Sensory Systems Practice Questions

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

C-nerve fibers. **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.

Q: What is the function of the stapedius muscle?

To protect the ear from loud sounds.. 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.

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

Localization of stimuli. **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).

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

Equilibrium. **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).

Q: What is the primary sensory function of the Pacinian corpuscle?

Vibration. **Explanation:** The **Pacinian corpuscle** is a rapidly adapting (phasic) mechanoreceptor located deep in the dermis and subcutaneous tissue. Its primary sensory function is the detection of **vibration** (specifically high-frequency vibration between 60–400 Hz) and rapid changes in mechanical displacement. **Why Vibration is Correct:** The unique "onion-like" lamellated structure of the Pacinian corpuscle acts as a mechanical filter. When a constant pressure is applied, the fluid between the lamellae redistributes, quickly neutralizing the stimulus. However, it responds vigorously to the onset and offset of pressure. This makes it exquisitely sensitive to repetitive, oscillating stimuli—clinically perceived as vibration. **Analysis of Incorrect Options:** * **A. Crude touch:** This is primarily mediated by **Meissner’s corpuscles** (fine touch) and free nerve endings. These signals are carried via the anterior spinothalamic tract. * **B. Deep pressure:** While Pacinian corpuscles respond to the *initiation* of pressure, **Ruffini endings** are the primary receptors for sustained deep pressure and skin stretch due to their slow-adapting nature. * **C. Pricking pain:** This is mediated by **A-delta fibers** (fast pain) and free nerve endings, not encapsulated mechanoreceptors. **High-Yield Clinical Pearls for NEET-PG:** * **Adaptation:** Pacinian corpuscles are the **fastest adapting** mechanoreceptors in the body. * **Pathway:** Vibration and proprioception are carried via the **Dorsal Column-Medial Lemniscal (DCML) pathway**. * **Clinical Testing:** Vibration sense is often the first modality lost in peripheral neuropathies (e.g., Diabetes Mellitus) and Vitamin B12 deficiency (Subacute Combined Degeneration of the spinal cord). It is tested using a **128 Hz tuning fork**.

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

Intracellular fluid (ICF). **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.

Q: Static equilibrium is due to which structure?

Macula. **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.

Q: Melanopsin is present in which type of retinal cell?

Ganglion cells. **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.

Q: Which of the following statements is NOT true regarding M cells?

Color contrast. In the visual pathway, the retinal ganglion cells are divided into two primary functional pathways: the **Magnocellular (M) pathway** and the **Parvocellular (P) pathway**. ### Why "Color Contrast" is the Correct Answer The **M cells** (Magnocellular) are specialized for detecting motion and depth. They are **color-blind** because they receive input from both rods and cones without discriminating between wavelengths. **Color contrast** and fine detail are the primary functions of **P cells** (Parvocellular), which are smaller and concentrated in the fovea. Therefore, stating that M cells handle color contrast is incorrect. ### Analysis of Other Options * **A. Larger cells:** This is true. "Magno" means large. M cells have larger cell bodies, thicker axons, and faster conduction velocities compared to P cells. * **C. High luminous contrast:** This is true. M cells are highly sensitive to low levels of light and small differences in luminance (brightness), making them ideal for vision in low-light conditions. * **D. Low spatial frequency:** This is true. Because M cells have large receptive fields, they are not good at resolving fine details (high spatial frequency) but are excellent at detecting global shapes and rapid changes (low spatial frequency). ### NEET-PG High-Yield Pearls * **M Cells (Magno):** "Where" pathway. Responsible for **M**otion, **M**omentum, and **M**onochrome vision. They project to the ventral two layers of the Lateral Geniculate Nucleus (LGN). * **P Cells (Parvo):** "What" pathway. Responsible for **P**oint-to-point detail (acuity) and **P**hoto-chromatic (color) vision. They project to the dorsal four layers of the LGN. * **K Cells (Konio):** Located between layers of the LGN; primarily involved in blue-yellow color vision.

Q: In dark adaptation, what occurs?

Pupillary dilatation. **Explanation:** Dark adaptation is the process by which the eyes increase their sensitivity to low light levels after moving from a bright environment to a dark one. This involves both neural and chemical changes. **Why Option C is Correct:** The immediate reflex response to entering a dark room is **pupillary dilatation (mydriasis)**. This is mediated by the sympathetic nervous system, which causes the dilator pupillae muscle to contract. Dilatation increases the amount of light entering the eye, allowing more photons to reach the retina to initiate the visual signal. **Analysis of Incorrect Options:** * **Option A & B:** While visual acuity (sharpness) is lower in the dark because we switch from foveal (cone) vision to peripheral (rod) vision, the *process* of dark adaptation is designed to **increase** light perception, not decrease it. The goal is to lower the visual threshold. * **Option D:** In the dark, **rhodopsin is resynthesized (regenerated)**, not broken down. Rhodopsin breakdown (bleaching) occurs upon exposure to light. The regeneration of rhodopsin is the slow phase of dark adaptation, taking about 20–30 minutes to reach maximum sensitivity. **NEET-PG High-Yield Pearls:** * **Biphasic Curve:** Dark adaptation follows a biphasic curve; the first 5–10 minutes represent **cone adaptation**, followed by the slower but more sensitive **rod adaptation**. * **Vitamin A:** Retinal (a derivative of Vitamin A) is essential for rhodopsin regeneration. Deficiency leads to **Nyctalopia** (night blindness), which is the failure of dark adaptation. * **Purkinje Shift:** During dark adaptation, the peak sensitivity of the eye shifts from longer wavelengths (yellow-green, 555 nm) to shorter wavelengths (blue-green, 505 nm).

Question 1

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

Accepted Answer

C-nerve fibers

Answer

A-alpha fibers

Answer

A-gamma fibers

Answer

Central itch center

Question 2

What is the function of the stapedius muscle?

Accepted Answer

To protect the ear from loud sounds.

Answer

To aid in hearing low-frequency sounds.

Answer

To aid in hearing high-frequency sounds.

Answer

To aid in hearing whispered words.

Question 3

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

Accepted Answer

Localization of stimuli

Answer

Sensation of pain

Answer

Sensation of temperature

Answer

Sensation of vibration

Question 4

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

Accepted Answer

Equilibrium

Answer

Taste

Answer

Touch

Answer

Smell

Question 5

What is the primary sensory function of the Pacinian corpuscle?

Accepted Answer

Vibration

Answer

Crude touch

Answer

Deep pressure

Answer

Pricking pain

Question 6

Endolymph most closely resembles which of the following body fluids?

Accepted Answer

Intracellular fluid (ICF)

Answer

Cerebrospinal fluid (CSF)

Answer

Extracellular fluid (ECF)

Answer

Plasma

Question 7

Static equilibrium is due to which structure?

Accepted Answer

Macula

Answer

Cupula

Answer

End organ of Corti

Answer

Cristae ampullae

Question 8

Melanopsin is present in which type of retinal cell?

Accepted Answer

Ganglion cells

Answer

Cones

Answer

Rods

Answer

Amacrine cells

Question 9

Which of the following statements is NOT true regarding M cells?

Accepted Answer

Color contrast

Answer

Larger cells

Answer

High luminous contrast

Answer

Low spatial frequency

Question 10

In dark adaptation, what occurs?

Accepted Answer

Pupillary dilatation

Answer

Decreased perception of vision

Answer

Decreased visual acuity

Answer

Breakdown of rhodopsin

Sensory Systems — MCQs

Sensory Systems — MCQs

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