Sensory Systems — MCQs
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A physiology experiment is conducted in which a glass microelectrode is inserted into a Pacinian corpuscle to record receptor potentials during different levels of stimulation. Increasing stimulus strength from 10 percent of maximum to 30 percent of maximum causes a 40 percent increase in the amplitude of the receptor potential. Increasing the stimulus potential from 70 percent of maximum to 90 percent of maximum is most likely to cause what increase in the amplitude of the receptor potential (in percent)?
A 39-year-old neurosurgeon picks up a scalpel, which activates numerous sensory receptors in her hand. An increase in which of the following best describes the basis for transduction of the sensory stimuli into nerve impulses?
Which sensory receptors are characterized by delayed recovery and least coverage?
Which nerve lesion could produce the condition where stimulation of the right cornea results in blinking of the left eye but not the right eye?
Which of the following are chemoreceptors?
Which of the following is NOT an action of Substrate P?
Substance P is increased in response to pain in the periphery, mediated by which of the following?
Golgi tendon organs are concerned with what?
All of the following are cellular components of taste buds except?
Ascending pain pathway is inhibited in the dorsal midbrain by which neurotransmitter?
Sensory Systems Indian Medical PG Practice Questions and MCQs
Question 121: A physiology experiment is conducted in which a glass microelectrode is inserted into a Pacinian corpuscle to record receptor potentials during different levels of stimulation. Increasing stimulus strength from 10 percent of maximum to 30 percent of maximum causes a 40 percent increase in the amplitude of the receptor potential. Increasing the stimulus potential from 70 percent of maximum to 90 percent of maximum is most likely to cause what increase in the amplitude of the receptor potential (in percent)?
- A. 10 (Correct Answer)
- B. 40
- C. 60
- D. 80
Explanation: ### Explanation The correct answer is **A. 10**. **1. Understanding the Concept: The Logarithmic Relationship** The relationship between stimulus intensity and receptor potential amplitude is **not linear**; it is **logarithmic** (Weber-Fechner Law). * At low intensities, a small increase in stimulus strength leads to a significant increase in the receptor potential. * As the stimulus intensity approaches its maximum, the receptor becomes "saturated." At this high-intensity range, further increases in stimulus strength result in progressively smaller increments in the receptor potential amplitude. In this scenario, moving from 10% to 30% (low range) yielded a 40% increase. However, moving from 70% to 90% (high range) occurs near the saturation point. Therefore, the resulting increase must be significantly **less than 40%**. Option A (10%) is the only value that reflects this physiological compression. **2. Analysis of Incorrect Options** * **B (40%):** This assumes a linear relationship, which is incorrect for sensory receptors. * **C & D (60% and 80%):** These assume an exponential relationship where sensitivity increases at higher intensities. In reality, receptors lose sensitivity at higher ranges to allow the detection of a vast range of stimulus intensities without over-stimulating the sensory nerve. **3. NEET-PG High-Yield Pearls** * **Pacinian Corpuscles:** These are rapidly adapting (phasic) mechanoreceptors sensitive to high-frequency vibration (250–350 Hz). * **Receptor Potential (Generator Potential):** It is a **graded potential**, not an "all-or-none" phenomenon. It does not have a refractory period. * **Mechanism:** Mechanical compression opens **mechanically-gated Na+ channels**, causing depolarization. If this potential reaches the threshold at the first node of Ranvier, an action potential is fired. * **Logarithmic Coding:** This allows the sensory system to have a wide "dynamic range," enabling us to distinguish between very weak stimuli while still being able to perceive very intense stimuli without the receptor saturating too quickly.
Question 122: A 39-year-old neurosurgeon picks up a scalpel, which activates numerous sensory receptors in her hand. An increase in which of the following best describes the basis for transduction of the sensory stimuli into nerve impulses?
- A. Activation of G protein
- B. Decreased ion permeability
- C. Decreased transmitter release
- D. Increased ion permeability (Correct Answer)
Explanation: ### Explanation **1. Why "Increased ion permeability" is correct:** The fundamental process of sensory transduction involves converting a physical stimulus (mechanical, thermal, or chemical) into an electrical signal (receptor potential). When the neurosurgeon touches the scalpel, mechanical deformation of the mechanoreceptors (like Meissner’s or Pacinian corpuscles) occurs. This physical stretch opens **mechanically-gated ion channels** in the receptor membrane. This leads to an **increased ion permeability**, primarily to sodium ($Na^+$) or calcium ($Ca^{2+}$) ions. The resulting influx of positive ions causes **depolarization** (the receptor potential). If this potential reaches a specific threshold, it triggers action potentials in the sensory nerve fiber. **2. Why the other options are incorrect:** * **A. Activation of G protein:** While some receptors (like olfactory or certain taste receptors) use G-protein coupled pathways, the primary mechanoreceptors in the hand involved in tactile discrimination rely on direct mechanical gating of ion channels for rapid signaling. * **B. Decreased ion permeability:** Transduction requires a flow of current to change the membrane potential. Decreasing permeability would typically lead to hyperpolarization or stabilization, preventing the generation of a nerve impulse. * **C. Decreased transmitter release:** This occurs in specific inhibitory pathways or in photoreceptors (where light causes a decrease in glutamate release). In cutaneous mechanoreceptors, the stimulus leads to depolarization and *increased* signaling. **3. High-Yield Clinical Pearls for NEET-PG:** * **Receptor Potential Characteristics:** It is a **graded potential** (not all-or-none), non-propagated, and its amplitude is proportional to the stimulus intensity. * **Rapidly Adapting Receptors:** Meissner’s (fine touch) and Pacinian (vibration) corpuscles. These are crucial for tasks requiring manual dexterity, like surgery. * **Slowly Adapting Receptors:** Merkel discs (pressure) and Ruffini endings (stretch). * **Law of Projection:** No matter where a sensory pathway is stimulated along its course to the cortex, the conscious sensation produced is referred to the location of the receptor (e.g., phantom limb pain).
Question 123: Which sensory receptors are characterized by delayed recovery and least coverage?
- A. Pacinian corpuscle
- B. Ruffini endings
- C. Meissner corpuscle
- D. Merkel's disc (Correct Answer)
Explanation: **Explanation:** The question tests the classification of mechanoreceptors based on their adaptation rates and receptive field sizes. **Why Merkel’s Disc is correct:** Merkel’s discs are **Slowly Adapting type 1 (SA1)** receptors. * **Delayed Recovery (Slow Adaptation):** Unlike rapidly adapting receptors that fire only at the start and end of a stimulus, Merkel’s discs continue to fire throughout the duration of a sustained stimulus. This "delayed recovery" to the baseline state allows for the perception of continuous pressure. * **Least Coverage (Small Receptive Field):** They have very small, highly localized receptive fields with well-defined borders. This allows for high **spatial resolution**, making them essential for tasks like reading Braille or feeling the texture of an object. **Why other options are incorrect:** * **Pacinian Corpuscles:** These are **Rapidly Adapting (RA2)** receptors with the **largest coverage** (large receptive fields). They detect high-frequency vibrations. * **Meissner Corpuscles:** These are **Rapidly Adapting (RA1)** receptors. While they have small receptive fields (low coverage), they adapt quickly to stimuli, unlike the "delayed recovery" seen in Merkel's. They detect point discrimination and low-frequency vibration (flutter). * **Ruffini Endings:** These are **Slowly Adapting (SA2)** receptors. While they show delayed recovery, they have **large receptive fields** (wide coverage) and detect skin stretch and joint rotation. **High-Yield Facts for NEET-PG:** * **Merkel’s Discs:** Located in the basal layer of the epidermis; highest density in fingertips; neurotransmitter released is **Serotonin**. * **Two-Point Discrimination:** Primarily mediated by Meissner’s and Merkel’s due to their small receptive fields. * **Deep Pressure/Vibration:** Mediated by Pacinian corpuscles (the most sensitive and fastest adapting).
Question 124: Which nerve lesion could produce the condition where stimulation of the right cornea results in blinking of the left eye but not the right eye?
- A. Left trigeminal nerve
- B. Left facial nerve
- C. Right trigeminal nerve
- D. Right facial nerve (Correct Answer)
Explanation: ### Explanation To solve this question, you must understand the **Corneal Reflex arc**: * **Afferent (Sensory):** Trigeminal nerve (CN V₁ - Ophthalmic division). * **Center:** Pons. * **Efferent (Motor):** Facial nerve (CN VII) – specifically the branch to the *orbicularis oculi* muscle. **Why Option D is Correct:** In this scenario, stimulating the **right cornea** successfully triggers a response in the **left eye**. This proves that the **right afferent (CN V)** and the **left efferent (CN VII)** are both intact. However, the **right eye** fails to blink. Since the sensory input was received and processed, the failure must lie in the motor output to the right side. Therefore, the **Right Facial Nerve (CN VII)** is lesioned. **Analysis of Incorrect Options:** * **A. Left Trigeminal Nerve:** A lesion here would prevent the reflex only when the *left* cornea is stimulated. Stimulating the right cornea would still result in a bilateral blink. * **B. Left Facial Nerve:** If this were lesioned, stimulating the right cornea would result in a blink of the *right* eye only (the opposite of the clinical presentation described). * **C. Right Trigeminal Nerve:** A lesion here would result in **no blink in either eye** when the right cornea is touched, as the sensory limb of the arc is broken. **High-Yield Clinical Pearls for NEET-PG:** * **Consensual Response:** Like the pupillary light reflex, the corneal reflex is bilateral. A stimulus in one eye should cause both eyes to blink. * **Bell’s Palsy:** Patients with CN VII palsy lose the efferent limb; they can feel the touch (CN V intact) but cannot perform the motor action of closing the eye. * **Acoustic Neuroma:** Often presents with a depressed corneal reflex early on due to pressure on the trigeminal nerve in the cerebellopontine angle.
Question 125: Which of the following are chemoreceptors?
- A. Rods
- B. Cones
- C. Taste buds (Correct Answer)
- D. Muscle spindle
Explanation: **Explanation:** Receptors are specialized structures that transduce specific environmental stimuli into electrical signals. They are classified based on the type of stimulus they detect. **Why Taste Buds are Correct:** **Taste buds** are classic examples of **chemoreceptors**. They detect chemical substances (tastants) dissolved in saliva. When these chemicals bind to specific receptors on the microvilli of gustatory cells, they trigger a receptor potential. Other examples of chemoreceptors include olfactory receptors (smell), carotid/aortic bodies (detecting $PaO_2$, $PaCO_2$, and pH), and hypothalamic glucose receptors. **Why Other Options are Incorrect:** * **Rods and Cones (Options A & B):** These are **photoreceptors** located in the retina. They respond to electromagnetic radiation (light) rather than chemical concentrations. Rods are responsible for scotopic (dim light) vision, while cones mediate photopic (bright light) and color vision. * **Muscle Spindles (Option D):** These are **mechanoreceptors** (specifically proprioceptors). They are sensitive to changes in muscle length and the rate of stretch, playing a vital role in the stretch reflex and posture maintenance. **High-Yield NEET-PG Pearls:** 1. **Classification by Stimulus:** * **Mechanoreceptors:** Pacinian corpuscles (pressure), Meissner’s (touch), Baroreceptors (blood pressure). * **Nociceptors:** Pain (free nerve endings). * **Thermoreceptors:** Cold and Warm receptors. 2. **Mnemonic for Taste:** The **Vagus (X)**, **Glossopharyngeal (IX)**, and **Facial (VII)** nerves carry taste sensations to the **Nucleus Tractus Solitarius (NTS)**. 3. **Receptor Potential:** Unlike action potentials, receptor potentials are **graded**, non-propagated, and do not follow the "all-or-none" law.
Question 126: Which of the following is NOT an action of Substrate P?
- A. Vasoconstriction (Correct Answer)
- B. Pain transmission
- C. Axon reflex
- D. Peristalsis
Explanation: **Explanation:** **Substance P** is an 11-amino acid neuropeptide belonging to the tachykinin family. It acts primarily through **NK1 receptors** and is a potent mediator of neurogenic inflammation. **Why Option A is correct:** Substance P is a potent **vasodilator**, not a vasoconstrictor. It induces vasodilation by acting directly on vascular smooth muscle and by stimulating the release of nitric oxide (NO) from the endothelium. This vasodilation contributes to the "flare" seen in the triple response of Lewis. **Why the other options are incorrect:** * **B. Pain transmission:** Substance P is the primary neurotransmitter released by **C-fibers** (slow pain) in the dorsal horn of the spinal cord. It modulates the transmission of nociceptive signals to the brain. * **C. Axon reflex:** When a peripheral nerve is stimulated, impulses travel orthodromically to the CNS and antidromically to other branches of the same neuron. This antidromic release of Substance P at the periphery causes local vasodilation and edema (neurogenic inflammation). * **D. Peristalsis:** In the enteric nervous system, Substance P acts as an excitatory neurotransmitter. It stimulates the contraction of intestinal smooth muscle, thereby promoting peristaltic activity. **High-Yield Clinical Pearls for NEET-PG:** * **Triple Response of Lewis:** Substance P is responsible for the **flare** (vasodilation) and **wheal** (increased capillary permeability/edema). * **Capsaicin:** Found in chili peppers, it causes the depletion of Substance P from sensory nerve endings, which is why it is used topically for post-herpetic neuralgia and osteoarthritis pain. * **Aprepitant:** A clinical antagonist of the NK1 receptor used as an antiemetic in chemotherapy-induced nausea and vomiting.
Question 127: Substance P is increased in response to pain in the periphery, mediated by which of the following?
- A. Mast cells
- B. Endothelium
- C. Plasma
- D. Nerve terminals (Correct Answer)
Explanation: **Explanation:** The correct answer is **Nerve terminals (Option D)**. This phenomenon is rooted in the concept of **Neurogenic Inflammation**. When peripheral nociceptors (specifically polymodal C-fibers) are stimulated by a painful stimulus, action potentials travel orthodromically toward the spinal cord. However, some impulses also travel **antidromically** (backward) along collateral branches of the same sensory nerve. Upon reaching the peripheral nerve terminals, these impulses trigger the release of neuropeptides, primarily **Substance P** and **Calcitonin Gene-Related Peptide (CGRP)**. * **Substance P** acts on local blood vessels to increase permeability, leading to **edema** (plasma extravasation). * **CGRP** causes potent **vasodilation**, resulting in the "flare" response of the Triple Response of Lewis. **Why other options are incorrect:** * **Mast cells (A):** While Substance P triggers mast cells to release histamine (furthering the inflammatory cycle), the mast cells themselves do not synthesize or release Substance P. * **Endothelium (B):** Endothelial cells are the *target* of Substance P (leading to gap junction formation and leakage), but they are not the source. * **Plasma (C):** Substance P is a neuropeptide synthesized in the dorsal root ganglion and transported to terminals; it is not a plasma-derived mediator like bradykinin or complement proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Triple Response of Lewis:** Consists of Red spot (capillary dilation), Flare (arteriolar dilation via axon reflex), and Wheal (exudation/edema). * **Substance P Receptor:** It acts via the **NK-1 (Neurokinin-1) receptor**. * **Axon Reflex:** This is a unique reflex that does not involve the CNS or a synapse; it occurs entirely within the peripheral branching of a single sensory neuron.
Question 128: Golgi tendon organs are concerned with what?
- A. Static length
- B. Dynamic length
- C. Muscle tension (Correct Answer)
- D. Muscle length
Explanation: **Explanation:** The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located in the tendons of skeletal muscles, arranged in **series** with the extrafusal muscle fibers. **Why Muscle Tension is Correct:** The GTO is primarily sensitive to the **force or tension** generated during muscle contraction. When a muscle contracts, it pulls on the tendon, compressing the nerve endings (Ib afferent fibers) within the GTO. This triggers a reflex arc (the inverse stretch reflex) that inhibits the alpha motor neuron of the same muscle, causing it to relax. This mechanism serves as a protective function to prevent tendon avulsion or muscle tearing under excessive loads. **Why Other Options are Incorrect:** * **Options A, B, and D (Muscle Length):** These are the primary functions of the **Muscle Spindle**. Muscle spindles are arranged in **parallel** with muscle fibers and detect changes in muscle length (static) and the rate of change in length (dynamic). They trigger the stretch reflex (e.g., knee jerk), which causes muscle contraction, whereas GTOs cause relaxation. **High-Yield NEET-PG Pearls:** * **Afferent Fiber Type:** GTOs use **Type Ib** sensory fibers; Muscle Spindles use **Type Ia** (primary) and **Type II** (secondary) fibers. * **Arrangement:** GTO = **Series** (measures tension); Muscle Spindle = **Parallel** (measures length). * **Reflex Type:** GTO is responsible for the **Inverse Stretch Reflex** (Autogenic Inhibition), which is a disynaptic inhibitory reflex. * **Clasp-Knife Response:** In upper motor neuron lesions, the sudden "give-way" of a spastic muscle is partly attributed to the activation of GTOs.
Question 129: All of the following are cellular components of taste buds except?
- A. Zymogenic Cells (Correct Answer)
- B. Basal Cell
- C. Sustentacular Cells
- D. Gustatory Cells
Explanation: **Explanation:** Taste buds are specialized ovoid sensory organs located primarily within the papillae of the tongue. They are composed of approximately 50–150 modified epithelial cells categorized into four distinct types based on their morphology and function. **Why Zymogenic Cells is the correct answer:** **Zymogenic cells** (also known as Chief cells) are found in the **stomach lining** (gastric glands), where they secrete pepsinogen. They are not a component of the sensory system or taste buds. **Analysis of other options:** * **Gustatory Cells (Type III):** These are the actual sensory receptors. They contain synaptic vesicles and release neurotransmitters (like serotonin) to stimulate the gustatory nerves. * **Sustentacular Cells (Type I & II):** These are support cells. Type I (Dark cells) act like glia, while Type II (Light cells) contain G-protein coupled receptors for sweet, bitter, and umami tastes. * **Basal Cells (Type IV):** These are undifferentiated stem cells located at the base of the taste bud. They continuously proliferate and differentiate to replace older taste cells every 10–14 days. **High-Yield Facts for NEET-PG:** * **Turnover:** Taste bud cells have a high turnover rate, being replaced every **10 to 14 days**. * **Innervation:** The anterior 2/3 of the tongue is supplied by the **Chorda Tympani** (CN VII); the posterior 1/3 by the **Glossopharyngeal nerve** (CN IX); and the epiglottis/pharynx by the **Vagus nerve** (CN X). * **Location:** Taste buds are most numerous in **circumvallate** and **foliate** papillae. Note that **filiform papillae** do NOT contain taste buds (they are purely mechanical).
Question 130: Ascending pain pathway is inhibited in the dorsal midbrain by which neurotransmitter?
- A. 5-HT (serotonin) (Correct Answer)
- B. Noradrenaline
- C. Substance P
- D. Glutamate
Explanation: ### Explanation The correct answer is **5-HT (serotonin)**. **Mechanism of Pain Inhibition** The body possesses an endogenous analgesia system that modulates pain perception. The primary center for this is the **Periaqueductal Gray (PAG)** in the dorsal midbrain. When stimulated, neurons from the PAG project to the **Nucleus Raphe Magnus (NRM)** in the medulla. The neurons originating from the NRM are **serotonergic (5-HT)**; they descend to the dorsal horn of the spinal cord to inhibit incoming pain signals (the "Gate Control" mechanism) by stimulating inhibitory enkephalin-releasing interneurons. **Analysis of Options:** * **A. 5-HT (Serotonin):** Correct. It is the primary neurotransmitter released by the descending fibers from the Raphe nuclei to inhibit pain transmission in the spinal cord. * **B. Noradrenaline:** While noradrenaline is involved in the descending inhibitory pathway (originating from the **Locus Coeruleus**), the classical pathway described from the dorsal midbrain/Raphe system is primarily serotonergic. * **C. Substance P:** This is a **pro-nociceptive** neurotransmitter. It is released by primary afferent fibers (A-delta and C fibers) in the dorsal horn to *transmit* pain signals, not inhibit them. * **D. Glutamate:** This is the major excitatory neurotransmitter in the CNS. It is involved in the fast transmission of pain signals at the first synapse in the dorsal horn. **High-Yield Clinical Pearls for NEET-PG:** * **PAG** is the "Command Center" for descending pain suppression. * **Enkephalins and Endorphins** act at the spinal level to cause pre- and post-synaptic inhibition of Substance P release. * **Tricyclic Antidepressants (TCAs)** are used in chronic pain management because they increase the availability of Serotonin and Noradrenaline in these inhibitory pathways.
Practice by Chapter
General Sensory Physiology
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Somatosensation
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Pain Physiology
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Taste and Smell
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