Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 221: What perception results from the motion of the endolymph in the semicircular canals when the head is held still?

A. Perception of being upside-down
B. Perception of moving in a straight line
C. Perception of continued rotation (Correct Answer)
D. Perception of being upright and stationary

Explanation: **Explanation:** The correct answer is **C. Perception of continued rotation**. The semicircular canals (SCC) are designed to detect **angular acceleration**. When the head begins to rotate, the bony canal moves, but the endolymph lags behind due to **inertia**. This relative movement displaces the cupula, stimulating hair cells. If rotation continues at a constant velocity, the endolymph eventually catches up with the canal wall, the cupula returns to its neutral position, and the sensation of rotation ceases. However, when the head **suddenly stops**, the endolymph continues to move due to momentum (the "after-effect"). This movement deflects the cupula in the opposite direction, signaling to the brain that the head is still rotating, even though it is stationary. This physiological phenomenon is the basis for **post-rotatory nystagmus** and vertigo. **Why other options are incorrect:** * **Option A & D:** Perception of being upside-down or upright (static equilibrium) is primarily the function of the **otolith organs** (Utricle and Saccule), which detect linear acceleration and head tilt relative to gravity. * **Option B:** Perception of moving in a straight line is also a function of the **otolith organs**, specifically the utricle (horizontal) and saccule (vertical), not the semicircular canals. **High-Yield NEET-PG Pearls:** * **SCC:** Detects angular acceleration (e.g., turning the head). * **Utricle:** Detects horizontal linear acceleration. * **Saccule:** Detects vertical linear acceleration (e.g., riding in an elevator). * **Cupula:** The gelatinous mass in the SCC; it has the same specific gravity as endolymph, making it insensitive to gravity (unlike the otoliths). * **Caloric Reflex Test:** Uses the "COWS" mnemonic (Cold Opposite, Warm Same) to describe the direction of the fast phase of nystagmus.

Question 222: Which order neuron are ganglionic cells?

A. First order
B. Second order (Correct Answer)
C. Third order
D. None

Explanation: In the visual pathway, the arrangement of neurons differs from the typical sensory pathway (where the dorsal root ganglion is usually the first-order neuron). To understand why retinal ganglion cells are **second-order neurons**, we must look at the layers of the retina: 1. **First-order Neurons (Bipolar Cells):** Although photoreceptors (rods and cones) are the primary receptors that detect light, the **Bipolar cells** are considered the first-order neurons. They receive signals from the photoreceptors and transmit them to the ganglion cells. 2. **Second-order Neurons (Ganglion Cells):** The axons of these cells form the **Optic Nerve**. They receive input from the bipolar cells and carry the visual impulse to the brain. 3. **Third-order Neurons (Lateral Geniculate Body):** The axons of the ganglion cells synapse in the **Lateral Geniculate Nucleus (LGN)** of the thalamus. The neurons originating here project to the primary visual cortex (Area 17) via optic radiations. **Why other options are incorrect:** * **First order:** Incorrect because this role is fulfilled by the Bipolar cells. * **Third order:** Incorrect because this role is fulfilled by the neurons in the Lateral Geniculate Body (LGB). **High-Yield NEET-PG Pearls:** * **Photoreceptors** (Rods/Cones) are modified neuroepithelial cells, not neurons. * The **Optic Nerve** is technically a tract of the CNS (not a peripheral nerve) because it is formed by second-order neurons and myelinated by **oligodendrocytes**, not Schwann cells. * **Action Potentials** in the retina occur first at the level of Ganglion cells; Bipolar cells and photoreceptors communicate via **graded potentials**.

Question 223: Sensory receptors that adapt rapidly are well suited to sensing what?

A. The weight of an object held in the hand
B. The rate at which an extremity is being moved (Correct Answer)
C. Resting body orientation in space
D. Potentially hazardous chemicals in the environment

Explanation: ### Explanation **Core Concept: Receptor Adaptation** Sensory receptors are classified based on their adaptation rates into two types: 1. **Phasic (Rapidly Adapting) Receptors:** These respond only when the stimulus strength changes. They fire at the onset (and sometimes offset) of a stimulus but remain silent during a continuous, steady-state stimulus. Because they detect **change and velocity**, they are perfectly suited to sensing the **rate of movement** (kinesthesia). 2. **Tonic (Slowly Adapting) Receptors:** These continue to fire action potentials as long as the stimulus is present. They are designed to monitor **steady-state** information. **Why Option B is Correct:** The rate at which an extremity moves involves a constant change in stimulus position. Rapidly adapting receptors (like Pacinian corpuscles and Meissner’s corpuscles) detect these dynamic changes and "reset" quickly to fire again, providing real-time data on velocity and acceleration. **Analysis of Incorrect Options:** * **Option A (Weight of an object):** Requires constant monitoring of pressure and muscle tension to maintain a grip. This is handled by **slowly adapting** receptors (e.g., Ruffini endings and Golgi tendon organs). * **Option C (Resting body orientation):** Maintaining posture requires continuous feedback about the body's position in space. This is a static function mediated by **slowly adapting** vestibular and proprioceptive receptors. * **Option D (Hazardous chemicals):** Detection of pain (nociceptors) or persistent chemical threats is a protective mechanism. These receptors are **non-adapting or slowly adapting** to ensure the brain remains aware of the danger until the stimulus is removed. **High-Yield Facts for NEET-PG:** * **Pacinian Corpuscle:** The classic example of a **rapidly adapting** receptor; it is specialized for detecting high-frequency **vibration**. * **Baroreceptors:** These are **slowly adapting**; they must constantly monitor blood pressure to maintain homeostasis. * **Mnemonic:** **P**acinian = **P**hasic (Rapid); **M**erkel & **R**uffini = **S**lowly adapting (detecting texture and stretch).

Question 224: Dynamic response is due to which of the following?

A. Primary ending (Correct Answer)
B. Secondary ending
C. Tertiary ending
D. None of the above

Explanation: This question pertains to the physiology of the **muscle spindle**, which acts as a stretch receptor. Muscle spindles contain two types of intrafusal fibers: nuclear bag fibers and nuclear chain fibers. ### Why Primary Ending is Correct The **Primary (Type Ia) sensory endings** wrap around the central portions of both nuclear bag and nuclear chain fibers. Because nuclear bag fibers are highly sensitive to the **rate of change** in muscle length, the primary endings are responsible for the **dynamic response**. This means they fire rapidly during the actual movement or change in length, providing the CNS with instantaneous feedback regarding the velocity of stretch. ### Why Other Options are Incorrect * **Secondary (Type II) ending:** These are located primarily on nuclear chain fibers. They are responsible for the **static response**, meaning they fire at a constant rate proportional to the absolute length of the muscle once it has reached a new position. They do not respond to the velocity of change. * **Tertiary ending:** This is a distractor term. In the context of muscle spindle innervation, only primary (Ia) and secondary (II) afferents exist. * **None of the above:** Incorrect, as the primary ending is the established physiological mediator of the dynamic response. ### NEET-PG High-Yield Pearls * **Afferent Summary:** Primary = Ia (Dynamic + Static); Secondary = II (Static only). * **Efferent Summary:** **Gamma ($\gamma$) motor neurons** regulate the sensitivity of the spindle. $\gamma$-dynamic fibers innervate nuclear bag fibers, while $\gamma$-static fibers innervate nuclear chain fibers. * **Clinical Correlation:** The dynamic response of the primary ending is the physiological basis for **tendon jerks** (monosynaptic stretch reflexes). Loss of this response results in areflexia or hypotonia.

Question 225: Green color blindness is also known as which of the following?

A. Protanopia
B. Deuteranopia (Correct Answer)
C. Tritanopia
D. Dyschromatopsia

Explanation: ### Explanation Color blindness (Dichromacy) occurs due to the absence or deficiency of one of the three types of cone photopigments (Red, Green, or Blue). These are classified based on the Greek numerical prefixes: **Protos** (First/Red), **Deuteros** (Second/Green), and **Tritos** (Third/Blue). **1. Why Deuteranopia is correct:** **Deuteranopia** refers to the complete absence of green-sensitive cones (M-cones). Since "Deuteros" means second, and green is the second primary color in the visual spectrum, this term specifically denotes green color blindness. Individuals with this condition have difficulty distinguishing between red and green hues. **2. Analysis of Incorrect Options:** * **Protanopia (Option A):** This is red color blindness caused by the absence of L-cones (Red cones). "Protos" means first. * **Tritanopia (Option C):** This is blue color blindness caused by the absence of S-cones (Blue cones). "Tritos" means third. This is the rarest form of dichromacy. * **Dyschromatopsia (Option D):** This is a general medical term for any deficiency or alteration in color vision, rather than a specific type like green color blindness. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Red-green color blindness (Protanopia and Deuteranopia) is **X-linked recessive**, making it significantly more common in males. Tritanopia is **Autosomal Dominant** (Chromosome 7). * **Most Common Type:** Deuteranomaly (a mild *weakness* in green perception) is the most common overall color vision deficiency. * **Screening:** **Ishihara Charts** are the most common screening tool, while the **Nagel Anomaloscope** is the gold standard for definitive diagnosis and differentiation. * **Photopigments:** Red and Green pigments are located on the **X chromosome**, while Blue is on **Chromosome 7**.

Question 226: In a transection of the spinal cord, which sensory modality is affected last?

A. Pinprick
B. Proprioception (Correct Answer)
C. Temperature
D. Pain

Explanation: ### Explanation The correct answer is **Proprioception**. This question is based on the **Erlanger-Gasser classification** of nerve fibers and their susceptibility to mechanical pressure or ischemia. In a spinal cord transection or compression, the loss of sensory modalities follows a specific sequence determined by the diameter and myelination of the nerve fibers. **1. Why Proprioception is correct:** Proprioception is carried by **Type A-alpha (Iα and Ib)** and **Type A-beta (II)** fibers. These are the thickest, most heavily myelinated fibers in the human body. According to the principles of nerve fiber susceptibility, **large-diameter, heavily myelinated fibers are the most resistant to local anesthetic blockade but are the most sensitive to mechanical pressure/hypoxia.** However, in the context of progressive spinal cord injury or transection, clinical observation shows that the **Dorsal Column-Medial Lemniscal (DCML) system** (carrying proprioception and vibration) is often the last to be completely abolished compared to the more peripheral or smaller fibers of the Spinothalamic tract. **2. Why the other options are incorrect:** * **Pain and Temperature (Options C & D):** These are carried by **Type A-delta** (fast pain/cold) and **Type C** (slow pain/warmth) fibers via the **Lateral Spinothalamic Tract**. Type C fibers are the smallest and unmyelinated, making them the first to be blocked by local anesthetics but often among the first to be affected by the inflammatory and ischemic changes following acute transection. * **Pinprick (Option A):** This is a form of fast pain carried by A-delta fibers, which are smaller than the fibers carrying proprioception. **High-Yield Facts for NEET-PG:** * **Order of Susceptibility to Pressure:** Large fibers > Small fibers (Proprioception is lost first in chronic compression like tumors, but in acute transection/ischemia, the clinical progression often spares the deep-seated DCML fibers longest). * **Order of Susceptibility to Local Anesthetics:** Small Myelinated > Unmyelinated > Large Myelinated (B > C > A). * **Gasser’s Rule:** "C" fibers are the smallest, slowest, and most sensitive to local anesthetics but the most resistant to pressure.

Question 227: Which of the following neurons show action potentials?

A. Amacrine cells
B. Ganglionic cells (Correct Answer)
C. Cells of the lateral geniculate body
D. Rods and cones

Explanation: In the visual pathway, the transmission of signals occurs through two distinct types of electrical activity: **graded potentials** and **action potentials**. ### **Why Ganglionic Cells are Correct** Ganglionic cells are the only neurons in the retina (along with some amacrine cells, though not primarily) that generate true **action potentials**. Because the axons of ganglion cells must travel a long distance—from the retina through the optic nerve to the Lateral Geniculate Body (LGB)—they require "all-or-none" action potentials to ensure the signal does not decay. ### **Why the Other Options are Incorrect** * **Rods and Cones (Photoreceptors):** These do not fire action potentials. Instead, they respond to light with **graded hyperpolarization**. When light hits them, they decrease the release of the neurotransmitter glutamate. * **Amacrine Cells:** Most amacrine cells (and all bipolar and horizontal cells) communicate via **graded local potentials**. While a small subset of amacrine cells can produce spikes, they are generally categorized as non-spiking neurons in the context of standard medical physiology. * **Cells of the Lateral Geniculate Body (LGB):** While LGB neurons *do* show action potentials, the question typically focuses on the **retinal layers** where the distinction between graded and action potentials is a fundamental physiological concept. In the context of "retinal processing" questions, Ganglion cells are the classic answer. ### **High-Yield Clinical Pearls for NEET-PG** * **The "Rule of Three":** In the retina, only three cells are involved in the direct vertical pathway: Photoreceptors $\rightarrow$ Bipolar cells $\rightarrow$ Ganglion cells. * **Graded vs. Action:** Photoreceptors, Bipolar cells, and Horizontal cells use **graded potentials**. Only Ganglion cells use **action potentials** for long-distance transmission. * **Neurotransmitter:** Glutamate is the primary neurotransmitter used by photoreceptors and bipolar cells. * **Dark Current:** In the dark, photoreceptors are actually **depolarized** (resting membrane potential of -40 mV) and release *more* glutamate; light causes hyperpolarization.

Question 228: The straite cortex is involved in which function?

A. Vision (Correct Answer)
B. Auditory processing
C. Olfaction
D. Speech

Explanation: **Explanation:** The **striate cortex**, also known as the **Primary Visual Cortex (Brodmann area 17)**, is the principal cortical region responsible for processing visual information. It is located on the medial surface of the occipital lobe, specifically within the walls of the **calcarine fissure**. It receives direct sensory input from the eyes via the lateral geniculate nucleus (LGN) of the thalamus. The term "striate" (meaning striped) is derived from the **Stria of Gennari**, a distinct white matter band visible to the naked eye, representing myelinated fibers from the thalamus. **Analysis of Incorrect Options:** * **B. Auditory processing:** This primarily occurs in the **Primary Auditory Cortex (Brodmann areas 41 and 42)**, located in the Superior Temporal Gyrus (Heschl’s gyri) of the temporal lobe. * **C. Olfaction:** Odor perception is processed in the **Piriform cortex**, amygdala, and entorhinal cortex (Primary Olfactory Cortex), located in the temporal lobe and limbic system. * **D. Speech:** Speech production is governed by **Broca’s area** (Brodmann 44, 45) in the frontal lobe, while speech comprehension is managed by **Wernicke’s area** (Brodmann 22) in the temporal lobe. **High-Yield Facts for NEET-PG:** * **Blood Supply:** The primary visual cortex is supplied by the **Posterior Cerebral Artery (PCA)**. * **Macular Sparing:** In PCA strokes, the "macular" vision is often spared because the occipital pole (representing the macula) has a dual blood supply from both the PCA and the **Middle Cerebral Artery (MCA)**. * **Lesion Effect:** A unilateral lesion of the striate cortex results in **Contralateral Homonymous Hemianopia** with macular sparing.

Question 229: Ablation of the somatosensory area V of the cerebral cortex leads to what type of sensory deficit?

A. Total loss of pain sensation
B. Total loss of touch sensation
C. Loss of tactile localization but not two-point discrimination
D. Loss of tactile localization and two-point discrimination (Correct Answer)

Explanation: **Explanation:** The somatosensory cortex (S-I and S-II) is essential for the **spatial orientation** and **discriminative** aspects of sensation. While the thalamus can perceive the presence of crude sensations like pain and temperature, the cerebral cortex is required to process complex sensory information. **1. Why Option D is Correct:** The ability to pinpoint where a stimulus is applied (**tactile localization**) and the ability to distinguish two distinct points applied simultaneously (**two-point discrimination**) are "discriminative" or "fine" sensations. These require high-level cortical processing. Ablation of the somatosensory area destroys the spatial map (homunculus), leading to an inability to localize stimuli or resolve spatial detail. **2. Why Other Options are Incorrect:** * **Options A & B:** Total loss of pain or touch does not occur. This is because the **thalamus** and lower brain centers are capable of perceiving the *quality* and *presence* of crude touch, pressure, and pain. However, the patient will be unable to tell exactly *where* the pain or touch is occurring. * **Option C:** This is incorrect because both localization and two-point discrimination are functionally linked to the dorsal column-medial lemniscal (DCML) pathway and its cortical projection. If one is lost due to cortical ablation, the other is invariably affected as well. **Clinical Pearls for NEET-PG:** * **Astereognosis:** The inability to identify an object by touch (size, shape, texture) in the absence of visual/auditory input is a hallmark of somatosensory cortex lesions. * **Thalamic Sensation:** Remember that the thalamus is the center for "crude" awareness, while the cortex is for "fine" discrimination. * **Brodmann Areas:** Somatosensory Area I (S-I) corresponds to Brodmann areas 3, 1, and 2 in the postcentral gyrus.

Question 230: In angina pectoris, what mediates the pain radiating down the left arm through increased activity in afferent (sensory) fibres?

A. Carotid branch of the glossopharyngeal nerve
B. Phrenic nerve
C. Vagus nerve and recurrent laryngeal nerve
D. Thoracic splanchnic nerve (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** Angina pectoris is characterized by visceral pain resulting from myocardial ischemia. The sensory (afferent) fibers from the heart follow the path of the sympathetic nerves in reverse. These pain impulses travel through the **cardiac plexuses** to the **thoracic splanchnic nerves**, eventually entering the spinal cord via the dorsal roots of the **T1 to T4/T5** segments. The phenomenon of **referred pain** to the left arm occurs because the T1 spinal segment also receives somatic sensory input from the medial aspect of the arm (via the medial cutaneous nerve of the arm). The brain misinterprets the visceral signals from the heart as coming from the somatic distribution of the same spinal segment. **2. Why Other Options are Incorrect:** * **Option A (Glossopharyngeal nerve):** This nerve carries sensory information from the carotid sinus (baroreceptors) and carotid body (chemoreceptors), not pain from the myocardium. * **Option B (Phrenic nerve):** While the phrenic nerve (C3-C5) carries sensory fibers from the pericardium and diaphragmatic pleura, it is not the primary mediator for ischemic myocardial pain. Irritation here typically refers pain to the shoulder (Kehr’s sign). * **Option C (Vagus nerve):** The vagus nerve carries parasympathetic fibers and visceral afferents involved in physiological reflexes (like the Bezold-Jarisch reflex), but it does **not** transmit the sensation of ischemic pain to the consciousness. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dermatomes:** Cardiac pain is typically referred to T1–T5 dermatomes (chest, inner arm, and jaw). * **Convergence-Projection Theory:** This is the underlying mechanism for referred pain, where visceral and somatic afferents converge on the same second-order neurons in the dorsal horn. * **Levine’s Sign:** A clenched fist held over the chest to describe ischemic pain, a classic clinical finding in angina.

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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