organ of Corti. It vibrates in response to sound waves but has no secretory function. * **D. Hair cells:** These are the sensory receptors (mechanoreceptors) of the auditory system. They do not secrete endolymph; rather, they are bathed in it and utilize its high K+ concentration for depolarization. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Endolymph is the only ECF in the body that resembles ICF (High K+). * **Absorption:** Endolymph is drained/absorbed by the **endolymphatic sac**. * **Meniere’s Disease:** Caused by the "endolymphatic hydrops" (excess accumulation of endolymph), leading to the triad of vertigo, sensorineural hearing loss, and tinnitus. * **Blood-Labyrinth Barrier:** The stria vascularis also serves as a protective barrier, similar to the blood-brain barrier.

Question 172: Extensive damage to the somatosensory area of the cerebral cortex results in which of the following sensory impairments? EXCEPT-

A. Pressure
B. Sensory localization
C. Exact weight determination
D. Pain (Correct Answer)

Explanation: **Explanation:** The somatosensory cortex (S1 and S2) is primarily responsible for the **discriminative aspects** of sensation rather than the mere perception of its presence. **Why Pain is the Correct Answer:** Pain perception is unique because it is highly primitive and mediated largely by the **thalamus** and the brainstem (reticular formation). While the somatosensory cortex helps in localizing the source of pain and interpreting its quality, the basic awareness of pain (nociception) remains intact even after extensive cortical damage. Therefore, pain is "spared" in terms of basic perception. **Analysis of Incorrect Options:** * **A. Pressure:** While crude touch can be perceived at the thalamic level, the fine grading and discrete perception of pressure require cortical processing. * **B. Sensory Localization:** This is a high-level discriminative function. The ability to pinpoint exactly where a stimulus is applied (topognosis) is lost without an intact somatosensory cortex. * **C. Exact Weight Determination:** This involves **Barognosis**. The ability to judge weights and distinguish between subtle differences in pressure or resistance is a complex cortical function that is abolished with cortical lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Thalamus:** The "Relay Center" where crude awareness of pain, temperature, and touch occurs. * **Stereognosis:** The ability to identify an object by touch without vision; it is one of the first functions lost in cortical lesions. * **Two-point discrimination:** A hallmark test for cortical integrity; it requires the somatosensory cortex to distinguish two close stimuli as separate. * **Astereognosis & Agraphesthesia:** These are classic signs of a parietal lobe (somatosensory) lesion.

Question 173: Light converts retinene to?

A. 11 cis
B. 11 trans (Correct Answer)
C. 13 cis
D. 13 trans

Explanation: **Explanation:** The core mechanism of visual transduction involves the photo-isomerization of the visual pigment, **Rhodopsin**. Rhodopsin consists of a protein called opsin and a light-sensitive chromophore called **retinene (11-cis retinal)**, which is a derivative of Vitamin A. **1. Why Option B is Correct:** In the resting state (darkness), retinene exists in the **11-cis** configuration. This shape allows it to fit snugly into the binding pocket of the opsin molecule. When a photon of light strikes the retina, it provides the energy required to break the double bond, causing the molecule to straighten out into the **all-trans retinal (11-trans)** configuration. This change in shape triggers a conformational change in opsin (forming Metarhodopsin II), which eventually leads to the closure of sodium channels and hyperpolarization of the photoreceptor. **2. Why the other options are incorrect:** * **Option A (11 cis):** This is the **pre-illumination** state. Light converts 11-cis *into* 11-trans; it does not produce 11-cis. * **Options C & D (13 cis/trans):** These isomers are not involved in the human visual cycle. 13-cis retinal is associated with certain bacterial rhodopsins (like bacteriorhodopsin) but plays no role in the vertebrate retina. **High-Yield Clinical Pearls for NEET-PG:** * **Wald’s Visual Cycle:** The process of regenerating 11-cis retinal from all-trans retinal occurs partly in the **Retinal Pigment Epithelium (RPE)**. * **Vitamin A Deficiency:** Leads to **Nyctalopia** (Night blindness) because Vitamin A is the precursor for 11-cis retinal. * **Dark Adaptation:** The time taken to regenerate rhodopsin stores (re-converting trans back to cis) determines the rate of dark adaptation.

Question 174: Loss of recognition of objects by touch is associated with a lesion of which structure?

A. Fasciculus gracilis
B. Lateral spinothalamic tract
C. Anterior column
D. Fasciculus cuneatus (Correct Answer)

Explanation: **Explanation:** The loss of recognition of objects by touch is known as **astereognosis**. This function requires the integration of fine touch, pressure, and conscious proprioception, all of which are carried by the **Dorsal Column-Medial Lemniscal (DCML) system**. **1. Why Fasciculus Cuneatus is correct:** The DCML system is divided into two tracts: the *Fasciculus Gracilis* (medial) and the *Fasciculus Cuneatus* (lateral). The Fasciculus Cuneatus carries sensory information from the **upper body (above T6)**, including the hands. Since object recognition (stereognosis) is primarily a function performed by the hands, a lesion in the Fasciculus Cuneatus leads to the inability to identify objects by touch. **2. Analysis of Incorrect Options:** * **Fasciculus Gracilis:** While part of the DCML, it carries sensations from the **lower body (below T6)** and lower limbs. While a lesion here would cause loss of vibration and position sense in the legs, it is not typically associated with the clinical testing of stereognosis. * **Lateral Spinothalamic Tract:** This tract carries **pain and temperature** sensations. A lesion here results in contralateral loss of pain and thermal perception. * **Anterior Column (Anterior Spinothalamic Tract):** This tract carries **crude touch and pressure**. It does not possess the high discriminative resolution required for stereognosis. **Clinical Pearls for NEET-PG:** * **Stereognosis** is a "cortical" sensation but requires intact dorsal columns to reach the parietal cortex. * **Rule of T6:** Gracilis is Medial (G comes before C) and handles the "Ground" (legs). Cuneatus is Lateral and handles the "Ceiling" (arms). * **Romberg’s Sign:** Positive in DCML lesions (sensory ataxia), distinguishing it from cerebellar ataxia. * **Tabes Dorsalis:** A classic high-yield condition involving bilateral destruction of the dorsal columns.

Question 175: The motivational-affect component of pain is carried by which pathway?

A. Paleospinothalamic tract (Correct Answer)
B. Neospinothalamic tract
C. Spinoreticular tract
D. Dorsal spinocerebellar tract

Explanation: ### Explanation Pain perception is divided into two distinct components: the **sensory-discriminative** (location, intensity, quality) and the **motivational-affective** (emotional response, unpleasantness, arousal). **1. Why Option A is Correct:** The **Paleospinothalamic tract** (Slow Pain/C-fibers) is responsible for the motivational-affective component. It projects to the **reticular formation, thalamic intralaminar nuclei, and the limbic system** (specifically the cingulate and insular cortex). These connections mediate the autonomic responses to pain and the emotional "suffering" associated with chronic or deep tissue injury. **2. Analysis of Incorrect Options:** * **B. Neospinothalamic tract:** This is the "Fast Pain" pathway (Aδ fibers). It projects directly to the **ventroposterolateral (VPL) nucleus** of the thalamus and then to the primary somatosensory cortex. It is responsible for the **sensory-discriminative** aspect (exact localization and sharp nature of pain). * **C. Spinoreticular tract:** While this tract is closely related to the paleospinothalamic system and contributes to arousal/alertness via the Reticular Activating System (RAS), the term "Paleospinothalamic" is the standard anatomical designation for the pathway carrying the emotional-affective component to the higher limbic centers. * **D. Dorsal spinocerebellar tract:** This pathway carries **unconscious proprioception** from the lower limbs to the cerebellum; it is not involved in pain transmission. ### High-Yield NEET-PG Pearls: * **Fast Pain:** Aδ fibers → Glutamate (Neurotransmitter) → Neospinothalamic tract. * **Slow Pain:** C fibers → Substance P (Neurotransmitter) → Paleospinothalamic tract. * **Limbic System Connection:** The emotional distress of pain is specifically linked to the **Anterior Cingulate Cortex**. * **Dual Projection:** Remember that most pain fibers in the paleospinothalamic tract terminate in the brainstem reticular formation before reaching the thalamus.

Question 176: The cell bodies of the only neurons in the retina whose axons exhibit a self-propagated action potential are located in which of the following layers?

A. Ganglion cell layer (Correct Answer)
B. Inner plexiform layer
C. Inner nuclear layer
D. Outer plexiform layer

Explanation: ### Explanation **1. Why the Ganglion Cell Layer is Correct:** In the retina, most cells (photoreceptors, bipolar, horizontal, and amacrine cells) communicate via **graded potentials** (local electronic conduction). These cells are small enough that signal transmission does not require an action potential to reach the next synapse. The **Ganglion cells** are the only neurons in the retina that generate **self-propagated action potentials**. This is a physiological necessity because their axons must travel a long distance—from the retina, through the optic nerve, to the lateral geniculate nucleus (LGN) in the thalamus. Graded potentials would decay over such distances; therefore, all-or-none action potentials are required for long-range signaling. **2. Why the Other Options are Incorrect:** * **Inner Nuclear Layer (C):** This layer contains the cell bodies of bipolar, horizontal, and amacrine cells. These cells utilize graded potentials to modulate signal intensity and contrast. * **Inner Plexiform Layer (B) & Outer Plexiform Layer (D):** These are synaptic layers, not cell body layers. The Inner Plexiform layer is where bipolar, amacrine, and ganglion cells synapse, while the Outer Plexiform layer is where photoreceptors synapse with bipolar and horizontal cells. **3. High-Yield Facts for NEET-PG:** * **Amacrine Cells:** While they primarily use graded potentials, some large amacrine cells can generate "spike-like" potentials, but they are not considered the primary "action potential" neurons of the retina. * **Müller Cells:** These are the principal glial cells of the retina, spanning almost the entire retinal thickness. * **Optic Nerve Composition:** The axons of the ganglion cells converge at the optic disc to form the optic nerve. * **Convergence:** There is significant convergence in the retina; roughly 126 million photoreceptors converge onto only 1.2 million ganglion cells.

Question 177: Transcutaneous electrical nerve stimulation is based on which of the following principles?

A. Allodynia
B. Referred pain
C. Central pain
D. Gate control theory of pain (Correct Answer)

Explanation: **Explanation:** **Gate Control Theory of Pain (Correct Answer):** Proposed by Melzack and Wall, this theory is the physiological basis for **Transcutaneous Electrical Nerve Stimulation (TENS)**. The "gate" is located in the **Substantia Gelatinosa (SG)** of the dorsal horn of the spinal cord. * Pain is carried by slow, unmyelinated **C-fibers**. * TENS stimulates large-diameter, myelinated **A-beta (Aβ) fibers** (touch/vibration). * These Aβ fibers activate inhibitory interneurons in the SG, which release enkephalins to inhibit the transmission of pain signals from C-fibers to the second-order projection neurons (T-cells). Essentially, non-painful input "closes the gate" to painful input. **Analysis of Incorrect Options:** * **A. Allodynia:** This refers to pain resulting from a stimulus that does not normally provoke pain (e.g., a light touch on sunburned skin). It is a feature of neuropathic pain, not a therapeutic principle. * **B. Referred Pain:** This occurs when pain is perceived at a site adjacent to or distant from the site of an injury (e.g., left arm pain during a myocardial infarction) due to the convergence of visceral and somatic afferents on the same spinal neurons. * **C. Central Pain:** This is pain initiated or caused by a primary lesion or dysfunction in the Central Nervous System (e.g., Thalamic Syndrome). **High-Yield Clinical Pearls for NEET-PG:** * **The "Gate" Location:** Lamina II of the Dorsal Horn (Substantia Gelatinosa). * **Inhibitory Neurotransmitter:** Glycine and Enkephalins are primarily involved in local spinal inhibition. * **Fiber Types:** Remember: **Aβ** (Large, Fast) inhibits **C** (Small, Slow). * **Other Applications:** Rubbing a bumped elbow or using vibration during an injection also works via the Gate Control Theory.

Question 178: Auditory impulses are relayed to which structure?

A. Inferior geniculate ganglion
B. Medial geniculate body (Correct Answer)
C. Medial lemniscus
D. Superior colliculus

Explanation: ### Explanation The auditory pathway is a classic high-yield topic for NEET-PG. To understand why the **Medial Geniculate Body (MGB)** is the correct answer, one must trace the flow of auditory information from the periphery to the cortex. **1. Why the Medial Geniculate Body is Correct:** The MGB is part of the thalamus and serves as the **obligatory relay station** for all auditory impulses before they reach the primary auditory cortex (Heschl’s gyrus, Areas 41 and 42). It receives input from the inferior colliculus via the brachium of the inferior colliculus and projects to the temporal lobe. **2. Analysis of Incorrect Options:** * **Inferior Geniculate Ganglion:** This is a distractor. There is no such structure. The *Geniculate Ganglion* is associated with the facial nerve (CN VII) and taste, while the *Spiral Ganglion* contains the first-order neurons for hearing. * **Medial Lemniscus:** This pathway carries **fine touch, vibration, and conscious proprioception** (DCML pathway) from the body to the Ventral Posterolateral (VPL) nucleus of the thalamus. The auditory equivalent is the *Lateral Lemniscus*. * **Superior Colliculus:** This midbrain structure is involved in **visual reflexes** and tracking. The *Inferior Colliculus* is the one involved in the auditory pathway. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Thalamic Relay:** **M**edial for **M**usic (Auditory); **L**ateral for **L**ight (Visual). * **Pathway Sequence (ECOLI):** **E**ighth Nerve → **C**ochlear Nuclei → **O**livary Complex (Superior) → **L**ateral Lemniscus → **I**nferior Colliculus → **M**GB → Auditory Cortex. * **Bilateral Representation:** Beyond the cochlear nuclei, auditory information ascends bilaterally. Therefore, a unilateral lesion proximal to the cochlear nuclei does not cause total deafness in one ear but rather leads to impaired sound localization.

Question 179: What is the approximate conduction velocity (in m/s) of nerve fibers carrying fast pain?

A. 120
B. 70
C. 30 (Correct Answer)
D. 2

Explanation: **Explanation:** Pain is transmitted via two distinct pathways involving different nerve fiber types. **Fast pain** (sharp, pricking, and well-localized) is carried by **Aδ (A-delta) fibers**. These are thin, myelinated fibers with conduction velocities ranging from **6 to 30 m/s**. Therefore, 30 m/s represents the upper limit of velocity for fast pain transmission. **Analysis of Options:** * **A (120 m/s):** This is the maximum velocity for **Aα (A-alpha)** fibers, which carry proprioception and motor signals to skeletal muscles. * **B (70 m/s):** This velocity is characteristic of **Aβ (A-beta)** fibers, which transmit touch and pressure sensations. * **C (30 m/s):** **Correct.** This is the conduction velocity for Aδ fibers responsible for fast pain and cold temperature. * **D (2 m/s):** This represents **Type C fibers** (0.5–2 m/s). These are small, unmyelinated fibers that carry **slow pain** (dull, aching, burning) and warmth. **High-Yield Clinical Pearls for NEET-PG:** * **Double Pain Sensation:** A single painful stimulus (like a pinprick) often results in a "double pain" perception—a fast, sharp flash (Aδ) followed by a slow, throbbing ache (C fibers). * **Neurotransmitters:** Glutamate is the primary neurotransmitter for fast pain (Aδ), while Substance P is associated with slow pain (C fibers). * **Erlanger-Gasser Classification:** Remember the hierarchy of velocity: **Aα > Aβ > Aγ > Aδ > B > C**. * **Susceptibility:** Type C fibers are most sensitive to local anesthetics, while Type A fibers are most sensitive to pressure/hypoxia.

Question 180: For detection of light, what is the minimum amount of light (quanta) that must reach the retina?

A. 1
B. 100 (Correct Answer)
C. 54
D. More than 500

Explanation: ### Explanation **1. Why Option B (100) is Correct:** The human eye is remarkably sensitive, but there is a significant difference between the sensitivity of a single photoreceptor and the threshold for conscious perception. While a single rod cell can be excited by a **single photon (1 quantum)** of light, the majority of photons entering the eye are lost due to reflection by the cornea or absorption by the lens and vitreous humor. Research (notably by Hecht, Shlaer, and Pirenne) established that for a human to actually "see" or detect a flash of light, approximately **100 quanta** must reach the retina. Out of these 100, only about **7 to 10** are actually absorbed by the rhodopsin in the rods to trigger a visual signal. **2. Why Other Options are Incorrect:** * **Option A (1):** This represents the physiological limit of a single rod cell's sensitivity, not the threshold for the entire visual system to perceive light. * **Option C (54):** This is a distractor value sometimes cited in older studies regarding the number of photons reaching the cornea, but it does not represent the standard threshold for retinal detection. * **Option D (More than 500):** This value is too high; the eye is far more efficient. At this level, light would be easily visible and well above the absolute threshold. **3. High-Yield Facts for NEET-PG:** * **Scotopic Vision:** Mediated by **Rods**; high sensitivity, low acuity, no color (used in dim light). * **Photopic Vision:** Mediated by **Cones**; low sensitivity, high acuity, color perception (used in bright light). * **Rhodopsin:** The visual pigment in rods. Its peak sensitivity is at a wavelength of approximately **500 nm**. * **Dark Adaptation:** Takes about 20–30 minutes to reach maximum sensitivity, primarily due to the slow regeneration of rhodopsin.

Practice by Chapter

General Sensory Physiology

Practice Questions

Somatosensation

Practice Questions

Pain Physiology

Practice Questions

Vision and Optics

Practice Questions

Retinal Physiology

Practice Questions

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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