Sensory Systems Practice Questions

Q: Touch sensation is carried by which nerve fibers?

Both A beta and A delta fibers. **Explanation:** The sensation of touch is mediated by two distinct types of nerve fibers depending on the nature of the stimulus: 1. **A-beta (Aβ) fibers:** These are large, myelinated fibers with fast conduction velocities. They carry **fine touch**, pressure, and vibration from specialized mechanoreceptors (like Meissner’s and Pacinian corpuscles). 2. **A-delta (Aδ) fibers:** These are small, thinly myelinated fibers. While primarily known for carrying "fast pain" and temperature, they also transmit **crude touch** (non-discriminative touch). Since "touch" as a general modality encompasses both fine and crude touch, **Option D** is the most accurate choice. **Analysis of Options:** * **Option A (A-beta only):** Incorrect because it excludes the crude touch component carried by A-delta fibers. * **Option B (A-delta only):** Incorrect because it excludes the primary fibers responsible for discriminative (fine) touch. * **Option C (C fibers):** Incorrect. C fibers are unmyelinated and primarily carry "slow pain," temperature, and itch. While a specific subset (C-tactile afferents) exists for "pleasant touch," they are not the primary mediators of standard touch sensation. **High-Yield Clinical Pearls for NEET-PG:** * **Erlanger-Gasser Classification:** Remember the order of fiber diameter and velocity: **Aα > Aβ > Aγ > Aδ > B > C**. * **Pathway:** Fine touch (Aβ) travels via the **Dorsal Column-Medial Lemniscal (DCML)** system, while crude touch (Aδ) travels via the **Anterior Spinothalamic Tract**. * **Susceptibility:** Large myelinated fibers (A type) are most sensitive to **pressure**, while small unmyelinated fibers (C type) are most sensitive to **local anesthetics**.

Q: Rhodopsin has a peak sensitivity to light of which wavelength?

505 nm. **Explanation:** **Rhodopsin**, also known as visual purple, is the photosensitive pigment found in the **rod cells** of the retina. It is a G-protein-coupled receptor (GPCR) consisting of the protein opsin bound to 11-cis-retinal. 1. **Why 505 nm is correct:** Rhodopsin is responsible for **scotopic vision** (vision under low-light conditions). While it can be activated by a range of wavelengths in the visible spectrum, its absorption spectrum peaks at approximately **505 nm**. This corresponds to the blue-green portion of the spectrum. At this wavelength, the probability of a photon being absorbed and triggering the photo-transduction cascade is highest. 2. **Why other options are incorrect:** * **100 nm:** This falls within the **Ultraviolet (UV-C)** range. Such short wavelengths are ionizing and are absorbed by the cornea and lens before reaching the retina; they do not stimulate visual pigments. * **355 nm:** This is in the **Ultraviolet (UV-A)** range. While some animals can see UV light, the human rhodopsin molecule does not have a peak sensitivity here. * **Photopic peaks:** For comparison, the three types of cones (responsible for color vision) have peak sensitivities at different wavelengths: Blue (437 nm), Green (533 nm), and Red (564 nm). **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin A Connection:** Rhodopsin is synthesized from Vitamin A (retinol). Deficiency leads to **Nyctalopia** (night blindness), the earliest clinical sign of Vitamin A deficiency. * **Wald’s Visual Cycle:** The process of rhodopsin bleaching (conversion of 11-cis-retinal to all-trans-retinal) and regeneration is a frequent exam topic. * **Dark Adaptation:** The "rod-cone break" in dark adaptation curves occurs because rods (rhodopsin) take longer to regenerate than cones but eventually achieve a much lower threshold for light detection.

Q: Processing of tactile stimulation occurs in Brodmann's area?

1, 2, 3. **Explanation:** The correct answer is **A (Brodmann’s areas 1, 2, 3)**. These areas constitute the **Primary Somatosensory Cortex (S1)**, located in the postcentral gyrus of the parietal lobe. This region is the primary destination for sensory information traveling via the Dorsal Column-Medial Lemniscal (DCML) pathway and the Spinothalamic tract. It is responsible for processing tactile sensations (touch, pressure), vibration, and proprioception. Specifically, Area 3 receives the bulk of thalamocortical projections, while Areas 1 and 2 are involved in higher-order processing like texture and shape recognition. **Analysis of Incorrect Options:** * **B (4, 6):** These represent the **Primary Motor Cortex (4)** and **Premotor/Supplementary Motor Cortex (6)** located in the frontal lobe. They are responsible for the execution and planning of voluntary motor movements, not sensory processing. * **C (44, 45):** These areas form **Broca’s Area** in the dominant hemisphere (usually left). They are responsible for motor speech production. * **D (41, 42):** These represent the **Primary Auditory Cortex**, located in the Superior Temporal Gyrus (Heschl’s gyri), responsible for processing sound. **High-Yield Clinical Pearls for NEET-PG:** * **Sensory Homunculus:** The body is represented upside-down in S1, with the face and hands having disproportionately large representations due to high receptor density. * **Lesion Effect:** A lesion in areas 1, 2, and 3 leads to **contralateral hemi-anesthesia** and loss of discriminative touch (astereognosis). * **Secondary Somatosensory Area (S2):** Located in the wall of the sylvian fissure; it handles less discrete sensory functions. * **Brodmann Area 5 & 7:** These are the **Sensory Association Areas**; lesions here lead to **Astereognosis** and **Amorphosynthesis** (neglect of the opposite side of the body).

Q: Which of the following sensations is NOT carried through the anterolateral system?

Proprioception. The sensory pathways of the spinal cord are divided into two major systems: the **Anterolateral System (ALS)** and the **Dorsal Column-Medial Lemniscal (DCML) System**. ### Why Proprioception is the Correct Answer **Proprioception** (position sense), along with fine touch, vibration, and two-point discrimination, is carried by the **DCML pathway**. These fibers enter the spinal cord and ascend ipsilaterally in the gracile and cuneate fasciculi before decussating in the medulla. Therefore, it is not a component of the anterolateral system. ### Why Other Options are Incorrect The Anterolateral System (Spinothalamic tract) primarily carries "protopathic" sensations. These fibers synapse in the dorsal horn, decussate immediately at the spinal level, and ascend in the anterior and lateral columns: * **Pain (C):** Carried via the Lateral Spinothalamic tract. * **Temperature (B):** Carried via the Lateral Spinothalamic tract. * **Crude touch (D):** Carried via the Anterior Spinothalamic tract (along with pressure). ### High-Yield Clinical Pearls for NEET-PG * **Brown-Séquard Syndrome:** A hemisection of the spinal cord results in **ipsilateral** loss of proprioception/vibration (DCML) and **contralateral** loss of pain/temperature (ALS) below the level of the lesion. * **Fiber Types:** Pain and temperature are carried by slow **A-delta and C fibers**, whereas proprioception is carried by fast, heavily myelinated **A-alpha and A-beta fibers**. * **Syringomyelia:** Classically affects the anterior white commissure first, leading to a "cape-like" loss of pain and temperature while sparing proprioception (dissociated sensory loss).

Q: Massage and application of liniments to painful areas in the body relieves pain due to which of the following mechanisms?

Inhibition by large myelinated fibers. ### Explanation The mechanism described in the question is based on the **Gate Control Theory of Pain**, proposed by Melzack and Wall. **1. Why Option D is Correct:** Pain signals are carried by small, slow, unmyelinated **C fibers** and lightly myelinated **A-delta fibers**. When we massage or apply liniments to a painful area, we stimulate **large-diameter myelinated A-beta fibers** (responsible for touch and vibration). These A-beta fibers activate **inhibitory interneurons** in the Substantia Gelatinosa of the spinal cord. These interneurons release GABA/enkephalins, which inhibit the transmission of pain signals from the first-order neurons to the second-order spinothalamic neurons. Essentially, the "gate" for pain is closed by the sensory input of touch. **2. Why Other Options are Incorrect:** * **Option A:** The endogenous analgesic system (involving the Periaqueductal Gray and Nucleus Raphe Magnus) provides descending inhibition from the brainstem to the spinal cord. While it modulates pain, it is not the primary mechanism triggered by local tactile stimulation like massage. * **Option B:** Endorphins are typically released by the descending system or higher centers, not by first-order neurons. First-order neurons are peripheral sensory neurons. * **Option C:** Glutamate and Substance P are **pro-nociceptive neurotransmitters** released by pain fibers (C and A-delta) to *transmit* pain. Their release would increase pain, not relieve it. **High-Yield Clinical Pearls for NEET-PG:** * **Gate Control Theory Location:** Occurs in the **Dorsal Horn** of the spinal cord (specifically Rexed Lamina II - Substantia Gelatinosa). * **TENS (Transcutaneous Electrical Nerve Stimulation):** Works on the same principle by stimulating A-beta fibers to alleviate chronic pain. * **Fiber Types:** Remember: **A-beta** (Touch/Large/Fast) inhibits **C-fibers** (Pain/Small/Slow).

Q: Epicritic perception of pain occurs at the level of which structure?

Somatosensory cortex areas 3, 1, 2. ### Explanation Pain perception is categorized into two distinct components: **Protopathic** (crude, emotional, and poorly localized) and **Epicritic** (fine, discriminative, and precisely localized). **1. Why Option B is Correct:** The **Somatosensory Cortex (S1), specifically Brodmann areas 3, 1, and 2**, is responsible for the **epicritic** aspect of pain. This includes the ability to localize the exact site of the stimulus, identify its intensity, and perceive its physical qualities (e.g., sharp vs. dull). This information is transmitted via the Neospinothalamic tract to the Ventroposterolateral (VPL) nucleus of the thalamus and then projected to S1. **2. Analysis of Incorrect Options:** * **Option A (Thalamus):** The thalamus is the site for **protopathic** perception. It allows for the conscious awareness of pain (the "hurt" feeling) but lacks the spatial resolution to pinpoint the exact location. * **Option C (Areas 5, 7):** These are the **Somatosensory Association Areas**. They are involved in higher-order processing, such as stereognosis and interpreting the significance of sensory input, rather than the primary perception of pain localization. * **Option D (Pulvinar):** This is a large nucleus in the posterior thalamus primarily involved in visual processing and attention, not the primary localization of pain. ### High-Yield NEET-PG Pearls: * **Fast Pain (A-delta fibers):** Corresponds to epicritic pain; uses the Neospinothalamic tract; terminates in the VPL thalamus and S1. * **Slow Pain (C fibers):** Corresponds to protopathic pain; uses the Paleospinothalamic tract; terminates in the Reticular formation and Intralaminar nuclei of the thalamus. * **S1 Organization:** Arranged as a **Sensory Homunculus**, where the size of the cortical representation is proportional to the density of sensory receptors (e.g., large area for fingertips and lips).

Q: Retinal output is mainly from which layer?

Ganglion cell layer. **Explanation:** The retina is organized into a complex hierarchy of ten layers that process visual information before sending it to the brain. **Why Option A is correct:** The **Ganglion cell layer** contains the cell bodies of retinal ganglion cells (RGCs). These are the final output neurons of the retina. Their axons converge at the optic disc to form the **optic nerve (Cranial Nerve II)**. Therefore, all visual information processed by the photoreceptors, bipolar cells, and horizontal/amacrine cells must pass through the ganglion cells to reach the lateral geniculate nucleus (LGN) of the thalamus. **Why the other options are incorrect:** * **Option B (Layer of rods and cones):** This is the outermost layer containing the outer and inner segments of photoreceptors. It is the site of **phototransduction** (converting light into electrical signals), not the output layer. * **Option C (Outer nuclear layer):** This layer contains the **cell bodies** of the rods and cones. * **Option D (Outer plexiform layer):** This is a layer of synapses where photoreceptors connect with bipolar and horizontal cells. It is a site of signal processing, not output. **High-Yield Facts for NEET-PG:** * **Vertical Pathway of Vision:** Photoreceptors $\rightarrow$ Bipolar cells $\rightarrow$ Ganglion cells. * **Action Potentials:** Most retinal cells (photoreceptors, bipolar cells) communicate via **graded potentials**. The **Ganglion cells** are the first cells in the visual pathway to fire true **action potentials**. * **Müller cells:** These are the principal glial cells of the retina, extending almost the entire thickness of the retina (from the inner to the outer limiting membrane). * **Fovea Centralis:** The area of highest visual acuity where internal layers are displaced laterally so light can strike photoreceptors (mostly cones) directly.

Question 1

Touch sensation is carried by which nerve fibers?

Accepted Answer

Both A beta and A delta fibers

Answer

A beta fibers

Answer

A delta fibers

Answer

C fibers

Question 2

Rhodopsin has a peak sensitivity to light of which wavelength?

Accepted Answer

505 nm

Answer

100 nm

Answer

355 nm

Answer

None of the above

Question 3

Sudden loud sound is more likely to damage the cochlea than prolonged sounds because?

Accepted Answer

There is a latent period before the attenuation reflex can occur

Answer

Basilar fibers are sensitive to sudden sounds but adapt to prolonged sounds

Answer

A sudden sound carries more energy

Answer

Tympanic membrane becomes flaccid during prolonged loud sounds

Question 4

Why is the blind spot on the retina not usually perceived?

Accepted Answer

Its location in the visual field is different in each eye

Answer

It is very small, below the ability of the sensory cells to detect

Answer

It is present only in very young children

Answer

Constant eye motion prevents the spot from remaining still

Question 5

Processing of tactile stimulation occurs in Brodmann's area?

Accepted Answer

1, 2, 3

Answer

4, 6

Answer

44, 45

Answer

41, 42

Question 6

Which of the following sensations is NOT carried through the anterolateral system?

Accepted Answer

Proprioception

Answer

Temperature

Answer

Pain

Answer

Crude touch

Question 7

Massage and application of liniments to painful areas in the body relieves pain due to which of the following mechanisms?

Accepted Answer

Inhibition by large myelinated fibers

Answer

Stimulation of endogenous analgesic system

Answer

Release of endorphins by first-order neurons in the brainstem

Answer

Release of glutamate and substance P in the spinal cord

Question 8

Epicritic perception of pain occurs at the level of which structure?

Accepted Answer

Somatosensory cortex areas 3, 1, 2

Answer

Thalamus

Answer

Somatosensory cortex areas 5, 7

Answer

Pulvinar

Question 9

Retinal output is mainly from which layer?

Accepted Answer

Ganglion cell layer

Answer

Layer of rods and cones

Answer

Outer nuclear layer

Answer

Outer plexiform layer

Question 10

Analysis of visual detail occurs in which secondary visual area?

Accepted Answer

Inferior ventral and medial regions of the occipital and temporal cortex

Answer

Brodmann's area 18

Answer

Frontal lobe

Answer

Occipitoparietal cortex

Sensory Systems — MCQs

Sensory Systems — MCQs

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