Sensory Systems Practice Questions

Q: Ablation of the somatosensory area V of the cerebral cortex leads to what type of sensory deficit?

Loss of tactile localization and two-point discrimination. **Explanation:** The somatosensory cortex (S-I and S-II) is essential for the **spatial orientation** and **discriminative** aspects of sensation. While the thalamus can perceive the presence of crude sensations like pain and temperature, the cerebral cortex is required to process complex sensory information. **1. Why Option D is Correct:** The ability to pinpoint where a stimulus is applied (**tactile localization**) and the ability to distinguish two distinct points applied simultaneously (**two-point discrimination**) are "discriminative" or "fine" sensations. These require high-level cortical processing. Ablation of the somatosensory area destroys the spatial map (homunculus), leading to an inability to localize stimuli or resolve spatial detail. **2. Why Other Options are Incorrect:** * **Options A & B:** Total loss of pain or touch does not occur. This is because the **thalamus** and lower brain centers are capable of perceiving the *quality* and *presence* of crude touch, pressure, and pain. However, the patient will be unable to tell exactly *where* the pain or touch is occurring. * **Option C:** This is incorrect because both localization and two-point discrimination are functionally linked to the dorsal column-medial lemniscal (DCML) pathway and its cortical projection. If one is lost due to cortical ablation, the other is invariably affected as well. **Clinical Pearls for NEET-PG:** * **Astereognosis:** The inability to identify an object by touch (size, shape, texture) in the absence of visual/auditory input is a hallmark of somatosensory cortex lesions. * **Thalamic Sensation:** Remember that the thalamus is the center for "crude" awareness, while the cortex is for "fine" discrimination. * **Brodmann Areas:** Somatosensory Area I (S-I) corresponds to Brodmann areas 3, 1, and 2 in the postcentral gyrus.

Q: Which of the following has a small representation in the somatosensory area of the cerebral cortex?

Trunk. ### Explanation The representation of body parts in the primary somatosensory cortex (S1, Postcentral Gyrus) is not proportional to the physical size of the body part, but rather to the **density of sensory receptors** and the **functional importance** of the tactile discrimination required. This concept is visually represented by the **Sensory Homunculus**. **Why the Trunk is the Correct Answer:** The **Trunk** (along with the legs and back) has a relatively low density of sensory receptors and large receptive fields. Since these areas do not require fine, discriminative touch or high spatial resolution, they occupy a very **small area** in the somatosensory cortex. **Analysis of Incorrect Options:** * **Lips (A) and Tongue (C):** These areas are critical for speech, mastication, and taste. They possess an extremely high density of sensory receptors, resulting in a disproportionately large representation in the homunculus (lateral aspect). * **Thumb (B):** The hands, particularly the thumb and index finger, are essential for fine motor tasks and precision grip. They have small receptive fields and high cortical representation to allow for stereognosis (identifying objects by touch). **High-Yield NEET-PG Pearls:** 1. **Location:** The primary somatosensory cortex corresponds to **Brodmann areas 3, 1, and 2**. 2. **Sequence:** From medial to lateral (longitudinal fissure to Sylvian fissure), the homunculus sequence is: *Toes → Leg → Trunk → Arm → Hand → Face → Tongue*. 3. **Blood Supply:** The medial part (lower limb) is supplied by the **Anterior Cerebral Artery (ACA)**, while the lateral part (face and upper limb) is supplied by the **Middle Cerebral Artery (MCA)**. 4. **Inversion:** The homunculus is "upside down," with the lower limbs represented medially and the face represented laterally.

Q: A person has defective blue color appreciation. What is the best name for this condition?

Tritanomalous. **Explanation:** Color vision is mediated by three types of cone photoreceptors, each sensitive to different wavelengths: **Long (Red)**, **Medium (Green)**, and **Short (Blue)**. Defects in these cones are classified using the Greek prefixes *Prot-* (First/Red), *Deuter-* (Second/Green), and *Trit-* (Third/Blue). **1. Why Tritanomalous is correct:** The term **"Anomalous Trichromacy"** refers to a condition where all three cone types are present, but one is functionally defective (shifted sensitivity), leading to "defective appreciation" rather than a total loss. Since the question specifies a defect in **blue** appreciation, the prefix **Trit-** (Blue) combined with **-anomalous** (defective/weak) makes **Tritanomalous** the most accurate description. **2. Why other options are incorrect:** * **Deuteranomalous:** Refers to defective appreciation of **green** light (the most common type of color blindness). * **Deuteranopia:** Refers to a complete **absence** of green cones (Dichromacy), not just defective appreciation. * **Tritanopia:** Refers to a complete **absence** of blue cones. While it involves blue color, "anomalous" specifically denotes a defect/weakness rather than a total lack. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Red-green defects (Protan/Deuteran) are **X-linked recessive** (more common in males). Blue defects (Tritan) are **Autosomal Dominant** and rare. * **Ishihara Charts:** The most common screening tool; however, they primarily detect red-green defects, not blue-yellow. * **Edridge-Green Lantern Test:** Used for occupational screening (e.g., Railways/Aviation). * **Nagel’s Anomaloscope:** The gold standard for definitive diagnosis and distinguishing between "anopia" and "anomaly."

Q: Which receptor gets stimulated in moderate cold?

CMR-1. **Explanation:** The perception of temperature is mediated by specialized ion channels known as **TRP (Transient Receptor Potential) channels**, which act as molecular thermometers. **1. Why CMR-1 is correct:** **CMR-1 (Cold and Menthol Receptor 1)**, also known as **TRPM8**, is the primary receptor stimulated by **moderate cold** (temperatures between 10°C and 25°C). It is a non-selective cation channel that allows the influx of $Ca^{2+}$ and $Na^+$ upon activation. Interestingly, it is also activated by **menthol**, which explains why minty substances produce a cooling sensation on the skin and tongue. **2. Why the other options are incorrect:** * **VR1 (TRPV1):** Also known as the Vanilloid Receptor 1, it is activated by **noxious heat** (>43°C) and **capsaicin** (the active component in chili peppers). It signals painful heat. * **VRL-1 (TRPV2):** This is the Vanilloid Receptor-Like protein 1. It has a much higher threshold and is activated by **extreme/intense heat** (>52°C). * **VR2:** This is an older nomenclature; however, in the context of TRP channels, other members like TRPV3 and TRPV4 are typically associated with warm temperatures (25°C–45°C), not cold. **Clinical Pearls for NEET-PG:** * **Paradoxical Cold:** When a cold-sensitive fiber is stimulated by a very high temperature (above 45°C), it may briefly signal a sensation of cold. * **TRPA1:** This receptor is activated by **extreme cold** (<18°C) and pungent chemicals like mustard oil; it is often associated with cold-induced pain. * **Fiber types:** Cold sensations are primarily carried by **Aδ (delta) fibers** (fast), while warmth is carried by **C fibers** (slow).

Q: Which extracellular fluid compartment is characterized by high potassium and low sodium concentrations?

Endolymph. **Explanation:** The correct answer is **Endolymph**. In the human body, extracellular fluids (ECF) typically resemble plasma, characterized by high sodium ($Na^+$) and low potassium ($K^+$) concentrations. The **endolymph**, found within the membranous labyrinth of the inner ear (scala media), is a unique exception. It is produced by the **stria vascularis** and is rich in $K^+$ (~150 mEq/L) and low in $Na^+$ (~1-5 mEq/L). This high potassium concentration creates a positive endocochlear potential (+80 mV), which provides the necessary electrochemical gradient for the depolarization of hair cells when mechanically stimulated. **Analysis of Incorrect Options:** * **Synovial fluid:** This is an ultrafiltrate of plasma supplemented with hyaluronic acid. Its electrolyte composition mirrors typical ECF (High $Na^+$, Low $K^+$). * **Ectolymph (Perilymph):** Perilymph fills the scala tympani and scala vestibuli. Its composition is similar to CSF and typical ECF, being rich in $Na^+$ and poor in $K^+$. * **Cerebrospinal fluid (CSF):** Produced by the choroid plexus, CSF is an ultrafiltrate of plasma. While it has lower $K^+$ and higher $Cl^-$ compared to plasma, it remains a high-sodium fluid. **High-Yield Clinical Pearls for NEET-PG:** * **Meniere’s Disease:** Caused by "Endolymphatic Hydrops" (excess accumulation of endolymph), leading to the triad of vertigo, sensorineural hearing loss, and tinnitus. * **Stria Vascularis:** Often referred to as the "battery of the ear" because it actively pumps $K^+$ into the endolymph via $Na^+-K^+$ ATPase pumps. * **Ionic Comparison:** Remember: **E**ndolymph is like **I**ntracellular fluid (High K+), while **P**erilymph is like **E**xtracellular fluid (High Na+).

Q: Sound transduction by the Organ of Corti is initiated by?

Potassium influx. **Explanation:** The transduction of sound in the **Organ of Corti** is a unique physiological process because it relies on **Potassium (K⁺) influx**, unlike most other excitable cells that rely on Sodium (Na⁺) influx for depolarization. **Why Potassium Influx is Correct:** The apical surfaces of hair cells (stereocilia) are bathed in **endolymph**, which is found in the scala media. Endolymph is unique because it has a very high K⁺ concentration and a positive endocochlear potential (+80 mV). When sound waves cause the basilar membrane to vibrate, the stereocilia bend toward the tallest kinocilium. This mechanical action opens **mechanically-gated K⁺ channels** (MET channels) at the tips. Due to the high electrochemical gradient, K⁺ rushes **into** the hair cell, causing depolarization. This leads to the opening of voltage-gated calcium channels and subsequent neurotransmitter release. **Why Other Options are Incorrect:** * **Sodium Influx/Efflux:** While Na⁺ influx drives depolarization in neurons and muscles, the endolymph is Na⁺-poor. Therefore, Na⁺ does not play a primary role in the initial transduction phase in the inner ear. * **Potassium Efflux:** K⁺ efflux occurs during the **repolarization** phase. After depolarization, K⁺ leaves the base of the hair cell to enter the **perilymph** (which is K⁺-poor), returning the cell to its resting state. **High-Yield NEET-PG Pearls:** * **Endolymph** resembles intracellular fluid (High K⁺, Low Na⁺) and is secreted by the **Stria Vascularis**. * **Perilymph** resembles extracellular fluid (High Na⁺, Low K⁺). * **Tip Links:** These are the protein filaments (containing cadherin-23) that connect stereocilia and pull the K⁺ channels open. * **Endocochlear Potential:** At +80 mV, it is the highest transepithelial potential in the body, providing the "battery" for hearing.

Q: Auditory transmission is via which pathway?

Lateral Lemniscus. **Explanation:** The auditory pathway is a complex, multisynaptic circuit that transmits sound information from the cochlea to the auditory cortex. The correct answer is **Lateral Lemniscus** because it represents the primary ascending tract of the auditory system in the brainstem. **Why Lateral Lemniscus is Correct:** After sound is transduced in the cochlea, signals travel via the vestibulocochlear nerve (CN VIII) to the cochlear nuclei. From there, fibers cross the midline (trapezoid body) and ascend as the **Lateral Lemniscus**. This tract carries auditory information from the superior olivary complex to the inferior colliculus. A high-yield mnemonic for the auditory pathway is **ECOLI**: **E**ighth nerve, **C**ochlear nuclei, **O**livary complex (superior), **L**ateral lemniscus, **I**nferior colliculus. **Why Other Options are Incorrect:** * **Lateral Geniculate Body (LGB):** This is the relay station for the **visual pathway**, not auditory. (Mnemonic: **L** for **L**ight). * **Medial Lemniscus:** This tract carries **fine touch, vibration, and conscious proprioception** (DCML pathway) from the body to the thalamus. * **Pretectal Nucleus:** This structure is involved in the **pupillary light reflex** circuit, located in the midbrain. **High-Yield Clinical Pearls for NEET-PG:** * **Medial Geniculate Body (MGB):** This is the thalamic relay station for hearing (Mnemonic: **M** for **M**usic). * **Bilateral Representation:** Above the level of the cochlear nuclei, auditory information is represented bilaterally. Therefore, a unilateral lesion of the lateral lemniscus or auditory cortex does not cause complete deafness in one ear but rather results in difficulty localizing sound. * **Primary Auditory Cortex:** Located in the superior temporal gyrus (Brodmann areas 41, 42).

Q: Which function is controlled by the striate cortex of the brain?

Vision. **Explanation:** The **striate cortex**, also known as the **Primary Visual Cortex (Brodmann area 17)**, is located in the occipital lobe, specifically along the calcarine fissure. It is termed "striate" because it contains the **Gennari line**, a distinct white matter band visible to the naked eye, representing myelinated fibers from the lateral geniculate nucleus (LGN) of the thalamus. Its primary function is the initial processing of visual information, including orientation, motion, and edge detection. **Analysis of Options:** * **Vision (Correct):** The striate cortex receives sensory input directly from the LGN. Damage to this area results in cortical blindness or contralateral homonymous hemianopia with macular sparing. * **Speech:** Controlled primarily by **Broca’s area** (Brodmann 44, 45) in the frontal lobe for production and **Wernicke’s area** (Brodmann 22) in the temporal lobe for comprehension. * **Balance:** Primarily regulated by the **cerebellum** and the vestibular system (vestibular nuclei in the brainstem). * **Auditory processing:** Located in the **Primary Auditory Cortex** (Brodmann areas 41, 42), also known as Heschl’s gyri, in the superior temporal lobe. **High-Yield NEET-PG Pearls:** * **Blood Supply:** The primary visual cortex is supplied by the **posterior cerebral artery (PCA)**. * **Macular Sparing:** In PCA strokes, the macula is often spared because the occipital pole has a dual blood supply from both the PCA and the **middle cerebral artery (MCA)**. * **Visual Association Areas:** Brodmann areas 18 and 19 are the extrastriate (association) areas responsible for interpreting visual images.

Question 1

Ablation of the somatosensory area V of the cerebral cortex leads to what type of sensory deficit?

Accepted Answer

Loss of tactile localization and two-point discrimination

Answer

Total loss of pain sensation

Answer

Total loss of touch sensation

Answer

Loss of tactile localization but not two-point discrimination

Question 2

Which of the following has a small representation in the somatosensory area of the cerebral cortex?

Accepted Answer

Trunk

Answer

Lips

Answer

Thumb

Answer

Tongue

Question 3

A person has defective blue color appreciation. What is the best name for this condition?

Accepted Answer

Tritanomalous

Answer

Deuteranomalous

Answer

Deuteranopia

Answer

Tritanopia

Question 4

Which receptor gets stimulated in moderate cold?

Accepted Answer

CMR-1

Answer

VR1

Answer

VRL-1

Answer

VR2

Question 5

Which extracellular fluid compartment is characterized by high potassium and low sodium concentrations?

Accepted Answer

Endolymph

Answer

Synovial fluid

Answer

Ectolymph

Answer

Cerebrospinal fluid (CSF)

Question 6

Sound transduction by the Organ of Corti is initiated by?

Accepted Answer

Potassium influx

Answer

Sodium influx

Answer

Sodium efflux

Answer

Potassium efflux

Question 7

Which of the following is true about the generator potential?

Accepted Answer

Its magnitude increases in direct proportion to the strength of the stimulus.

Answer

It is not graded.

Answer

It is an all-or-none phenomenon.

Answer

It is propagated.

Question 8

Which part of the retina is primarily involved in the process of dark adaptation?

Accepted Answer

Both rods and cones

Answer

Rods

Answer

Cones

Answer

Pigment epithelium

Question 9

Auditory transmission is via which pathway?

Accepted Answer

Lateral Lemniscus

Answer

Lateral Geniculate body

Answer

Medial Lemniscus

Answer

Pretectal Nucleus

Question 10

Which function is controlled by the striate cortex of the brain?

Accepted Answer

Vision

Answer

Speech

Answer

Balance

Answer

Auditory processing

Sensory Systems — MCQs

Sensory Systems — MCQs

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