Sensory Systems Practice Questions

Q: Ascending pain pathway is inhibited in the dorsal midbrain by which neurotransmitter?

5-HT (serotonin). ### Explanation The correct answer is **5-HT (serotonin)**. **Mechanism of Pain Inhibition** The body possesses an endogenous analgesia system that modulates pain perception. The primary center for this is the **Periaqueductal Gray (PAG)** in the dorsal midbrain. When stimulated, neurons from the PAG project to the **Nucleus Raphe Magnus (NRM)** in the medulla. The neurons originating from the NRM are **serotonergic (5-HT)**; they descend to the dorsal horn of the spinal cord to inhibit incoming pain signals (the "Gate Control" mechanism) by stimulating inhibitory enkephalin-releasing interneurons. **Analysis of Options:** * **A. 5-HT (Serotonin):** Correct. It is the primary neurotransmitter released by the descending fibers from the Raphe nuclei to inhibit pain transmission in the spinal cord. * **B. Noradrenaline:** While noradrenaline is involved in the descending inhibitory pathway (originating from the **Locus Coeruleus**), the classical pathway described from the dorsal midbrain/Raphe system is primarily serotonergic. * **C. Substance P:** This is a **pro-nociceptive** neurotransmitter. It is released by primary afferent fibers (A-delta and C fibers) in the dorsal horn to *transmit* pain signals, not inhibit them. * **D. Glutamate:** This is the major excitatory neurotransmitter in the CNS. It is involved in the fast transmission of pain signals at the first synapse in the dorsal horn. **High-Yield Clinical Pearls for NEET-PG:** * **PAG** is the "Command Center" for descending pain suppression. * **Enkephalins and Endorphins** act at the spinal level to cause pre- and post-synaptic inhibition of Substance P release. * **Tricyclic Antidepressants (TCAs)** are used in chronic pain management because they increase the availability of Serotonin and Noradrenaline in these inhibitory pathways.

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: The gate control theory of pain is primarily related to which anatomical structure?

Substantia gelatinosa. **Explanation:** The **Gate Control Theory**, proposed by Melzack and Wall, suggests that pain perception is not a direct result of pain fiber activation but is modulated by a "gate" mechanism in the spinal cord. **Why Substantia Gelatinosa (SG) is correct:** The "gate" is anatomically located in the **Substantia Gelatinosa (Lamina II)** of the dorsal horn of the spinal cord. When large-diameter, non-nociceptive sensory fibers (A-beta) are stimulated (e.g., by rubbing a wound), they activate inhibitory interneurons within the SG. These interneurons release inhibitory neurotransmitters (like Enkephalins) that cause **presynaptic inhibition** of the pain-carrying T-cells (Transmission cells). This effectively "closes the gate," preventing pain signals from reaching the brain. **Analysis of Incorrect Options:** * **A-delta fibers:** These are myelinated fibers responsible for "fast pain" (sharp, localized). While they participate in the circuit, they are the *input*, not the anatomical site of the gate itself. * **C-fibers:** These are unmyelinated fibers responsible for "slow pain" (dull, aching). Stimulation of C-fibers actually "opens" the gate by inhibiting the SG interneurons. * **Free nerve endings:** These are peripheral receptors (nociceptors) where the pain signal originates; they are not involved in the central modulation described by the theory. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Application:** Transcutaneous Electrical Nerve Stimulation (**TENS**) and dorsal column stimulation work on the principle of Gate Control Theory by stimulating A-beta fibers. * **Neurotransmitter:** **Enkephalin** is the primary endogenous opioid involved in the SG interneurons. * **Lamina Architecture:** Remember that the Substantia Gelatinosa corresponds specifically to **Rexed Lamina II**.

Q: In the Electroretinogram (ERG), 'A' waves correspond to which retinal layer?

Rods and cones. ### Explanation The **Electroretinogram (ERG)** is a diagnostic test that measures the electrical response of various cell types in the retina to a light stimulus. It represents a composite waveform of the potential changes occurring across the retinal layers. **Why Rods and Cones are correct:** The **'a' wave** is the first major component of the ERG. It is a **negative deflection** that originates from the **photoreceptors (rods and cones)**. Specifically, it represents the hyperpolarization of these cells in response to light, which leads to a decrease in the release of the neurotransmitter glutamate. **Analysis of Incorrect Options:** * **Nerve fiber layer (Option B):** The ERG does **not** record activity from the ganglion cells or the nerve fiber layer. To assess these, a Pattern ERG (pERG) or Visual Evoked Potential (VEP) is required. * **Amacrine cells (Option C):** These cells, along with bipolar and Muller cells, contribute to the **'b' wave** (a large positive deflection). The 'b' wave is the most clinically significant part of the ERG. * **Pigment epithelium (Option D):** The Retinal Pigment Epithelium (RPE) is primarily responsible for the **'c' wave**, which is a prolonged positive wave following the 'b' wave. The RPE is also specifically evaluated using **Electro-oculography (EOG)**. **High-Yield Clinical Pearls for NEET-PG:** * **Wave Sequence:** 'a' wave (Photoreceptors) $\rightarrow$ 'b' wave (Bipolar/Muller cells) $\rightarrow$ 'c' wave (RPE). * **Early Receptor Potential (ERP):** Occurs even before the 'a' wave; it originates from the bleaching of rhodopsin. * **Clinical Utility:** ERG is most useful in diagnosing **Retinitis Pigmentosa** (where the ERG is typically "extinguished" or flat) and **Night Blindness**. * **Arden Index:** Associated with EOG (Normal > 1.85); it measures the ratio of light peak to dark trough.

Q: What is the function of the stria vascularis?

Endolymph secretion. **Explanation:** The **stria vascularis** is a highly vascularized layer of stratified epithelium located on the lateral wall of the cochlear duct (scala media). Its primary physiological role is the **secretion of endolymph** and the maintenance of its unique ionic composition. 1. **Why Option B is correct:** Unlike most extracellular fluids, endolymph is rich in potassium ($K^+$) and low in sodium ($Na^+$). The stria vascularis acts as a "metabolic engine," utilizing $Na^+/K^+$ ATPase pumps to actively transport potassium into the scala media. This creates the **endocochlear potential** (+80 mV), which provides the necessary driving force for sensory hair cell transduction. 2. **Why Options A & C are incorrect:** Perilymph, which fills the scala vestibuli and scala tympani, is chemically similar to extracellular fluid (high $Na^+$, low $K^+$). It is primarily formed as an ultrafiltrate of blood plasma or from cerebrospinal fluid (via the cochlear aqueduct), not by the stria vascularis. 3. **Why Option D is incorrect:** Cerebrospinal fluid (CSF) is produced by the choroid plexus in the brain's ventricles. While CSF communicates with the perilymphatic space through the cochlear aqueduct, it is not secreted within the cochlea. **High-Yield Clinical Pearls for NEET-PG:** * **Meniere’s Disease:** Caused by the distension of the endolymphatic space (endolymphatic hydrops), often due to impaired resorption by the endolymphatic sac. * **Waardenburg Syndrome:** Can involve sensorineural hearing loss due to the absence of melanocytes in the stria vascularis, which are essential for its secretory function. * **Presbycusis:** "Sensory" presbycusis involves hair cell loss, but "Strial" (metabolic) presbycusis is specifically due to atrophy of the stria vascularis.

Question 1

Ascending pain pathway is inhibited in the dorsal midbrain by which neurotransmitter?

Accepted Answer

5-HT (serotonin)

Answer

Noradrenaline

Answer

Substance P

Answer

Glutamate

Question 2

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 3

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 4

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 5

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 6

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 7

The gate control theory of pain is primarily related to which anatomical structure?

Accepted Answer

Substantia gelatinosa

Answer

A-delta fibers

Answer

C-fibers

Answer

Free nerve endings

Question 8

In the Electroretinogram (ERG), 'A' waves correspond to which retinal layer?

Accepted Answer

Rods and cones

Answer

Nerve fiber layer

Answer

Amacrine cells

Answer

Pigment epithelium

Question 9

What is the function of the stria vascularis?

Accepted Answer

Endolymph secretion

Answer

Perilymph secretion

Answer

Both perilymph and endolymph secretion

Answer

Cerebrospinal fluid secretion

Question 10

What is the primary function of the middle ear bones?

Accepted Answer

Impedance matching for efficient sound transmission to the inner ear

Answer

Amplification of sound intensity

Answer

Reduction of sound intensity

Answer

Protecting the inner ear from loud noises

Sensory Systems — MCQs

Sensory Systems — MCQs

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