Sensory Systems Practice Questions

Q: Sharp pain is transmitted by which type of nerve fibers?

Ad. **Explanation:** Pain perception is categorized into two distinct pathways based on the speed of transmission and the type of nerve fiber involved. **Why Aδ (A-delta) is correct:** Aδ fibers are **thinly myelinated**, medium-diameter axons that conduct impulses at a velocity of 6–30 m/s. They are responsible for **"fast pain"** or **"first pain."** This is characterized as sharp, pricking, and well-localized. These fibers primarily terminate in Rexed laminae I and V of the spinal cord dorsal horn and utilize glutamate as their primary neurotransmitter. **Analysis of Incorrect Options:** * **Aα (Alpha):** These are the thickest, most heavily myelinated fibers. They carry proprioceptive information from muscles and joints and serve as motor neurons to skeletal muscles. * **Aβ (Beta):** These are large, myelinated fibers that transmit non-noxious stimuli such as touch, pressure, and vibration. According to the **Gate Control Theory**, stimulation of Aβ fibers can inhibit pain transmission at the spinal level. * **C Fibers:** These are **unmyelinated**, small-diameter fibers that conduct slowly (0.5–2 m/s). They transmit **"slow pain"** or **"second pain,"** which is described as dull, aching, burning, and poorly localized. They utilize Substance P as a neurotransmitter. **NEET-PG High-Yield Pearls:** * **Fast Pain (Aδ):** Sharp, localized, glutamate-mediated, Neospinothalamic tract. * **Slow Pain (C):** Dull, diffuse, Substance P-mediated, Paleospinothalamic tract. * **Susceptibility:** **Type B** fibers are most sensitive to local anesthetics; **Type C** are most sensitive to local anesthetics among sensory fibers; **Type A** are most sensitive to pressure/hypoxia.

Q: Depolarization in hair cells of the organ of Corti occurs due to:

Movement of the basilar membrane. **Explanation:** The **Organ of Corti** is the transducer organ of hearing. The process of sensory transduction begins when sound waves cause pressure changes in the perilymph, leading to the **vibration of the basilar membrane**. 1. **Why Option A is correct:** When the basilar membrane moves upward, it creates a **shearing force** between the hair cell stereocilia and the overlying tectorial membrane. This mechanical displacement is the primary trigger that initiates the entire electrical cascade of hearing. 2. **Why Option B is incorrect:** Depolarization occurs when **shorter stereocilia move toward the taller ones**. This stretches the "tip links" (cadherin-23 proteins), which mechanically opens **MET (Mechano-Electrical Transducer) channels**. Movement in the opposite direction (taller to shorter) leads to hyperpolarization. 3. **Why Option C is incorrect:** Unlike most excitable cells, hair cell depolarization is mediated by an **influx of Potassium (K+)**, not Sodium. The stereocilia are bathed in **endolymph**, which is uniquely high in K+ (produced by the stria vascularis). When MET channels open, K+ flows down its electrochemical gradient into the hair cell. **High-Yield NEET-PG Pearls:** * **Endocochlear Potential:** The endolymph has a positive potential of **+80 mV**, the highest resting potential in the body, which provides the driving force for K+ influx. * **Tip Links:** These are the molecular "springs" that open the K+ channels. * **Glutamate:** This is the primary excitatory neurotransmitter released at the base of the hair cell following depolarization and subsequent Ca2+ influx. * **Outer vs. Inner Hair Cells:** Inner hair cells (single row) are primarily responsible for sensory transduction, while outer hair cells (three rows) act as "cochlear amplifiers" via the protein **prestin**.

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: Which nucleus controls the pupillary reflex?

Edinger Westphal nucleus. The **Edinger-Westphal (EW) nucleus** is the correct answer as it serves as the parasympathetic preganglionic nucleus for the oculomotor nerve (CN III). ### 1. Why Edinger-Westphal Nucleus is Correct The pupillary light reflex involves a four-neuron arc. When light hits the retina, the signal travels via the optic nerve to the **pretectal nucleus** in the midbrain. From there, fibers project bilaterally to the **Edinger-Westphal nuclei**. The EW nucleus sends parasympathetic fibers via the oculomotor nerve to the **ciliary ganglion**, which then innervates the **sphincter pupillae** muscle, causing pupillary constriction (miosis). ### 2. Why Other Options are Incorrect * **Nucleus Tractus Solitarius (NTS):** This is the primary sensory nucleus for visceral afferents (taste via CN VII, IX, X and baroreceptor/chemoreceptor inputs via CN IX, X). * **Nucleus Ambiguus:** This is a motor nucleus that gives rise to branchial efferent fibers of CN IX and X, controlling muscles of the pharynx, larynx, and upper esophagus (swallowing and phonation). * **Dorsal Vagus Nucleus:** This provides parasympathetic innervation to the thoracic and abdominal viscera (heart, lungs, and GI tract), not the eye. ### 3. High-Yield Clinical Pearls for NEET-PG * **Consensual Reflex:** Light in one eye causes bilateral constriction because the pretectal nucleus sends fibers to **both** the left and right EW nuclei. * **Argyll Robertson Pupil:** Seen in neurosyphilis; the pupil "accommodates but does not react" to light. This is due to a lesion in the pretectal area, sparing the accommodation pathway. * **Hutchinson’s Pupil:** A dilated, non-reactive pupil caused by compression of the oculomotor nerve (and its superficial parasympathetic fibers) by an uncal herniation.

Q: Which of the following is a chemoreceptor?

Osmoreceptors. **Explanation:** **Correct Answer: A. Osmoreceptors** Osmoreceptors, primarily located in the **anterior hypothalamus** (specifically the OVLT and SFO), are specialized **chemoreceptors**. They function by detecting changes in the chemical composition of the blood—specifically the **osmotic pressure** or solute concentration of the extracellular fluid. When plasma osmolarity increases, these receptors shrink, triggering the release of ADH (Vasopressin) and the sensation of thirst to maintain water homeostasis. **Analysis of Incorrect Options:** * **B. Rods & Cones:** These are **photoreceptors** located in the retina. They convert electromagnetic radiation (light) into electrical signals (phototransduction). * **C. Hair Cells:** These are **mechanoreceptors** found in the cochlea (for hearing) and vestibular apparatus (for balance). They respond to mechanical displacement caused by sound waves or head movement. * **D. Merkel’s Disc:** These are **mechanoreceptors** located in the basal layer of the epidermis. They are slow-adapting (Type I) receptors that respond to steady pressure and texture (fine touch). **High-Yield Clinical Pearls for NEET-PG:** * **Other Chemoreceptors:** Peripheral chemoreceptors (Carotid and Aortic bodies) detect $PO_2$, $PCO_2$, and $pH$; Central chemoreceptors (Medulla) detect $H^+$ concentration in the CSF. * **Classification Tip:** Always classify receptors by their **adequate stimulus**. If the stimulus is a chemical change (ions, gases, osmolarity), it is a chemoreceptor. * **Baroreceptors:** These are mechanoreceptors (not chemoreceptors) that detect "stretch" in blood vessel walls to regulate blood pressure.

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).

Question 1

Sharp pain is transmitted by which type of nerve fibers?

Accepted Answer

Ad

Answer

Aa

Answer

Ab

Answer

C

Question 2

Depolarization in hair cells of the organ of Corti occurs due to:

Accepted Answer

Movement of the basilar membrane

Answer

Movement of taller stereocilia toward the shorter ones

Answer

Influx of Na+

Answer

None of the above

Question 3

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 4

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 5

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 6

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 7

Which nucleus controls the pupillary reflex?

Accepted Answer

Edinger Westphal nucleus

Answer

Nucleus tractus solitarius

Answer

Nucleus ambiguus

Answer

Dorsal vagus nucleus

Question 8

Which of the following is a chemoreceptor?

Accepted Answer

Osmoreceptors

Answer

Rods & cones

Answer

Hair cells

Answer

Merkel's disc

Question 9

Processing of tactile stimulation occurs in Brodmann's area?

Accepted Answer

1, 2, 3

Answer

4, 6

Answer

44, 45

Answer

41, 42

Question 10

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

Accepted Answer

Proprioception

Answer

Temperature

Answer

Pain

Answer

Crude touch

Sensory Systems — MCQs

Sensory Systems — MCQs

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