Sensory Systems Practice Questions

Q: Synaptic transmission between pain fibers from the skin and spinal cord neurons is mediated by which of the following neurotransmitters?

Substance P. ### Explanation **Correct Answer: B. Substance P** **Mechanism:** Pain sensation (nociception) is transmitted from the periphery to the dorsal horn of the spinal cord via two types of primary afferent fibers: **A-delta (fast pain)** and **C fibers (slow pain)**. * **Substance P**, a neuropeptide belonging to the tachykinin family, is the primary neurotransmitter released by **C fibers** at the synapse with second-order neurons in the *Substantia Gelatinosa* (Lamina II). * It acts via **NK-1 receptors** to produce slow, prolonged excitatory postsynaptic potentials (EPSPs), facilitating the transmission of chronic or dull aching pain. * *Note:* Glutamate is also released (primarily by A-delta fibers) for rapid, acute pain transmission. **Analysis of Incorrect Options:** * **A. Acetylcholine:** This is the primary neurotransmitter at the neuromuscular junction and in preganglionic autonomic fibers, but it is not the mediator for primary pain afferents in the spinal cord. * **C. Endorphins:** These are endogenous opioids that **inhibit** pain transmission. They act pre-synaptically and post-synaptically in the dorsal horn to hyperpolarize neurons and decrease the release of Substance P (the "Gate Control Theory"). * **D. Somatostatin:** While found in some sensory neurons, its primary role is inhibitory (e.g., inhibiting growth hormone or GI secretions). It is not the principal mediator of pain transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Capsaicin:** Depletes Substance P from peripheral sensory neurons; used topically for post-herpetic neuralgia and diabetic neuropathy. * **Fast Pain vs. Slow Pain:** Fast pain uses **Glutamate** (A-delta fibers); Slow pain uses **Substance P** (C fibers). * **Lamina II:** The *Substantia Gelatinosa* is the specific site in the spinal cord where these pain fibers synapse.

Q: A 25-year-old student with 20/20 vision looks up from his book to view his girlfriend sitting on the other side of the room. Which of the following is most likely to occur when the student changes his view from his book to his girlfriend?

Thinner lens, relaxation of ciliary muscle. ### Explanation The physiological process described in this scenario is the **reversal of the accommodation reflex**. **1. Why Option D is Correct:** When the student shifts his gaze from a near object (his book) to a distant object (his girlfriend on the other side of the room), the eye must decrease its refractive power. * **Ciliary Muscle Relaxation:** To focus on distant objects, the ciliary muscle relaxes. * **Zonular Tension:** This relaxation increases the tension on the **suspensory ligaments (Zonules of Zinn)**. * **Lens Flattening:** The increased tension pulls the lens periphery, causing the lens to become **thinner** and flatter (less convex). This increases the focal length, allowing distant images to focus precisely on the retina. **2. Why Other Options are Incorrect:** * **Options A & B (Thicker lens):** A thicker, more spherical lens is required for **near vision** (accommodation) to increase refractive power. Shifting to a distant object requires the opposite. * **Option C (Contraction of ciliary muscle):** Contraction of the ciliary muscle reduces tension on the zonules, allowing the lens to bulge (become thicker). This occurs when shifting from a distant object to a near one. **3. NEET-PG High-Yield Pearls:** * **The Accommodation Triad:** When shifting from far to near vision, three things happen: 1) Lens thickening (increased curvature), 2) Pupillary constriction (miosis), and 3) Convergence of eyeballs. * **Innervation:** The ciliary muscle is supplied by **parasympathetic fibers** via the **Short Ciliary Nerves** (from the Edinger-Westphal nucleus/Cranial Nerve III). * **Presbyopia:** With age, the lens loses elasticity, making it difficult to become "thick" enough for near vision, even when the ciliary muscle contracts. * **Far Point:** For a normal emmetropic eye, the "far point" is infinity; at this distance, the ciliary muscle is fully relaxed.

Q: Which of the following stimulates the macula?

All of the above. The **maculae** are the sensory receptors located within the **otolith organs** (the Utricle and Saccule) of the inner ear. They are specifically designed to detect changes in static equilibrium and linear movement. ### **Explanation of the Correct Answer** The correct answer is **D (All of the above)** because the macula functions through the displacement of **otoconia** (calcium carbonate crystals) embedded in a gelatinous membrane. 1. **Gravity (Option A):** In a static position, gravity pulls on the otoconia, providing the brain with information about the head's orientation relative to the vertical axis. 2. **Head Position Change (Option B):** Tilting the head changes the angle of gravitational pull on the macula, causing the otolithic membrane to shift and stimulate hair cells. 3. **Linear Acceleration (Option C):** The inertia of the otoconia causes them to lag behind during horizontal (utricle) or vertical (saccule) movement, stimulating the hair cells. ### **Clinical Pearls & High-Yield Facts for NEET-PG** * **Utricle vs. Saccule:** The Utricle detects **horizontal** linear acceleration (e.g., a car moving forward), while the Saccule detects **vertical** linear acceleration (e.g., riding an elevator). * **Striola:** This is the central curved ridge in the macula. Hair cells are arranged in opposite directions on either side of the striola. * **BPPV Connection:** Benign Paroxysmal Positional Vertigo occurs when otoconia from the **utricular macula** become displaced into the semicircular canals (most commonly the posterior canal). * **Angular Acceleration:** Remember that **Angular (rotational) acceleration** is sensed by the **Crista Ampullaris** in the Semicircular Canals, NOT the macula. This is a common distractor in NEET-PG questions.

Q: Impulses generated in the taste buds of the tongue reach the cerebral cortex via which structure?

Thalamus. **Explanation:** The correct answer is **Thalamus**. In the sensory pathway for gustation (taste), the thalamus acts as the essential relay station before information reaches the primary gustatory cortex. **The Pathway:** 1. **First-order neurons:** Taste impulses from the tongue are carried by the Facial (CN VII), Glossopharyngeal (CN IX), and Vagus (CN X) nerves. 2. **Second-order neurons:** These fibers synapse in the **Nucleus Tractus Solitarius (NTS)** in the medulla. Axons from the NTS ascend via the central tegmental tract. 3. **Third-order neurons:** These are located in the **Ventral Posteromedial (VPM) nucleus of the Thalamus**. From here, impulses are projected to the gustatory cortex (Insula and frontal operculum). **Why other options are incorrect:** * **Internal capsule:** While taste fibers pass through the posterior limb of the internal capsule to reach the cortex, it is a white matter tract (conduit), not the relay structure where the synapse occurs. * **Cervical spinal nerve:** These carry general somatic sensation from the neck and back of the head, not special visceral afferents like taste. * **Trigeminal nerve:** This nerve carries general sensations (touch, pain, temperature) from the anterior 2/3rd of the tongue via the lingual nerve, but it does not carry taste impulses. **High-Yield NEET-PG Pearls:** * **VPM Nucleus:** Remember the mnemonic **"VPM = Very Polar Mouth"** (relays sensory/taste from the face/mouth). In contrast, the **VPL** relays sensations from the **L**imbs/body. * **Ageusia:** Loss of taste sensation. * **NTS:** Known as the "sensory nucleus" for all visceral afferents (Taste, Baroreceptors, Chemoreceptors).

Q: In near vision, what change occurs in the eye?

The depth of focus increases.. To understand the changes in the eye during near vision, we must look at the **Near Reflex triad**: Accommodation, Convergence, and Miosis. ### **Why the correct answer is right:** **B. The depth of focus increases:** During near vision, the pupils constrict (**Miosis**). A smaller pupil acts like a pinhole camera, reducing the size of the blur circles on the retina. This physical phenomenon increases the **depth of focus** (the range over which an object can be moved without appearing out of focus), allowing for sharper vision of near objects. ### **Why the other options are wrong:** * **A. The power of the lens decreases:** Incorrect. During accommodation, the ciliary muscle contracts, causing the lens to become more spherical (convex). This **increases** the refractive power of the lens to focus divergent rays from near objects onto the retina. * **C. The lateral recti contract:** Incorrect. To maintain single binocular vision at a near point, the eyes must turn inward. This requires the contraction of both **medial recti** (Convergence), not the lateral recti. * **D. The zonular tension increases:** Incorrect. When the ciliary muscle contracts for near vision, it moves forward and inward, which **relaxes the suspensory ligaments (zonules)**. This relaxation allows the elastic lens capsule to bulge and increase its curvature. ### **High-Yield Clinical Pearls for NEET-PG:** * **Accommodation Mechanism (Helmholtz Theory):** Ciliary muscle contraction → Zonular relaxation → Increased lens curvature (mostly anterior) → Increased dioptric power. * **Presbyopia:** A physiological loss of accommodation due to age-related decrease in lens elasticity and ciliary muscle efficiency. * **Argyll Robertson Pupil:** A classic neuro-ophthalmic condition where the **Light Reflex is lost but the Accommodation Reflex is present** (Prostitute's Pupil), often seen in neurosyphilis.

Q: Which body part is represented superiorly and medially within the postcentral gyrus?

Lower limb. ### Explanation The **postcentral gyrus** of the parietal lobe houses the **Primary Somatosensory Cortex (Brodmann areas 3, 1, and 2)**. The spatial distribution of body parts in this area follows a specific pattern known as the **Sensory Homunculus**. **1. Why the Lower Limb is Correct:** The sensory homunculus is an "upside-down" representation of the body. The **lower limb (leg and foot)** and the **perineum** are represented on the **medial surface** of the hemisphere and the most **superior (dorsal)** aspect of the postcentral gyrus. These areas are supplied by the **Anterior Cerebral Artery (ACA)**. **2. Analysis of Incorrect Options:** * **A. Upper Limb:** The hand and arm are represented on the **superolateral** aspect of the postcentral gyrus. The hand, particularly the thumb and fingers, occupies a disproportionately large area due to high receptor density. * **C. Abdomen:** The trunk and abdomen are located on the upper part of the **lateral surface**, inferior to the lower limb representation but superior to the upper limb. * **D. Genitalia:** While genitalia are also represented medially, they are located most inferiorly on the medial wall, often below the representation of the toes. In the context of "superior and medial," the lower limb is the primary landmark. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Supply:** A stroke involving the **Anterior Cerebral Artery (ACA)** leads to sensory and motor deficits primarily in the **contralateral lower limb**. In contrast, a **Middle Cerebral Artery (MCA)** stroke affects the face and upper limb. * **Disproportionate Representation:** The size of the cortical area is not proportional to the size of the body part, but to the **density of sensory receptors** (e.g., the lips and hands have the largest representation). * **Sequence (Medial to Lateral):** Toes → Foot → Leg → Hip → Trunk → Upper Limb → Face → Intra-abdominal.

Q: A person with defective blue color appreciation is called?

Tritanomalous. ### Explanation The question tests the nomenclature of color vision deficiencies based on the **Trichromatic Theory**. Human color vision relies on three photopigments (cones): Erythrolabe (Red/Long-wave), Chlorolabe (Green/Medium-wave), and Cyanolabe (Blue/Short-wave). **Why Tritanomalous is correct:** The suffix **"-anomalous"** refers to a deficiency or "weakness" in a specific color pigment (trichromacy is preserved, but one pigment is altered). The prefix **"Trit-"** refers to the third pigment, which is **Blue**. Therefore, a person with **Tritanomalous** vision has a defective appreciation of blue light due to an altered spectral sensitivity of the blue cones. **Analysis of Incorrect Options:** * **Deuteranomalous (A):** This refers to a "green weakness." It is the most common type of color vision deficiency. * **Deuteranopia (B):** The suffix **"-anopia"** indicates a complete absence of a pigment. Deuteranopia is the total absence of green-sensitive cones ("green blindness"). * **Tritanopia (C):** This is the total absence of blue-sensitive cones ("blue blindness"). While it involves the blue spectrum, the question specifies "defective appreciation" (weakness) rather than total absence. **NEET-PG High-Yield Pearls:** 1. **Inheritance:** Most color blindness (Red-Green) is **X-linked recessive**. However, **Tritan** defects (Blue-Yellow) are rare and typically **Autosomal Dominant** (encoded on Chromosome 7). 2. **Kollner’s Rule:** * Outer retinal diseases/macular degeneration usually cause **Blue-Yellow** defects. * Optic nerve diseases (except Glaucoma) usually cause **Red-Green** defects. 3. **Screening:** **Ishihara Charts** are the most common screening tool but are primarily designed to detect Red-Green defects, not Blue-Yellow. 4. **Gold Standard:** The **Nagel Anomaloscope** is the definitive test to differentiate between dichromacy and anomalous trichromacy.

Question 1

Synaptic transmission between pain fibers from the skin and spinal cord neurons is mediated by which of the following neurotransmitters?

Accepted Answer

Substance P

Answer

Acetylcholine

Answer

Endorphins

Answer

Somatostatin

Question 2

A 25-year-old student with 20/20 vision looks up from his book to view his girlfriend sitting on the other side of the room. Which of the following is most likely to occur when the student changes his view from his book to his girlfriend?

Accepted Answer

Thinner lens, relaxation of ciliary muscle

Answer

Thicker lens, contraction of ciliary muscle

Answer

Thicker lens, relaxation of ciliary muscle

Answer

Thinner lens, contraction of ciliary muscle

Question 3

Which of the following stimulates the macula?

Accepted Answer

All of the above

Answer

Gravity

Answer

Head position change

Answer

Linear acceleration

Question 4

Somatosensory area I has the largest representation for which body part?

Accepted Answer

Arm

Answer

Leg

Answer

Back

Answer

Head

Question 5

Stereoanesthesia is due to a lesion of which of the following?

Accepted Answer

Cerebral cortex

Answer

Nucleus Gracilis

Answer

Nucleus cuneatus

Answer

Spinothalamic tract

Question 6

Impulses generated in the taste buds of the tongue reach the cerebral cortex via which structure?

Accepted Answer

Thalamus

Answer

Internal capsule

Answer

Cervical spinal nerve

Answer

Trigeminal nerve

Question 7

In near vision, what change occurs in the eye?

Accepted Answer

The depth of focus increases.

Answer

The power of the lens decreases.

Answer

The lateral recti contract.

Answer

The zonular tension increases.

Question 8

Which body part is represented superiorly and medially within the postcentral gyrus?

Accepted Answer

Lower limb

Answer

Upper limb

Answer

Abdomen

Answer

Genitalia

Question 9

A person with defective blue color appreciation is called?

Accepted Answer

Tritanomalous

Answer

Deuteranomalous

Answer

Deuteranopia

Answer

Tritanopia

Question 10

Bilateral destruction of the auditory cortex in humans causes what type of hearing impairment?

Accepted Answer

Deficiency in the interpretation of sounds

Answer

Almost total deafness

Answer

Hearing defect for higher frequency sounds

Answer

Inability for the orientation of sounds

Sensory Systems — MCQs

Sensory Systems — MCQs

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