Sensory Systems Practice Questions

Q: Stimulation of which brain area can modulate the sensation of pain?

Periaqueductal gray. **Explanation:** The **Periaqueductal Gray (PAG)**, located in the midbrain, is the primary control center for the **descending pain modulation system**. When stimulated, the PAG activates a descending pathway that inhibits pain transmission at the level of the spinal cord (the "Gate Control" mechanism). **Mechanism:** 1. Stimulation of the PAG activates the **Nucleus Raphe Magnus** (medulla). 2. These neurons release **Serotonin** onto the dorsal horn of the spinal cord. 3. This triggers the release of **Enkephalins** (endogenous opioids) from local interneurons. 4. Enkephalins cause pre- and post-synaptic inhibition of Type C and Type Aδ pain fibers, effectively "closing the gate" to pain signals. **Analysis of Incorrect Options:** * **A. Superior olivary complex:** Part of the **auditory pathway** in the pons; it is involved in sound localization. * **B. Locus ceruleus:** The primary site for **Norepinephrine** synthesis in the brain; while it has minor roles in stress and attention, it is not the primary modulator of pain compared to the PAG. * **D. Amygdala:** Part of the **limbic system**; it processes emotions (fear and aggression) rather than direct sensory modulation of pain. **High-Yield Clinical Pearls for NEET-PG:** * **Neurotransmitters:** The descending inhibitory pathway primarily uses **Serotonin** (from Raphe nuclei) and **Norepinephrine**. * **Opiate Receptors:** The PAG is densely packed with opioid receptors, which is why morphine and endogenous opioids are highly effective in this region. * **Surgical Application:** Deep brain stimulation (DBS) of the PAG is sometimes used for the management of chronic, intractable pain.

Q: A 10-year-old girl with type I diabetes develops a neuropathy limited to sensory neurons with free nerve endings. Quantitative sensory testing will reveal higher-than-normal thresholds for the detection of which of the following?

Temperature. **Explanation:** The core of this question lies in matching specific sensory receptors with the modalities they transmit. **Free nerve endings** are the most basic type of sensory receptor and are primarily responsible for detecting **pain (nociception)** and **temperature (thermal sensation)**. They are also involved in crude touch and itch. 1. **Why Temperature is Correct:** Thermoreceptors (for both cold and heat) are histologically classified as free nerve endings. If a neuropathy selectively affects these endings, the patient will require a stronger stimulus (a higher threshold) to perceive changes in temperature. 2. **Why Other Options are Incorrect:** * **Fine Touch (A):** This is primarily mediated by **Meissner’s corpuscles** (in non-hairy skin) and **Merkel discs**. These are encapsulated or specialized receptors, not free nerve endings. * **Vibration (B):** High-frequency vibration is detected by **Pacinian corpuscles**, while low-frequency vibration is detected by Meissner’s corpuscles. * **Pressure (C):** Deep pressure is sensed by **Pacinian corpuscles** and sustained pressure by **Ruffini endings**. **High-Yield NEET-PG Pearls:** * **A-delta fibers:** Thinly myelinated; carry "fast" pain and cold temperature. * **C fibers:** Unmyelinated; carry "slow" pain and warm temperature. Both terminate as free nerve endings. * **Large Myelinated Fibers (A-beta):** Carry vibration, proprioception, and fine touch. These are usually affected first in common diabetic "glove and stocking" neuropathy, but this specific question describes a selective "small fiber" involvement. * **Mnemonic:** **P**acinian = **P**ressure/vibration; **M**eissner = **M**oving touch/vibration.

Q: A colorblind male has decreased vision for red light, which appears very light compared to other colors. Which of the following is the likely anomaly in him?

Protanomaly. ### Explanation **1. Why Protanomaly is the Correct Answer:** Color vision is mediated by three types of cone pigments: erythrolabe (red), chlorolabe (green), and cyanolabe (blue). In **Protanomaly**, there is a functional but **defective red-cone pigment** (L-cone). Because the sensitivity of the red cones is shifted toward shorter wavelengths, red light appears dimmer, less saturated, and "lighter" or greyish compared to normal vision. This specific clinical presentation—dimness or decreased vision for red light—is the hallmark of protan defects. **2. Analysis of Incorrect Options:** * **B. Deuteranomaly:** This is the most common type of color blindness. It involves a **defective green-cone pigment** (M-cone). While it causes red-green confusion, it does not typically result in the "dimming" of red light seen in protanopes. * **C. Tritanomaly:** This is a rare condition involving a **defective blue-cone pigment** (S-cone). Patients have difficulty distinguishing between blue and yellow. * **D. Butanomaly:** This is a **fictitious term** and does not exist in medical literature. **3. Clinical Pearls for NEET-PG:** * **Terminology:** Suffix **"-anomaly"** means the pigment is present but defective (Trichromat); suffix **"-anopia"** means the pigment is completely absent (Dichromat). * **Prefixes:** *Prot-* (1st/Red), *Deuter-* (2nd/Green), *Trit-* (3rd/Blue). * **Inheritance:** Red-green color blindness (Protan/Deuteran) is **X-linked recessive**, making it significantly more common in males. Tritan defects are usually autosomal dominant. * **Diagnosis:** The **Ishihara Pseudoisochromatic Plate** test is the most common screening tool, though it primarily detects red-green defects. The **Nagel Anomaloscope** is the gold standard for distinguishing between dichromacy and anomalous trichromacy.

Q: What is true about visceral pain?

It is poorly localized.. ### Explanation **Correct Answer: A. It is poorly localized.** Visceral pain originates from internal organs and is characterized by its diffuse nature. This occurs because the density of sensory receptors in the viscera is significantly lower than in the skin. Furthermore, visceral afferent fibers from different organs often converge onto the same second-order neurons in the spinal cord (multisegmental innervation), making it difficult for the cerebral cortex to pinpoint the exact site of origin. **Analysis of Incorrect Options:** * **B. It resembles "fast pain":** Visceral pain is typically described as "slow pain" (dull, aching, or burning). Fast pain is sharp, pricking, and localized, characteristic of cutaneous (skin) injury. * **C. It is mediated by B fibers:** Visceral pain is primarily mediated by **unmyelinated C fibers** (slow pain) and some small myelinated **A-delta fibers**. B fibers are preganglionic autonomic fibers and do not transmit pain. * **D. It shows relatively rapid adaptation:** Pain receptors (nociceptors) are **non-adapting** or very slowly adapting. This is a protective mechanism; if pain adapted quickly, the body would ignore ongoing tissue damage. **High-Yield Clinical Pearls for NEET-PG:** 1. **Referred Pain:** Because visceral and somatic nociceptors converge on the same dorsal horn neurons (Dermatomal Rule), visceral pain is often felt in a somatic structure (e.g., Kehr’s sign: splenic rupture causing left shoulder pain). 2. **Stimuli for Visceral Pain:** Unlike the skin, the viscera are insensitive to cutting or burning. The primary triggers are **distension** (stretch), **ischemia**, **inflammation**, and **smooth muscle spasm**. 3. **Autonomic Association:** Severe visceral pain is frequently accompanied by autonomic responses like nausea, vomiting, and changes in blood pressure.

Q: Mydriasis is due to?

Contraction of radial muscles. **Explanation:** The size of the pupil is determined by the balance between two smooth muscles in the iris: the **sphincter pupillae** (circular muscle) and the **dilator pupillae** (radial muscle). **1. Why the correct answer is right:** **Mydriasis** refers to the dilation of the pupil. This occurs when the **radial muscles (dilator pupillae)** of the iris contract. These muscles are under **sympathetic control** (via alpha-1 adrenergic receptors). When they contract, they pull the inner edge of the iris outward, increasing the pupillary diameter to allow more light to enter the eye (the "fight or flight" response). **2. Why the incorrect options are wrong:** * **Relaxation of radial muscle:** This would lead to passive narrowing of the pupil, not dilation. * **Contraction of ciliary muscle:** The ciliary muscle controls the curvature of the lens for **accommodation**, not pupil size. Contraction of the ciliary muscle relaxes the suspensory ligaments, making the lens more globular for near vision. * **Relaxation of ciliary muscle:** This occurs during far vision to flatten the lens; it has no direct effect on pupillary diameter. **3. NEET-PG High-Yield Pearls:** * **Miosis (Pupillary Constriction):** Caused by the contraction of **circular muscles** (sphincter pupillae) via **parasympathetic** fibers (Cranial Nerve III). * **Nerve Supply:** Sympathetic supply for mydriasis originates from the **Ciliospinal center of Budge (C8-T2)**. * **Pharmacology Link:** * **Atropine** causes mydriasis by blocking parasympathetic action. * **Phenylephrine** causes mydriasis by stimulating alpha-1 receptors on radial muscles. * **Horner’s Syndrome:** Characterized by miosis due to loss of sympathetic supply to the radial muscles.

Q: What is the peak sensitivity of retinal rods and cones pigment in the most refractive eyes?

498 nm and 555 nm respectively. ### Explanation The human retina contains two types of photoreceptors—rods and cones—which contain photopigments sensitive to specific wavelengths of light. This question tests the fundamental physiological concept of **spectral sensitivity**. **1. Why Option D is Correct:** * **Rods:** These contain the pigment **Rhodopsin**, which is responsible for scotopic (night) vision. Its peak sensitivity is approximately **498 nm** (often rounded to 500 nm in some texts), which falls in the blue-green spectrum. * **Cones:** These are responsible for photopic (daylight) and color vision. While there are three types of cones (S, M, and L), the **overall peak sensitivity** of the combined cone system (photopic luminosity curve) is approximately **555 nm**, which corresponds to a yellowish-green hue. **2. Why Other Options are Incorrect:** * **Option A:** 260 nm is in the Ultraviolet (UV) range, which is largely absorbed by the cornea and lens to protect the retina. * **Option B:** 460 nm and 480 nm are in the blue spectrum. While S-cones (blue cones) peak near 420-440 nm, these values do not represent the peak for rods or the overall cone system. * **Option C:** 398 nm is near the violet/UV border, and 550 nm is close to the cone peak but the rod value is incorrect. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Purkinje Shift:** This is the shift in peak sensitivity from 555 nm (cones) to 505/498 nm (rods) as lighting conditions change from daylight to twilight. * **Vitamin A:** Retinal (the aldehyde form of Vitamin A) is the prosthetic group for all photopigments. Deficiency leads to **Nyctalopia** (night blindness) because rods are affected first. * **Tritanopia:** Rare blue-yellow color blindness (S-cone deficiency). * **Deuteranopia:** Most common form of color blindness (M-cone/green deficiency).

Q: Endolymph is secreted by which structure?

Stria vascularis. **Explanation:** The correct answer is **Stria vascularis**. **1. Why Stria Vascularis is Correct:** The stria vascularis is a highly vascularized layer of stratified epithelium located on the lateral wall of the **cochlear duct (scala media)**. It is the primary site for the production and secretion of **endolymph**. Unlike most extracellular fluids, endolymph is unique because it is rich in **Potassium (K+)** and low in Sodium (Na+), resembling intracellular fluid. The stria vascularis maintains this high positive potential (+80 mV, known as the endocochlear potential) which is essential for the transduction of sound by hair cells. **2. Why Other Options are Incorrect:** * **Basilar membrane:** This is a structural framework that supports the Organ of Corti. It vibrates in response to sound waves but has no secretory function. * **Cochlear duct:** While the endolymph is *contained* within the cochlear duct (scala media), the duct itself is a space. The specific structure within it that secretes the fluid is the stria vascularis. * **Hensen cells:** These are supportive cells located lateral to the outer hair cells in the Organ of Corti. They provide structural support but do not secrete endolymph. **3. High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Endolymph (High K+, Low Na+); Perilymph (High Na+, Low K+ — similar to ECF/CSF). * **Absorption:** Endolymph is drained/absorbed by the **endolymphatic sac**. * **Meniere’s Disease:** Caused by the distension of the membranous labyrinth due to overproduction or decreased absorption of endolymph (**Endolymphatic Hydrops**), leading to vertigo, tinnitus, and sensorineural hearing loss. * **Blood Supply:** The stria vascularis is supplied by the **labyrinthine artery** (usually a branch of the AICA).

Question 1

Stimulation of which brain area can modulate the sensation of pain?

Accepted Answer

Periaqueductal gray

Answer

Superior olivary complex

Answer

Locus ceruleus

Answer

Amygdala

Question 2

A 10-year-old girl with type I diabetes develops a neuropathy limited to sensory neurons with free nerve endings. Quantitative sensory testing will reveal higher-than-normal thresholds for the detection of which of the following?

Accepted Answer

Temperature

Answer

Fine touch

Answer

Vibration

Answer

Pressure

Question 3

Joint position sense in the left lower limb is impaired in all EXCEPT which of the following?

Accepted Answer

Left thalamic lesion

Answer

Left dorsal column lesion in the thoracic region

Answer

Left cerebellar lesion

Answer

Right lenticulostriate artery thrombosis

Question 4

A colorblind male has decreased vision for red light, which appears very light compared to other colors. Which of the following is the likely anomaly in him?

Accepted Answer

Protanomaly

Answer

Dutanomaly

Answer

Tritanomaly

Answer

Butanomaly

Question 5

During accommodation, what changes occur in the lens?

Accepted Answer

Reduction in the radius of curvature of the anterior surface of the lens only

Answer

Reduction in the radius of curvature of the posterior surface of the lens only

Answer

Increase in the radius of curvature of the posterior surface of the lens only

Answer

Both the radius of curvature of the anterior and posterior surfaces reduce

Question 6

What is true about visceral pain?

Accepted Answer

It is poorly localized.

Answer

It resembles "fast pain" produced by noxious stimulation of the skin.

Answer

It is mediated by B fibers in the dorsal root of the spinal nerve.

Answer

It shows relatively rapid adaptation.

Question 7

Mydriasis is due to?

Accepted Answer

Contraction of radial muscles

Answer

Relaxation of ciliary muscle

Answer

Relaxation of radial muscle

Answer

Contraction of ciliary muscle

Question 8

What is the peak sensitivity of retinal rods and cones pigment in the most refractive eyes?

Accepted Answer

498 nm and 555 nm respectively

Answer

260 nm and 500 nm respectively

Answer

460 nm and 480 nm respectively

Answer

398 nm and 550 nm respectively

Question 9

Which of the following is the basis for referred pain?

Accepted Answer

Some visceral pain signals and pain signals from the skin provide convergent input to a common set of neurons in the dorsal horn

Answer

Visceral pain signals and pain signals from the skin synapse with separate populations of neurons in the dorsal horn

Answer

Visceral pain transmission and pain transmission from the skin are received by a common set of neurons in the thalamus

Answer

Visceral pain signals are rarely of sufficient magnitude to exceed the threshold of activation of dorsal horn neurons

Question 10

Endolymph is secreted by which structure?

Accepted Answer

Stria vascularis

Answer

Basilar membrane

Answer

Cochlear duct

Answer

Hensen cell

Sensory Systems — MCQs

Sensory Systems — MCQs

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