Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 261: Mitral and periglomerular cells are seen in which of the following structures?

A. Olfactory bulb (Correct Answer)
B. Primary visual cortex
C. Geniculate body
D. Medulla

Explanation: **Explanation:** The correct answer is **A. Olfactory bulb**. The olfactory bulb is the primary relay station for the sense of smell. It contains a complex arrangement of neurons organized into layers. The **Mitral cells** are the primary output neurons of the olfactory bulb; their dendrites receive input from olfactory sensory neurons in structures called **glomeruli**, and their axons form the olfactory tract. **Periglomerular cells** are inhibitory interneurons located around the glomeruli that modulate the signal transmission between sensory neurons and mitral cells, enhancing odor discrimination through lateral inhibition. **Analysis of Incorrect Options:** * **B. Primary Visual Cortex:** Located in the occipital lobe (Brodmann area 17), it is characterized by **Stellate and Pyramidal cells**. It features the "Stria of Gennari," but does not contain mitral cells. * **C. Geniculate Body:** The Lateral Geniculate Body (vision) and Medial Geniculate Body (hearing) are part of the thalamus. They consist of **magnocellular and parvocellular layers**, not glomerular structures. * **D. Medulla:** Contains various nuclei (e.g., Nucleus Tractus Solitarius, Dorsal Motor Nucleus of Vagus), but the specific architecture of mitral and periglomerular cells is unique to the olfactory system. **High-Yield NEET-PG Pearls:** * **Tufted cells** are another type of output neuron in the olfactory bulb, similar to mitral cells but located more superficially. * **Granule cells** are the most numerous cells in the olfactory bulb; they are GABAergic interneurons that lack axons. * The olfactory system is unique because it is the **only sensory pathway** that reaches the cerebral cortex without a mandatory relay in the thalamus. * **Anosmia** (loss of smell) is an early clinical marker in neurodegenerative diseases like Parkinson’s and Alzheimer’s.

Question 262: Auditory cortex is present in which area?

A. Area 41 (Correct Answer)
B. Area 42
C. Area 44
D. Area 48

Explanation: ### Explanation The **Auditory Cortex** is located in the temporal lobe, specifically on the superior temporal gyrus (Heschl’s gyri). In Brodmann’s classification, the auditory system is divided into primary and secondary areas. **1. Why Area 41 is Correct:** **Brodmann Area 41** is the **Primary Auditory Cortex (AI)**. It receives direct input from the medial geniculate body (MGB) of the thalamus via the auditory radiations. It is organized tonotopically (mapping of different sound frequencies) and is essential for the conscious perception of sound. **2. Analysis of Incorrect Options:** * **Area 42:** This is the **Secondary Auditory Cortex (AII)**. While it is involved in processing auditory information, it serves as an association area that surrounds Area 41. In the context of "The Auditory Cortex" in standard medical exams, Area 41 is the primary designation. * **Area 44:** This corresponds to **Broca’s Area** (specifically the pars opercularis) located in the frontal lobe. It is responsible for motor speech production, not auditory perception. * **Area 48:** This is the **Retrosubicular area**, part of the hippocampal formation involved in memory and navigation, having no direct role in primary auditory processing. **3. High-Yield Clinical Pearls for NEET-PG:** * **Location:** Heschl’s gyri (Superior Temporal Gyrus). * **Thalamic Relay:** The **M**edial Geniculate Body is for **M**usic (Auditory), whereas the **L**ateral Geniculate Body is for **L**ight (Visual). * **Wernicke’s Area:** Located in **Area 22**; essential for comprehension of speech. Damage leads to sensory aphasia (fluent but meaningless speech). * **Unilateral Lesion:** A lesion of one primary auditory cortex does not cause deafness because auditory pathways are **bilateral**; it primarily results in difficulty localizing sound.

Question 263: Which of the following types of nerve fibers carry pain?

A. A alpha
B. A beta
C. A gamma
D. A delta (Correct Answer)

Explanation: ### Explanation Pain sensation is transmitted to the central nervous system via two specific types of primary afferent fibers: **A-delta (Aδ)** and **C fibers**. **1. Why A-delta is correct:** A-delta fibers are thin, myelinated axons that conduct impulses at a moderate velocity (6–30 m/s). They are responsible for **"fast pain"**—the sharp, pricking, and well-localized sensation felt immediately after an injury. They primarily respond to mechanical and thermal stimuli. (Note: C fibers, which are unmyelinated, carry "slow pain" characterized by dull, aching, and poorly localized sensations). **2. Why the other options are incorrect:** * **A-alpha (Aα):** These are the largest and fastest fibers. They primarily carry **proprioception** (from muscle spindles and Golgi tendon organs) and somatic motor impulses to skeletal muscles. * **A-beta (Aβ):** These large, myelinated fibers carry sensations of **fine touch, pressure, and vibration**. According to the "Gate Control Theory," stimulation of Aβ fibers can actually inhibit pain transmission in the spinal cord. * **A-gamma (Aγ):** These are motor fibers that innervate the **intrafusal fibers** of the muscle spindle, regulating muscle tone and sensitivity. **Clinical Pearls & High-Yield Facts:** * **Erlanger-Gasser Classification:** Fibers are classified by diameter and velocity (A > B > C). * **Susceptibility:** * **Hypoxia:** Affects Type A fibers first. * **Pressure:** Affects Type A fibers first (e.g., "Saturday Night Palsy"). * **Local Anesthetics:** Affect Type C fibers first (smallest diameter). * **Neospinothalamic tract:** Pathway for fast pain (Aδ). * **Paleospinothalamic tract:** Pathway for slow pain (C fibers).

Question 264: All of the following are concerned with the auditory pathway, except:

A. Trapezoid body
B. Medial geniculate body
C. Genu of internal capsule (Correct Answer)
D. Lateral lemniscus

Explanation: The auditory pathway is a complex multisynaptic circuit extending from the cochlea to the primary auditory cortex. To answer this question, one must trace the flow of auditory information: **Why "Genu of internal capsule" is the correct answer:** The auditory radiation (the final relay in the pathway) passes through the **sublentiform part of the posterior limb** of the internal capsule to reach the Superior Temporal Gyrus (Heschl’s gyrus). The **Genu** of the internal capsule contains **corticobulbar fibers** (motor fibers to cranial nerve nuclei) and is not involved in the transmission of sensory auditory signals. **Analysis of incorrect options (Auditory structures):** * **Trapezoid Body:** Located in the lower pons, this consists of decussating fibers from the ventral cochlear nuclei. it is the first site where binaural localization occurs. * **Lateral Lemniscus:** This is the primary ascending tract of the auditory system in the brainstem, carrying fibers from the superior olivary complex and cochlear nuclei to the inferior colliculus. * **Medial Geniculate Body (MGB):** Located in the thalamus, the MGB acts as the "thalamic relay station" for hearing. It receives input from the inferior colliculus and projects to the auditory cortex. **High-Yield NEET-PG Pearls:** * **Mnemonic for Auditory Pathway:** **E.C.O.L.I. M.A.** (**E**xternal ear, **C**ochlear nuclei, **O**livary complex (superior), **L**ateral lemniscus, **I**nferior colliculus, **M**edial geniculate body, **A**uditory cortex). * **Inferior Colliculus** is the center for auditory reflexes, while the **Superior Colliculus** is for visual reflexes. * **MGB vs. LGB:** Remember **M**edial for **M**usic (Auditory) and **L**ateral for **L**ight (Visual).

Question 265: Which vitamin deficiency causes spinocerebellar degeneration?

A. Vitamin A
B. Vitamin D
C. Vitamin E (Correct Answer)
D. Vitamin B12

Explanation: **Explanation:** **Vitamin E (Alpha-tocopherol)** is a potent lipid-soluble antioxidant that protects neuronal membranes from oxidative damage caused by free radicals. The **posterior columns** (responsible for vibration and position sense) and the **spinocerebellar tracts** (responsible for coordination) are particularly sensitive to oxidative stress. Deficiency leads to a clinical syndrome characterized by progressive ataxia, loss of deep tendon reflexes, and loss of proprioception, mimicking the neurological presentation of Friedreich’s Ataxia. **Analysis of Incorrect Options:** * **Vitamin A:** Deficiency primarily affects the eyes (night blindness, xerophthalmia) and epithelial integrity, but does not cause spinal cord degeneration. * **Vitamin D:** Deficiency leads to Rickets (children) and Osteomalacia (adults) due to impaired calcium and phosphate metabolism, affecting bone health rather than sensory tracts. * **Vitamin B12:** While B12 deficiency causes **Subacute Combined Degeneration (SCD)** of the spinal cord, it primarily involves the posterior columns and **lateral corticospinal tracts** (upper motor neuron signs). While ataxia occurs, the classic association with isolated spinocerebellar degeneration in the context of fat malabsorption points toward Vitamin E. **High-Yield Clinical Pearls for NEET-PG:** * **Etiology:** Vitamin E deficiency is usually secondary to **fat malabsorption** (e.g., Celiac disease, Cystic Fibrosis, Abetalipoproteinemia). * **Key Finding:** Look for **hemolytic anemia** alongside neurological symptoms in Vitamin E deficiency (due to increased fragility of RBC membranes). * **Differential:** If a question mentions "megaloblastic anemia + neurological signs," choose **B12**. If it mentions "malabsorption + ataxia + normal hematocrit/hemolysis," think **Vitamin E**.

Question 266: Umami taste is due to:

A. Glutamate (Correct Answer)
B. Sodium
C. H+
D. K+

Explanation: **Explanation:** The word **Umami** is derived from Japanese, meaning "delicious" or "savory." It represents one of the five primary taste modalities and is triggered specifically by **L-glutamate** (an amino acid) and certain nucleotides like guanylate and inosinate. **1. Why Glutamate is Correct:** The sensation of Umami is mediated by G-protein coupled receptors (GPCRs), specifically the **T1R1 + T1R3** heterodimer receptor. When glutamate binds to these receptors, it activates a second messenger cascade (PLC-β2 and IP3), leading to the release of intracellular calcium and subsequent depolarization. This taste is characteristic of protein-rich foods, MSG (monosodium glutamate), and aged cheeses. **2. Analysis of Incorrect Options:** * **Option B (Sodium):** Responsible for **Salty** taste. It acts via ENaC (Epithelial Sodium Channels) which allow direct influx of Na+ ions into the taste cell. * **Option C (H+):** Responsible for **Sour** taste. Protons (H+) act by entering through OTOP1 channels or by blocking K+ channels, leading to depolarization. * **Option D (K+):** While potassium salts can sometimes taste bitter or salty, K+ is not a primary trigger for a specific taste modality; rather, the closure of K+ channels is a mechanism involved in sour and bitter transduction. **High-Yield Facts for NEET-PG:** * **Taste Receptors:** * GPCRs: Sweet (T1R2+T1R3), Bitter (T2R family), Umami (T1R1+T1R3). * Ion Channels: Salty (ENaC), Sour (OTOP1). * **Agraphia/Ageusia:** Loss of taste sensation. * **Innervation:** Anterior 2/3 of tongue (Chorda tympani - CN VII); Posterior 1/3 (Glossopharyngeal - CN IX); Epiglottis/Pharynx (Vagus - CN X). * **Primary Gustatory Cortex:** Located in the **Insula** and frontal operculum.

Question 267: Proprioception is lost when which part of the spinal cord is injured?

A. Posterior column (Correct Answer)
B. Lateral column
C. Anterior column
D. None of the above

Explanation: The **Posterior Column-Medial Lemniscus (PCML) pathway** is the primary sensory tract responsible for conveying conscious proprioception (position sense), fine touch, vibration, and two-point discrimination. ### Why Posterior Column is Correct: The posterior column (composed of the **Fasciculus Gracilis** and **Fasciculus Cuneatus**) contains the central processes of first-order neurons. These fibers ascend ipsilaterally until they reach the medulla. Because these specific modalities are anatomically localized to the dorsal (posterior) aspect of the spinal cord, any injury to this region results in a loss of proprioception and vibration sense below the level of the lesion. ### Why Other Options are Incorrect: * **Lateral Column:** This region primarily contains the **Lateral Spinothalamic Tract** (pain and temperature) and the **Lateral Corticospinal Tract** (motor control). While the dorsal spinocerebellar tract (unconscious proprioception) is located here, "proprioception" in clinical exams typically refers to the conscious position sense carried by the posterior columns. * **Anterior Column:** This area houses the **Anterior Spinothalamic Tract** (crude touch and pressure) and the **Anterior Corticospinal Tract**. Injury here does not affect proprioception. ### NEET-PG High-Yield Pearls: * **Romberg’s Test:** A positive Romberg sign (swaying when eyes are closed) indicates a loss of conscious proprioception (sensory ataxia), often due to posterior column damage. * **Tabes Dorsalis:** A classic neurosyphilis manifestation characterized by the destruction of posterior columns. * **Vitamin B12 Deficiency:** Causes Subacute Combined Degeneration (SCD), which involves both the **Posterior Columns** and **Lateral Columns**. * **Brown-Séquard Syndrome:** Hemisection of the cord results in **ipsilateral** loss of proprioception and **contralateral** loss of pain/temperature.

Question 268: Which of the following is not a test for the integrity of the 9th cranial nerve?

A. Gag reflex
B. Palate symmetry
C. Taste
D. Tongue protrusion (Correct Answer)

Explanation: The 9th cranial nerve (Glossopharyngeal nerve) is a mixed nerve responsible for sensory, motor, and parasympathetic functions. Understanding its distribution is key to differentiating it from other cranial nerves. **Why Tongue Protrusion is the Correct Answer:** Tongue protrusion is a test for the **12th cranial nerve (Hypoglossal nerve)**. The hypoglossal nerve provides motor supply to all intrinsic and extrinsic muscles of the tongue, except the palatoglossus (supplied by the Vagus nerve). When the 12th nerve is damaged, the tongue deviates toward the side of the lesion upon protrusion. **Explanation of Incorrect Options:** * **Gag Reflex:** The Glossopharyngeal nerve provides the **sensory (afferent) limb** of the gag reflex, while the Vagus nerve (CN X) provides the motor (efferent) limb. * **Palate Symmetry:** While primarily a test for the Vagus nerve (CN X), the Glossopharyngeal nerve contributes to the pharyngeal plexus. In clinical practice, the elevation of the palate and uvula position are used to assess the integrity of the lower cranial nerves (IX and X) together. * **Taste:** The Glossopharyngeal nerve carries taste sensations from the **posterior 1/3rd of the tongue**. (Note: The anterior 2/3rd is supplied by the Chorda tympani branch of the Facial nerve). **High-Yield Clinical Pearls for NEET-PG:** * **Glossopharyngeal Neuralgia:** Characterized by paroxysmal, severe pain in the throat, ear, and base of the tongue, often triggered by swallowing. * **Secretomotor Function:** CN IX provides parasympathetic supply to the **parotid gland** via the otic ganglion. * **Sensory Supply:** It supplies the carotid body and carotid sinus (chemoreceptors and baroreceptors).

Question 269: What causes the depolarization of the cochlea?

A. Potassium influx (Correct Answer)
B. Potassium efflux
C. Sodium influx
D. Sodium efflux

Explanation: **Explanation:** In the auditory system, the mechanism of depolarization is unique compared to most other excitable cells. The correct answer is **Potassium (K⁺) influx**. 1. **Mechanism of Depolarization:** The hair cells of the cochlea are bathed in two different fluids. The apical portion (stereocilia) is submerged in **endolymph**, which is unusually rich in K⁺ and has a high positive potential (+80 mV). When sound waves cause the stereocilia to bend toward the tallest kinocilium, mechanically gated K⁺ channels (MET channels) open. Because the concentration of K⁺ is higher in the endolymph than inside the hair cell, K⁺ flows **into** the cell down its electrochemical gradient, causing depolarization. 2. **Why other options are incorrect:** * **Potassium efflux:** In most neurons, K⁺ leaving the cell causes repolarization. In hair cells, K⁺ efflux occurs at the *base* of the cell (into the perilymph) to reset the membrane potential, but it does not cause the initial depolarization. * **Sodium influx/efflux:** While Na⁺ influx causes depolarization in typical nerve and muscle cells, the endolymph is Na⁺-poor. Therefore, sodium does not play a primary role in the receptor potential of cochlear hair cells. **High-Yield NEET-PG Pearls:** * **Endolymph** resembles intracellular fluid (High K⁺, Low Na⁺) and is secreted by the **Stria Vascularis**. * **Perilymph** resembles extracellular fluid (High Na⁺, Low K⁺). * The potential difference between endolymph and hair cells is approximately **150 mV**, the highest transepithelial potential difference in the body. * **Endocochlear Potential:** The +80 mV charge of the endolymph is essential for hearing; its loss leads to deafness.

Question 270: Which of the following statements is true regarding the Helmholtz theory of accommodation?

A. Ciliary muscles relax.
B. Zonules contract.
C. Ciliary muscles move posteriorly.
D. There is a decrease in the equatorial diameter of the lens. (Correct Answer)

Explanation: The **Helmholtz Theory (Capsular Theory)** is the most widely accepted mechanism for ocular accommodation. It describes how the eye increases its refractive power to focus on near objects. ### **Mechanism of Accommodation (The Correct Answer)** When focusing on a near object, the **ciliary muscles contract**. This contraction moves the ciliary body **anteriorly and inward** (towards the lens). This movement reduces the tension on the **suspensory ligaments (zonules)**. With the zonular tension relaxed, the inherent elasticity of the lens capsule allows the lens to become more **spherical (convex)**. As the lens bulges to become more "round," its **equatorial diameter decreases**, while its anteroposterior diameter increases, thereby increasing its dioptric power. ### **Analysis of Incorrect Options** * **A & B: Ciliary muscles relax / Zonules contract:** During accommodation, the ciliary muscle **contracts**, which paradoxically **slackens (relaxes)** the zonules. Zonules are passive fibers; they do not "contract" like muscles. * **C: Ciliary muscles move posteriorly:** In Helmholtz’s model, the ciliary muscle moves **anteriorly and centripetally** to release tension on the lens. Posterior movement would increase tension, flattening the lens (far vision). ### **High-Yield Clinical Pearls for NEET-PG** * **Presbyopia:** Age-related loss of accommodation due to decreased lens elasticity and hardening of the lens nucleus (sclerosis), making it difficult for the lens to become spherical even when zonules relax. * **Innervation:** Accommodation is mediated by **Parasympathetic fibers** via the **3rd Cranial Nerve (Oculomotor)**, originating from the **Edinger-Westphal nucleus**. * **The Near Triad:** Accommodation occurs simultaneously with **Miosis** (pupillary constriction) and **Convergence** of the eyeballs.

Practice by Chapter

General Sensory Physiology

Practice Questions

Somatosensation

Practice Questions

Pain Physiology

Practice Questions

Vision and Optics

Practice Questions

Retinal Physiology

Practice Questions

Visual Pathways and Processing

Practice Questions

Auditory System

Practice Questions

Vestibular System

Practice Questions

Taste and Smell

Practice Questions

Sensory Integration

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free