Sensory Systems Practice Questions

Q: Light converts retinene to?

11 trans. **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.

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

Fasciculus cuneatus. **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.

Q: 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?

Ganglion cell layer. ### 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.

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

Gate control theory of pain. **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.

Q: Which of the following cells exhibits the highest transmembrane voltage difference?

Hair cell. **Explanation:** The correct answer is **Hair cell** (Option B). This question refers to the **transmembrane voltage difference**, which is the electrical potential gradient across a cell membrane. **1. Why Hair Cells?** Inner and outer hair cells in the cochlea are unique because they are bathed in two different fluids. Their apical surface is exposed to **endolymph** (high $K^+$, $+80$ mV potential), while their basal surface is bathed in **perilymph** (low $K^+$, $0$ mV potential). The intracellular potential of a hair cell is approximately $-40$ to $-70$ mV. Therefore, the potential difference across the apical membrane (the **endocochlear potential** gradient) is roughly **120–150 mV** ($+80$ mV minus $-70$ mV). This is the highest electrical gradient found in any cell in the human body and serves as the primary driving force for sensory transduction. **2. Why other options are incorrect:** * **Smooth muscle cell (A):** These have a relatively low and unstable resting membrane potential (RMP), typically between **$-40$ to $-60$ mV**. * **Purkinje fiber (C):** These exhibit a high RMP of about **$-90$ mV**, but this is still significantly lower than the combined gradient seen in hair cells. * **Skeletal muscle fiber (D):** These have a stable RMP of approximately **$-80$ to $-90$ mV**, which is standard for excitable tissues but does not exceed the hair cell's gradient. **Clinical Pearls for NEET-PG:** * **Endolymph** is unique because it is an extracellular fluid that resembles intracellular fluid (High $K^+$, Low $Na^+$). It is secreted by the **Stria Vascularis**. * The **Endocochlear Potential (+80 mV)** is essential for hearing; its loss (e.g., due to loop diuretics like Furosemide affecting the stria vascularis) leads to sensorineural deafness. * **Highest RMP magnitude** in a single-fluid environment: Purkinje fibers/Skeletal muscle ($\approx -90$ mV).

Q: Auditory impulses are relayed to which structure?

Medial geniculate body. ### 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.

Q: Which special sense has no relay in the hypothalamus?

Smell. **Explanation:** The correct answer is **Smell (Olfaction)**. **Why Smell is the Correct Answer:** Olfaction is unique among the special senses because it is the only sensory modality that reaches the cerebral cortex (specifically the primary olfactory cortex in the temporal lobe) **without first relaying in the Thalamus**. While the question mentions the "Hypothalamus," it is a common high-yield point in medical exams to test the bypass of the **Thalamic relay station**. Olfactory neurons project directly from the olfactory bulb via the olfactory tract to the limbic system and piriform cortex. This direct connection explains why smells are so potently linked to memory and emotion. **Why Other Options are Incorrect:** * **Vision:** Visual impulses from the retina travel via the optic nerve and tract to relay in the **Lateral Geniculate Nucleus (LGN)** of the Thalamus before reaching the visual cortex. * **Hearing:** Auditory signals relay in the **Medial Geniculate Nucleus (MGN)** of the Thalamus (part of the "C-O-L-M-A" pathway) before reaching the auditory cortex. * **Taste:** Gustatory pathways from the tongue relay in the **Ventral Posteromedial (VPM) nucleus** of the Thalamus before projecting to the gustatory cortex. **High-Yield Clinical Pearls for NEET-PG:** * **Thalamic Relay Mnemonic:** **L**GN is for **L**ight (Vision); **M**GN is for **M**usic (Hearing). * **Olfactory Pathway:** It is the only sense that bypasses the Thalamus initially, though it may project to the Thalamus (Mediodorsal nucleus) *after* reaching the primary cortex for odor discrimination. * **Anosmia:** Loss of smell is often an early sign of neurodegenerative diseases like Parkinson’s or head trauma involving the cribriform plate.

Q: Posterior dorsal columns transmit which of the following sensory modalities?

Fine touch and vibration. The **Dorsal Column-Medial Lemniscal (DCML) pathway** is a major sensory tract responsible for transmitting highly localized and discriminative sensations. ### 1. Why Option A is Correct The DCML pathway carries "epicritic" sensations. These include: * **Fine Touch (Tactile Discrimination):** Ability to localize touch and perform two-point discrimination. * **Vibration:** Detected by Pacinian corpuscles. * **Conscious Proprioception:** Sense of position and movement of body parts. * **Stereognosis:** Ability to identify objects by touch. These fibers are large, myelinated, and fast-conducting. They ascend ipsilaterally in the spinal cord (Fasciculus Gracilis and Cuneatus) and decussate in the medulla. ### 2. Why Other Options are Incorrect * **Option B (Pain and Temperature):** These are transmitted by the **Lateral Spinothalamic Tract**. * **Option C (Crude Touch):** This is transmitted by the **Anterior Spinothalamic Tract**. Together with pain/temperature, these form the Anterolateral System. * **Option D (Unconscious Proprioception):** This is transmitted to the cerebellum via the **Spinocerebellar Tracts** (Dorsal and Ventral). ### 3. High-Yield Clinical Pearls for NEET-PG * **Tabes Dorsalis:** A late stage of neurosyphilis that specifically involves the destruction of dorsal columns, leading to sensory ataxia and a positive **Romberg’s sign**. * **Brown-Séquard Syndrome:** In a spinal cord hemisection, there is **ipsilateral** loss of dorsal column sensations (fine touch/vibration) and **contralateral** loss of pain/temperature. * **Somatotopy:** In the dorsal columns, fibers from the lower limbs are medial (**Fasciculus Gracilis**), while fibers from the upper limbs are lateral (**Fasciculus Cuneatus**). Remember: *"Gracilis is Ground (legs)."*

Question 1

Light converts retinene to?

Accepted Answer

11 trans

Answer

11 cis

Answer

13 cis

Answer

13 trans

Question 2

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

Accepted Answer

Fasciculus cuneatus

Answer

Fasciculus gracilis

Answer

Lateral spinothalamic tract

Answer

Anterior column

Question 3

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

Accepted Answer

Paleospinothalamic tract

Answer

Neospinothalamic tract

Answer

Spinoreticular tract

Answer

Dorsal spinocerebellar tract

Question 4

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?

Accepted Answer

Ganglion cell layer

Answer

Inner plexiform layer

Answer

Inner nuclear layer

Answer

Outer plexiform layer

Question 5

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

Accepted Answer

Gate control theory of pain

Answer

Allodynia

Answer

Referred pain

Answer

Central pain

Question 6

Which of the following cells exhibits the highest transmembrane voltage difference?

Accepted Answer

Hair cell

Answer

Smooth muscle cell

Answer

Purkinje fiber

Answer

Skeletal muscle fiber

Question 7

Auditory impulses are relayed to which structure?

Accepted Answer

Medial geniculate body

Answer

Inferior geniculate ganglion

Answer

Medial lemniscus

Answer

Superior colliculus

Question 8

Which special sense has no relay in the hypothalamus?

Accepted Answer

Smell

Answer

Vision

Answer

Hearing

Answer

Taste

Question 9

In humans, auditory receptors act by a phenomenon called 'Masking'. Which of the following best describes the mechanism related to this phenomenon?

Accepted Answer

Relative or absolute refractoriness

Answer

All or none

Answer

Beneficial effect

Answer

Post-tetanic potentiation

Question 10

Posterior dorsal columns transmit which of the following sensory modalities?

Accepted Answer

Fine touch and vibration

Answer

Pain and temperature

Answer

Crude touch

Answer

Unconscious proprioception

Sensory Systems — MCQs

Sensory Systems — MCQs

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