Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 31: What does 'phantoguesia' mean?

A. Decreased sensation of taste
B. Disruption of taste sensation
C. Increased sensation of taste
D. Alternation of taste sensation (Correct Answer)

Explanation: **Explanation:** **Phantogeusia** (derived from 'phantom' and 'geusia') refers to the perception of a taste sensation in the absence of any external stimulus. In the context of NEET-PG, it is classified as an **alteration of taste sensation**, specifically a gustatory hallucination. Patients often describe a persistent, usually unpleasant (bitter or metallic) taste despite having an empty mouth. **Analysis of Options:** * **Option D (Correct):** It is an **alteration** because the sensory perception does not align with reality. It is a qualitative disturbance rather than a quantitative one. * **Option A (Incorrect):** Decreased sensation of taste is termed **Hypogeusia**. * **Option B (Incorrect):** Disruption or distortion of an existing taste stimulus (e.g., everything tastes salty) is termed **Dysgeusia**. * **Option C (Incorrect):** Increased sensitivity to taste is termed **Hypergeusia**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Ageusia:** Total loss of taste sensation (commonly seen in COVID-19 or Zinc deficiency). 2. **Common Causes:** Phantogeusia is frequently associated with **Zinc deficiency**, temporal lobe epilepsy (as an aura), side effects of drugs (like Metronidazole or Clarithromycin), and burning mouth syndrome. 3. **Innervation:** Remember the **7, 9, 10 rule** for taste: Anterior 2/3 of tongue (CN VII via Chorda Tympani), Posterior 1/3 (CN IX), and Epiglottis/Pharynx (CN X). 4. **Pathway:** Taste fibers terminate in the **Nucleus Tractus Solitarius (NTS)** in the medulla.

Question 32: Pacinian corpuscles are?

A. Rapidly adapting receptors (Correct Answer)
B. Slowly adapting receptors
C. Fine touch receptors
D. None of the above

Explanation: **Explanation:** **Pacinian corpuscles** are large, encapsulated mechanoreceptors located deep in the dermis and subcutaneous tissues. They are the classic example of **Rapidly Adapting (Phasic) Receptors**. 1. **Why Option A is correct:** Pacinian corpuscles respond only at the onset and offset of a stimulus (the "on-off" response). When a constant pressure is applied, the fluid within the concentric lamellae redistributes, dissipating the energy and causing the receptor potential to cease despite the continued stimulus. This makes them specialized for detecting **vibration** (high frequency: 200–300 Hz) and rapid changes in mechanical displacement. 2. **Why other options are incorrect:** * **Option B:** Slowly adapting (Tonic) receptors, such as **Merkel discs** and **Ruffini endings**, continue to fire action potentials as long as a stimulus is maintained. They signal the duration and intensity of a stimulus. * **Option C:** Fine touch (tactile discrimination) is primarily the domain of **Meissner’s corpuscles** and **Merkel discs**, which have small receptive fields and are located superficially in the skin. Pacinian corpuscles have large receptive fields with poorly defined boundaries, making them unsuitable for fine spatial localization. **High-Yield Facts for NEET-PG:** * **Most Rapidly Adapting:** Pacinian corpuscles are the fastest adapting receptors in the body. * **Vibration Sensing:** They are the primary receptors for high-frequency vibration. (Note: Meissner’s corpuscles detect low-frequency vibration/flutter). * **Structure:** They consist of a central nerve terminal surrounded by multiple layers of fibroblasts and fluid (onion-like appearance). * **Receptive Field:** They have a **large receptive field**, meaning they can detect stimuli over a wide area but with low precision.

Question 33: The sensation of itch from the skin is carried by which type of nerve fiber?

A. A-alpha fibers
B. A-gamma fibers
C. C-nerve fibers (Correct Answer)
D. Central itch center

Explanation: **Explanation:** The sensation of **itch (pruritus)** is a complex sensory perception primarily mediated by specific subsets of **unmyelinated C-nerve fibers**. These fibers are slow-conducting and possess free nerve endings in the superficial layers of the skin (epidermis and dermo-epidermal junction). When chemical mediators like histamine or proteases are released, they activate "pruriceptors" on these C-fibers, which then transmit the signal to the dorsal horn of the spinal cord via the spinothalamic tract. **Analysis of Options:** * **C-nerve fibers (Correct):** These are small-diameter, unmyelinated fibers with the slowest conduction velocity (0.5–2 m/s). They are the primary carriers for both "slow pain" and itch. * **A-alpha fibers:** These are the largest, most heavily myelinated fibers. They carry information regarding proprioception and somatic motor function; they do not carry pain or itch. * **A-gamma fibers:** These are medium-sized myelinated fibers that specifically innervate the intrafusal fibers of muscle spindles (motor function), not sensory cutaneous stimuli. * **Central itch center:** While the brain (thalamus and somatosensory cortex) processes itch, the question asks for the **nerve fiber** responsible for carrying the sensation from the skin to the CNS. **High-Yield NEET-PG Pearls:** * **Histaminergic vs. Non-histaminergic:** Itch is categorized into histamine-mediated (common in urticaria) and non-histaminergic (common in chronic conditions like cholestasis or atopic dermatitis), both primarily using C-fibers. * **The Scratch-Itch Cycle:** Scratching relieves itch by activating **A-beta fibers** (mechanoreceptors), which trigger inhibitory interneurons in the spinal cord to "gate" the itch signal (Gate Control Theory). * **Specific Pathway:** Itch is transmitted via the **Lateral Spinothalamic Tract**, similar to pain and temperature.

Question 34: What is the function of the stapedius muscle?

A. To protect the ear from loud sounds. (Correct Answer)
B. To aid in hearing low-frequency sounds.
C. To aid in hearing high-frequency sounds.
D. To aid in hearing whispered words.

Explanation: The **stapedius muscle**, the smallest skeletal muscle in the human body, is the primary effector of the **Acoustic Reflex** (Tympanic Reflex). ### **Explanation of the Correct Answer** When the ear is exposed to high-intensity sounds (usually >70–80 dB), the stapedius muscle contracts reflexively. It is innervated by the **Stapedial branch of the Facial Nerve (CN VII)**. Upon contraction, it pulls the neck of the stapes posteriorly, tilting it and increasing the stiffness of the ossicular chain. This reduces the transmission of vibrational energy to the cochlea, thereby **protecting the delicate hair cells of the inner ear from noise-induced damage.** ### **Analysis of Incorrect Options** * **B & C:** The stapedius reflex actually **attenuates** sound transmission, particularly for low-frequency sounds (below 1000 Hz). It does not "aid" in hearing specific frequencies but rather filters out background noise to improve signal-to-noise ratios. * **D:** Whispered words are low-intensity sounds. The stapedius muscle remains relaxed during quiet sounds to allow maximum sensitivity of the ossicular chain. ### **NEET-PG High-Yield Pearls** * **Innervation:** Stapedius is supplied by **CN VII** (Facial Nerve), while the Tensor Tympani is supplied by **CN V3** (Mandibular Nerve). * **Hyperacusis:** Damage to the facial nerve proximal to the stapedial branch leads to paralysis of the stapedius. This results in **hyperacusis** (normal sounds appearing uncomfortably loud) because the protective dampening mechanism is lost. * **Latency:** The reflex has a latency of 40-160 ms, meaning it cannot protect the ear against sudden, impulsive sounds like explosions or gunshots.

Question 35: A lesion in the somatosensory cortex will affect which of the following functions?

A. Sensation of pain
B. Sensation of temperature
C. Localization of stimuli (Correct Answer)
D. Sensation of vibration

Explanation: **Explanation:** The **Somatosensory Cortex (S1)**, located in the postcentral gyrus (Brodmann areas 3, 1, and 2), is primarily responsible for the **processing and interpretation** of sensory information rather than its mere perception. **Why "Localization of stimuli" is correct:** The hallmark of the somatosensory cortex is its **somatotopic organization** (the Sensory Homunculus). While the thalamus can perceive the presence of a sensation, the cortex is required for **spatial discrimination** and precise localization (topognosis). A lesion here impairs the ability to pinpoint exactly where a stimulus is applied and disrupts complex discriminative tasks like stereognosis and two-point discrimination. **Why the other options are incorrect:** * **A & B (Pain and Temperature):** These are "crude" sensations. The **Thalamus** is the primary center for the conscious perception of pain and temperature. Even if the cortex is destroyed, a patient can still feel pain and heat/cold, though they cannot localize the source or judge the intensity accurately. * **D (Vibration):** While vibration is carried via the Dorsal Column-Medial Lemniscal (DCML) pathway to the cortex, basic perception of vibration can still occur at the subcortical level (thalamus). The cortex is more essential for the *fine-tuning* and *localization* of that vibration. **High-Yield Clinical Pearls for NEET-PG:** * **Thalamic Syndrome (Dejerine-Roussy):** Results in "thalamic pain"—exaggerated, agonizing pain on the contralateral side. * **Astereognosis:** The inability to identify an object by touch (e.g., a key in a pocket), a classic sign of a parietal/somatosensory cortex lesion. * **Sensory Homunculus:** Remember that the **lower limb** is represented on the medial surface (supplied by the Anterior Cerebral Artery), while the **face and upper limb** are on the lateral surface (Middle Cerebral Artery).

Question 36: Injury to cranial nerve VIII affects which of the following functions?

A. Taste
B. Equilibrium (Correct Answer)
C. Touch
D. Smell

Explanation: **Explanation:** **Cranial Nerve VIII (Vestibulocochlear Nerve)** is a purely sensory nerve consisting of two distinct components: the **vestibular division** and the **cochlear division**. 1. **Why Equilibrium is Correct:** The vestibular division originates from the semicircular canals, saccule, and utricle of the inner ear. It carries information regarding linear and angular acceleration (balance) to the brainstem. Therefore, an injury to CN VIII disrupts the transmission of these signals, leading to vertigo, loss of balance, and impaired **equilibrium**. The cochlear division handles hearing; thus, CN VIII is responsible for both hearing and balance. 2. **Why Other Options are Incorrect:** * **Taste (A):** Taste sensation is mediated by CN VII (anterior 2/3 of the tongue), CN IX (posterior 1/3), and CN X (epiglottis/pharynx). * **Touch (C):** General somatic sensation (touch, pain, temperature) from the face is primarily carried by the **Trigeminal Nerve (CN V)**. * **Smell (D):** Olfaction is the sole function of the **Olfactory Nerve (CN I)**. **High-Yield Clinical Pearls for NEET-PG:** * **Acoustic Neuroma (Vestibular Schwannoma):** A benign tumor of the Schwann cells of CN VIII, typically presenting with unilateral sensorineural hearing loss, tinnitus, and equilibrium issues. If it occurs bilaterally, it is a hallmark of **Neurofibromatosis Type 2 (NF2)**. * **Location:** CN VIII exits the brainstem at the **cerebellopontine (CP) angle** and enters the internal acoustic meatus along with CN VII. * **Testing:** Equilibrium is clinically assessed using the **Romberg Test** or caloric reflex testing (COWS).

Question 37: Endolymph most closely resembles which of the following body fluids?

A. Cerebrospinal fluid (CSF)
B. Intracellular fluid (ICF) (Correct Answer)
C. Extracellular fluid (ECF)
D. Plasma

Explanation: **Explanation:** The correct answer is **Intracellular fluid (ICF)**. The inner ear contains two distinct fluids: **perilymph** and **endolymph**. Endolymph is unique because, unlike most extracellular fluids in the body, it is characterized by a **high potassium (K⁺) concentration** and a **low sodium (Na⁺) concentration**. This ionic composition is chemically similar to the cytoplasm of a cell (Intracellular Fluid). The high potassium concentration in the endolymph is maintained by the **stria vascularis** in the cochlear duct. This creates a strong electrochemical gradient (the endocochlear potential of +80 mV) that allows K⁺ to flow into the hair cells during mechanotransduction, leading to depolarization. **Analysis of Incorrect Options:** * **A. Cerebrospinal fluid (CSF):** CSF is high in sodium and low in potassium. It is chemically similar to perilymph, not endolymph. * **C. Extracellular fluid (ECF):** Most ECFs (like interstitial fluid) are sodium-rich. Endolymph is an "atypical" ECF because its ionic profile mimics ICF. * **D. Plasma:** Plasma is a component of ECF with high sodium and protein content; it does not resemble the high-potassium environment of the endolymph. **NEET-PG High-Yield Pearls:** * **Endolymph:** High K⁺, Low Na⁺. Found in the **Scala Media** (membranous labyrinth). * **Perilymph:** High Na⁺, Low K⁺. Found in the **Scala Vestibuli** and **Scala Tympani**. It is continuous with CSF via the cochlear aqueduct. * **Meniere’s Disease:** Caused by "Endolymphatic Hydrops" (excess accumulation of endolymph), leading to the triad of vertigo, sensorineural hearing loss, and tinnitus.

Question 38: Which sensation is NOT affected in a lesion of the posterior column of the spinal cord?

A. Romberg's sign
B. Temperature sense (Correct Answer)
C. Vibration sense
D. Ataxia

Explanation: The **Posterior Column-Medial Lemniscus (PCML) pathway** is responsible for transmitting fine touch, vibration, pressure, and conscious proprioception. To answer this question, one must distinguish between the functions of the PCML and the **Anterolateral System (Spinothalamic Tract)**. ### 1. Why Temperature sense is the correct answer: Temperature and pain sensations are carried by the **Lateral Spinothalamic Tract**, not the posterior columns. These fibers decussate (cross over) at the level of the spinal cord within 1-2 segments of entry. Therefore, a lesion isolated to the posterior column will leave temperature and pain perception intact. ### 2. Analysis of Incorrect Options: * **Vibration sense:** This is a primary modality carried by the posterior columns (specifically via the fasciculus gracilis and cuneatus). Loss of vibration is often the earliest sign of posterior column involvement. * **Romberg’s sign:** This is a clinical test for **sensory ataxia**. When posterior columns are damaged, the patient loses conscious proprioception (position sense). They can maintain balance with eyes open (using visual input), but sway or fall when eyes are closed. Thus, a positive Romberg’s sign *is* a feature of posterior column lesions. * **Ataxia:** Specifically "sensory ataxia," which occurs because the brain lacks feedback regarding the position of limbs in space. This leads to a characteristic "stamping gait." ### 3. High-Yield Clinical Pearls for NEET-PG: * **Tabes Dorsalis:** A classic neurosyphilis manifestation involving bilateral destruction of posterior columns. * **Subacute Combined Degeneration (SCD):** Caused by Vitamin B12 deficiency; involves both the posterior columns and lateral corticospinal tracts. * **Rule of Thumb:** If the question mentions "Fine touch, Vibration, or Position," think **PCML**. If it mentions "Pain or Temperature," think **Spinothalamic Tract**.

Question 39: Static equilibrium is due to which structure?

A. Macula (Correct Answer)
B. Cupula
C. End organ of Corti
D. Cristae ampullae

Explanation: **Explanation:** The vestibular apparatus of the inner ear is responsible for maintaining equilibrium and posture. It consists of the semicircular canals and the otolith organs (utricle and saccule). **1. Why Macula is Correct:** The **Macula** is the sensory epithelium found within the **utricle and saccule**. It contains hair cells embedded in a gelatinous membrane weighted with calcium carbonate crystals called **otoconia**. These structures are sensitive to gravity and linear acceleration. Because they detect the orientation of the head relative to gravity when the body is stationary, they are the primary receptors for **static equilibrium**. **2. Why the other options are incorrect:** * **Cupula:** This is a gelatinous, cap-like structure located within the ampulla of the semicircular canals. It lacks otoconia and is displaced by endolymph movement during head rotation. * **Cristae ampullae:** These are the sensory organs located in the semicircular canals. They detect **angular (rotational) acceleration**, which is a form of dynamic equilibrium, rather than static equilibrium. * **End organ of Corti:** Located in the cochlea, this is the sensory organ for **hearing**, not equilibrium. **High-Yield NEET-PG Pearls:** * **Utricle:** Detects horizontal linear acceleration (e.g., moving in a car). * **Saccule:** Detects vertical linear acceleration (e.g., riding in an elevator). * **BPPV (Benign Paroxysmal Positional Vertigo):** Caused when otoconia from the macula become dislodged and enter the semicircular canals (most commonly the posterior canal). * **Scarpa’s Ganglion:** The vestibular ganglion where the first-order neurons for equilibrium are located.

Question 40: Melanopsin is present in which type of retinal cell?

A. Cones
B. Rods
C. Amacrine cells
D. Ganglion cells (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Ganglion cells**. **Underlying Medical Concept:** While rods and cones are the primary photoreceptors for vision, a small subset (approximately 1-3%) of retinal ganglion cells are intrinsically photosensitive. These are known as **intrinsically photosensitive Retinal Ganglion Cells (ipRGCs)**. They contain the photopigment **melanopsin**, a G-protein-coupled receptor. Unlike rods and cones, which provide image-forming vision, ipRGCs are primarily responsible for non-image-forming visual functions, most notably the **entrainment of circadian rhythms** via the suprachiasmatic nucleus (SCN) and the **pupillary light reflex**. **Analysis of Incorrect Options:** * **A & B (Cones and Rods):** These are the classical photoreceptors. Rods contain **rhodopsin** (for scotopic/night vision), and cones contain **photopsins** (for photopic/color vision). They do not contain melanopsin. * **C (Amacrine cells):** These are interneurons in the retina that modulate signals between bipolar cells and ganglion cells. They are involved in lateral inhibition and temporal signal processing but are not intrinsically photosensitive. **High-Yield Facts for NEET-PG:** * **Pathway:** ipRGCs transmit signals via the **retinohypothalamic tract** to the SCN (the master biological clock). * **Spectral Sensitivity:** Melanopsin is most sensitive to **blue light** (wavelength ~480 nm), which is why blue light exposure from screens significantly impacts sleep-wake cycles. * **Clinical Pearl:** Even in some patients with total blindness (loss of rods/cones), the pupillary light reflex and circadian synchronization may remain intact if the ipRGC layer is preserved.

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

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