Sensory Systems Practice Questions

Q: For a sensory nerve fiber connected to a Pacinian corpuscle on the palmar surface of the right hand, where is the synaptic connection with the subsequent neuron in the corresponding sensory pathway located?

The right dorsal column nucleus. ### Explanation **1. Why Option A is Correct:** The Pacinian corpuscle is a mechanoreceptor that detects vibration and pressure. These sensations are carried by **large, myelinated A-beta fibers** via the **Dorsal Column-Medial Lemniscus (DCML) pathway**. A unique feature of the DCML pathway is that the **first-order neurons** (primary afferents) do not synapse in the spinal cord. Instead, their axons enter the dorsal column and ascend **ipsilaterally** (on the same side) to the medulla oblongata. For the upper limb (hand), these fibers travel in the **Fasciculus Cuneatus** and make their first synaptic connection with **second-order neurons** in the **Cuneate Nucleus** (a dorsal column nucleus) on the **right side**. **2. Why Other Options are Wrong:** * **Option B:** The DCML pathway remains ipsilateral until it reaches the medulla. Decussation (crossing over) only occurs *after* the first synapse in the dorsal column nuclei via internal arcuate fibers. * **Options C & D:** Synapsing in the dorsal horn of the spinal cord is characteristic of the **Anterolateral System** (Spinothalamic tract), which carries pain and temperature. Fibers for fine touch and vibration bypass the dorsal horn gray matter entirely. **3. High-Yield Clinical Pearls for NEET-PG:** * **Somatotopy:** In the dorsal columns, fibers from the lower limbs (Sacral/Lumbar) are medial (**Fasciculus Gracilis**), while fibers from the upper limbs (Thoracic/Cervical) are lateral (**Fasciculus Cuneatus**). Remember: *"Gracilis is near the Grass (legs)."* * **Lesion Localization:** A lesion of the dorsal columns in the spinal cord results in **ipsilateral** loss of vibration and proprioception below the level of the lesion. * **Tabes Dorsalis:** A classic NEET-PG topic where neurosyphilis causes selective destruction of the dorsal columns, leading to sensory ataxia and a positive Romberg's sign.

Q: Non-noxious stimuli are perceived as painful in which of the following conditions?

Allodynia. The correct answer is **Allodynia**. This phenomenon is defined as the perception of pain resulting from a stimulus that does not normally provoke pain (non-noxious stimuli) [1], [2]. **1. Why Allodynia is Correct:** Allodynia occurs due to central sensitization or structural remodeling of the nervous system [1], [2]. In this state, low-threshold mechanoreceptors (Aβ fibers), which normally signal light touch, begin to activate the pain pathways in the spinal cord. A classic clinical example is a patient with post-herpetic neuralgia or fibromyalgia who experiences intense pain from the light touch of clothing or a gentle breeze [2]. **2. Analysis of Incorrect Options:** * **Hyperalgesia:** This is an **increased response** to a stimulus that is *normally* painful [1]. The stimulus is noxious, but the perceived intensity of pain is exaggerated. * **Paresthesia:** This refers to abnormal sensations (such as "pins and needles," tingling, or numbness) that occur **spontaneously** in the absence of an external stimulus. It is usually not described as painful. * **Hyperpathia:** This is a clinical symptom wherein a painful syndrome is characterized by an abnormally exaggerated subjective response to a stimulus, especially a **repetitive** stimulus, often with an increased threshold (the stimulus must be stronger to be felt, but once felt, it is excruciating). **High-Yield Clinical Pearls for NEET-PG:** * **Dysesthesia:** An unpleasant, abnormal sensation (painful paresthesia), whether spontaneous or evoked. * **Mechanism:** Allodynia involves **Aβ fibers** (touch), whereas Hyperalgesia involves sensitized **C-fibers** and **Aδ fibers** (nociceptors). * **Clinical Context:** These terms are frequently tested in the context of **Neuropathic Pain** (e.g., Diabetic Neuropathy, Trigeminal Neuralgia).

Q: Smooth muscle is not pain-sensitive to which of the following stimuli?

Cutting. **Explanation:** The sensitivity of internal organs (viscera) differs significantly from that of the skin [1]. Smooth muscle, which forms the walls of hollow viscera, is innervated by **visceral afferent fibers** rather than somatic nerves. **1. Why "Cutting" is the Correct Answer:** Visceral pain receptors (nociceptors) are not sensitive to mechanical trauma like **cutting, burning, or crushing**. During surgical procedures, if a patient is under local anesthesia for the abdominal wall, the surgeon can cut or cauterize the intestine without the patient feeling pain. This is because these stimuli do not trigger the specific physiological receptors present in smooth muscle. **2. Why the Other Options are Incorrect:** The primary triggers for visceral pain are **tension and ischemia**. * **Distension & Stretching (Options B & C):** Rapid stretching of the smooth muscle wall (e.g., an obstructed ureter or a distended bladder) stimulates mechanoreceptors, leading to intense, often "colicky" pain [2]. * **Torsion (Option D):** Twisting of a viscus (e.g., volvulus or ovarian torsion) causes both extreme stretching of the mesentery and ischemia (loss of blood supply), both of which are potent triggers for visceral pain. **Clinical Pearls for NEET-PG:** * **Localization:** Visceral pain is typically **dull, aching, and poorly localized** because visceral afferent fibers are sparse and enter the spinal cord at multiple levels [1]. * **Referred Pain:** Because visceral afferents travel alongside sympathetic fibers to the same spinal cord segments as somatic nerves, pain is often "referred" to the overlying dermatome [2]. * **High-Yield Fact:** The only visceral structure highly sensitive to all stimuli (including cutting) is the **parietal layer** of serous membranes (parietal pleura/peritoneum) because it is innervated by somatic nerves [1].

Q: Prosopagnosia is associated with the lesion of which area?

Fusiform gyrus and lingual gyrus. **Explanation:** **Prosopagnosia**, also known as "face blindness," is a cognitive disorder characterized by the inability to recognize familiar faces, while other aspects of visual processing and intellectual functioning remain intact. **1. Why Option D is Correct:** The visual processing of objects follows two main pathways: the dorsal "where" stream and the **ventral "what" stream**. The ventral stream travels from the primary visual cortex to the inferior temporal lobe. A specialized area within this stream, located in the **fusiform gyrus** (specifically the Fusiform Face Area or FFA) and the adjacent **lingual gyrus**, is dedicated to facial recognition. Bilateral lesions (or sometimes right-sided lesions) in these areas result in the inability to link a face to a specific identity. **2. Why Other Options are Incorrect:** * **A. Cingulate cortex gyrus:** Part of the limbic system involved in emotion processing, learning, and memory, but not visual recognition. * **B. Posterior parietal cortex:** Part of the dorsal stream ("where" pathway). Lesions here lead to **spatial neglect**, optic ataxia, or Bálint’s syndrome, rather than recognition deficits. * **C. Hippocampus, amygdala, and entorhinal cortex:** These structures are central to memory consolidation and emotional responses (Klüver-Bucy syndrome). While they help remember *who* a person is, they do not perform the primary visual synthesis of a face. **Clinical Pearls for NEET-PG:** * **Ventral Stream:** Occipital to Temporal (Object/Face recognition). * **Dorsal Stream:** Occipital to Parietal (Spatial orientation/Motion). * **Achromatopsia:** Often co-occurs with prosopagnosia due to damage in the nearby V4 area of the lingual/fusiform gyri. * **Anton’s Syndrome:** Cortical blindness where the patient denies they are blind (occipital lobe lesion).

Q: Which of the following nerves serves as an afferent in the mediation of the corneal reflex?

Nasociliary nerve. **Explanation:** The **corneal reflex** (or blink reflex) is an involuntary blinking of the eyelids elicited by stimulation of the cornea. It is a polysynaptic reflex arc consisting of: 1. **Afferent limb:** Trigeminal nerve (Ophthalmic division - V1). 2. **Center:** Pons. 3. **Efferent limb:** Facial nerve (CN VII), which supplies the orbicularis oculi muscle to close the eyelid. **Why Nasociliary Nerve is Correct:** The **Nasociliary nerve** is a major branch of the Ophthalmic nerve (V1). It gives off the **long ciliary nerves**, which provide sensory innervation directly to the cornea. When the cornea is touched, impulses travel through the long ciliary nerves to the nasociliary nerve, then to the trigeminal ganglion, making it the specific afferent mediator. **Analysis of Incorrect Options:** * **A & B (Lacrimal and Frontal nerves):** While these are branches of the Ophthalmic nerve (V1), they supply the lacrimal gland, forehead, and upper eyelid skin, respectively. They do not provide sensory fibers to the cornea. * **D (Auriculotemporal nerve):** This is a branch of the Mandibular nerve (V3). it supplies the TMJ, external auditory meatus, and temple region, playing no role in the corneal reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Consensual Response:** Stimulating one eye causes bilateral blinking. A unilateral absent response helps localize lesions (e.g., CN V lesion = no response in either eye; CN VII lesion = no response on the affected side). * **Contact Lens Users:** May have a diminished or absent corneal reflex due to decreased corneal sensitivity. * **Reflex Center:** The sensory nucleus of the trigeminal nerve in the **Pons** communicates with the facial nerve nuclei via interneurons.

Q: Which of the following are rapidly adapting receptors?

Pacinian corpuscle. **Explanation:** Mechanoreceptors in the skin are classified based on their adaptation rates: **Rapidly Adapting (RA)** receptors respond only at the onset and offset of a stimulus (detecting change/velocity), while **Slowly Adapting (SA)** receptors provide a continuous discharge during a sustained stimulus (detecting pressure/intensity). **Why Pacinian Corpuscle is Correct:** The **Pacinian corpuscle** is the most rapidly adapting receptor in the body. It is located in the deeper dermis and is specialized for detecting **high-frequency vibrations** (60–300 Hz) and rapid changes in mechanical displacement. Its rapid adaptation is due to its onion-like encapsulated structure, which filters out steady pressure, allowing only transient energy to reach the nerve ending. **Analysis of Incorrect Options:** * **Merkel cells (Option A):** These are **Slowly Adapting Type I (SAI)** receptors. They are located in the basal layer of the epidermis and are responsible for detecting fine touch, edges, and texture. * **Free nerve endings (Option C):** These primarily function as nociceptors (pain) and thermoreceptors. While their adaptation varies, they are generally not classified under the classic rapid mechanoreceptor category. * **Ruffini endings (Option D):** These are **Slowly Adapting Type II (SAII)** receptors. They respond to skin stretch and are crucial for perceiving finger position and grip. **NEET-PG High-Yield Pearls:** * **Meissner’s Corpuscles:** Also rapidly adapting, but detect **low-frequency vibrations** (flutter) and are located in the dermal papillae of hairless skin. * **Mnemonic for Adaptation:** **P**acinian and **M**eissner are **P**rompt (**Rapid**); **M**erkel and **R**uffini are **S**teady (**Slow**). * **Receptive Fields:** Meissner and Merkel have small, well-defined borders (Type I); Pacinian and Ruffini have large, ill-defined borders (Type II).

Q: The bitter taste is mediated by the action of which of the following?

G protein. **Explanation:** The sensation of taste (gustation) is mediated by specific receptors on the tongue that utilize different signal transduction pathways. **Why G protein is correct:** Bitter, sweet, and umami tastes are mediated by **G-protein coupled receptors (GPCRs)**. Specifically, bitter taste involves the **T2R family** of receptors. When a bitter tastand binds to these receptors, it activates a specialized G-protein called **Gustducin**. This triggers the phospholipase C (PLC-β2) pathway, increasing intracellular calcium, which leads to the release of neurotransmitters (primarily ATP) via hemichannels to activate gustatory afferent nerves. **Why the other options are incorrect:** * **Guanyl cyclase:** This is involved in cGMP signaling (e.g., Nitric Oxide or ANP pathways) but does not play a primary role in taste transduction. * **Tyrosine kinase:** These receptors are typically associated with growth factors and insulin signaling, not sensory perception of taste. * **Epithelial Na+ channel (ENaC):** These channels are responsible for **Salty** taste. Sodium ions enter the taste cell directly through amiloride-sensitive ENaC channels, causing depolarization. **High-Yield Facts for NEET-PG:** 1. **Salty & Sour:** Use **Ion Channels** (Salty: ENaC; Sour: H+ sensitive channels/OTOP1). 2. **Sweet, Bitter, & Umami:** Use **GPCRs** (Sweet: T1R2+T1R3; Umami: T1R1+T1R3; Bitter: T2R family). 3. **Gustducin:** The specific G-protein associated with bitter, sweet, and umami. 4. **Innervation:** Anterior 2/3 of the tongue is supplied by the Chorda Tympani (CN VII); Posterior 1/3 by the Glossopharyngeal nerve (CN IX). 5. **AGUESIA:** Loss of taste sensation; **DYSGEUSIA:** Distorted taste.

Q: Tapping the patellar tendon with a reflex hammer produces a brief contraction of the knee extensors. What is the cause of this muscle contraction?

Muscle spindle activation. **Explanation:** The patellar reflex (knee-jerk) is a classic example of a **monosynaptic stretch reflex**. When the patellar tendon is tapped, it causes a rapid, transient stretch of the quadriceps muscle. 1. **Why the correct answer is right:** The primary sensory receptors for detecting muscle stretch are the **muscle spindles** (intrafusal fibers). When the quadriceps is stretched, the muscle spindles are activated, sending afferent impulses via **Type Ia nerve fibers** to the spinal cord. These fibers synapse directly (monosynaptically) with **alpha-motor neurons** in the anterior horn, which then trigger the contraction of the extrafusal muscle fibers of the quadriceps. 2. **Why the incorrect options are wrong:** * **Elastic rebound:** This is a physical property of tissue, not a neural reflex. The force of contraction in a reflex is far greater than any simple elastic recoil. * **Golgi tendon organ (GTO) response:** GTOs (Type Ib fibers) respond to **muscle tension**, not stretch. Their activation typically causes *autogenic inhibition* (relaxation) to prevent tendon avulsion, which is the opposite of the observed contraction. * **Muscle spindle unloading:** This occurs during muscle **contraction** (shortening), which actually decreases the firing rate of the spindle. It is the *loading* (stretching) that triggers the reflex. **Clinical Pearls for NEET-PG:** * **Reflex Arc Components:** Receptor (Spindle) → Afferent (Ia) → Center (L3-L4 spinal cord) → Efferent (Alpha-motor neuron) → Effector (Quadriceps). * **Gamma-motor neurons:** These regulate the sensitivity of the muscle spindle. High gamma-discharge leads to hyperreflexia. * **Jendrassik Maneuver:** A reinforcement technique used to accentuate deep tendon reflexes by increasing the excitatory drive to the spinal motor pool.

Question 1

For a sensory nerve fiber connected to a Pacinian corpuscle on the palmar surface of the right hand, where is the synaptic connection with the subsequent neuron in the corresponding sensory pathway located?

Accepted Answer

The right dorsal column nucleus

Answer

The left dorsal column nucleus

Answer

The dorsal horn of the right side of the spinal cord

Answer

The dorsal horn of the left side of the spinal cord

Question 2

Non-noxious stimuli are perceived as painful in which of the following conditions?

Accepted Answer

Allodynia

Answer

Hyperalgesia

Answer

Paresthesia

Answer

Hyperpathia

Question 3

Smooth muscle is not pain-sensitive to which of the following stimuli?

Accepted Answer

Cutting

Answer

Distension

Answer

Stretching

Answer

Torsion

Question 4

Prosopagnosia is associated with the lesion of which area?

Accepted Answer

Fusiform gyrus and lingual gyrus

Answer

Cingulate cortex gyrus

Answer

Posterior parietal cortex

Answer

Hippocampus, amygdala and entorhinal cortex

Question 5

Which of the following nerves serves as an afferent in the mediation of the corneal reflex?

Accepted Answer

Nasociliary nerve

Answer

Lacrimal nerve

Answer

Frontal nerve

Answer

Auriculotemporal nerve

Question 6

The Weber-Fechner Law states that the magnitude of stimulus strength perceived is approximately proportional to the logarithm of the stimulus intensity?

Accepted Answer

Magnitude of stimulus strength perceived is approximately proportional to the log of the intensity of stimulus strength.

Answer

Magnitude of stimulus strength perceived is directly proportional to the intensity of stimulus strength.

Answer

Threshold of receptor is directly proportional to stimulus strength.

Answer

Threshold of receptor is inversely proportional to stimulus strength.

Question 7

Which of the following are rapidly adapting receptors?

Accepted Answer

Pacinian corpuscle

Answer

Merkel cells

Answer

Free nerve endings

Answer

Ruffini endings

Question 8

The bitter taste is mediated by the action of which of the following?

Accepted Answer

G protein

Answer

Guanyl cyclase

Answer

Tyrosine kinase

Answer

Epithelial Na+ channel

Question 9

A lesion in the posterior column of the spinal cord will affect which of the following modalities?

Accepted Answer

Temperature sense

Answer

Vibration sense

Answer

Fine touch

Answer

Crude touch sensation

Question 10

Tapping the patellar tendon with a reflex hammer produces a brief contraction of the knee extensors. What is the cause of this muscle contraction?

Accepted Answer

Muscle spindle activation

Answer

Elastic rebound of muscle connective tissue

Answer

Golgi tendon organ response

Answer

Muscle spindle unloading

Sensory Systems — MCQs

Sensory Systems — MCQs

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