Sensory Systems Practice Questions

Q: Posterior dorsal columns transmit which of the following sensory modalities?

Fine touch and vibration. The **Dorsal Column-Medial Lemniscal (DCML) pathway** is a major sensory tract responsible for transmitting highly localized and discriminative sensations. ### 1. Why Option A is Correct The DCML pathway carries "epicritic" sensations. These include: * **Fine Touch (Tactile Discrimination):** Ability to localize touch and perform two-point discrimination. * **Vibration:** Detected by Pacinian corpuscles. * **Conscious Proprioception:** Sense of position and movement of body parts. * **Stereognosis:** Ability to identify objects by touch. These fibers are large, myelinated, and fast-conducting. They ascend ipsilaterally in the spinal cord (Fasciculus Gracilis and Cuneatus) and decussate in the medulla. ### 2. Why Other Options are Incorrect * **Option B (Pain and Temperature):** These are transmitted by the **Lateral Spinothalamic Tract**. * **Option C (Crude Touch):** This is transmitted by the **Anterior Spinothalamic Tract**. Together with pain/temperature, these form the Anterolateral System. * **Option D (Unconscious Proprioception):** This is transmitted to the cerebellum via the **Spinocerebellar Tracts** (Dorsal and Ventral). ### 3. High-Yield Clinical Pearls for NEET-PG * **Tabes Dorsalis:** A late stage of neurosyphilis that specifically involves the destruction of dorsal columns, leading to sensory ataxia and a positive **Romberg’s sign**. * **Brown-Séquard Syndrome:** In a spinal cord hemisection, there is **ipsilateral** loss of dorsal column sensations (fine touch/vibration) and **contralateral** loss of pain/temperature. * **Somatotopy:** In the dorsal columns, fibers from the lower limbs are medial (**Fasciculus Gracilis**), while fibers from the upper limbs are lateral (**Fasciculus Cuneatus**). Remember: *"Gracilis is Ground (legs)."*

Q: Which of the following best describes the "blind spot" of the eye?

The exit point of the optic nerve. **Explanation:** The **blind spot**, or **physiological scotoma**, corresponds to the **Optic Disc**. This is the specific anatomical site where the axons of the retinal ganglion cells converge to form the **optic nerve (Cranial Nerve II)** and exit the eyeball. **1. Why the Correct Answer is Right:** At the optic disc, there is a complete absence of photoreceptors (both rods and cones). Because no light-sensitive cells exist here, any light falling on this area cannot be transduced into neural signals, resulting in a "blind" area in the visual field. **2. Why the Other Options are Wrong:** * **Option A:** The blind spot is located approximately **15 degrees lateral (temporal)** to the central point of vision (fixation point). It is not 5 degrees. * **Option C:** The blind spot contains **no photoreceptors** at all. The area containing only one type of photoreceptor (densely packed cones) is the **fovea centralis**, which is responsible for maximum visual acuity. * **Option D:** The optic disc is actually the entry and exit point for the **central retinal artery and vein**. Therefore, it is highly vascularized, not devoid of vessels. **High-Yield Facts for NEET-PG:** * **Location:** The optic disc is located **medial (nasal)** to the fovea centralis in the retina, but it projects to the **lateral (temporal)** visual field. * **Clinical Correlation (Papilledema):** Increased intracranial pressure (ICP) causes swelling of the optic disc, known as papilledema. On perimetry, this manifests as an **enlargement of the blind spot**. * **Glaucoma:** Pathological "cupping" of the optic disc is a hallmark of glaucomatous optic neuropathy.

Q: Slow pain in nerves is carried by which nerve fibers?

C fibers. ### Explanation **Correct Answer: D. C fibers** **Understanding the Concept:** Pain perception is transmitted via two distinct pathways based on the type of nerve fiber involved. **Slow pain** (also described as dull, aching, burning, or chronic pain) is transmitted by **Type C fibers**. These are small-diameter, **unmyelinated** axons with slow conduction velocities (0.5–2.0 m/s). Because they lack myelin, the signal travels significantly slower than in myelinated fibers. Slow pain is typically poorly localized and felt after the initial sharp sensation. **Analysis of Incorrect Options:** * **B. Delta (Aδ) fibers:** These are responsible for **fast pain** (sharp, pricking, acute pain). They are small, thinly myelinated fibers that conduct impulses much faster (6–30 m/s) than C fibers, allowing for rapid localization of injury. * **A. Alpha (Aα) fibers:** These are the thickest and fastest myelinated fibers. they primarily carry **proprioception** and somatic motor signals, not pain. * **C. Beta (Aβ) fibers:** These are large, myelinated fibers that carry sensations of **fine touch, pressure, and vibration**. According to the "Gate Control Theory," stimulation of Aβ fibers can actually inhibit pain transmission from C fibers in the spinal cord. **High-Yield Clinical Pearls for NEET-PG:** * **Double Pain Sensation:** A single painful stimulus (like a stubbed toe) often produces a "double" sensation: a sharp, immediate flash (Aδ fibers) followed a second later by a throbbing ache (C fibers). * **Neurotransmitters:** Fast pain (Aδ) primarily uses **Glutamate** (rapid action), while slow pain (C fibers) uses **Substance P** (prolonged action). * **Termination:** Aδ fibers terminate in Laminae I and V of the dorsal horn; C fibers terminate primarily in **Laminae II and III (Substantia Gelatinosa)**. * **Sensitivity:** C fibers are the most sensitive to local anesthetics, while Aα fibers are the most sensitive to pressure/hypoxia.

Q: Light touch on a patient's face causes severe pain. This is best described as?

Allodynia. **Explanation:** The correct answer is **Allodynia**. **1. Why Allodynia is correct:** Allodynia is defined as the perception of pain resulting from a stimulus that does not normally provoke pain. In this clinical scenario, "light touch" (a non-noxious stimulus) is perceived as "severe pain." This occurs due to central sensitization, where low-threshold Aβ fibers (which normally carry touch) begin to activate the pain pathways in the spinal cord or trigeminal nucleus. This is a hallmark feature of neuropathic pain conditions, such as Trigeminal Neuralgia. **2. Why other options are incorrect:** * **Paresthesia:** Refers to abnormal sensations (like "pins and needles") that are not necessarily painful and can occur spontaneously without an external stimulus. * **Dysesthesia:** An unpleasant, abnormal sensation (either spontaneous or evoked) that is always disagreeable but does not specifically require the stimulus to be non-noxious. It is a broader term than allodynia. * **Peripheral neuropathy:** This is a general clinical diagnosis referring to damage to peripheral nerves, which can *cause* symptoms like allodynia, but it is not the descriptive term for the symptom itself. **3. High-Yield NEET-PG Pearls:** * **Hyperalgesia:** An exaggerated or increased response to a stimulus that is *normally* painful (e.g., a pinprick feeling like a knife stab). * **Hyperpathia:** An explosive, painful reaction to a repetitive stimulus, often with an increased threshold. * **Mechanism:** Allodynia involves **Aβ fibers**, whereas hyperalgesia primarily involves sensitized **C-fibers** and **Aδ fibers**. * **Clinical Correlation:** Allodynia is frequently tested in the context of Post-herpetic neuralgia and Migraine (cutaneous allodynia).

Q: Vibration sense is carried by which of the following structures?

Pacinian corpuscles. **Explanation:** The correct answer is **Pacinian corpuscles**. Vibration sense (pallesthesia) is mediated by specialized mechanoreceptors that are sensitive to high-frequency mechanical stimuli. **Pacinian corpuscles** are rapidly adapting (RA II) receptors located deep in the dermis and subcutaneous tissue. They have a large receptive field and are specifically tuned to detect high-frequency vibrations (ranging from 60–400 Hz, with peak sensitivity around 250 Hz). When a vibratory stimulus is applied, the concentric lamellae of the Pacinian corpuscle deform, triggering an action potential in the sensory nerve fiber. **Analysis of Incorrect Options:** * **Meckel’s disks:** These are slowly adapting (SA I) receptors located in the basal layer of the epidermis. They are responsible for detecting static touch, pressure, and vertical indentation (e.g., feeling the edges of an object). * **Ruffini corpuscle end-organs:** These are slowly adapting (SA II) receptors found in the deep dermis. They respond primarily to skin stretch and joint rotation. * **Free nerve endings:** These are non-specialized endings that primarily mediate pain (nociception) and temperature (thermoreception). **Clinical Pearls for NEET-PG:** * **Pathway:** Vibration sense is carried via the **Dorsal Column-Medial Lemniscal (DCML) pathway**. * **Clinical Testing:** Vibration is often the first sense lost in peripheral neuropathies (e.g., Diabetic neuropathy) and is tested using a **128 Hz tuning fork** over bony prominences. * **Meissner’s Corpuscles:** These also detect vibration, but at much lower frequencies (30–40 Hz), often described as "flutter." * **Rule of Thumb:** If the question specifies "high-frequency vibration," the answer is always Pacinian corpuscles.

Q: What is the neurotransmitter employed by rods and cones?

Glutamate. **Explanation:** The correct answer is **Glutamate**. In the retina, photoreceptors (rods and cones) are specialized neuroepithelial cells that maintain a constant release of the excitatory neurotransmitter **glutamate** in the dark. **Why Glutamate is Correct:** Photoreceptors exhibit a unique physiological behavior called the "dark current." In the absence of light, rods and cones are relatively depolarized (approx. -40 mV), leading to the continuous release of glutamate onto bipolar cells. When light strikes the retina, it triggers a G-protein coupled cascade (involving transducin and phosphodiesterase) that closes sodium channels, causing **hyperpolarization**. This hyperpolarization *decreases* the release of glutamate. Thus, the signal for light is actually a reduction in glutamate secretion. **Analysis of Incorrect Options:** * **A. Dopamine:** In the retina, dopamine is primarily secreted by **amacrine cells** and interplexiform cells. it plays a role in light adaptation and circadian rhythms, but it is not the primary transmitter for photoreceptors. * **B. Serotonin:** While found in some invertebrate visual systems, it is not the neurotransmitter for mammalian rods and cones. * **D. Nitric Oxide:** This acts as a retrograde gaseous signaling molecule in various parts of the CNS but does not serve as the primary neurotransmitter for the first synapse in the visual pathway. **High-Yield NEET-PG Pearls:** * **The Visual Paradox:** Photoreceptors are the only sensory receptors that **hyperpolarize** in response to an adequate stimulus (light). * **Bipolar Cells:** Glutamate has different effects depending on the receptor: it inhibits **On-center** bipolar cells (via mGluR6 receptors) and excites **Off-center** bipolar cells (via AMPA/Kainate receptors). * **Vitamin A:** Retinal, a derivative of Vitamin A, is the prosthetic group of rhodopsin; its deficiency leads to Nyctalopia (night blindness).

Question 1

In humans, auditory receptors act by a phenomenon called 'Masking'. Which of the following best describes the mechanism related to this phenomenon?

Accepted Answer

Relative or absolute refractoriness

Answer

All or none

Answer

Beneficial effect

Answer

Post-tetanic potentiation

Question 2

Posterior dorsal columns transmit which of the following sensory modalities?

Accepted Answer

Fine touch and vibration

Answer

Pain and temperature

Answer

Crude touch

Answer

Unconscious proprioception

Question 3

Which of the following best describes the "blind spot" of the eye?

Accepted Answer

The exit point of the optic nerve

Answer

Located 5 degrees lateral to the central point of vision

Answer

Contains only rods and thus has monochromatic vision

Answer

Contains no blood vessels

Question 4

Slow pain in nerves is carried by which nerve fibers?

Accepted Answer

C fibers

Answer

Alpha fibers

Answer

Delta fibers

Answer

Beta fibers

Question 5

Light touch on a patient's face causes severe pain. This is best described as?

Accepted Answer

Allodynia

Answer

Paresthesia

Answer

Dysesthesia

Answer

Peripheral neuropathy

Question 6

Vibration sense is carried by which of the following structures?

Accepted Answer

Pacinian corpuscles

Answer

Meckel's disks

Answer

Ruffini corpuscle end-organs

Answer

Free nerve endings

Question 7

What is the consequence of removing one side of the auditory cortex?

Accepted Answer

Decreased sound localization

Answer

Total hearing loss

Answer

Mild decrease in hearing

Answer

Decreased sound interpretation

Question 8

When stereocilia of an auditory hair cell are deflected in the appropriate direction, potassium channels open in the apical membrane of the cell and?

Accepted Answer

Potassium ions flow into the cell, depolarizing the cell.

Answer

Potassium ions flow out of the cell, hyperpolarizing the cell.

Answer

Potassium ions flow out of the cell, depolarizing the cell.

Answer

Potassium ions flow into the cell, hyperpolarizing the cell.

Question 9

What is the neurotransmitter employed by rods and cones?

Accepted Answer

Glutamate

Answer

Dopamine

Answer

Serotonin

Answer

Nitric oxide

Question 10

Direction of sound is differentiated by which structure?

Accepted Answer

Inferior colliculus

Answer

Auditory cortex

Answer

Medial geniculate body

Answer

Lateral geniculate body

Sensory Systems — MCQs

Sensory Systems — MCQs

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