Sensory Systems Practice Questions

Q: Which of the following tracts is concerned with pain and temperature sensation?

Lateral spinothalamic tract. **Explanation:** The **Lateral Spinothalamic Tract (LSTT)** is the primary ascending pathway for **pain and temperature** sensations. The pathway begins with first-order neurons in the dorsal root ganglion, which synapse in the dorsal horn (Substantia Gelatinosa). Second-order neurons decussate (cross over) at the same spinal level through the anterior white commissure and ascend in the lateral funiculus to the thalamus (VPL nucleus). **Analysis of Options:** * **A. Pyramidal tract:** This is a **descending motor pathway** (Corticospinal tract) responsible for voluntary motor control, not sensory perception. * **B. Anterior spinothalamic tract:** This tract primarily carries **crude touch and pressure** sensations. While related to the LSTT, it is functionally distinct. * **C. Lateral spinothalamic tract (Correct):** Specifically dedicated to nociception (pain) and thermoreception (temperature). * **D. Dorsal spinocerebellar tract:** This pathway carries **unconscious proprioception** from the lower limbs to the cerebellum to coordinate posture and gait. **High-Yield Clinical Pearls for NEET-PG:** * **Brown-Séquard Syndrome:** A hemisection of the spinal cord results in **contralateral** loss of pain and temperature (due to the LSTT crossing at the spinal level) and **ipsilateral** loss of vibration/proprioception (Dorsal columns cross at the medulla). * **Syringomyelia:** This condition involves a cyst in the central canal that compresses the anterior white commissure, leading to a classic **"cape-like" bilateral loss** of pain and temperature, while sparing touch (dissociated sensory loss). * **Rexed Laminae:** The second-order neurons of the LSTT primarily originate in Laminae I, IV, and V.

Q: Sour taste is mediated by which of the following?

H+ ions permeable to ENaCs. **Explanation:** The perception of taste (gustation) involves different transduction mechanisms for the five primary taste qualities. **Sour taste** is triggered by acids, specifically the concentration of **H+ ions**. **Why Option C is Correct:** Sour taste transduction occurs when H+ ions (protons) interact with the apical membrane of taste cells. These ions act in two ways: 1. They directly enter the cell through **Amiloride-sensitive Sodium Channels (ENaCs)** or specialized proton channels (like OTOP1). 2. They bind to and block **K+ selective channels**, preventing K+ efflux. Both mechanisms lead to depolarization of the taste cell, opening of voltage-gated Ca2+ channels, and neurotransmitter release. **Analysis of Incorrect Options:** * **Option A (Na+ selective channel):** While ENaCs are involved in sour taste, they are the primary mediators for **Salty taste** when they transport Na+ ions. * **Option B (G-protein coupled receptors):** GPCRs (specifically T1R and T2R families) mediate **Sweet, Bitter, and Umami** tastes via the second messenger IP3 and release of intracellular Ca2+. * **Option D (Glutamate receptors):** These are specifically associated with **Umami (savory) taste**, triggered by L-glutamate (found in MSG). **High-Yield NEET-PG Pearls:** * **Salty & Sour:** Use **Ion Channels** (Direct depolarization). * **Sweet, Bitter, Umami:** Use **GPCRs** (Metabotropic receptors). * **Taste Pathway:** Anterior 2/3 of tongue (CN VII via Chorda tympani) → Posterior 1/3 (CN IX) → Epiglottis/Pharynx (CN X) → Nucleus Tractus Solitarius (NTS) → Thalamus (VPM nucleus) → Gustatory Cortex (Insula). * **Agraphia/Ageusia:** Loss of taste sensation, often tested in the context of Zinc deficiency or cranial nerve lesions.

Q: What is true about the electroretinogram?

The c-wave is a positive wave.. ### Explanation: Electroretinogram (ERG) The Electroretinogram (ERG) is a recording of the electrical potentials generated by the retina in response to a light stimulus. It consists of three primary waves: **a, b, and c**. **Why Option D is Correct:** The **c-wave** is a prolonged, **positive** deflection. It is the slowest component of the ERG and is generated by the **Pigment Epithelium** of the retina. It reflects the metabolic activity and ionic changes (specifically potassium levels) in the subretinal space following light exposure. **Why the Other Options are Incorrect:** * **Option A:** The **a-wave** is the first component and is a **negative** wave. It represents the "receptor potential." * **Option B:** The **a-wave** arises from the **photoreceptors** (rods and cones), not the pigment epithelium. * **Option C:** The **b-wave** is a large positive wave that arises from the **inner retinal layers**, specifically the **Bipolar cells** (with contributions from Müller cells), not the rods and cones themselves. --- ### High-Yield Facts for NEET-PG: * **Sequence of Waves:** 1. **a-wave (Negative):** Photoreceptors (Rods/Cones). 2. **b-wave (Positive):** Bipolar cells/Müller cells. (This is the most clinically significant wave). 3. **c-wave (Positive):** Retinal Pigment Epithelium (RPE). 4. **d-wave (Positive):** Occurs at the cessation of light (off-response). * **Clinical Utility:** ERG is used to diagnose generalized retinal disorders. It is **normal in Macular Degeneration** (because the macula is a small area) but **extinguished (flat)** in **Retinitis Pigmentosa** and Vitamin A deficiency. * **EOG (Electro-oculogram):** Measures the standing potential between the front and back of the eye; it is the gold standard for diagnosing **Best’s Disease**.

Q: Which cells are the first-order neurons for visual sensations?

Bipolar cells. In the visual pathway, the transmission of signals follows a specific anatomical sequence. To identify the "order" of neurons, we look for the cells that conduct the first nerve impulses toward the brain. ### **Why Bipolar Cells are the Correct Answer** In the retina, **Rods and Cones** (photoreceptors) are specialized neuroepithelial cells, not neurons. They act as **sensory receptors** that convert light into electrical potentials (hyperpolarization). The **Bipolar cells** receive this signal and are the first cells in the pathway to function as true neurons, transmitting the impulse to the next layer. Therefore, they are classified as the **First-order neurons**. ### **Analysis of Incorrect Options** * **A. Rods and Cones:** These are the **photoreceptors**. While they initiate the visual process through phototransduction, they are considered receptors rather than first-order neurons in the classical neuroanatomical hierarchy. * **C. Ganglion Cells:** These are the **Second-order neurons**. Their axons converge to form the optic nerve, which carries the signal to the Lateral Geniculate Body (LGB). * **LGB (Lateral Geniculate Body):** Though not an option here, it is important to remember that the neurons in the LGB are the **Third-order neurons**, which then project to the primary visual cortex. ### **High-Yield Facts for NEET-PG** * **The Exception:** In most sensory systems (like touch or pain), the first-order neuron is located in the Dorsal Root Ganglion. In vision, the first three neurons of the pathway are uniquely located **entirely within the retina**. * **Action Potentials:** Rods, cones, and bipolar cells do **not** fire action potentials; they communicate via **graded potentials**. The **Ganglion cells** are the first cells in the visual pathway to fire true action potentials. * **Neurotransmitter:** Glutamate is the primary neurotransmitter released by photoreceptors in the dark.

Q: Which of the following statements regarding the lateral geniculate body is FALSE?

P cells are responsible for detecting movement.. The **Lateral Geniculate Body (LGB)** is the primary relay center for visual information from the retina to the primary visual cortex. Understanding the functional segregation of its layers is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer (Option C)** Option C is **FALSE** because it swaps the functions of P and M cells. * **P cells (Parvocellular pathway):** These cells are located in layers 3, 4, 5, and 6 of the LGB. They have small receptive fields and are responsible for **color vision, fine detail, and shape (high spatial resolution).** * **M cells (Magnocellular pathway):** These cells are located in layers 1 and 2. They have large receptive fields and are specialized for detecting **movement, flicker, and depth perception (high temporal resolution).** ### **Analysis of Other Options** * **Option A (True):** The LGB is indeed a sensory nucleus of the **thalamus** (specifically the metathalamus), acting as the "gatekeeper" for visual signals. * **Option B (True):** As part of the Magnocellular pathway, M cells are crucial for processing motion and stereopsis (**depth perception**). ### **High-Yield NEET-PG Pearls** * **LGB Layer Organization:** * **Layers 1, 4, 6:** Receive input from the **contralateral** eye (nasal retina). * **Layers 2, 3, 5:** Receive input from the **ipsilateral** eye (temporal retina). * *Mnemonic: "See (C) the 1, 4, 6" (C for Contralateral).* * **Koniocellular (K) Cells:** These are found between the M and P layers and are involved in blue-yellow color vision. * **Lesion:** A lesion in the LGB results in **contralateral homonymous hemianopia** with macular sparing (though sparing is more characteristic of cortical lesions).

Q: Most of the afferents from the lateral geniculate body terminate in which layer of the visual cortex?

Layer 4. **Explanation:** The primary visual cortex (V1 or Brodmann area 17) is organized into six distinct horizontal layers. The **Lateral Geniculate Body (LGB)** of the thalamus serves as the primary relay station for visual information. In the sensory systems, it is a fundamental physiological principle that most **thalamocortical afferents** terminate in **Layer 4** (the internal granular layer). In the visual cortex, Layer 4 is highly developed and further subdivided. Specifically, the parvocellular and magnocellular fibers from the LGB terminate predominantly in **Layer 4C**. This layer is rich in stellate cells, which receive the sensory input before distributing it to other layers for processing. **Analysis of Incorrect Options:** * **Layer 1 (Molecular Layer):** Contains mostly dendrites and axons from other layers; it is a site for modulatory input rather than primary sensory termination. * **Layers 2 & 3 (External Granular/Pyramidal):** These layers primarily function in **cortico-cortical communication**, sending projections to other areas of the cerebral cortex. * **Layers 5 & 6 (Internal Pyramidal/Multiform):** These are the **output layers**. Layer 5 sends projections to subcortical structures (like the superior colliculus), and Layer 6 provides **feedback projections** back to the Thalamus (LGB). **High-Yield NEET-PG Pearls:** * **Histology:** The visual cortex is called the "Striate Cortex" because the dense termination of afferents in Layer 4 creates a visible white line known as the **Stria of Gennari**. * **Functional Rule:** Remember the "Input-Output" rule: **Layer 4 is Input** (from Thalamus), **Layer 5/6 is Output** (to subcortical/thalamus). * **LGB Structure:** Layers 1-2 are Magnocellular (motion), while 3-6 are Parvocellular (color/detail). All eventually target Layer 4 of the cortex.

Q: What is the typical membrane potential of a hair cell?

-60 mV. **Explanation:** The resting membrane potential (RMP) of a hair cell (the sensory receptor of the auditory and vestibular systems) is approximately **-60 mV**. Unlike typical neurons (RMP -70 mV) or skeletal muscle (RMP -90 mV), hair cells exist in a unique ionic environment. The apical portion of the hair cell is bathed in **endolymph** (high $K^+$, +80 mV potential), while the basal portion is bathed in **perilymph** (low $K^+$, 0 mV potential). This creates a massive electrical gradient of 140 mV across the stereocilia, allowing for rapid depolarization when mechanically gated $K^+$ channels open. **Analysis of Options:** * **A (-20 mV):** This is too depolarized for a resting state. Hair cells may reach this potential during active stimulation, but it is not the baseline. * **B (-40 mV):** While some texts cite a range of -40 to -60 mV for specific vestibular cells, **-60 mV** is the standard physiological value taught for the cochlear hair cells in major textbooks like Guyton and Ganong. * **D (-80 mV):** This is closer to the RMP of large nerve fibers or cardiac myocytes. Hair cells are more depolarized than these cells at rest to allow for "tonic" neurotransmitter release. **High-Yield Clinical Pearls for NEET-PG:** * **Endocochlear Potential:** The endolymph has a positive potential of **+80 mV**, the highest transepithelial potential in the body, maintained by the **Stria Vascularis**. * **Tip Links:** These protein filaments connect stereocilia; their stretching opens **MET (Mechano-Electrical Transducer)** channels. * **Unique Depolarization:** In hair cells, **$K^+$ influx** causes depolarization (unlike neurons where $Na^+$ influx causes it). * **Outer Hair Cells (OHC):** Act as "cochlear amplifiers" via the protein **Prestin**, which provides electromotility.

Q: Metabotropic glutamate receptor 4 (mGluR4) is associated with which taste sensation?

Umami. **Explanation:** The correct answer is **Umami (D)**. Taste transduction occurs through specific receptors on the tongue. **Umami**, the savory taste associated with L-glutamate (found in MSG and protein-rich foods), utilizes two primary types of G-protein coupled receptors (GPCRs): 1. **T1R1 + T1R3 heterodimer:** The primary receptor for most umami-tasting amino acids. 2. **mGluR4 (metabotropic Glutamate Receptor 4):** A truncated version of this receptor is specifically expressed in taste buds and functions as a specialized umami sensor. **Analysis of Incorrect Options:** * **Sweet (A):** Mediated by a heterodimer of **T1R2 + T1R3** GPCRs. These respond to sugars, saccharin, and certain proteins. * **Sour (B):** Mediated by ion channels, specifically the **OTOP1 (Otopetrin-1)** proton channel, which detects the acidity (H+ ions) of substances. * **Bitter (C):** Mediated by the **T2R** family of GPCRs. There are approximately 30 different T2R receptors to detect a wide variety of potentially toxic bitter substances. **High-Yield Facts for NEET-PG:** * **Salt taste** is primarily mediated by **ENaC** (Epithelial Sodium Channels). * **Gustducin** is the specialized G-protein involved in the transduction of Sweet, Bitter, and Umami tastes. * **Ageusia** is the loss of taste, while **Dysgeusia** is a distorted sense of taste (often a side effect of drugs like Captopril or Metronidazole). * The **anterior 2/3** of the tongue is supplied by the Chorda Tympani (CN VII), and the **posterior 1/3** by the Glossopharyngeal nerve (CN IX).

Q: Semicircular canals are stimulated by?

Rotation. **Explanation:** The vestibular apparatus in the inner ear is responsible for maintaining equilibrium and posture. It consists of the **Semicircular Canals (SCC)** and the **Otolith organs** (Utricle and Saccule). **Why Rotation is Correct:** The Semicircular Canals are specifically designed to detect **angular (rotational) acceleration**. There are three canals (Anterior, Posterior, and Lateral) oriented at right angles to each other, allowing for the detection of rotation in any plane. When the head rotates, the **endolymph** (fluid) within the canals moves due to inertia, which displaces the **cupula** in the ampulla. This bends the hair cells, triggering a neural impulse. **Why Other Options are Incorrect:** * **Gravity & Linear Acceleration (Options A & B):** These are detected by the **Otolith organs**. The **Utricle** detects horizontal linear acceleration, while the **Saccule** detects vertical linear acceleration (including gravity). These organs contain calcium carbonate crystals called *otoconia* that shift in response to linear movement. * **Sound (Option D):** Sound waves are processed by the **Cochlea**, specifically the Organ of Corti, which is the sensory organ for hearing, not balance. **High-Yield Clinical Pearls for NEET-PG:** * **BPPV (Benign Paroxysmal Positional Vertigo):** Caused by displaced otoconia from the utricle entering the semicircular canals (most commonly the **Posterior canal**). It is diagnosed by the **Dix-Hallpike maneuver** and treated with the **Epley maneuver**. * **Caloric Reflex Test:** Used to test SCC function. Remember the mnemonic **COWS**: **C**old **O**pposite, **W**arm **S**ame (refers to the direction of the fast phase of nystagmus). * **Scarpa’s Ganglion:** The sensory ganglion for the vestibular nerve.

Question 1

Which of the following tracts is concerned with pain and temperature sensation?

Accepted Answer

Lateral spinothalamic tract

Answer

Pyramidal tract

Answer

Anterior spinothalamic tract

Answer

Dorsal spinocerebellar tract

Question 2

Sour taste is mediated by which of the following?

Accepted Answer

H+ ions permeable to ENaCs

Answer

Na+ selective channel (ENac)

Answer

G - protein coupled receptors

Answer

Glutamate receptors

Question 3

What is true about the electroretinogram?

Accepted Answer

The c-wave is a positive wave.

Answer

The a-wave is a positive wave.

Answer

The a-wave arises from the pigmented epithelium.

Answer

The b-wave arises from rods and cones.

Question 4

Which cells are the first-order neurons for visual sensations?

Accepted Answer

Bipolar cells

Answer

Rods and cones

Answer

Ganglion cells

Answer

None of the above

Question 5

Which of the following statements regarding the lateral geniculate body is FALSE?

Accepted Answer

P cells are responsible for detecting movement.

Answer

It is part of the thalamus.

Answer

M cells are responsible for depth perception.

Answer

None of the above.

Question 6

Most of the afferents from the lateral geniculate body terminate in which layer of the visual cortex?

Accepted Answer

Layer 4

Answer

Layer 1

Answer

Layers 2 & 3

Answer

Layers 5 & 6

Question 7

What is the typical membrane potential of a hair cell?

Accepted Answer

-60 mV

Answer

-20 mV

Answer

-40 mV

Answer

-80 mV

Question 8

Odorant molecules bind to which type of receptors?

Accepted Answer

G protein-coupled membrane receptors

Answer

Cyclic AMP (cAMP)

Answer

JAK kinase pathway

Answer

Nuclear factor (NF) receptors

Question 9

Metabotropic glutamate receptor 4 (mGluR4) is associated with which taste sensation?

Accepted Answer

Umami

Answer

Sweet

Answer

Sour

Answer

Bitter

Question 10

Semicircular canals are stimulated by?

Accepted Answer

Rotation

Answer

Gravity

Answer

Linear acceleration

Answer

Sound

Sensory Systems — MCQs

Sensory Systems — MCQs

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